Site Testing Of The Soil For House Foundation Design

My architectural and design company, from Bangkok in Thailand, has arranged at my request to send a team to the house site in Pakchong to test the soil so that the foundation design of our planned retirment house can be finalised.

The test will be what is technically termed a “Standard Penetration Test” (SPT) and will start on 11th or 13 th September 2010 and a Report will be produced.

For more detailed information about how to determine Ground Bearing Pressure (GBP) and how to correctly size house foundations refer to the Thai House Foundation Design web page.

Disclaimer

What follows below is what I have learned about this soil testing procedure so far and no doubt as the investigation proceeds I will learn more.

Please take this article in the manner in which it was written – In good faith and with limited knowledge. I am no expert in this subject and cannot take any responsibility for any events that may occur by you taking any action as a result of reading this article.

Remember that all this is new to me – this is the first time I have had a house designed anywhere – let alone in Thailand!

Always have a properly qualified engineer to design your foundations for you!

What The Standard Penetration Test Will Achieve

In simple terms, the (SPT) test will determine the Ground Bearing Capacityof the soil, in other words how much weight the soil can support. This is measured (in Thailand) in tons per square meter. (Ton/m2). In the metric system the units are kN/m2.

From this, the size of the foundations necessary to carry the weight of the house can be calculated. Of course the weight of the house when contructed and in use has to be determined first.

How Important Is It To Have This Soil Test Carried Out?

To me it’s vital. After all that’s why I am paying for the test to be carried out. As I mentioned above I’m no expert in these matters so I am proceeding cautiously. I have been told by a civil engineer here in the UK that there are many reasons why the ground bearing pressure can vary significantly even on the same plot of land. This can be because the type of soil varies form location to location. Another big factor influencing the soil bearing pressure is whether the soil is compacted i.e. undisturbed or whether is is loose i.e. it is is soil that has been placed on the lad – often to fill a hole. In our case in PakchongI do know that the soil is agricultural land so it will have been ploughed to a certain depth.

However, this test is not always performed when designing house foundation and there are several reasons for this.

  • Sometimes it is not necessary to undertakea soil test for an individual property because the load bearing capacity of the soil may already be know for the region and held on record either with builders or piling companies or in the local Land Office. This information may have come from tests undertaken by other companies for other projects.
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  • Another reason that a Standard Penetration Test may not be required on a small house is that the load bearing pressure of the soil can be approximately determined by a knowledgeable geotechnical engineer from a knowledge of the soil classification. Tables of bearing capacities of various types of soil (such as rock, clay, gravel, sand etc) are readily available. In fact the British Building Regulations Approved Document A at table 10 gives the foundation sizes for various types of ground for houses of various weights.
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    So a good geotechnical/civil engineer with some local knowledge may be able to estimate a suitable safe size for foundations for a small house like the house we are planning to build in Pakchong. In case there is any suspicion of the type of ground, the foundations can be slightly oversized. This may be an unnecessary expense but the additional cost may only be a small percentage of the total house cost.

Design Procedures And Responsibility For Foundation Design In Thailand

I have been advised that it is common in Thailand not to finalise the size of the house foundations before appointing a construction company to build the house. In this case the design company makes an assessment of the foundation design based on ‘common practice’. The builder then takes responsibility for the foundation design and obtains the load bearing capacity of the soild to do so, either from local knowledge, existing data or by means of soil tests.

To me, this has several disadvantages and issues.

  1. The design company has to sign the drawings and provide the calculations for the foundation design. I don’t see how that can be done if the ground bearing capacity of the soil is not known.
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  2. The Or Bor Tor (The land office in Thailand where the house plans are submitted to apply for a building permit) will check the structural calculations. If these are not based on a knowledge of the actual load bearing capacity (that the Or BorTor Officer may well know because he deals with the applications daily) the calculations may not be accepted and a building permit may not be issued.
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  3. The local builder may not have the necessary knowledge and skills to determine the load bearing capacity and to re-design the foundations if necessary. Yes, a large company may have a qualified enginer, but I’m expecting to find a small, local builder in Pakchong who may not have the necessary expertise. By the way, I’m told that there is a Thai regulation that applies for buildings over 150 square metres and it stipulates that the builder must have a qualified and knowledgeable engineer to supervise the project to control the construction process. Well, judging by the coments on so many Thai websites, that expertise is often not applied very well!
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  4. I will be unable to award a construction contract with a known fixed scope, price and timescale and will be at the mercy of the builder when it comes to price and programme.
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  5. Responsibilities become blurred. the designer is effectively passing the responsibility for foundation design on the the builder. The builder may not accept this.
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  6. Extention to the programme. I am having the soil bearing pressure tested and the foundation design finalised in parallel with the preparation for the construction drawings. If this work is handed over to the builder there will be a delay to the construction programme while it is carried out.
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  7. Design costs will be more expensive. The design company has spent time designing foundations and making drawings, including steel reinforcement details. This will may have to be repeated by the builder and he will charge me for it. Double work. Double cost.
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  8. I may not be in Thailand at the time of the build. The works will be supervised by my wife, Kanyah, who does not have building knowledge and she would not understand the commercial implications of undertaking the soil test and the foundation (re)design. Yes, we will appoint an inspector to take care of the technical details of the build, but he will not be expected to manage the builder’s activities.

More Information On Foundation Design

You may not be interested in how to design house foundations and I can well understand that. But you ought to understand how foundations are designed so that you can decide whether to design your house the way I am and let the design company finalise the house foundation design or to do it the common Thai way and let the builder do it.

I have therefore written an article about foundation design and the effect of land bearing pressure for the complete novice.


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