- The term ‘soil’ in soil engineering is defined as an unconsolidated material, composed of solid particles, produced by the disintegration of rocks.
- The void space between the particles may contain air, water or both.
- The soil particles may contain organic matter.
2) Types of Soil
Different Types of Soils Found in India.
- Alluvial Soils: formed by the deposition of sediments by rivers.
- Black Cotton Soils: are made up of volcanic rocks and lava-flow.
- Laterite Soils: formed by decomposition of rock, and accumulation of iron oxide and aluminum oxide.
- Dessert Soils: generally sandy and deficient in organic matter.
- Marine deposits: are mainly clay, also contains organic matter and have very low shear strength.
3) Classification of Soils
i) Purpose of soil classification
- To arrange various types of soils into groups according to their engineering or various other characteristics.
- Soil possessing similar characteristics can be placed in the same group.
ii) Soil Classification Systems
- Particle Size Classification
- Textural Classification
- Highway Research Board (HRB) Classification
- Unified Soil Classification System (USCS)
- Indian Standard Classification System (ISCS)
iii) Particle Size Classification
- Soils arranged according to grain sizes.
- Terms such as gravel, sand, silt and clay are used to indicate grain
- These terms are used only as designation of particle sizes.
- They do not signify naturally occurring soil sizes.
- Naturally occurring soil are mixture of particles of different sizes.
iv) I.S. Classification of Grain Size
4) Indian Standard Classification System (ISCS) IS:1498-1970
i) Classification Group Symbols
5) Soil investigation
- Determination of surface and subsurface soil conditions and features in an area of proposed construction that may influence the design and construction and address expected post construction problems.
6) Purpose of soil investigation
- The site investigation provides first hand information for;
- Selection of foundation type.
- Design of foundations.
- Contractors to quote realistic and competitive tenders.
- Planning construction techniques.
- Selection of appropriate construction equipment (especially for excavation and foundations).
- Feasibility studies of the site.
- Study of environmental impacts of the proposed construction.
7) Soil Investigations for Building Foundations
Soil investigations involve the following steps:
- Planning the details and sequence of operations
- Collection of soil samples from the field
- Conducting all field tests for determining the strength and compressibility characteristics of the soil.
- Study of ground water level conditions and collection of water samples for chemical analysis
- Geophysical exploration if necessary
- Testing in the laboratory of all samples of soil, rock, and water
- Preparation of drawings and charts
- Analysis of the results of the tests
- Preparation of report
8) Methods of Soil Investigations
The normal methods of soil investigations are:
- Test pits
- The first step in this connection is the inspection of the site and its vicinity to get a preliminary idea of the site conditions.
- This includes the study of the existing buildings in the neighborhood and if possible the type of their foundations.
- The cuts made in the nearby areas should also be looked into.
- The subject can be discussed with those persons who were associated in constructing buildings in the surroundings with regard to their experiences and difficulties encountered by them.
ii) Test pits
- Test pits are dug by hand or by excavating machines.
- The size of the pit should be such that a person can easily enter the pit and have a visual inspection.
- Both disturbed and undisturbed soil samples are collected from the pit for detailed analysis.
- This will give a rough idea of the underlying soil.
- In this, a steel bar of 25 to 40 mm (1 inch to 1.5 inch) in diameter is driven into the ground until a hard stratum is met with.
- The bar is driven by a hammer.
- The bar is then drawn out at intervals and the soil sticking to the bar is examined to get an idea of the type of the soil.
- An experienced workman can assess the nature of the soil by observing the way the rod is penetrated into the soil.
- In this process, bore holes are made in the ground and the soil samples collected.
- Boring helps in obtaining
- Extent of each strata of soil/rock
- Nature of each stratum and the engineering properties of the soils.
- Location of ground water table.
- The depth and number of boreholes will depend upon the type of the structure and nature of the soil as obtained from preliminary examination.
- The depth of boreholes is governed by the depth of the soil affected by the loading.
- As a rough estimate, it is advisable to investigate the subsoil to a depth of at least twice the width of the anticipated largest size of the foundation.
- In case of a pile foundation, the depth of boring should extend into the bearing stratum.
9) Bearing capacity of soil
- The bearing capacity of a soil is defined as the capacity of the subsoil to support the load of the structure without yielding.
- The bearing capacity of the soil depends upon the characteristics such as cohesion, friction, and unit weight.
- The bearing capacity can be determined in the field and also from the results of tests conducted in the laboratory on the soil samples.
- The maximum load is obtained from the graph between the settlement and load.
- Safe bearing capacity of soil = [Ultimate bearing capacity of soil] / [Factor of safety]
- On completion of a structure there may be some displacement (or settlement) in the position of the foundation.
- This settlement should be as uniform as possible as differential settlement can affect the building to a very large extent.
- The maximum differential settlement should not exceed 25 mm in case of foundations on sandy soil and 40 mm in case of foundations on clayey soil.
- In case of non-cohesive soils, such as sand and gravel, the allowable bearing capacity should be reduced by 50%, provided that the water table is above or near the bearing surface of the soil.
- The bearing capacity of shrinkable soils can be taken as 50kN/m2 in the absence of the site data.
10) Allowable Bearing Pressure
Maximum bearing pressure that can be applied on the soil satisfying two fundamental requirements:
- Bearing capacity with adequate factor of safety – net safe bearing capacity
- Settlement within permissible limits (critical in most cases) – net safe bearing pressure
11) Artificial methods to improve bearing capacity of soil
- By increasing the depth of foundation.
- By draining the sub-soil water.
- By compacting the soil.
- By confining the soil mass.
- By cement grouting.
- By injecting chemicals like silicates etc.
12) Field and Laboratory Testing
The commonly adopted field tests are:
- Geophysical methods- (seismic refraction method and Electrical resistivity method)
- Standard penetration test,
- Static cone penetration test,
- Plate load test,
- Vane shear test, and
- Pressure meter test.
Plate Load Test – IS:1888-1982
A set of laboratory tests are required to be done to obtain the soil parameters for the design of foundation. These tests are:
- Classification tests
- Chemical Tests
- Compaction Tests
- Shear strength and triaxial tests
- Consolidation Tests
- Permeability Tests
- Rock Tests
13) General Summary of Soil Investigation
Ranking of Soil for foundations (from best to unsuitable):
- Sand & Gravel – Best
- Medium & Hard Clays – Good
- Silts & Soft Clays – Poor
- Organic Silt and Clays – Undesirable
- Peat – Unsuitable