2)indirect methods

Sounding methods Standard penetration Test Static Cone Penetration Test Dynamic Cone Penetration Test

This method is also used to skin friction values which is used to determine the length of the piles

The cone is pushed only by thrust and not by driving

In order to find out the cone resistance , the cone alone is pushed

Later the cone and sleeve is pushed together to find out the combined frictional and point resistance of the cone.

Hydraulic gauges are used for measuring pressure developed

Frictional resistance = Combined resistance – Cone resistance

Modified Cone penetrometer is known as Refined Dutch Cone

Cone penetration resistance is denoted as qc in kN/m2

Unlike SPT, this method is also suitable for clayey deposits

Unsuitable for gravels and dense sand. For such soil dynamic Cone penetration is used

Point cone resistance Vs SPT Correlation

Type of Soil Qc (kN/m2) VsSPT ‘N’

Gravel 800 to 1000Sands 500 to 600Silty sands 300 to 400Silts and clayey silts 200

Cone and Friction assembly for CPT

Dynamic Cone Penetration Test IS 4968 –part I and II 1976

Cone is Driven by means of hammer blows No. of blows required for every 100 mm penetration is

measured No. of blows required for 300mm penetration is taken as

dynamic cone penetration resistance Height of Fall of hammer : 750 mm Weight of Hammer : 65 kg

Cone Diameter : i)50mm (without Bentonite slurry) ii)65mm (with Bentonite slurry)

Cone used for DCPT

Typical Test Set up for DCPT

For 50 mm Diameter

Depth Ncr Vs SPT ‘N’(50 mm Dia)

3m 1.5 N3m to 6m 1.75 N> 6m 2.0 N

For 65 mm Diameter

Depth Ncr Vs SPT ‘N’(50 mm Dia)Upto 4m 1.54 to 9 m 1.75>9m 2N

Geophysical methodsGeophysical methods are used for

preliminary investigations, are quick and results are obtained rapidly.

Seismic Refraction Electrical Resitivity

Seismic refraction method Shock waves are created either using sledge

hammer or using small explosives These shock waves travel through the ground and

are picked up by the detector called geophones Geo phones are the devices that detects vibrations

and converts them into measurable electric signals. The travel time of elastic waves are automatically

recorded by seismograph The waves which travel along the ground surface

are called primary waves or direct waves. These arefirst recorded at the geophones

The waves which travel to the inner surface will get refracted if the inner layers are denser. These waves are called refracted waves

As the distance between the source and the geophones increases and if the inner layers are denser, the waves are refracted fast and they reach the geophones earlier

The distance at which both the direct and refracted waves reach the geophones simultaneously is known as critical distance which depends on depth and velocity ratio

Seismic refraction method

Typical Wave Velocities for Different Materials (IS: 1892-1979 Appendix B)

A graph is plotted between distance and the time. The reciprocal of slope of the curve gives velocity

There is a break in the graph which shows critical distance after which the graph becomes flatter

The thickness H1 of the top layer is given by, if V1 and V2 are the velocities of upper and lower layer and dc is the critical distance.

Distance Vs Time Graph

Limitations This method can not be used in the case

where hard layer overlies a soft layer This cannot be applicable for concrete or

asphalt pavements, irregular surfaces, surface with discontinunities like faults, joints etc and for frozen surfaces

Electrical Resitivity method

Electrical resistivity is the property of the material to hinder the passage of electricity. It differs based on the nature of the material and percentage of water content

Dry soil and rocks possess more resistivity than saturated sand

The Wenner configuration with four equally spaced electrodes is popularly used.

Four electrodes are placed at definite spacing in the ground. Electric current of 50 to 100 milliamperes is passed between the two outer electrodes.

The potential drop is measured between the two inner electrode using volt meter

Wenner configuration for electrical resistivity

Typical Values of Electrical Resistivity of Soils and Rocks(1 to 8 from IS: 1892-1979 Appendix B)

Electrical profiling and Electrical sounding are the two types of field procedure

Electrical profiling are used for establishing boundaries between different types of materials

Electrical sounding are used to find variation of soil strata with depth

The potential drop at each electrode is plotted against spacing. A change in the curve indicates the change of strata

Electrode arrangement for electrical profiling and electrical

sounding

Applications This method is used to find the depth of

water table. This has practical application in prospecting for sand and gravel deposits or ore deposits.

This is useful in soil investigation for major civil Engineering construction.