Why do a Geotechnical site investigation

Good soil information can lead to substantial savings and tighter foundation design because the design parameters are well defined. The more comprehensive the information, the more economically can a good structure be built. The object is to build a good building cheaply and not to build a cheap building.

Geo-technical site investigations may cost up to 2% of the total project cost, but is very necessary as geo-technical engineers often say: you will pay for your geo-technical investigation anyway!

GEOTECHNICAL INVESTIGATION PROCEDURE

Initially one should find geological information for a specific site about the original rock type of the area and the soils which are the weathered rock. This can be done at either the government printer or your local geological board.

If in doubt regarding the characteristics of the soil, consult an engineer or take soil samples from a test hole to a soil laboratory for analysis. Based on the results of this analysis, a recommendation can be made regarding the best foundation structure to use.

CLASSIFICATION OF SOIL TYPES

Sand:

Sand consists of discrete particles normally visible to the naked eye. Sand is clearly distinguishable by the presence of gritty particles which do not break down when rubbed on the palm of the hand.

Silt:

Silt consists of fine particles. In general silt particles are barely felt when rubbed with water on the palm. When this wetted soil is placed on the tongue, the particles can be felt grating the against the enamel of the teeth.

Clay:

Clay consists of very fine particles. The particles are flaky and when rubbed on the palm with water has a soapy or greasy feeling. No sensation of grittiness can be detected when tasted.

   

Gravel:

Gravel consists of fragments of rock. The shape of the particles should be described as: rounded shape, oval shape, rounded corners, corners slightly bevelled, corners sharp or irregular.

Boulders:

Boulders are fragments of rock larger than 200mm.

Once the soil is classified and the composition of the grain particles are determined, one can make a recommendation as to which type of foundation or pile structure should be used.

There are various types of clay and not all of them may be regarded as bad. Some are soft but are still stable. Collapsible soils are usually not very dense because of the high proportion of voids present.

Heaving or active clays contain minerals which cause the soil mass to react strongly to changes regarding moisture content. These clays will swell when more moisture is added and will shrink as they dry out. The treatment of soft soils is usually to make the foundations wider to reduce the pressure to an acceptable level so that the soil will be able to withstand the load with little or no settlement. Should the soil prove to be too soft or wet then a raft foundation or piling is recommended. A pile is a concrete column driven into the earth which either carries the load down to a stronger soil or rock below or by hanging in friction on the soil around its shank.

RECOMMENDATIONS FOR PROBLEM SOILS

Clay soils

Clay is generally smooth and has no visible particles, it retains water and does not drain well. The presence of clay is sometimes indicated by cracks on the surface of the ground. This type of soil could pose a problem although some soft clays are quite stable. Heaving clay is very problematic and changes a lot when the moisture content changes.

Recommendation: One should lay a compacted hardcore (of broken bricks, stones etc.) under the concrete foundations but for heaving clay a raft foundation will often have to be used; also, depending on the extremeness of the heaving clay, the site can be thoroughly soaked once the trenches have been dug and building can then begin when it has dried sufficiently. This ‘sealing-in’ of the moisture greatly reduces subsoil movement, minimize the wet-dry cycle and the structure will only have to copy with minimal movement. However it is essential that a structural engineer should be consulted when dealing with heaving clay.

Collapsing soils

Low density soils contain voids, which often cause them to collapse and cause serious cracks to appear in the walls.

Recommendation: Wider foundations can be laid or piling can be used.

Soft soil

Most buildings are subject to some settling but soft soils will in some, although rare, instances provoke the kind of settlement that causes the entire structure to sink.

Recommendation: Foundations should be widened in order to reduce pressure to a level that will hold the load. Brickforce should also be laid on every 3rd brick course depending on the severity of the soft soil.

Sandy soil

This type of soil feels gritty and has no plasticity and has good drainage properties. Generally, sandy soil is not a problem but on a steeply sloping site, the weathered soil from the hill gradually slides down to the bottom and one might have collapsing soil problems.

Recommendation: The area where is to be built can be re-filled with a stable soil type to a depth decided by an engineer or one could use friction piling or end load bearing piling if the soil type below is suitable. One can also make used of wider foundations. It is also common practice to flood the ground once the trenches have been dug and then to compact thoroughly. This improves cohesion and makes the soil considerably more stable to build on.

FOUNDATION CONSIDERATIONS

Foundations are important to ensure that the loads of the building above are transferred to solid ground capable of supporting these loads. Thus, the foundations should be able to carry the weight of the building without uneven settlement taking place, which would result in cracking and potential collapse.

The topsoil on the site usually contains all kinds of vegetable matter which is easily compressed and would not be suitable for foundations, but is valuable as a top dressing for gardens and may be disposed of in this matter.

You should avoid at all costs any site where there is a chance of ground or shallow sub-surface water remaining close to the base of the building, therefore:

1. Avoid sites that can be flooded or be left in damp conditions

2. Make sure that water coming off the roof and walls of the building is moved quickly away from the base of the walls. This can be done by compacting a protective sloping pavement (apron).

CONCLUSION

One should be very careful when building on unstable soils. When inadequate foundations cause cracking of walls or other problems to occur, you will have to spend a lot of money to rectify the problem, and it will not only reduce your property’s market value in future, also most house insurance policies exclude coverage against subsidence and landslip damage caused by unstable soils. Never take any chances, consult a qualified geo-technical engineer when you are uncertain of your stand’s soil conditions.

Please note: This document should be used as a guide only and the author accepts no responsibility for losses incurred as a result of advice followed in this document. The foundations of your proposed dwelling should always be discussed with your engineer.