Tag: Site Exploration

Methods to Improve Bearing Capacity of soils

Some of the methods to improve bearing capacity of soils:

  1. Increasing the depth of the footing is the simplest method of improve the bearing capacity of soil, This method is restricted to sites where the sub-soil water level is much below and deep excavations do not increase the cost of foundations disproportionately.
  2. Drainage is a well known method to improve the bearing capacity of certain soils. Drains (with open joints) are laid in trenches just at the footing base. The sub-soil water thus collected is drained out through a system of pipe drains provided outside the external walls of the building.
  3. By blending granular material, like sand, gravel or crushed stone into the natural soil by ramming. The layer of soil thus formed is much stronger and is of improved bearing capacity.
  4. By confining the soil in an enclosed area with the help of sheet piles. This method is used with advantage in shallow foundations in sandy soils.
  5. By driving sand piles. This method is based on the principle of reducing the void volume of the natural soil. Holes are made in the soft soil with the help of wooden piles or other means and then sand is filled in the holes and rammed. These are called sand piles. Bearing capacity of soft soil can be appreciably improved by driving sand piles at close spacing.

 

Penetration Test Method Determine Bearing Capacity of Soils

Penetration Test Method – Standard Split Spoon Sampler

The equipment used for this test may be a standard split spoon sampler, a cone or other specially shaped tool. The test consists of measuring the resistance offered by the soil to the penetration of the test-tool under dynamic or static loading. The values of resistance, also known as N values, are co-related with properties of soil, such as density, bearing capacity etc. Thus, based on information about N values, it is possible to determine the bearing capacity of soil by use of standard graphs or tables. In this split spoon sampler method, a hole 55 to 150 mm in diameter is made in the ground with the help of suitable equipment for conducting the test. It may sometimes be necessary to use casing or drilling mud where the soil at site is sandy, soft clay or other such type where the sides of the hole are likely to cave in. At sites where casing is used to keep the walls of the test hole stable, it should not be driven below the level at which the test is to be made or soil sample is to be taken. The distance between end of casing and bottom of bore hole should not be more than 150 mm.The casing should preferably be sunk in ground by turning it slowly instead of driving it so as to ensure that it does not disturb the soil and hence the test is performed on undisturbed soil.

Standard Split Spoon Sampler

Standard Split Spoon Sampler

A standard split spoon sampler consists of a thick wall tube having outer diameter of 50.8 mm and internal diameter of 35 mm and a length of  6OO mm. The tube has a drive shoe attached to its bottom and coupling head at top to accommodate the drill rod, used for testing. The drill rod is coupled to sampler head and the sampler is lowered into the clean hole, made in the ground in advance. The sampler is driven into the undisturbed soil at the bottom of the hole, with the help of a driving weight assembly consisting of a driving head and a 650 N weight with 750 mm  free fall. The blows from the driving weight fall on the drill rod which drives the sampler into the soil. The sampler is first driven through 150 mm in the hole. This is known as seating drive. The sampler is then driven further through 300 mm and the number of blows required for 300mm penetration are recorded. This number of blows is termed as penetration resistance of soil and is represented by symbol N.

Setup for Driving the penetrometer in the ground

Setup for Driving the Penetrometer in the ground


The tests are made and samples are taken out at every change in stratum or at intervals of 15 m whichever is less.

In case of very fine or silty saturated sand, it is seen that although they have less value of bearing capacity, yet, due to apparent increase in resistance that such type of soil offer to penetration of the sampler, the N values obtained by this test are more and as such Terzaghi and Peck recommend adoption of an equivalent penetration resistance (Ns) in place of actually observed value of N, where N is greater than 15.

Ns is given by formula,

                                          Ns = 15 + 0.5(N-15)

Depth and Spacing of Trial Pits or Bore Holes

Depth and Spacing of Trial Pits or Bore Holes

The location and the spacing of trial pits or bore holes for a particular site require special consideration. The pits or the bore holes should be so located so as to give adequate information in respect of changes in properties of the underlying strata with depth. The number and spacing of the test pits or bore holes to be adopted for a site will depend upon the area of the plot as well as the type of structure to he built. For a plot of 0.4 hectare we may have one trial pit or bore hole near each corner and one in the middle. For smaller size of plot and for ordinary structure, one trial pit or bore hole near the centre should be sufficient. The depth of the pit or bore hole will depend upon the characteristics of the soil as well as the type of structure, its shape, size and loading condition. As a thumb rule, its depth should be one and a half times the probable width of the footing or 1.5 m whichever is more. In case of weak soils, however, the test pits or the bore holes should he taken to a depth at which the loads can be carried by the soil without undesirable settlement.

Geo-Physical Method of Soil Exploration

Geo-Physical Method of Soil Exploration

Geo-physical methods are used when soil exploration is to be carried out over large area and where speed is of prime importance. These soil exploration methods are based on principle that physical properties like electrical conductivity, elasticity or seismicity, magnetic susceptibility, density etc. vary for different types of soils. There are four  soil exploration methods of geo physical survey, namely, (1) Seismic refraction method (it) Electrical resistivity method, (iit Magnetic method and (ivy Gravitational method. However, out of these only two methods namely (1) Seismic refraction method and (ii) Electrical resistivity method are widely used.

(i) Seismic Refraction Method: This soil exploration method is based on the principle that sound waves travel faster in rock than in soil. This is on account of the fact that velocity of sound waves is different in different media. In this method shock waves  (or sound waves of vibration) are created into the soil at ground level or at a certain depth below it, either by striking a plate on the soil with the hammer or by exploding small charge in the soil. The shock waves so produced travel down in the sub-soil strata and get refracted after striking a hard rock surface below. The refracted or radiated shock waves are picked up by the vibration detector (also known as geophone) where the time of travel of the shock waves gets recorded. Knowing the time of travel of the primary and refracted waves at various geophones, tune and distance graphs are drawn based on which it is possible to evaluate the depth of various strata in the sub-soil. Different materials such as clay, gravel, silt rock, hard rock etc. have characteristics seismic velocities and hence it is possible to establish their identity in the sub-soil based on time distance graph.

Seismic Refraction Method

Seismic Refraction Method

(ii) Electrical Resistivity Method: This soil exploration method is based on the principle that each soil has different electrical resistivity, depending upon the type of soil, its water content, compaction and composition. Thus saturated soil has lower electrical resistivity as compared to loose dry gravel or solid rock. In this method 4 electrodes are driven in the ground at equal distance apart and in a straight line. The distance between two electrodes being the depth of exploration or depth up to which the ground resistance is to he measured. A current is passed between the two outer electrodes and the potential drop between the inner electrodes is measured by use of potentiometer.

Electrical Resistivity Method

Electrical Resistivity Method

The mean resistivity is calculated by the following formula:-

P = 2 π D (E/I)

Where,

P = mean resistivity (ohm.cm)
D = distance between electrodes (cm)
E =potential drop between inner electrodes (volts)
I
= current flowing between outer electrodes (amperes)

Average value of resistivity for various types of soils have already been established based on tests. Thus on knowing the values of change in mean resistivity of sub-soil strata at site, it is possible to establish the nature and distribution of different type of soils in the formation.

Boring – Types of Boring

Boring – Types of Boring

The types of boring methods commonly adopted for soil exploration are as under :-

(a) Auger boring
(b) Shell and Auger boring
(c) Wash boring
(d) Percussion boring
(e)
Rotary boring

(a) Auger boring:

The examination of the sub-soil conditions for simple buildings to be erected in clayey or sandy soil can be best performed by a post hole auger. The auger is held vertically and is driven into the ground by rotating its handle by applying leverage. The auger is pressed down during the process of rotation. At every 30 cm of depth penetrated, the auger is taken out and the samples of the soils are collected separately for examination. This method can be conveniently used for soil penetration up to 15 m depth. The type of augers commonly used are shown below. For deeper holes or in grounds where gravel, boulders or comp act material is present, this method is not adopted.

Types of Augers

(b) Shell and auger boring:

In this method different type of tools have to be adopted for boring. In case of soft to stiff clay, cylindrical auger consisting of a hollow tube of 75 to 200mm in diameter with a cutting edge at its bottom is used. In case of various stiff and hard clay, shells with cutting edge or teeth at lower end are to be adopted while in case of sandy soil, shells or sand, pumps are used for boring. By this method it is possible to make vertical boring up to 200 mm in diameter and 25 m in depth by use of a hand rig. By use of mechanical rig it is possible to extend the depth of the bore hole up to 50 m. The samples of the soil are recovered at regular intervals (or whenever there is a change in strata) for conducting tests in laboratory for identification of soils and establishing properties of the sub-soil strata at various depths.

(c) Wash boring:

For test boring over 3 meter in depth, this method can be conveniently used. In this method a hollow steel pipe known as casing pipe or drive pipe is driven into the ground for a certain depth. Then a pipe usually known as water jet pipe or wash pipe, which is shorter in diameter, is lowered into the casing pipe. At its upper end, the wash pipe is connected to water supply system while the lower end of the pipe is contracted so as to produce jet action. Water under considerable pressure is forced down the wash pipe. The hydraulic pressure displaces the material immediately below the pipe and the slurry thus formed is forced up through the annular space between the two pipes. The slurry is collected and samples of material encountered are obtained by settlement. In this process the particles of finer material like clay, loam etc. do not settle easily and the larger and heavy particles of the soil may not be brought up at all. Moreover, the exact position of a material in the formation cannot be easily be located. However the change of stratification can be guessed from the rate of progress of driving the casing pipe as well as the color of slurry flowing out. Yet the results obtained by wash boring process give fairly good information about the nature of the sub-soil strata. This method can be adopted in soft to stiff cohesive soils and fine sand.

wash boring installation

Wash Boring Installation

(d) Percussion boring:

This method consists of breaking up of the sub-strata by repeated blows from a bit or chisel. The material thus pulverized is converted into slurry by pouring water in the bore. At intervals the slurry is bailed out of the hole and dried for examination. This method can be adopted in rocks and soils having boulders. However this method is not recommended for loose sand or clayey soils.

(e) Rotary drilling:

When rocks or hard pans are to be penetrated for examination, core drilling is done to get undisturbed samples of the formation. In this process a hole is made by rotating a hollow steel tube having a cutting bit at its base. The cutting bit makes an annular cut in the strata and leaves a cylindrical core of the material in the hollow tube. Two types of cutting bits are generally used, namely, diamond bit and shot bit. Diamond bit consists of industrial diamonds set in the face of the bit and in shot bit, chilled shot is used as an abrasive to cut the hard pan. When core samples of small diameter are needed, diamond bit is preferred.

Subsurface Soundings

Subsurface Soundings

The method of subsurface soundings consists of measuring the variation in the resistance offered by the soil with the depth by means of a tool known as penetrometer. The penetrometer may consist of a 50 mm diameter mild steel cone fitted loosely to a steel rod or it may be a tool known as Standard Split Spoon Sampler. The sampler can be split into two parts longitudinally for removal of the soil sample which get filled up in its tube when, it is driven in the ground. The penetrometer is driven in the ground with the help of blows from a 650 N weight falling from a height of 750 mm. The number of blows required to drive the penetrometer into the ground through a distance of 300 mm is termed as Standard Penetration Resistance or SPT of the soil and it is designated as N. The ‘N’ values of soil are thus determined at different depths. The resistance offered by the soil to penetration is co-related with the engineering properties of soil like density, consistency (strength), permeability and bearing capacity. Curves co-relating the N values with bearing capacity of certain types of soil have been drawn and are available for ready reference.

Subsurface Soundings

Probing

Probing

In the probing method of site exploration, a steel bar of 25 to 40 mm. in diameter with a pointed end is driven in the ground until a hard sub-stratum is reached. The bar is driven by allowing it to fall vertically under its own self weight or by means of a drop hammer. At intervals the bar is drawn out and some idea of the nature of the soil is obtained by examining the soil sticking to the sides of the bar. Probing method is suitable to be performed in places where the soil is soft like clay, gravel or sand. A rough idea of the nature of strata is obtained from the number of blows required to drive the rod inside the ground. Experienced workmen engaged in the execution of the driving operation may be able to judge the soil at various depths by reading the way the rod penetrates into the ground. Probing method is not always to be relied upon completely. More definite analysis should be done to ascertain the results arrived at by this method.

Test Pits – Soil Exploration

Test Pits

The commonly used method to find the nature of the subsoil strata is to dig a hole and see. The holes which are large enough to permit the entry of persons for inspection are called ‘Test Pits’. The pits are normally square in plane and are dug by hand or by excavating equipment. The dimensions of the pit depend largely upon the depths up to which the excavation is to be made. In cohesion-less soils, the sides of the test pit are sharply sloped. In cohesive soils, at depth below 3 m, bracing is required to keep the sides of the pit vertical.These are comparatively expensive and hence they are only used for structure having shallow foundations (up to 3 meters).

Test Pits

Purpose of Site Exploration

The purpose of site exploration is to collect complete details of the site to enable the designer to take following decisions,

(i) To fix the value of the safe bearing capacity of soil.
(ii) To select an economical yet safe type of foundation.
(iii) To fix the depth up to which the foundation must be taken inside the ground.
(iv) To predict the likely settlement of the selected foundation and to make allowance, for the same in the design.
(v) To know the underground water level and if needed, to decide upon the method to be adopted to solve the ground water problem.
(vi) To forecast the difficulties which are likely to be encountered due to nature of the sub-soil during construction and to take advance actions in this regard.