Building Advice – Soil Sampling for Foundations

Building Advice – How to Sample Soil When Building Foundations

The first part of the process of extending a property or building a new house will involve the laying of the foundations to support the structure. Important decisions will be made depending on the ground conditions that are found. So what is the process to determine the type of foundations required?

Initially a structural engineer will assess the location of the work to be carried out as he should have a good idea of the type of soil and the height of the water table from reviewing existing records. The next step will involve digging trial pits or making bore holes from which there will be two main types of soil samples that can be taken. Disturbed soil samples will only provide a certain amount of information such as the type of soil and whether there are any pollutants. Undisturbed samples are obtained by driving a thin-walled tube into the soil, using a hand auger its aim is to represent as closely as possible the in-situ structure and water content of the soil. Using an auger avoids compressing the contents, as the purpose of this sample is to determine such things as the compressive nature of the soil and to see if there are layers in the sample that may cause disturbance to the soil above each layer. Good sampling can make the difference between certain construction methods being judged possible or not.

Hand auguring to provide soil sampling up to 10 m deep

Hand auguring to provide soil sampling up to 10 m deep

A structural engineer will look at the soil sampling data and determine the correct type and depth of foundation to suit the proposed building.

There are very few homes built in areas where soil conditions are perfect. The main reason for building a home is often determined by factors completely unrelated to soil conditions, such as affordability, availability, proximity and demand.

or deep soil sampling a rig similar to this is needed

or deep soil sampling a rig similar to this is needed

AN OVERVIEW OF SOIL

Expansive clay soils

Much of the south of England geology consists of Clay. They are either in shallow surface layers or deeper formations, often tens of meters thick. Clay will swell in volume when wet and Shrink again if dry- this is known as Shrink – Swell. It occurs within the first 5m from the surface. Older buildings with Spread of shallow strip foundations – See our article on Foundations.  Are very susceptible to this seasonal movement. The clay can move up to 150mm in both the vertical & horizontal directions. This displacement can cause serious damage to the structure of buildings. Foundations that are built in the vicinity of trees are particularly prone to damage by the roots undermining them, and localised drying of the clay.

If you have trees close to your building, do not rush out and cut them down, this could cause the ground to swell in a short period of time causing more damage. Get the advice of a tree surgeon, and have them removed over a period of time in a managed way.

By using piled foundations, that are set deep into the clay will avoid the first 5m where the problems can occur. This will provide a more stable foundation on which to build.

Clay rich soil will shrink in dry conditions

Clay rich soil will shrink in dry conditions

Select fill

This type of soil is defined as a sandy loam and is not affected as much by moisture variations and can be ideal for the support of foundations. It is possible for surrounding erosion to affect the bearing capacity of this type of soil.

Sand

Sand is not affected by the change in moisture conditions, but can be greatly affected by surrounding conditions such as cracking in the soil or water movement which can carry the sand away, thus affecting the stability of the sand.

Checking the soil where you plan to build the foundations is important

Checking the soil where you plan to build the foundations is important

Rock

Rock can be an ideal foundation to build on as long as there is a consistency throughout the foundation; however low density or shale rock is not ideal. If the structure is supported by various soil conditions, such as rock on one half and expansive clay on the other, then the foundation system would need to be designed to allow for the differentials in their movement.

Testing soil for pollutants is becoming more commonplace in view of the number of new constructions on brownfield or industrial land. Should contaminants be found as a result of former industries carried out on the site it may be necessary to have the contaminated soil removed and cleaned. This can be extremely expensive and time consuming so it would be important to determine these factors before purchasing such a site.

When it is necessary to have soil testing carried out there are specialist geotechnical companies that will do the work. They will initially conduct a desktop survey, which requires looking at historical data to determine any factors that may impact on the report or influence the nature of the tests. Then a field study will be done which involves using a specialist boring machine. These samples are analyzed in laboratory conditions to avoid contaminating the samples with false data and a report is prepared as a basis for determining what is necessary.

Much of the south of England geology consists of Clay. They are either in shallow surface layers or deeper formations, often tens of meters thick. Clay will swell in volume when wet and Shrink again if dry- this is known as Shrink – Swell. It occurs within the first 5m from the surface. Older buildings with Spread of shallow strip foundations – See our article on Foundations.  Are very susceptible to this seasonal movement. The clay can move up to 150mm in both the vertical & horizontal directions. This displacement can cause serious damage to the structure of buildings. Foundations that are built in the vicinity of trees are particularly prone to damage by the roots undermining them, and localised drying of the clay.

If you have trees close to your building, do not rush out and cut them down, this could cause the ground to swell in a short period of time causing more damage. Get the advice of a tree surgeon, and have them removed over a period of time in a managed way.

By using piled foundations, that are set deep into the clay will avoid the first 5m where the problems can occur. This will provide a more stable foundation on which to build.