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Foundation Design For Houses In Thailand

Here is a complete mini-treatise on how to ensure that the foundations for your retirement house in Thailand are correctly sized and positioned based on a knowledge of the ground bearing pressure and soil type and stratification obtained by a Site Soil Investigation.

Why I Recommend That You Have Your Design Company To Undertake An On-Site Soil Investigation For Your House Foundation Design

Apparently it is common practice in Thailand for the builder to check the design of the foundations as made by your architectural design company and to change the design if he thinks necessary.

This is not a practice I would recommend and on the Site Testing Of The Soil For House Foundation Design page I give eight good reasons why not. I recommend that you ask your architect to arrange for the engineers to undertake a site soil investigation to make sure that the foundation design is correct. On this page I give some of the reasons for making that recommendation.

A Disclaimer

These notes are made based on the advice of a civil engineer and after studying the report presented by my geotechnical engineer for the site soil investigation they undertook on our land in Pakchong. I have also studied other technical texts on the subject of foundation design and geotechnical site soil investigation methods and procedures.

However, please note that I am not a civil engineer and not trained or qualified in soil testing or foundation design. Please therefore note that this information is presented in good faith and I cannot be held liable for any decisions you make as a consequence of reading this document. You are advised to take professional advice in any decisions you make with regards to site testing of soil and foundation design.

Classifications of Soils

Apparently, there are three main classifications of soils under the Unified Soil Classification system (USCS):-

1) Cohesionless soils such as sand, gravel and ballast.

2) Cohesive soils such as clays and silts.

3) Other classifications such as a mix of those above, rock and made ground.

The general method of testing each of those classifications is different.

• For Cohesionless soils the normal test is a “Standard Penetration Test” (SPT). It is also possible to get some indications from SPT of the shear strength in cohesive soils.
• For Cohesive soils a “Field Vane Shear Test” (SVST) or lab test of an undisturbed soil sample called an Unconfined Compression Test (UCT) is also  used.
• Another type of test is called the Plate Test.

Of course the above is an over-simplification, but is just provided to show make you familiar with the common terminology.

Ground Bearing Capacity

The size of the foundations for houses in Thailand is usually determined by what is called the Ground Bearing Pressure (GBP). That is a measure of the pressure the ground can safely take from the weight of a building.

The Standard Penetration Test (STP) is commonly used by geotechnical engineers in Thailand to measure the Ground Bearing Pressure (GBP) of the soil on the site where a house is to be built.

Why Sound Foundation Design Is So Very Important in Thailand

Of course, in every country, the foundations for a house are important but here are good reasons why in Thailand foundation design needs special attention.

The type of soil and it’s bearing capacity can have a dramatic impact on the size and type of foundations for you house and these factors are commonly dictated by location.

Poor Soil Bearing Capacity

The soil in many areas of Thailand has a very low ‘bearing capacity’, in other words it cannot take much weight before the foundations start to sink into the soil.

For example, the soil in Bangkok is a soft clay and piled foundations are usually used. In the case of small buildings piles of up to 600 mm diameter and 28 metres deep are used and for high rise buildings piles of 800 mm diameter to a depth of 35 metres up to a diameter of 1,500 mm to a depth of 60 meters are used.

‘Up Country” where the land is firmer, Pad foundations are common. So the type of soil and the safe ground bearing pressure obviously are important in the design of building foundations.

But is it always necessary to have site soil test carried out to determine the safe ground bearing pressure?

Whether to have a site soil test carried out or not depends upon several factors.

Often, the safe ground bearing pressure is known for a particular area from tests undertaken for previous projects. Construction companies,  piling companies and builders often have this information. Also the local Or Bor Tor office may have details of the soil in various locations and it’s safe bearing pressure.

It also depends upon the size of you project. For a multi-story building the type, design and size of the foundations will be crucial top the cost and safety of the building and a good knowledge not only of ground bearing pressure but also of the nature of the sub-soil will be required. But for a small house of the type you might be considering to build to retire to in Thailand a lesser degree of knowledge would be adequate.

A Practical Example Of Foundation Design For A House In Pakchong, Thailand

To give a practical example, let’s look at the foundation design for my house to be built in Pakchong.

I was surprised by the weight of my small house which sits on 16 posts and each post has a square concrete  pad foundation. The structural calculations from the engineer in Thailand indicated a design weight of 20.6 tons per post. That’s 330 tons for the whole house which I find staggering. I did get a civil engineer her e in the UK to check that and he concurred that the 20.6 tons was probably of the right order. Note that my civil engineer pointed out that the weight of the soil above each foundation pad has to be added to the weight of the house.

Anyway, the weight on each post is 20.6 tons and the pad foundation for each post is 1.2 m by 1.2m, or 1.44 m². That means that the pressure on the soil is 14.3 ton/m².

The structural calculations indicate that the safe bearing pressure of the soil is 15 ton/m² if the foundations are at 1.5 m deep.

If the foundations were any smaller, the house could sink!

Here is a table of typical safe bearing pressures for various types of soil:-

 The results from the site soil survey for my house indicate that there is a layer of “stiff to very stiff clayey silt” down to 1.5 to 2 m after which the soil is “hard silty clay”. The safe bearing pressures for the two layers are given as 18 to 25 ton and 26 to 35 ton respectively as determined by a Standard Penetration Test (STP) undertaken during the site soil investigation.

You can see in the table above that the ground bearing pressure for silts is extremely low (<7.5 ton/m²) and for stiff clay it is 15 to 30 ton/m².

The Danger With Made Up Land

Sometimes houses are built on what is called “Made Ground”. This is land that has been deposited by the builder or land owner rather than land that has been undisturbed since it’s formation millions of years ago. This is typically done for example to fill in a low part of the land or to raise it’s level for some other reason.

If you look at the table above under “made ground”, the last entry, the bearing capacity is to be determined by soil test. This is very important because made ground has an extremely low ground bearing capacity. I particularly recommend that you take professional advice on your foundation design if this type of land is applicable to the location where you are planning to build your house.

The Effects Of Flooding

Many areas of Thailand are prone to flooding in the rainy season cause by a combination of flat land and poor or no drainage. If the soil is clay, there is little natural drainage. The flooding saturates the soil and reduces it’s allowable bearing pressure.

When silt is saturated with water it becomes like a fluid and it’s already low safe bearing pressure is reduced further.

When Kanyah was talking to our future neighbours at Pakchong they told her that the land in that area is prone to flooding in the rainy season. When this was explained to our structural engineer, they suggested that it would be safer to put the foundations at 2 m deep to sit on the clay, rather than to sit in the silt.

The Importance Of The Site Soil Survey

From the above you will now understand that the site soil survey was essential to determine the actual safe bearing capacity of the soil and to determine the safe depth of the foundations.

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