Soil Exploration

We have already studied the need for and the methods of collecting topographic, geological and hydrologic data vis-a-vis a building site. A knowledge of the soil at the site is important not only in terms of its engineering  but also in terms of its relevance to natural life systems such as vegetation, animal and insect life etc. We will emphasize in this section the engineering properties of the surface soil, vis-a-vis a given site. 

Structures are heavy and their heavy weight must be borne by the land upon which they are built. But the substructure of the soil and earth is not always what it seems to be from the surface. Therefore, before the construction of a building can begin, samples of the soil, through borings, will have to be taken and analysed. No matter what the structure is, it must be supported by a sturdy foundation. This foundation links the structure to the earth. A well-designed and well-built foundation will always help protect the structure from shifting or sagging. It will be thus, realised that soil exploration in respect of a site is an important activity which is bound to influence the site selection. 

The following factors are important for site exploration : 

1. Depth to seasonal high water table 
2. Depth to bed rock 
3. Drainage characteristics 
4. Suitability for the functioning of septic tanks, excavation and grading 
5. Value as foundation material 
6. Susceptibility to compaction 
7. Susceptibility to erosion 
8. The pH rating 
9. Soil fertility 

We will present a brief review, only of the items which will aid the engineer in proper site selection. 

General Considerations 

The type of site explorations to be carried out is dependent on the loads transmitted to the ground and the functions of a building. These factors can vary widely. The following are presented as guidelines (Krymine and Judol, 1957) in this regard : 

• Generally for lighter structures, the depth to which the investigations are carried is limited, whereas for heavier structures it is generally necessary to explore the entire depth of soil covering the rock and even to penetrate into the rock, if necessary. 
• The drilling work should be limited to a strictly necessary number of holes of reasonable depth. 
• Laboratory soil and rock testing are to be considered as integral part of the exploratory programme. 
• Besides the soil and rock exploration proper, all information is to be obtained concerning the earth work to be done in connection with the building. 
• The magnitude and especially, the cost of the exploratory depends on the importance and cost of the building. 
• The nature of exploration depend on the proposed type of foundation such as spread footings, mat or raft, caissons, piles etc. 
• The safe capacity may have to be obtained from load tests for important structures, if the behaviour of underlying soil is suspected, even if these be expensive. 
• Some foundation problems are common to all kinds of buildings such as excavation problems: unstable foundation material and ground water problems. These may have to be considered at the very initial stages. 
• Extensive geotechnical investigqtions for ordinary residential buildings may not be necessary. Simple trial pits or bore holes are adequate along with the observation of adjoining buildings. 
• Commercial buildings are characterised by a heavy concentration of loads, generally transmitted to the foundation by columns. Many of them have deep basements, a fact that calls for a careful ground water investigations. The ideal programme would be to have one bore hole at every column location, but ordinarily sufficient information about stratification may be obtained from the bore holes drilled at the building comers and at the locations of the interior columns which will carry the heaviest loads. In soils, suspected to be of highly variable nature, additional holes for exact correlation of the data should be drilled. 
• Industrial buildings include warehouses, factories and garages. In such buildings floor loads may be very heavy and individual footings for heavy machines may be required. In manufacturing plants, besides the usual dead and live loads, it may be necessary to consider vibration effects. Particularly sensitive to vibrations are relatively loose sands and gravels, and compaction of these materials by vibrations is responsible for settlement of footings placed on them. Because of the large site of these buildings sufficient bore holes should be drilled to locate possible critical changes in sub-surface materials. High water table is of concern, since it may cause uplift on the floor slabs when they are placed directly on the soil, and cause buoyancy on the footings or interfere with excavation. 
• In the case of power plants and pumping stations, intense influence of vibrations, and their sensitivity to relative settlements are important. Sub- surface investigations should be very detailed and thorough. All bore holes should go to a depth equivalent to if times the width of a power plant. All faults, shear- zones, weathered zones, closely spaced changes in rock type etc, should be studied thoroughly with the help of sufficient number of bore holes. 
• Building foundations on fills have become necessary because of shortage of desirable sites in urban areas. Bed fills, including city dumps, reclaimed lands at the ocean shores and swamps and shallow water areas, permanently or periodically flooded, are to be carefully investigated. Old fills generally have a hard crust and may support a very light building; it is advisable to remove the quest and some of the underlying material to a depth of at least 1 meter and then place a new fill. Generally, field explorations should provide samples to test for shear failure and possible settlement of the fill. If piles are used to support a building, they should be driven through the fill to a firm bearing stratum.

Description of Soils (Peck,  et a1 1974) 

Engineers must know the principal terms which describe soils. Gravel, sand, silt and clay are the common terms used. Most natural soils consist of a mixture of the two or more of these constituents and many contain an admixture of organic material in a partly decomposed state. The mixture is given the name of the constituent that appears to hatre the maximum influence on its behavior and the other constituents are indicated by adjectives. Thus, a silty clay has predominantly the properties of a clay, but contains a significant amount of silt. 

Gravels, greater than (4.75  mm particle size) and sands (4.75  to 0.75 mm) are known as coarse grained soils, while silts (0.75  to 0.002  mm) and clay are fine grained soils. Clay is predominantly an aggregate of microscopic and sub-microscopic flaky shaped crystalline minerals. It is characterised by the typical colloidal properties of plasticity, cohesion and the ability to absorb ions. These properties are exhibited over a wide range of water content. Clay particles are less than 0.002  mm in size. 

Behavior of Soil Types 

Gravels do not shrink gr swell and have a high bearing capacity unless they are free to flow laterally. Next to rocks, gravels form ideal foundation materials. 

Sands do not swell or shrink too much, in comparison, when moisture content changes. Sand is a good foundation material when it is dense and confined (prevented from flowing out). 

Silt has a slight tendency to swell or shrink. Foundation an silty soil is likely to settle significantly. 

Clay cannot be drained off easily. Settlement takes place gradually and may persist for years. Clay shrinks when dried. Capillary action is significant in clayey so?ls and it is difficult to construct and support foundation of buildings in a purely clayey soil. 

When there are layers of these soil types underneath building foundations the resultant behaviour is complex and requires the services of a soil mechanics specialist to predict the behaviours. 

Safe Bearing Capacity (NBC 1983) 

Safe bearing capacity of a soil is the maximum intensity of loading that the soil will safely carry without the risk of shear failure, irrespective of any settlement that may occur. 

Allowable Bearing Pressure 

Allowable bearing pressure is the maximum allowable loading intensity on the soil for a particular foundation, taking into account the safe bearing capacity, the amount and kind of settlement that is prescribed and the ability of the structure to take up such settlements.