Site investigation and sub-soil exploration

The process of investigating/inspecting site or field to collect necessary data and also to check the quality of work done is called site investigation.

The necessity of site investigation and sub-soil exploration

a. To select the types and depth of foundation for a given structure.

b. To determine the bearing capacity of the soil.

c. To determine the probable maximum and differential settlement of soil.

d. To estimate the groundwater level and to determine the properties of water.

e. To predict the lateral earth pressure against retaining wall and bridge abutments.

f. To select a suitable construction technique.

g. To predict and solve potential foundation problems.

h. To determine the suitability of a soil as a construction material.

i. To investigate the safety of the existing structure and to suggest the remedial measures.

Types of site investigation (exploration)

a. Reconnaissance
b. Preliminary exploration
c. Detailed exploration

a. Reconnaissance

             Site reconnaissance is the first step in the subsurface exploration program it includes a site visit and studies the map and other available records. It helps to decide to be adopted, types of sample to be taken and lab testing or in site testing.

b. Preliminary exploration (General exploration)

             The aim of preliminary exploration is to determine the depth, thickness, extent and composition of each soil strata at the site and depth of bedrock, groundwater table etc. The preliminary exploration is generally in the form of few boring or tests pits. These are conducted with cone penetration and sounding rods to obtain information about the strength and compressibility of the soil.

c. Detailed exploration

             The purpose of detailed exploration is to determine the engineering properties of soil by extensive boring, sampling and testing of the sample in a laboratory.

              Field test such as vane shear test, plate load test and permeability test are conducted to determine the properties of soil in the natural state. Detailed exploration is required only for large scale construction which requires huge investment and develops technique, where small scale construction may not require detailed exploration.

Method of exploration

1. Open Excavation

              In this method of exploration, an open excavation is done to inspect the sub-strata. This method can be divided into two categories:

i. Pit and trench 

ii. Drift and shaft

i. Pit and trench

              Pits and trenches are excavated to inspect the strata the size of pits should be sufficient to provide necessary working space of 1.2 m * 1.2 m at bottom of the pit . Shallow pit up to 3-meter depth can be made without providing any lateral support. For depth greater than 6 m boreholes are more economical than the open pit. Deep pits should be ventilated to provide fresh air and water should be properly drained out.

ii. Drift and shafts

             Drift is a horizontal tunnel made in the hillside to determine the nature and structure of the geological formation. The drift should have a minimum clear dimension of 1.5 m width and 2 m height in hard rock. if the ground is unable to support the load, supports have to be provided at roof and side. This method is expensive and rarely used.

             The shaft is long sized vertical holes made in the geological formation. These may be rectangular or circular in section minimum width/ diameter of the shaft is 2.4 m.

2. Boring

             When the depth of exploration is more, boring method is used. A vertical borehole is drilled into the ground to get the information about the sub-soil strata. Samples are taken from the borehole and tested in the laboratory . Depending upon the type of soil and the purpose of boring, the following method of boring are used for drilling the hole;

i. Auger boring

ii. wash boring 

iii. Rotary drilling

iv. Percussion drilling

v. Core boring

Leave a Comment

Your email address will not be published. Required fields are marked *