Soil investigation is often neglected or rejected by most clients on the basis of cost, despite the fact that the cost of carrying out a soil investigation is very little compared to the cost of the project
Soil investigation is done for various purposes. In engineering, soil investigation is very necessary.
It is essential to investigate the soil of the selected plot on which a structure will be constructed.
Based on soil investigation a soil report is prepared for the purpose of designing the building foundation. When an engineer designs building foundation he/she must carefully read the report and design the foundation based on the data provided in the report.

Soil investigation is required for the following purposes -
To know the allowable bearing capacity of foundation for proposed building.
To know the depth and type of foundation for the proposed building.
To know the allowable passive resistance for the foundation of proposed building.
To know the type, grading and nature of soil.
To know the ground water level.
Typical steps of soil investigation
Soil investigation involves following steps –
 Details planning for the sequence of operations.
 Collecting the samples of soil from the plot.
 Determining the soil characteristics by conducting field tests.
 Study the condition of ground water level.
 Collecting ground water sample for chemical analysis.
 Soil exploration.
 Testing all collected samples in the laboratory.
 Preparation of drawings and charts
 Analysis the test results.
 Preparing report

Depending on the load type, soil investigation can be limited to Penetrometer teats only for relatively good soil and light loads. For heavier soil, a combination of both Penetrometer tests and borehole tests are required.
Methods of Soil Investigations

The normal methods of soil investigations are:
• Inspection
• Test pits
• Probing
• Boring

Inspection

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.
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.

Probing
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 thesoil. An experienced workman can assess the nature of the soil by observing the way the rod is penetrated into the soil.
Boring
In this process, bore holes are made in the ground and the soil samples collected. Boring helps in obtaining 1) extent of each strata of soil/rock 2) nature of each stratum and the engineering properties of the soils 3) location of ground water table. The depth and number of boreholes will depend upon the type of the structure and nature of thesoil 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.
• Methods of Boring for Soil Investigations
The different methods of boring are:
Auger Boring
The examination of the soil for ordinary buildings can be done by a post hole auger. The auger is held vertically and is driven into the ground by rotating its handle. At every 30 cm of depth, the auger is taken out and the soil samples collected.
Wash Boring
Wash boring is commonly used for boring in difficult soil. The hole is advanced by an auger and then a casing pipe is pushed to prevent the sides from caving in. A stream of water under pressure is forced through the rod into the hole. The loosened soil in suspension in water is collected in a tub.

Percussion Boring
In this method, the substrata is broken by repeated blows by a bit or chisel. Water is circulated in the hole and then the slurry is bailed out of the hole.
Core Drilling
When rocks are to be penetrated for examination, core drilling is resorted to. In this process, a hole is made by rotating a hollow steel tube having a cutting bit at its end.
• Bearing Capacity
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.
• Field and Laboratory Testing
Field Tests
The commonly adopted field tests are:
Standard penetration test, Dynamic cone penetration test, Static cone penetration test, Plate load test, Vane shear test, and Pressure meter test. The Penetrometer test is carried out most times locally using the 2 ½ Ton testing equipment. The tests are usually terminated when the end and side resistance against the cone and the rods make further penetration difficult at a certain depth. Such depth is taken down. The basic principle of a penetrometer test is driving a rod into the ground and the resistance on the tip of the rod is measured by a hydraulic system.

Laboratory Tests
A set of laboratory tests are required to be done to obtain the soil parameters for the design of foundation. These tests are: Shear strength, Compressibility, Permeability, Chemical and Mineralogical Composition, and Soil Classification.
A list of relevant laboratory tests for geotechnical investigation of soil are as follows:
Determination of moisture content and bulk density: In general, bulk density is determined by soil texture and modified by soil structure. Within any textural class a certain range in bulk density is expected and whether, within this range, bulk density is relatively low or high depends on the degree of structural development. Whereas texture is not affected by soil management, soil structure is a fragile property that can deteriorate with intensive cultivation, exposure to raindrops and machinery traffic.
1. Atterberg’s limits
2. Particle size distribution by sieve analysis
3. Unconfined compression testing

The results of these laboratory tests are presented in graphical form. The level at which water is encountered is clearly indicated on the test reports.These results are clearly analysed by the soil consultant and recommendation on foundation types are made. Methods

• Synopsis
The foundation is that part of the building which serves as a base and transmits the load to the soil. Before a foundation is designed, it is necessary to investigate the characteristics of the underlying soil. There are different methods to assess these characteristics.