Method of Soil Exploration

2Apr, 2020

The technical investigation by which the necessary information regarding various soil properties is obtain to enable the engineer in charge to design a safe and economical foundation is refer as sub surface investigation and Then, the various method of soil exploration are :

The method of soil exploration:

Inspection
Probing
Boring
Trail pit method

(i) Inspection : It is the first step and should be taken before carrying out the design and construction of the structure. In this, the study is make of all nearby quarries or cuts, resisting structure, all strata expose in the old pits, unlined wells etc. to determine the nature of the soil at various depths. The useful information which can be gather from inspection is to obtain data regarding sulphates, contents in soil, chances of land movements, changes in water table, type of soil whether soft or hard or marshy or water log.

(ii) Probing : In this, a rod about 25 mm in diameter is driven in the ground. It has a point end when it is taken out, the soil which sticks on its sides gives us the idea about the strata available.

(iii) Boring : In this, a hole is bore in the ground for taking soil samples. The exploration of soil below water table is usually very difficult to perform by means of test pits, Therefore, test pits are replaced by boring methods.

Method of boring

(a) Auger boring : It is the device which helps in advancing a bore hole into the ground. This is sample and an efficient method of soil investigation, Auger are use for clayey to silty soils.

Auger boring gives fairly representative disturb samples and a correct idea of different soil layers. The relative stiffness of the soil is indicate from the effort apply to rotate the auger. Auger is operate either by hand or by power. Hand operate augers are suitable for soft soils and up to a depth of 6 meters whereas power operate augers are suitable up to a depth of 9 meters. The most common augers in use are:

Post hole augers ( for general use )
Gravel augers ( for gravelly soils )
Screw augers ( for stiff clay )
Power or mechanically operate auger ( for greater depths and when boring is require in a comparatively short time )

(b) Wash boring : Wash boring is a quick and inexpensive method of deep boring. It is the best boring where sufficient water is available for the proper working of the water jet plant and also for underground formations consisting of sand, gravel and clayey formations and suitable for obtaining samples of soil as heavier soil particles remain under suspension. Small thickness of important soil layers also remain undetected. The change of stratification can be judge from the rate of progress and color of wash water.

(c) Rotary boring : This process of boring is also know as Mechanical boring. It is use for very deep boring specially when the soil consists of boulders or rock. This is the most fastest method of boring and the bore holes range from 10 to 75 cm.

(d) Percussion boring : In this, a drilling rod is raise up and down alternately in the bore hole. It is use in most of the soils and rocks. It is consider to be the most useful method in penetrating through coarse gravel deposits. They cannot be use for soil consisting of loose sand or fine sand.

Trail Pit Method

(iv) Trail pit method : The method in which a rectangular pit is excavate at the construction site in order to obtain soil samples at different levels is refer as trail pit method or open test pit method.

It is one of the simplest and most common method for investigating the sub-soil in its natural and undisturbed state. This method is limit to small depths up to 3 m only.

This method is very expensive and slow for depths exceeding 3 m and for cuts below water table due to proper bracing of sheet pilling or the use of caissons. If the depth exceeds 6 m, then, boring are prefer.