Building Technology – Soil Investigation 1

Building Technology – Soil Investigation Soil investigation and assessment forms part of the preliminary investigative work carried out as part of the feasibility studies of a proposed building. The principal objectives of soil investigation can be summarised as follows: To determine the suitability of the soil for the proposed project. To examine the ground conditions so that the most appropriate type of foundation for a proposed building can be selected. To determine the difficulties that may arise during the construction process and period. The information required is found using a variety of methods which may include desktop studies, walkover survey, aerial photographs and the taking of soil samples. 2

Building Technology – Soil Investigation Soil Types  In simple terms, there are four main classifications of soil type:   Rock Cohesive soil (eg clays) Granular or non – cohesive soil (eg sands or gravels) Organic soil (eg peats) Fill or made ground. 3

Building Technology – Soil Investigation As part of the investigation soil on the site will be assessed for a variety of different properties including:   Origin Compressive and shear strength Particle size distribution Moisture content Shrinkability Fill or made ground. 4

Building Technology – Soil Investigation Compressive strength is the ability of the soil to resist compaction under an applied load. The pressure exerted on the soil depends upon the area of the soil foundation and the size of the applied load. The pressure decreases with the depth below the foundation. 5

Building Technology – Soil Investigation A number of other important factors will also need to be investigated, including:   Ground water Contaminants Sulphates Radon Methane Fill or made ground. 6

Building Technology – Soil Investigation Ground water 7

Building Technology – Soil Investigation Problems of water in the subsoil: A high water table can cause flooding during wet periods. Subsoil water can cause problems during excavation works by its natural tendency to flow into the voids created by the excavation activities. It can cause unacceptable humidity around finished structures. 8

Building Technology – Soil Investigation Soil Contaminants Contaminants that may be injurious to health or to the building may be found in a variety of different locations, particularly when a site is being redeveloped (a brownfield site). Some examples of typical problem sites include: Asbestos works, chemical works, printing works, gas works, landfill sites, metal mines, oil storage depots, sewage works, tanneries. Part C of the Building Regulations 2000 gives guidance on how to deal with contaminants, but specialised advice should always be sought. 9

Building Technology – Soil Investigation Sulphates Sulphates in soils cause problems in that sulphate solutions attack concrete. One of the constituents of cement, Tricalcium Aluminate reacts with sulphate solutions to produce Calcium Sulphoaluminate. Expansion occurs during this reaction thus causing the concrete to crack and spall. Sulphates occur naturally in certain clays. The most abundant are calcium sulphate (gypsum), magnesium sulphate (epsom salt), and sodium sulphate. Sulphates also often occur in industrial wastes. Soils are classified into five categories of sulphate content, with Class 5 being the most severe. The amount of groundwater present is also significant. The sulphate content is determined by site or laboratory tests. Where attack is considered likely, the usual form of protection is to use concrete made with sulphate resisting cement.   1010

Building Technology – Soil Investigation Radon  Radon is a radioactive gas that occurs naturally in certain types of soil. Some areas of the country, including parts of Cardiff have high levels. Exposure to radon for long periods increases the risk of developing lung cancer. The preferred method of protection is to incorporate an airtight barrier below the ground floor. Mechanical ventilation may also be required in some cases. 11

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Building Technology – Soil Investigation Methane Methane and other organic gases occur due to the action of bacteria on bio-degradable material in the soil. It is therefore commonly found on or near landfill sites. It is an asphyxiant, will burn readily and can explode in air. Expert advice is always needed when methane contamination is considered possible. A ventilated void below the ground floor may be a solution, but if high concentrations are present then further measures will be required.   13

Building Technology – Soil Investigation Planning the Investigation Identify the depths of investigation required at different locations around the site. Boreholes should always penetrate completely through made ground or infilling. Identify suitable in-situ and laboratory testing methods for the expected soil conditions and the parameters required. Decide on the number of exploratory holes and the sampling and testing frequency, making allowances for the presence of unforeseen ground or groundwater conditions. 14

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Building Technology – Soil Investigation   Types of Sample Soil samples can be classified according to their quality. The quality of the sample relates to how representative the sample is of the undisturbed natural soil conditions. The higher the quality of the sample, the greater the range of tests which can be carried out on it. There are five classes of quality, with Class 1 being the highest. Classes 1 and 2 are generally referred to as “undisturbed” samples, and Classes 3, 4 and 5 are referred to as “disturbed” samples. Suitable equipment must be employed to produce the required sample type. In non-cohesive soils it may prove impossible to obtain undisturbed samples. 16