Construction Advice

[chiang mai]

I need some technical advice regarding the construction of a retaining wall, here's the situation:

I'm helping the wife's family build a small home in a village and we have already built up the low lying land to the same height as the road. The next step in this process is to build a retaining wall to hold the raised building plot in place. My common sense and limited knowledge on this subject suggests strongly that the wall should comprise a concrete footing of about 70 x 60 containing reinforcing mesh with a wall of hollow 15mm concrete blocks backfilled with re-bar and concrete. I've used this design in a past life and it produces a strong wall.

Thai family "experts" and neighbors want to use the traditional Thai walling approach using 2 metre concrete posts (15 x 15mm) containing 9mm steel. These would be spaced every three metres and connected by two horizontal cement cross beams and in filled with three rows of concrete block. The posts would be buried in the ground 0.50 metres deep, encased in cement and a one metre support post added at the base at a 45 degree angle on the in filled side - this angled support beam seems to prevent pull rather than push.

The Thai approach, for lack of a better description, costs about one third more than my design but that is not a major factor. My concern is that the Thai design does not appear to be a retaining wall of any significance although it might useful be deployed as a barrier wall to keep sheep out!

The wall will run for about 70 metres and the raised side of the land will be about one metre higher than the other side. In the longer term the wall may be built higher so as to provide some privacy and security and I would guess the total finished height might be around 2.5 metres above ground.

My question quite simply is, will the Thai design work in any reasonable sort of way? I've tried explaining (unsuccessfully) to them the physics of downward and outward pressure but they are convinced their proposal is suitable. Any and all experienced and qualified comments will be gratefully received. Thanks.

[Diablo Bob]

I need some technical advice regarding the construction of a retaining wall, here's the situation:

I'm helping the wife's family build a small home in a village and we have already built up the low lying land to the same height as the road. The next step in this process is to build a retaining wall to hold the raised building plot in place. My common sense and limited knowledge on this subject suggests strongly that the wall should comprise a concrete footing of about 70 x 60 containing reinforcing mesh with a wall of hollow 15mm concrete blocks backfilled with re-bar and concrete. I've used this design in a past life and it produces a strong wall.

Thai family "experts" and neighbors want to use the traditional Thai walling approach using 2 metre concrete posts (15 x 15mm) containing 9mm steel. These would be spaced every three metres and connected by two horizontal cement cross beams and in filled with three rows of concrete block. The posts would be buried in the ground 0.50 metres deep, encased in cement and a one metre support post added at the base at a 45 degree angle on the in filled side - this angled support beam seems to prevent pull rather than push.

The Thai approach, for lack of a better description, costs about one third more than my design but that is not a major factor. My concern is that the Thai design does not appear to be a retaining wall of any significance although it might useful be deployed as a barrier wall to keep sheep out!

The wall will run for about 70 metres and the raised side of the land will be about one metre higher than the other side. In the longer term the wall may be built higher so as to provide some privacy and security and I would guess the total finished height might be around 2.5 metres above ground.

My question quite simply is, will the Thai design work in any reasonable sort of way? I've tried explaining (unsuccessfully) to them the physics of downward and outward pressure but they are convinced their proposal is suitable. Any and all experienced and qualified comments will be gratefully received. Thanks.

The Thai system MIGHT work! Comment below.....

1) I assume you meant centimeters and not milimeters in your dimensions. Wall height exceeds post height in the Thai design?

2) Factors that have to be considered are the height of the wall, in neither design do you mention the use of horizontal reinforcement which will become critical due to the height of the wall, vertical steel keeps the wall on the foundation but does little to keep the wall from overturning. Vertical steel does prevent the wall from Kicking out, or sliding off the foundation.

3) Using the Thai design, there is no simple way to physically attach horizontal steel to the column. The bracing while a good idea, will provide absolutely no support to the wall, just the column. Unless you provide attachment of the wall steel to the columns, you will be seeing vertical deformation within a few years and maybe even failure.

4) When designing a retaining wall the main consideration is the horizontal force at the bottom of the wall. This is a factor based upon the depth of fill. The highest horizontal loads are at the bottom of the wall. As compacted soils become saturated (Rain!) they increase in weight thus increasing this horizontal load. Ways to reduce this horizontal load are to provide drainage at the bottom of the wall. The problem with most of the native soils are that they are clays and silts which retain significant amounts of water (gravel - 3-6%, Clay 25%+). Consider sloping the building pad from the top of the foundation and backfill this zone with gravel and also provide PVC drainage sleeves in the wall.

5) Even in your design, I would provide for coulmns. This provides for intermediate support for the wall. Not having the columns increases the potential for failure at the corners or a wall that bows in the center.

6) Lastly, regardless of what design you opt for, I would use 12mm rebar. The 9mm has a tendency to "stretch" when under load. Oh, and forget mesh in the foundation, use a 12mm matt. You are not building a house slab, you are building a foundation.

There is an old saying in structural engineering, "A building is only as good as it's foundation!"

[Rimmer]

This wall did not have sow poon's just footings. When it rained the raised land filled with water and over it went.

Use Sow Poon's half a meter in the ground and relieve the water pressure on the high side with drains.

[Rimmer]

Another picture of the wall that fell down, (not mine I hasten to add) If I remember correctly the steel was about knitting needle size and just bent.

One view of my wall before I put a drain in showing the build up water at the base.

One view of the drain at the base to relieve the pressure.

One view showing my 1 mtr land fill from the low side. (outside)

In retrospect I should have done as DB suggests and used a plastic pipe with holes in it laid in a bed of granite chips for the drain but the one in the picture is working up to date.

Make sure you have good size cast feet on the sow poons not just the small ready made ones that you can buy

Listen to what DB is saying he obviously knows about construction, I only know what worked for me.

[Rimmer]

The wall that came down.

Other view.

[chiang mai]

Many many thanks to Diablo Bob and Rimmer for some outstanding professional advice, I am extremely grateful.

[tutsiwarrior]

drainage, drainage, drainage!!!...don't put up anything unless you design for a rainwater load that is considerable. we put up a roof terrace onna foundation that was designed for a roof only...now we gotta steel raft, concrete slabs and finishing concrete. I told the builder, make sure that the mother drains after a rain...and it looks good...gotta wait for the rain to come next year to test it.

people don't unnerstand that a cubic meter of water weighs 1000 kgs (2200lbs)...what the metric system is based upon...if your structure cannot handle the load then LOOK OUT...

from tutsiwarrior...not a structural engineer but a genius nonetheless...

[chiang mai]

I need some technical advice regarding the construction of a retaining wall, here's the situation:

I'm helping the wife's family build a small home in a village and we have already built up the low lying land to the same height as the road. The next step in this process is to build a retaining wall to hold the raised building plot in place. My common sense and limited knowledge on this subject suggests strongly that the wall should comprise a concrete footing of about 70 x 60 containing reinforcing mesh with a wall of hollow 15mm concrete blocks backfilled with re-bar and concrete. I've used this design in a past life and it produces a strong wall.

Thai family "experts" and neighbors want to use the traditional Thai walling approach using 2 metre concrete posts (15 x 15mm) containing 9mm steel. These would be spaced every three metres and connected by two horizontal cement cross beams and in filled with three rows of concrete block. The posts would be buried in the ground 0.50 metres deep, encased in cement and a one metre support post added at the base at a 45 degree angle on the in filled side - this angled support beam seems to prevent pull rather than push.

The Thai approach, for lack of a better description, costs about one third more than my design but that is not a major factor. My concern is that the Thai design does not appear to be a retaining wall of any significance although it might useful be deployed as a barrier wall to keep sheep out!

The wall will run for about 70 metres and the raised side of the land will be about one metre higher than the other side. In the longer term the wall may be built higher so as to provide some privacy and security and I would guess the total finished height might be around 2.5 metres above ground.

My question quite simply is, will the Thai design work in any reasonable sort of way? I've tried explaining (unsuccessfully) to them the physics of downward and outward pressure but they are convinced their proposal is suitable. Any and all experienced and qualified comments will be gratefully received. Thanks.

The Thai system MIGHT work! Comment below.....

1) I assume you meant centimeters and not milimeters in your dimensions. Wall height exceeds post height in the Thai design?

2) Factors that have to be considered are the height of the wall, in neither design do you mention the use of horizontal reinforcement which will become critical due to the height of the wall, vertical steel keeps the wall on the foundation but does little to keep the wall from overturning. Vertical steel does prevent the wall from Kicking out, or sliding off the foundation.

3) Using the Thai design, there is no simple way to physically attach horizontal steel to the column. The bracing while a good idea, will provide absolutely no support to the wall, just the column. Unless you provide attachment of the wall steel to the columns, you will be seeing vertical deformation within a few years and maybe even failure.

4) When designing a retaining wall the main consideration is the horizontal force at the bottom of the wall. This is a factor based upon the depth of fill. The highest horizontal loads are at the bottom of the wall. As compacted soils become saturated (Rain!) they increase in weight thus increasing this horizontal load. Ways to reduce this horizontal load are to provide drainage at the bottom of the wall. The problem with most of the native soils are that they are clays and silts which retain significant amounts of water (gravel - 3-6%, Clay 25%+). Consider sloping the building pad from the top of the foundation and backfill this zone with gravel and also provide PVC drainage sleeves in the wall.

5) Even in your design, I would provide for coulmns. This provides for intermediate support for the wall. Not having the columns increases the potential for failure at the corners or a wall that bows in the center.

6) Lastly, regardless of what design you opt for, I would use 12mm rebar. The 9mm has a tendency to "stretch" when under load. Oh, and forget mesh in the foundation, use a 12mm matt. You are not building a house slab, you are building a foundation.

There is an old saying in structural engineering, "A building is only as good as it's foundation!"

First and foremost, thank you for taking the time to give such a superb description of the design problems. If I may I would like to feedback the revised design which will hopefully take on board all your comments and to add clarification where necessary - please let me know if I have got this wrong or have misunderstood anything:

1) Yes I did mean to say cm's and not mm's when referring to the dimensions of blocks and posts.

2) using the Thai design the concrete posts and cross beams will now comprise four strand 12 mm steel. The posts will be buried in the ground to a depth of 0.50 metres and will be secured with concrete. The posts will initially rise 120 cm above ground, just high enough to act as a retaining wall. The steel at the top of the posts will be left exposed in the short term which will allow them to be extended at a later date to a maximum overall height above ground of 2.5 metres on the unfilled side of the land.

3) the horizontal cross beams will measure 30 cm by 3 metres and will contain four strands of 12 mm steel. The steel at the ends of these beams will be extended and exposed forming hooks which will then be capable of being inserted into coresponding holes on the upright posts. The lowest cross beam will sit at ground level and the three courses of 12cm block will sit on top of it. The second and final cross beam will be fixed in the same way as the first and will sit on top of the block. This will give a finished height of 120 cm, sufficient to retain the raised ground. At some point in the future the height of the wall may be raised by adding additional courses of block and post extensions, up to a maximum overall height of 2.5 metres.

4) drainage tubes will be placed through the lowest level of block every two metres and gravel poured on the inside of the wall to faciltate drainage.

5) ideally one would place vertical steel reinforcing bars through all concrete blocks in addition to filling the blocks with concrete. (I haven;t quite figured out how to do this yet since the only 15cm blocks I have been able find are only open on one side - perhaps the answer here is to use two 7.5 cm blocks side by side instead of one single 15 cm block since the smaller ones are open at top and bottom).

I think that's everything but please let me know if I missed anything.

Thanks again

[gburns57au]

you may also want to consider using a waterproofing membrane on the in fill side to prevent moisture and mould seeping through.

[Diablo Bob]

First and foremost, thank you for taking the time to give such a superb description of the design problems. If I may I would like to feedback the revised design which will hopefully take on board all your comments and to add clarification where necessary - please let me know if I have got this wrong or have misunderstood anything:

1) Yes I did mean to say cm's and not mm's when referring to the dimensions of blocks and posts.

2) using the Thai design the concrete posts and cross beams will now comprise four strand 12 mm steel. The posts will be buried in the ground to a depth of 0.50 metres and will be secured with concrete. The posts will initially rise 120 cm above ground, just high enough to act as a retaining wall. The steel at the top of the posts will be left exposed in the short term which will allow them to be extended at a later date to a maximum overall height above ground of 2.5 metres on the unfilled side of the land.

3) the horizontal cross beams will measure 30 cm by 3 metres and will contain four strands of 12 mm steel. The steel at the ends of these beams will be extended and exposed forming hooks which will then be capable of being inserted into coresponding holes on the upright posts. The lowest cross beam will sit at ground level and the three courses of 12cm block will sit on top of it. The second and final cross beam will be fixed in the same way as the first and will sit on top of the block. This will give a finished height of 120 cm, sufficient to retain the raised ground. At some point in the future the height of the wall may be raised by adding additional courses of block and post extensions, up to a maximum overall height of 2.5 metres.

4) drainage tubes will be placed through the lowest level of block every two metres and gravel poured on the inside of the wall to faciltate drainage.

5) ideally one would place vertical steel reinforcing bars through all concrete blocks in addition to filling the blocks with concrete. (I haven;t quite figured out how to do this yet since the only 15cm blocks I have been able find are only open on one side - perhaps the answer here is to use two 7.5 cm blocks side by side instead of one single 15 cm block since the smaller ones are open at top and bottom).

I think that's everything but please let me know if I missed anything.

Thanks again

OK, Here are my comments/questions so far:

1) I need more information on your columns. I am assuming that they are precast not poured in-place. Do they come standard with holes? If so, what is the vertical spacing of the holes? Do the holes go all the way through the column? What is the diameter of the holes?

2) You mention using 4 strands of 12mm rebar, I don't quite follow the term "exposed" and when you say "forming hooks & inserted", I start to shudder! What are these strands of rebar hooking too?

3) You don't have to construct a Rolex here! The horizontal reinforcement does not have to be in a concrete beam! IF you really want a concrete beam you would only need 20cm max (Vertical), but then you would need to use stirrups to seperate the individual strands, which will cause you problems at the column connection. Inasmuch as this beam is not structurally bridging (a lintel) and carrying a significant vertical load, you only need to notch the block in the web to depress the rebar. In addition you don't need 4 strands of 12mm, just use a single 16mm strand.

4) Rebar wouldn't be required in every vertical cell, if the block is staggered, every other cell, but still grout every cell.

5) Drainage tubes under the 1st course of block.

6) I had forgotten about the Thai block. Using 2 (7.5cm) blocks in lieu of 1 (15cm) block won't work in my opinion. You need the volume of the cell filled with concrete to resist that horizontal force being exerted on the wall by the retained soil. Let me ponder that issue!

The idea here is to not only have a structurally sound wall, but not get it to expensive or time consuming. There is another method used that can cut down the vertical component (And horizontal component). I will run some calculations to see what would be required then I will get back to you.

Get me some answers on #1 & #2 above, I will ponder #6 and the idea.

Edited January 7, 2007 by Diablo Bob

[Daffy D]

I know very little about construction so any comments I make here are just personal observation.

I am a fairly proficient DIY kind of guy and have always “over engineered” my projects.

When I have tried to get the same standard of work done here it has been like head banging a brick wall. Always long list of reasons why it does not need to be done this way. So for the sake of a reasonably quieter life I now just give in and let them get on with it.

The thing is that in any third word type country labor is much, much cheaper than in Farang land. So what happens is you get a job done cheaply, everybody is happy but we know they will have to come back and redo the job again and again.

In the end you will have paid the same if not more over the years than having the job done correctly in the first place. TOT.

It is very difficult if not impossible to get any job done to your satisfaction if it goes against the local “expert”. If you insist to do it your way he will loose face and sabotage the work so you will have to call him back anyway thus proving that his way would have been right.

You can’t win.

D.D.

(I am not indigenous people knocking, this is true all over)

[chiang mai]

First and foremost, thank you for taking the time to give such a superb description of the design problems. If I may I would like to feedback the revised design which will hopefully take on board all your comments and to add clarification where necessary - please let me know if I have got this wrong or have misunderstood anything:

1) Yes I did mean to say cm's and not mm's when referring to the dimensions of blocks and posts.

2) using the Thai design the concrete posts and cross beams will now comprise four strand 12 mm steel. The posts will be buried in the ground to a depth of 0.50 metres and will be secured with concrete. The posts will initially rise 120 cm above ground, just high enough to act as a retaining wall. The steel at the top of the posts will be left exposed in the short term which will allow them to be extended at a later date to a maximum overall height above ground of 2.5 metres on the unfilled side of the land.

3) the horizontal cross beams will measure 30 cm by 3 metres and will contain four strands of 12 mm steel. The steel at the ends of these beams will be extended and exposed forming hooks which will then be capable of being inserted into coresponding holes on the upright posts. The lowest cross beam will sit at ground level and the three courses of 12cm block will sit on top of it. The second and final cross beam will be fixed in the same way as the first and will sit on top of the block. This will give a finished height of 120 cm, sufficient to retain the raised ground. At some point in the future the height of the wall may be raised by adding additional courses of block and post extensions, up to a maximum overall height of 2.5 metres.

4) drainage tubes will be placed through the lowest level of block every two metres and gravel poured on the inside of the wall to faciltate drainage.

5) ideally one would place vertical steel reinforcing bars through all concrete blocks in addition to filling the blocks with concrete. (I haven;t quite figured out how to do this yet since the only 15cm blocks I have been able find are only open on one side - perhaps the answer here is to use two 7.5 cm blocks side by side instead of one single 15 cm block since the smaller ones are open at top and bottom).

I think that's everything but please let me know if I missed anything.

Thanks again

Please see my answers in line below:

OK, Here are my comments/questions so far:

1) I need more information on your columns. I am assuming that they are precast not poured in-place. Do they come standard with holes? If so, what is the vertical spacing of the holes? Do the holes go all the way through the column? What is the diameter of the holes?

Reply: The family had proposed a neighbor construct these using moulds from his place of work hence I cannot answer any questions regarding the spacing of holes. On reflection it seems that I will almost certainly buy these from a commercial supplier.

2) You mention using 4 strands of 12mm rebar, I don't quite follow the term "exposed" and when you say "forming hooks & inserted", I start to shudder! What are these strands of rebar hooking too?

Reply: The horizontal beams are three metres long and from what I understand from a farang builder here in Thailand the way these cross beams are connected to the upright posts is by using the ends of the steel to form hooks into the upright posts. I have never seen these for myself, only a rough sketch so that I could understand the principle. I has presumed there would be a corresponding hole in the upright that allowed the steel to mate. I shall go to a builders yard here in CM tomorrow and look deeper into this aspect.

3) You don't have to construct a Rolex here! The horizontal reinforcement does not have to be in a concrete beam! IF you really want a concrete beam you would only need 20cm max (Vertical), but then you would need to use stirrups to seperate the individual strands, which will cause you problems at the column connection. Inasmuch as this beam is not structurally bridging (a lintel) and carrying a significant vertical load, you only need to notch the block in the web to depress the rebar. In addition you don't need 4 strands of 12mm, just use a single 16mm strand.

Reply; please see the above answer. I had got the impression that the horizontal was in fact a form of lintel since it bears the downward weight of the load of block above. That point notwithstanding a single 16 mm strand it shall be.

4) Rebar wouldn't be required in every vertical cell, if the block is staggered, every other cell, but still grout every cell.

Reply: every second cell for vertical rebar it is. By grout every cell I presume you mean fill each cell with concrete? This being the case it still escapes me as to how it is possible to fill each cell AND insert vertical rebar given that the blocks are sealed at the bottom. Were they hollow all the way through it would be possible to do the concrete fill of the blocks and insert the vertical rebar once the wall had been constructed. I guess I'll have to do a search for entirely hollow blocks.

5) Drainage tubes under the 1st course of block.

Reply: I assume this is a question? We had spoken earlier about the need to vent the land filled side of water hence the use of PVC pipe lengths run through the wall to accomplish this. The lowest possible point to place these is above the bottom beam which would place them in the first course of blocks.

6) I had forgotten about the Thai block. Using 2 (7.5cm) blocks in lieu of 1 (15cm) block won't work in my opinion. You need the volume of the cell filled with concrete to resist that horizontal force being exerted on the wall by the retained soil. Let me ponder that issue!

The idea here is to not only have a structurally sound wall, but not get it to expensive or time consuming. There is another method used that can cut down the vertical component (And horizontal component). I will run some calculations to see what would be required then I will get back to you.

Get me some answers on #1 & #2 above, I will ponder #6 and the idea.

Once again, many thanks for your patience.

[Diablo Bob]

I know very little about construction so any comments I make here are just personal observation.

I am a fairly proficient DIY kind of guy and have always “over engineered” my projects.

When I have tried to get the same standard of work done here it has been like head banging a brick wall. Always long list of reasons why it does not need to be done this way. So for the sake of a reasonably quieter life I now just give in and let them get on with it.

The thing is that in any third word type country labor is much, much cheaper than in Farang land. So what happens is you get a job done cheaply, everybody is happy but we know they will have to come back and redo the job again and again.

In the end you will have paid the same if not more over the years than having the job done correctly in the first place. TOT.

It is very difficult if not impossible to get any job done to your satisfaction if it goes against the local “expert”. If you insist to do it your way he will loose face and sabotage the work so you will have to call him back anyway thus proving that his way would have been right.

You can’t win.

D.D.

(I am not indigenous people knocking, this is true all over)

Daffy

I know where you are coming from, but let me throw this out for thought.

All to often, we from the Western world discount the opinions and experience of the "locals", since we are from the West and know the proper methods etc. etc. etc., how wrong we are......

Over the years, I have learned that the solution is in what I call "manipulated compromise", where I know what I want and I keep an open mind as to how we get to the end result, aka. my final requirements. Most of the time it is just a matter of taking a method I would like to see used and posing the method into a question, "Have you tried it this way?", "What do you think about trying it this way?". The What do you think method is always a sure fire way to get it done your way....... When you bring the local into the decision making process in their own mind, the amount of manipulation possible is staggering.

I have/am supervising engineering staffs of 25-50 engineers from a variety of backgrounds and nations, most of which we would consider third-world. I have found that when you put them into a creative environment the solutions to problems are not only creative but cost effective as well. Often, I find the solutions so simple, but the technology so ancient that it isn't even covered in the Western curriculum anymore........

Don't demand that it has to be done this way......... keep an open mind on the methods and you will be surprised by the results.

[Diablo Bob]

First and foremost, thank you for taking the time to give such a superb description of the design problems. If I may I would like to feedback the revised design which will hopefully take on board all your comments and to add clarification where necessary - please let me know if I have got this wrong or have misunderstood anything:

1) Yes I did mean to say cm's and not mm's when referring to the dimensions of blocks and posts.

2) using the Thai design the concrete posts and cross beams will now comprise four strand 12 mm steel. The posts will be buried in the ground to a depth of 0.50 metres and will be secured with concrete. The posts will initially rise 120 cm above ground, just high enough to act as a retaining wall. The steel at the top of the posts will be left exposed in the short term which will allow them to be extended at a later date to a maximum overall height above ground of 2.5 metres on the unfilled side of the land.

3) the horizontal cross beams will measure 30 cm by 3 metres and will contain four strands of 12 mm steel. The steel at the ends of these beams will be extended and exposed forming hooks which will then be capable of being inserted into coresponding holes on the upright posts. The lowest cross beam will sit at ground level and the three courses of 12cm block will sit on top of it. The second and final cross beam will be fixed in the same way as the first and will sit on top of the block. This will give a finished height of 120 cm, sufficient to retain the raised ground. At some point in the future the height of the wall may be raised by adding additional courses of block and post extensions, up to a maximum overall height of 2.5 metres.

4) drainage tubes will be placed through the lowest level of block every two metres and gravel poured on the inside of the wall to faciltate drainage.

5) ideally one would place vertical steel reinforcing bars through all concrete blocks in addition to filling the blocks with concrete. (I haven;t quite figured out how to do this yet since the only 15cm blocks I have been able find are only open on one side - perhaps the answer here is to use two 7.5 cm blocks side by side instead of one single 15 cm block since the smaller ones are open at top and bottom).

I think that's everything but please let me know if I missed anything.

Thanks again

Please see my answers in line below:

OK, Here are my comments/questions so far:

1) I need more information on your columns. I am assuming that they are precast not poured in-place. Do they come standard with holes? If so, what is the vertical spacing of the holes? Do the holes go all the way through the column? What is the diameter of the holes?

Reply: The family had proposed a neighbor construct these using moulds from his place of work hence I cannot answer any questions regarding the spacing of holes. On reflection it seems that I will almost certainly buy these from a commercial supplier.

2) You mention using 4 strands of 12mm rebar, I don't quite follow the term "exposed" and when you say "forming hooks & inserted", I start to shudder! What are these strands of rebar hooking too?

Reply: The horizontal beams are three metres long and from what I understand from a farang builder here in Thailand the way these cross beams are connected to the upright posts is by using the ends of the steel to form hooks into the upright posts. I have never seen these for myself, only a rough sketch so that I could understand the principle. I has presumed there would be a corresponding hole in the upright that allowed the steel to mate. I shall go to a builders yard here in CM tomorrow and look deeper into this aspect.

3) You don't have to construct a Rolex here! The horizontal reinforcement does not have to be in a concrete beam! IF you really want a concrete beam you would only need 20cm max (Vertical), but then you would need to use stirrups to seperate the individual strands, which will cause you problems at the column connection. Inasmuch as this beam is not structurally bridging (a lintel) and carrying a significant vertical load, you only need to notch the block in the web to depress the rebar. In addition you don't need 4 strands of 12mm, just use a single 16mm strand.

Reply; please see the above answer. I had got the impression that the horizontal was in fact a form of lintel since it bears the downward weight of the load of block above. That point notwithstanding a single 16 mm strand it shall be.

4) Rebar wouldn't be required in every vertical cell, if the block is staggered, every other cell, but still grout every cell.

Reply: every second cell for vertical rebar it is. By grout every cell I presume you mean fill each cell with concrete? This being the case it still escapes me as to how it is possible to fill each cell AND insert vertical rebar given that the blocks are sealed at the bottom. Were they hollow all the way through it would be possible to do the concrete fill of the blocks and insert the vertical rebar once the wall had been constructed. I guess I'll have to do a search for entirely hollow blocks.

5) Drainage tubes under the 1st course of block.

Reply: I assume this is a question? We had spoken earlier about the need to vent the land filled side of water hence the use of PVC pipe lengths run through the wall to accomplish this. The lowest possible point to place these is above the bottom beam which would place them in the first course of blocks.

6) I had forgotten about the Thai block. Using 2 (7.5cm) blocks in lieu of 1 (15cm) block won't work in my opinion. You need the volume of the cell filled with concrete to resist that horizontal force being exerted on the wall by the retained soil. Let me ponder that issue!

The idea here is to not only have a structurally sound wall, but not get it to expensive or time consuming. There is another method used that can cut down the vertical component (And horizontal component). I will run some calculations to see what would be required then I will get back to you.

Get me some answers on #1 & #2 above, I will ponder #6 and the idea.

Once again, many thanks for your patience.

When are you planning on constructiong this wall? The reason I ask is that I think I should do a few drawings to make sure we are on the same page so to speak. I am not comfortable with the precast columns for various reasons, but high on my list is a positive connection is not achieved by hooking rebar.

If memory serves me correctly, you are only looking at 2-3cm of concrete on the bottom of the block. Just use a chipping hammer to break this out.......in lieu of that you could drill a hole to accept the vertical rebar and grout each layer of block before moving up to the next course.

But let me get a few drawings together and I will PM you when I get done.

[chownah]

I've been following this design procedure and I'm starting to get a bit worried.

Chinag mai, you want to put a post in the ground to a depth of 50 cm and space them every 3 metres along a retaining wall. I put my barb wire fence posts in the ground 50 cm and during the rainy season you can shake them back and forth which enlarges the holes they are in and although I've never tried I'm reasonably sure that if I tried hard enough I could push one over...but not sure since I've never tried it. You talk about "securing" the posts with concrete....I'm not sure what this means....you can not use concrete to "glue" posts into the soil so I'm not sure if your method of "securing" them will work.

Also, saying that replacing 4 strands of 12mm rebar with one strand of 16mm will be fine and that it really doesn't have to be placed in a concrete beam is really scary for reasons that I would discuss but only if you want me to since it would be a sort of lengthy discussion.

At any rate I'm a bit worried....but....there is one really good thing on your side. Have you ever dug a hole or a ditch here in Thailand in good old northern Thai soil? If you have you might have noticed that if you dig a perfectly verticle cut it will stay there and not fall down. I have a hand dug well in my back yard that is over 3m deep with verticle sides and it has been there for about 10 years and basically the walls are still verticle with no sloughing off of any dirt. How can this be? When they dug the well they left the dirt right around the hole so there is a berm of earth surrounding the hole so all the rain flow away form the hole and not into it. Most of the dirt around where I live in the north will hold fast to a verticle cut and stay that way unless inundated with water. This fact is why the Thai people can build such flimsy retaining walls and mostly they don't fall down...if the earth at the wall is sloped away from the wall (on BOTH sides) and the earth around the wall is not inundated with water then you really don't need so much to keep the wall upright.

I think the bracing that the Thai people want to do is a good idea.....I really think that the direction you are heading in will be overly expensive and not as effective...but then you have not finished your design process yet and maybe I should have waited before chiming in. I could discuss wall design theory a bit but it would take alot of typing on my end and I'd rather not take the time if it would just bore you.

Chownah

[Diablo Bob]

I've been following this design procedure and I'm starting to get a bit worried.

Chinag mai, you want to put a post in the ground to a depth of 50 cm and space them every 3 metres along a retaining wall. I put my barb wire fence posts in the ground 50 cm and during the rainy season you can shake them back and forth which enlarges the holes they are in and although I've never tried I'm reasonably sure that if I tried hard enough I could push one over...but not sure since I've never tried it. You talk about "securing" the posts with concrete....I'm not sure what this means....you can not use concrete to "glue" posts into the soil so I'm not sure if your method of "securing" them will work.

Also, saying that replacing 4 strands of 12mm rebar with one strand of 16mm will be fine and that it really doesn't have to be placed in a concrete beam is really scary for reasons that I would discuss but only if you want me to since it would be a sort of lengthy discussion.

At any rate I'm a bit worried....but....there is one really good thing on your side. Have you ever dug a hole or a ditch here in Thailand in good old northern Thai soil? If you have you might have noticed that if you dig a perfectly verticle cut it will stay there and not fall down. I have a hand dug well in my back yard that is over 3m deep with verticle sides and it has been there for about 10 years and basically the walls are still verticle with no sloughing off of any dirt. How can this be? When they dug the well they left the dirt right around the hole so there is a berm of earth surrounding the hole so all the rain flow away form the hole and not into it. Most of the dirt around where I live in the north will hold fast to a verticle cut and stay that way unless inundated with water. This fact is why the Thai people can build such flimsy retaining walls and mostly they don't fall down...if the earth at the wall is sloped away from the wall (on BOTH sides) and the earth around the wall is not inundated with water then you really don't need so much to keep the wall upright.

I think the bracing that the Thai people want to do is a good idea.....I really think that the direction you are heading in will be overly expensive and not as effective...but then you have not finished your design process yet and maybe I should have waited before chiming in. I could discuss wall design theory a bit but it would take alot of typing on my end and I'd rather not take the time if it would just bore you.

Chownah

Chownah,

A few points.....

1) My statement didn't say he didn't need to use a beam, what I said was that he notch the block place the rebar into the notch, then further on I indicated that ALL the cells in the block be grouted. In the USA we manufacture a block that is called a Bond Beam which has the notch already in place, when horizontal rebar is placed into the notch and you grout, you have effectively constructed a beam, without forming. The end result is the same. The reasoning behind using a single 16mm bar in lieu of 4 (12mm) bars is simple, the horizontal force on the wall doesn't require more than a single 16mm bar. I have seen retaining walls constructed in excess of 7m using this technique and they are still doing the job.

2) Water has to go somewhere, he is going to have to slope the soil away from his buildings foundation, and he could slope the water away from the top of his retaining wall, but at the end of the day the underlaying soil will still absorb water. Putting in a gravel drain facilitates the evacuation of the water from around a cohesive soil and removes the water as part of the equation. in my experience, I have found that rain water is probably a lesser problem than landscaping water. You can slope all you want, but when the homeowner decides to put in trees and flower beds you are opened up to deep penetrations of water which could be destructive.

3) The bracing is a great idea, but of absolutely no benifit unless the wall is positively attached to the columns.

[chownah]

Diablo Bob,

You said

The horizontal reinforcement does not have to be in a concrete beam!

this is what made me sceptical....now I see that you want to use a bond beam...which is a concrete beam built inside a row of blocks....but this will be difficult with the 7 cm thick blocks readily available in Thailand. The thicker the concrete beam the more reasonable the use of a 16mm central bar becomes but with a 7 cm overall thickness there doesn't seem to be enough concrete on either side of the bar to provide much compressive strength especially given how weak Thai concrete usually is and also how weak the concrete blocks are...not to mention the poor quality mortar that is used in stacking the blocks. A 16 mm bar inside a 7 cm beam would mostly provide just the bending strength of the bar which would mean a really flexible beam which would have pretty good ultimate strength but which could deflect quite a bit before developing that strength and this deflection would tend to allow lateral movement of the wall which would shift the center of gravity to the side which would create a force tending to topple the wall.....it might work though but I think two bars one near each face of the wall would provide superior strength and rigidity.

I'm not surprised that a retaining wall could be built with 16mm rebar....as I said the dirt in northern Thailand will stand up vertically by itself if kept from being inundated with water on BOTH sides. The top beam in alot of Thai walls are four strands of #2 which is about 6 millimetres but then I've heard of people using just 2 strands of #2 and it working....with the dirt here you could almost do a layer of stucco and it would work to hold up the dirt if you kept it dry.

So far the only thing I've seen keeping this wall from falling down is 50 cm of post in the ground somehow "secured" with concrete......not knowing what "secured" means I'm a bit skeptical as to how this would work given that I put wire fence posts in the ground 55 cm and when the ground is wet I can shake them back and forth enought to enlarge their hole so that they wobble like a drunk...they're so lose that I can then just grab ahold of one and pull it out of the ground!!!! Since the posts seem to be the only thing resisting overturning I think its important to figure out how to "secure" them to the ground better than just 50 cm in the ground and some unspecified method of "securing" them....but given the strength of the soil in northern Thailand and its ability to stand straight up when kept reasonably dry (just think of my well and its verticle dirt walls which have not eroded appreciably in ten years) it might work if you can keep the water away from the wall.

Maybe your design will work fine....it just that there are details that don't make sense to me based on what I've learned about structures.

Chownah

[Daffy D]

Daffy

I know where you are coming from, but let me throw this out for thought.

All to often, we from the Western world discount the opinions and experience of the "locals", since we are from the West and know the proper methods etc. etc. etc., how wrong we are......

Over the years, I have learned that the solution is in what I call "manipulated compromise", where I know what I want and I keep an open mind as to how we get to the end result, aka. my final requirements. Most of the time it is just a matter of taking a method I would like to see used and posing the method into a question, "Have you tried it this way?", "What do you think about trying it this way?". The What do you think method is always a sure fire way to get it done your way....... When you bring the local into the decision making process in their own mind, the amount of manipulation possible is staggering.

I have/am supervising engineering staffs of 25-50 engineers from a variety of backgrounds and nations, most of which we would consider third-world. I have found that when you put them into a creative environment the solutions to problems are not only creative but cost effective as well. Often, I find the solutions so simple, but the technology so ancient that it isn't even covered in the Western curriculum anymore........

Don't demand that it has to be done this way......... keep an open mind on the methods and you will be surprised by the results.

D B

I did not mean to give the impression of being dictatorial and demanding things be done my way. Not always easy to convey the tone of something when writing.

In a former life I too have supervised various nationals in various countries and fully understand the power of creative suggestion. But sometimes you just get fed up with constantly “negotiating” to get things done properly.

D.D.

[Diablo Bob]

Diablo Bob,

You said

The horizontal reinforcement does not have to be in a concrete beam!

this is what made me sceptical....now I see that you want to use a bond beam...which is a concrete beam built inside a row of blocks....but this will be difficult with the 7 cm thick blocks readily available in Thailand. The thicker the concrete beam the more reasonable the use of a 16mm central bar becomes but with a 7 cm overall thickness there doesn't seem to be enough concrete on either side of the bar to provide much compressive strength especially given how weak Thai concrete usually is and also how weak the concrete blocks are...not to mention the poor quality mortar that is used in stacking the blocks. A 16 mm bar inside a 7 cm beam would mostly provide just the bending strength of the bar which would mean a really flexible beam which would have pretty good ultimate strength but which could deflect quite a bit before developing that strength and this deflection would tend to allow lateral movement of the wall which would shift the center of gravity to the side which would create a force tending to topple the wall.....it might work though but I think two bars one near each face of the wall would provide superior strength and rigidity.

I'm not surprised that a retaining wall could be built with 16mm rebar....as I said the dirt in northern Thailand will stand up vertically by itself if kept from being inundated with water on BOTH sides. The top beam in alot of Thai walls are four strands of #2 which is about 6 millimetres but then I've heard of people using just 2 strands of #2 and it working....with the dirt here you could almost do a layer of stucco and it would work to hold up the dirt if you kept it dry.

So far the only thing I've seen keeping this wall from falling down is 50 cm of post in the ground somehow "secured" with concrete......not knowing what "secured" means I'm a bit skeptical as to how this would work given that I put wire fence posts in the ground 55 cm and when the ground is wet I can shake them back and forth enought to enlarge their hole so that they wobble like a drunk...they're so lose that I can then just grab ahold of one and pull it out of the ground!!!! Since the posts seem to be the only thing resisting overturning I think its important to figure out how to "secure" them to the ground better than just 50 cm in the ground and some unspecified method of "securing" them....but given the strength of the soil in northern Thailand and its ability to stand straight up when kept reasonably dry (just think of my well and its verticle dirt walls which have not eroded appreciably in ten years) it might work if you can keep the water away from the wall.

Maybe your design will work fine....it just that there are details that don't make sense to me based on what I've learned about structures.

Chownah

Chownah

I too don't think the 75mm block will work......

Haven't addressed the foundation yet, would appreciate your input. Not knowing the bearing capacity of the soil nor the water table nor the local geomorphology it is pretty much a crap shoot. Regardless, it will be substancial. And I agree about the post hole foundation, it will need to be integrated into the continuous wall footing somehow...

Another area that disturbs me is the precast concrete posts and how to integrate them into this wall where they contribute to the load capacity. Woke up at 3:30am (Hence why I am on right now) with an idea, what do you think about using them externally! No positive attachment to the wall itself just the foundation and placing them externally with the brace to resist any turning moment? The problem I see in using the precast posts is that any reinforcement they have is ((4) 8-10mm cage?) unconnected to the foundation and the horizontal wall steel, and as you say built with normally below standard concrete.

Will continue ferreting out information on this end..... please keep monitoring the progress and PLEASE shoot out any ideas and concerns, the assistance is appreciated and more heads is better than one. You know the old saying, "An engineer is a person who can look through a keyhole with both eyes open!"

DB

[chiang mai]

This wall did not have sow poon's just footings. When it rained the raised land filled with water and over it went.

Use Sow Poon's half a meter in the ground and relieve the water pressure on the high side with drains.

Thanks for the pictures, they will be very useful when talking to the family about their "expert" ideas. If I was going to adopt the post and bar method I had always intended to bury the posts at least half a metre, not to do so is asking for failure. Many thanks for your help.

[chiang mai]

First and foremost, thank you for taking the time to give such a superb description of the design problems. If I may I would like to feedback the revised design which will hopefully take on board all your comments and to add clarification where necessary - please let me know if I have got this wrong or have misunderstood anything:

1) Yes I did mean to say cm's and not mm's when referring to the dimensions of blocks and posts.

2) using the Thai design the concrete posts and cross beams will now comprise four strand 12 mm steel. The posts will be buried in the ground to a depth of 0.50 metres and will be secured with concrete. The posts will initially rise 120 cm above ground, just high enough to act as a retaining wall. The steel at the top of the posts will be left exposed in the short term which will allow them to be extended at a later date to a maximum overall height above ground of 2.5 metres on the unfilled side of the land.

3) the horizontal cross beams will measure 30 cm by 3 metres and will contain four strands of 12 mm steel. The steel at the ends of these beams will be extended and exposed forming hooks which will then be capable of being inserted into coresponding holes on the upright posts. The lowest cross beam will sit at ground level and the three courses of 12cm block will sit on top of it. The second and final cross beam will be fixed in the same way as the first and will sit on top of the block. This will give a finished height of 120 cm, sufficient to retain the raised ground. At some point in the future the height of the wall may be raised by adding additional courses of block and post extensions, up to a maximum overall height of 2.5 metres.

4) drainage tubes will be placed through the lowest level of block every two metres and gravel poured on the inside of the wall to faciltate drainage.

5) ideally one would place vertical steel reinforcing bars through all concrete blocks in addition to filling the blocks with concrete. (I haven;t quite figured out how to do this yet since the only 15cm blocks I have been able find are only open on one side - perhaps the answer here is to use two 7.5 cm blocks side by side instead of one single 15 cm block since the smaller ones are open at top and bottom).

I think that's everything but please let me know if I missed anything.

Thanks again

Please see my answers in line below:

OK, Here are my comments/questions so far:

1) I need more information on your columns. I am assuming that they are precast not poured in-place. Do they come standard with holes? If so, what is the vertical spacing of the holes? Do the holes go all the way through the column? What is the diameter of the holes?

Reply: The family had proposed a neighbor construct these using moulds from his place of work hence I cannot answer any questions regarding the spacing of holes. On reflection it seems that I will almost certainly buy these from a commercial supplier.

2) You mention using 4 strands of 12mm rebar, I don't quite follow the term "exposed" and when you say "forming hooks & inserted", I start to shudder! What are these strands of rebar hooking too?

Reply: The horizontal beams are three metres long and from what I understand from a farang builder here in Thailand the way these cross beams are connected to the upright posts is by using the ends of the steel to form hooks into the upright posts. I have never seen these for myself, only a rough sketch so that I could understand the principle. I has presumed there would be a corresponding hole in the upright that allowed the steel to mate. I shall go to a builders yard here in CM tomorrow and look deeper into this aspect.

3) You don't have to construct a Rolex here! The horizontal reinforcement does not have to be in a concrete beam! IF you really want a concrete beam you would only need 20cm max (Vertical), but then you would need to use stirrups to seperate the individual strands, which will cause you problems at the column connection. Inasmuch as this beam is not structurally bridging (a lintel) and carrying a significant vertical load, you only need to notch the block in the web to depress the rebar. In addition you don't need 4 strands of 12mm, just use a single 16mm strand.

Reply; please see the above answer. I had got the impression that the horizontal was in fact a form of lintel since it bears the downward weight of the load of block above. That point notwithstanding a single 16 mm strand it shall be.

4) Rebar wouldn't be required in every vertical cell, if the block is staggered, every other cell, but still grout every cell.

Reply: every second cell for vertical rebar it is. By grout every cell I presume you mean fill each cell with concrete? This being the case it still escapes me as to how it is possible to fill each cell AND insert vertical rebar given that the blocks are sealed at the bottom. Were they hollow all the way through it would be possible to do the concrete fill of the blocks and insert the vertical rebar once the wall had been constructed. I guess I'll have to do a search for entirely hollow blocks.

5) Drainage tubes under the 1st course of block.

Reply: I assume this is a question? We had spoken earlier about the need to vent the land filled side of water hence the use of PVC pipe lengths run through the wall to accomplish this. The lowest possible point to place these is above the bottom beam which would place them in the first course of blocks.

6) I had forgotten about the Thai block. Using 2 (7.5cm) blocks in lieu of 1 (15cm) block won't work in my opinion. You need the volume of the cell filled with concrete to resist that horizontal force being exerted on the wall by the retained soil. Let me ponder that issue!

The idea here is to not only have a structurally sound wall, but not get it to expensive or time consuming. There is another method used that can cut down the vertical component (And horizontal component). I will run some calculations to see what would be required then I will get back to you.

Get me some answers on #1 & #2 above, I will ponder #6 and the idea.

Once again, many thanks for your patience.

When are you planning on constructiong this wall? The reason I ask is that I think I should do a few drawings to make sure we are on the same page so to speak. I am not comfortable with the precast columns for various reasons, but high on my list is a positive connection is not achieved by hooking rebar.

If memory serves me correctly, you are only looking at 2-3cm of concrete on the bottom of the block. Just use a chipping hammer to break this out.......in lieu of that you could drill a hole to accept the vertical rebar and grout each layer of block before moving up to the next course.

But let me get a few drawings together and I will PM you when I get done.

Construction will only start when I have an agreed plan and not before so that is not a concern. I had thought about knocking out the bottom of the block but from the brief look I have taken they may well be too brittle to allow for that. I'm off to the builders yard this morning to see if I can't find an alternate product. Shall also buy a couple of blocks and see if the bottom can be knocked out with shattering the block. As for drilling a hole and grouting each layer before moving up: that would only work if you use one short length of vertical rebar per layer of block and tie each short length to the next as you move up through the layers. Otherwise the grout in the lower layers would have set by the time all three (or seven) layers had been completed hence you cannot insert a single continuous vertical rod down through all the layers.

[chiang mai]

I've been following this design procedure and I'm starting to get a bit worried.

Chinag mai, you want to put a post in the ground to a depth of 50 cm and space them every 3 metres along a retaining wall. I put my barb wire fence posts in the ground 50 cm and during the rainy season you can shake them back and forth which enlarges the holes they are in and although I've never tried I'm reasonably sure that if I tried hard enough I could push one over...but not sure since I've never tried it. You talk about "securing" the posts with concrete....I'm not sure what this means....you can not use concrete to "glue" posts into the soil so I'm not sure if your method of "securing" them will work.

Also, saying that replacing 4 strands of 12mm rebar with one strand of 16mm will be fine and that it really doesn't have to be placed in a concrete beam is really scary for reasons that I would discuss but only if you want me to since it would be a sort of lengthy discussion.

At any rate I'm a bit worried....but....there is one really good thing on your side. Have you ever dug a hole or a ditch here in Thailand in good old northern Thai soil? If you have you might have noticed that if you dig a perfectly verticle cut it will stay there and not fall down. I have a hand dug well in my back yard that is over 3m deep with verticle sides and it has been there for about 10 years and basically the walls are still verticle with no sloughing off of any dirt. How can this be? When they dug the well they left the dirt right around the hole so there is a berm of earth surrounding the hole so all the rain flow away form the hole and not into it. Most of the dirt around where I live in the north will hold fast to a verticle cut and stay that way unless inundated with water. This fact is why the Thai people can build such flimsy retaining walls and mostly they don't fall down...if the earth at the wall is sloped away from the wall (on BOTH sides) and the earth around the wall is not inundated with water then you really don't need so much to keep the wall upright.

I think the bracing that the Thai people want to do is a good idea.....I really think that the direction you are heading in will be overly expensive and not as effective...but then you have not finished your design process yet and maybe I should have waited before chiming in. I could discuss wall design theory a bit but it would take alot of typing on my end and I'd rather not take the time if it would just bore you.

Chownah

I am grateful for any input you can provide on this subject but it does seem that DB has taken the bit between his teeth and is off at a gallop, for which I am very grateful. As for me and my knowledge: I'm a systems and telecoms person and know very little about the subject at hand, fact is I feel currently like I am slipping the same way a poorly designed and constructed wall might! But I do understand that people who are expert in this subject will want to question and challenge other experts so please feel free to do so. Whilst I had initially thought that answers here might be simple I now realize that I perhaps ought to be paying for plans to be drawn up but was hoping to avoid that.

[Diablo Bob]

I am grateful for any input you can provide on this subject but it does seem that DB has taken the bit between his teeth and is off at a gallop, for which I am very grateful. As for me and my knowledge: I'm a systems and telecoms person and know very little about the subject at hand, fact is I feel currently like I am slipping the same way a poorly designed and constructed wall might! But I do understand that people who are expert in this subject will want to question and challenge other experts so please feel free to do so. Whilst I had initially thought that answers here might be simple I now realize that I perhaps ought to be paying for plans to be drawn up but was hoping to avoid that.

Chiang Mai,

Expert! Far from it, I stopped working in this segment of engineering years ago for a segment of the industry that paid money not peanuts. Remember, an expert is ANYBODY more than 50 miles from home.

[chownah]

Some ideas.

I think of the wall as being a 2.5 metre wall which must be made to also retain 1.2 metres of fill. Around here a 2.5 metre wall which does not hold back any dirt would usually be built with cast in place posts that are 15 cm square or up to 20 cm square. The excavation for the foundation for the post would typically be 50 cm deep and 50 cm square....although in Thailand 50 cm can somtimes be 40 cm if no one is looking but I keep an honest 50 cm for the depth and the size of the foundation. No rebar is placed in this foundation except for the verticle steel from the column which has a 90 degree bend at its base, extends 20 cm laterally into the foundation concrete and has a hook on the end....20 cm AFTER the hook is made not 20 cm and then make the hook so it really only extends 10 cm. The steel for the column is usually called "song hune tem" which means "number two full"....this should be about 6mm in diameter......I suggest always measuring the rebar before buying it....the actual truth is that two days ago I went to buy some rebar for the kitchen I'm building and I paid for "see hune tem" which is "number 4 full" and should be 12mm rebar and paid the price for it but they tried to give me "see hune bouw" which is "number 4 light" which is only about 10mm....could be an honest mistake...but regardless you want to be sure to get the right size and my experience is that the only way to be sure is to measure before buying.

The four verticle bars in the colums are bent and hooked and cast into the foundation....they are also enclosed in squares of rebar that are placed about every 20 cm up and down all along the lenght of the column. These are usually made from "see hune tem" (number 2 full) or "see hune bauw" (number two light) but I stick with the full because it only costs about half a baht more per metre so why not buy only full since if you have some light stuff around it will inevitably get put in as a verticle (its kind of hard to tell the difference especially if you have a "mai pen rai" attitude) so in my view its best to just not have any of the light around to cause problems.....unless you are strapped for cash in which case you can save a very few baht. I put one of these squares in the foundation around the verticles about 3cm from the top surface although its probably not necessary. By the way...when most Thais make these squares...if the square is 10cm on a side they will cut a piece of rebar about 45 cm long and when it is bent the ends will just barely overlap by bending just a bit around one of the corners....this is not good....you want the ends to go around the corner rebar and then extend toward the center of the column about 4 or 5 cm.....to help visualize this the bends at the end should be 135 degree bends and have a straight section after the bend about 4 cm long....I think that this is important but on the other hand most Thais don't do it this way and most walls don't fall down...so go figure. By the way the squares are usually placed every 20 cm along the length of the post.

Anyway after the footing is cast with the verticle steel embedded (along with one of the squares) and the concrete has set then the next step is to pour a short piece of column (placing probably one square in it) which might be only 10 or 20 cm tall to get you up to the level of the bottom of the lowest beam which I'll call the grade beam. After this is hard (the next day) then the steel for the beam is placed and the beam forms are built and the beam is poured....when the beam is hard (the next day) then the column can be formed and poured all the way to the top with squares every 20 cm or so.

So this is how a post is constructed around here for a 2.5 metre tall wall which does not act as a retaining wall....it is built to basically just hold up a wall.....my idea is that you could do your project with precast posts that would be as tall as needed to get you to the top of your fill but then getting good connections with the beams would be more of a problem....with cast in place you place your steel and then it all gets cast securely so you have a better assurance of a stronger connection.....but Thais sometimes use precast and the walls don't really fall down very often....so go figure.

Also, I figure that if you go with a foundation like the one described that will be adequate to keep the post from sinking from the weight of the wall and resist any overturning caused from a non-retaining type wall....then....if you add the braces like the Thai people suggested you will be adding the extra strength required to restrain the overturning forces imposed by the fill.

Key point: If you eventually decide you want to go with precast then I think that the braces could be connected with the beams on both sides of the post where it could be securely cast in the grout instead of using holes in the precast post....or...as an altenative....you could construct the brace with one long piece of steel that wraps around the post so that the brace has two strands running its length created by wrapping one piece around the post.....come concrete would need to be cast around the post to cover the steel wrap which shouldn't be too difficult to do but might be ugly. How do the Thai people make this connection when they do it?

All for now....maybe more later.

Chownah

Edited January 8, 2007 by chownah

[Grover]

Remember, an expert is ANYBODY more than 50 miles from home.

My new sig....

[tuky]

Great thread.

Lets put it in the Housing forum now where it will generate even more interest.

[Gary A]

I have 240 meters of 2 meter high wall. What I can tell you is that it has a foundation that is below the original fill. NONE of the verticle posts are pre-cast. They are all poured in place and anchored to the 8mm rebar in the foundation. Each post has 4 pieces of the same 8mm rebar. Every other post has 90 degree stabilizers about 25 cm wide a meter out from the foundation to prevent the whole thing from leaning or tipping. They also have 4 pieces of rebar. The entire length has a cap on top that has 2 pieces of the same rebar. I wish now I had used 4 pieces because after about 3 years I have noticed some cracks in the cap. I think that the 2 pieces have stretched. The drainage is no problem because of 2 inch PVC spaced along the fence through the block to allow water to drain out to the low side.

[chownah]

I'm back for a bit more.

To recap.....a wall around where I live that retains no dirt usually are constructed with posts which have a 50 cm square footing (maybe 40 cm if no one is looking) and the bottom of the footing is 50 cm below grade (or 40 cm if noone is looking)...and the posts are cast in place usually with 4 pieces of #2 vertically with squares of #2 around these 4 pieces (called "confinement steel" sometimes) placed about every 20 cm.......and I suggest that if this design is followed and then the braces like the Thai folk like to use are added to withstand the forces made on the wall by the dirt (trying to tip the wall over) then you should have a post foundation that will hold the wall up adequately and not overturn.

Some more details....the bottom of the post footing (footing = foundation here) should be in undistrubed earth....it should not be sitting on any fill since the fill will settle in the years to come and this could destabilize your wall. Ideally the bottom of the footing should be 20 cm into the undisturbed earth but sitting on undisturbed earth is probably good enough and for people like me who like to overbuild go the extra 20 cm. If you go down 50 cm and you are still in the fill dirt then just go down further until you get to undistrubed earth and you'll just have to make the post a bit longer but that doesn't matter....if for some reason you can't or don't want to go so deep then pack the dirt below the footing as hard as you can get it....a 2 or 3 metre long 2x4 held vertically and used like a ram can get dirt really hard if used with vigor and perseverence....if the dirt is too wet you won't be able to compact it....lets hope that you can get to undisturbed earth easily. Also, the best thing is for the lowest beam (I'll call it the "grade beam") to be sitting on undisturbed earth because it has to hold up the parts of the wall between the posts and you don't want the earth settling under the grade beam unless you want to make a full on beam capable of carrying the weight of the wall with no support from underneath....if you can't get the grade beam sitting on undisturbed earth then I would use more steel in the grade beam or dig a hole midway between posts down to undisturbed earth and build a footer and support post under the beam....this will cut your span lenght in half and it shouldn't really cost much...but it is more digging....boy do me and my workers get tired of digging sometimes!!! Let's just hope that you can build your grade beam on undisturbed soil...and its even better if you can build it 10 or 20 cm into the undisturbed earth......the Thai people don't worry about these things too much and mostly thier walls don't fall down....go figure. By the way, if you can't put your grade beam on undisturbed earth then you could compact the earth under the grade beam like under the post footing.....I wish you could rent a "jumping jack" here in my neighborhood....its a mechanical compactor that is operated by one person standing up...it has handle bars attached to a big foot that is gasoline driven so that it bounces up and down with great force....it works great for compacting soil....except for if the soil is to wet in which case you can not compact it until it dries out.

I've said alot about a very small topic....if this is too much detail I can skip them and try to go for the highlights....what do you think? Any particular thing I've said that I should clarify?...all of it?...oh damnn!

Chownah

[Diablo Bob]

OK, Let the fun begin.......

I have been pondering the block issue so today while avoiding work, I did an internet search on masonry retaining walls. Somewhere around page 25 of results, I ran across this little gem of a design.

This design is by the North Carolina Brick Association, not block, but brick. This is a proffesional association representing the brick manufacturers. The design is conservative to say the least, but inasmuch as it's a "Cookbook" design not supported by geotechnical data, the design assumes the worst possible geologic condition. Please not that there are no columns in the design. The height of wall can be up to 3m.

Type_1_Wall.bmp

Type_1_Wall_Specifications_3.bmp

Type_1_Wall_Specifications_2.bmp

Type_1_Wall_Specifications.bmp

Now, this is in English Standard units, which some of us use and some remember from the good old days pre-metric era. The way I see it, the actual retaining wall is in the grouted space between the brick, the brick is just acting as a concrete form, but I might be wrong.

BEFORE attempting this at home boys and girls, certain items still need to be defined, such as:

1) If you decide to use the 75mm block in lieu of brick, while the compressive strength of a masonry block NORMALLY exceeds that of a clay fired brick, we are not sure that it does in Thailand. The block has an open cell that in my opinion would have to be fully grouted to make the block solid.

2) I don't know if the brick in Thailand is clay fired, and more importantly does it make a difference. I will continue my search.

3) They recommend an expansion joint every 5 meters.

4) In the drawing, dimension W (width of the wall) is a minimum value.

During my search, I ran across a paper that jolted a memory from many years ago. If you have the space to play with, backfill the front face of the retaining wall. This reduces the load on the foundation and wall. This can create problems though for your weep holes.

OK, Fire away..........

Edited January 8, 2007 by Diablo Bob

[chiang mai]

OK, Let the fun begin.......

I have been pondering the block issue so today while avoiding work, I did an internet search on masonry retaining walls. Somewhere around page 25 of results, I ran across this little gem of a design.

This design is by the North Carolina Brick Association, not block, but brick. This is a proffesional association representing the brick manufacturers. The design is conservative to say the least, but inasmuch as it's a "Cookbook" design not supported by geotechnical data, the design assumes the worst possible geologic condition. Please not that there are no columns in the design. The height of wall can be up to 3m.

Type_1_Wall.bmp

Type_1_Wall_Specifications_3.bmp

Type_1_Wall_Specifications_2.bmp

Type_1_Wall_Specifications.bmp

Now, this is in English Standard units, which some of us use and some remember from the good old days pre-metric era. The way I see it, the actual retaining wall is in the grouted space between the brick, the brick is just acting as a concrete form, but I might be wrong.

BEFORE attempting this at home boys and girls, certain items still need to be defined, such as:

1) If you decide to use the 75mm block in lieu of brick, while the compressive strength of a masonry block NORMALLY exceeds that of a clay fired brick, we are not sure that it does in Thailand. The block has an open cell that in my opinion would have to be fully grouted to make the block solid.

2) I don't know if the brick in Thailand is clay fired, and more importantly does it make a difference. I will continue my search.

3) They recommend an expansion joint every 5 meters.

4) In the drawing, dimension W (width of the wall) is a minimum value.

During my search, I ran across a paper that jolted a memory from many years ago. If you have the space to play with, backfill the front face of the retaining wall. This reduces the load on the foundation and wall. This can create problems though for your weep holes.

OK, Fire away..........

I don't know what the rest of the world thinks about this but I need at least overnight to begin to understand it. Er, um, so you had a lazy day then!