STANDARD PENETRATION TEST

Generally used to determine the bearing capacity of sand and gravel soil.

A split spoon sampler is used. It is a sampler tube which can be split open longitudinally after sampling (Fig A).Coss

section is shown in Fig B.

Fig A

Fig B

A split spoon sampler is driven by a hammer of 65 kg falling from 750mm.

Number of blows required for each 150mm is recorded. See Fig. C.

Number of blows for 300mm penetration is called N value.

The number of blows for the first 150mm is neglected because the top soil may be loose will not simulate the actual

ground condition.

Corrections in N value.:

1. Over burden Correction: An important feature of SPT is the influence of effective over burden pressure on the N

count. Sand can exhibit different N value at different depths even though its relative density is constant. It was

concluded by engineers that significant feature affecting the N value is relative density and not effective over burden.

In order to remove the effect overburden in N value this correction is applied.

There are number of relations for over burden correction.

1. NC over burden (N value after overburden correction) = N observed * Coverburden (correction factor)

There are number of equations for calculating over burden correction.

Coverburden =350/( б+70). This is applied when б is less than 280kN/m2

Here ‘б’ = effective overburden pressure.

There are many other empirical formulas for calculating over burden correction factor.

IS 2131-1987 gives a graph between effective over burden pressure and correction factor.

2. Dilatancy correction: In saturated fine and silty sand the N value can be altered by low permeability of soil.

When SPT is conducted the soil is subjected to impact load – a dynamic load. In saturated soil this load application is

under undrained condition and generates pore water pressure. Hence to eliminate the effect of pore water pressure

dilatancy correction is applied. Dilatancy correction is applied to soils whose N value is less than 15.

N C DILATANCY = 15 + (NC over burden – 15)/2

Note: Overburden correction is applied first and then correction for dilatancy.

Importance of SPT ‘N’ value : N value represents compression and shear strength parameters of the soil. N value is

correlated to shear strength parameters (Cohesion and angle of internal friction). Design of foundations can be done

directly using N value.

Problem:

FIELD PERMEABILITY TEST

Flow of water through soil or any porous media (in saturated condition) follows Darcy’s law. It states that velocity of

flow of liquid (water) through a saturated porous medium is directly proportional to the hydraulic gradient (i) causing

the flow.

Ie. V α i

or V=ki

Here k is called coefficient of permeability. K is an important parameter in soil mechanics especially in consolidation

analysis, seepage analysis etc.

K is determined in lab by either constant head parameter test (used for gravelly sand) and by falling head parameter

test (used for silty sand), Covered in soil mechanics course.

Determination of k in the field

In field k is determined by pumping out test or by pumping in tests.

1. PUMPING OUT TEST

The test can be used to measure the average k value of a stratum of soil below water table effectively up to depths

of about 45m.

A casing of about 40cm (main well or pumping well) is driven to bedrock or to impervious stratum. 2 Observation

wells (may be more) are placed at distance r1 and r2 from the main well. (See fig.) The casing of the main well and

the observation well are perforated.

The test consists of pumping water from the central well @ a measured rate q, and observing the resulting

drawdown in ground water level by means of observation wells.

The min. Distance between the main well and observation wells should be ten times the radius of the pumping

well and at least one of the observation wells in each row should be at a radial distance greater than twice the

thickness of the ground being tested.

Calculation of K:

Let h be the depth of water at radial distance r. The area of the vertical cylindrical surface of radius r and

depth h through which water flows is

A = 2*π*r*h

The hydraulic gradient is i = dh/dr

As per Darcy's law the rate of inflow into the well when the water levels in the wells remain stationary is : q

= kiA.

Substituting for A and i in

q =K (dh/dr)*2* π*r*h

Rearranging the terms, we have

dr/r= (2 π k h dh)/q

The integral of the equation within the boundary limits is

Equation for K after rearranging :

Radius of Influence R^ Based on experience, Sichardt (1930) gave an equation for estimating the radius of influence

for the stabilized flow condition as

Ri = 3000D0*(k)^0.5& meters.

where DQ = maximum drawdown in meters

k = hydraulic conductivity in m/sec

PROBLEM:

Pumping in test

Where bedrock level is very deep or where the permeabilities of different strata are required the pumping in test can be

used, A casing, perforated for a meter or so at its end, is driven into the ground. At intervals during the driving the rate

of flow required to maintain a constant head in casing is determined and a measure of soil’s permeability obtained.

GEOPHYSICAL METHODS

The determination of the nature of the subsurface materials through the use of borings and test pits can be time-

consuming and expensive. Considerable interpolation between checked locations is normally required to arrive at an

area-wide indication of the conditions. Geophysical methods involve the technique of determining subsurface

materials by measuring some physical property of the materials, and through correlations, using the values obtained

for identifications. Most geophysical methods determine conditions over large distances and can be used to obtain

rapid results. Thus, these are suitable for investigating large areas quickly, as in preliminary investigations.

A number of methods have been devised, but are mostly useful in the study of geologic structure and exploration for

mineral wealth. However, two methods have been found to be useful for site investigation in the geotechnical

engineering profession. They are the seismic refraction and the electrical resistivity methods. Although these have

proven to be reliable, there are certain limitations as to the data that may be got; hence, spot checking with boringsand

pits has to be necessarily undertaken to complement the data obtained by the geophysical methods.

1. Seismic Refraction:

When a shock or impact is made at a point on or in the earth, the resulting seismic (shock or sound) waves travel

through the surrounding soil at speeds related to their elastic characteristics.

The velocity is given by:

where, v = velocity of the shock wave,

E = modulus of elasticity of the soil, g = acceleration due to gravity,

γ = density of the soil, and

C = a dimensionless constant involving Poissons’s ratio.

The magnitude of the velocity is determined and is utilised to identify the material.

A shock may be created with a sledge hammer hitting a strike plate placed on the ground or by detonating a small

explosive charge at or below the ground surface. The radiating shock waves are picked up by detectors, called ‘geophones’, placed in a line at increasing distances, d1, d2, ..., from the origin of the shock (The geophone is actually

a transducer, an electromechanical

device that detects vibrations and converts them into measurable electric signals). The time required for the elastic wave to reach each geophone is automatically recorded by a ‘seismograph’. Some of the waves, known as direct or

primary waves, travel directly from the source along the ground surface or through the upper stratum and are picked

up first by the geophone.

If the sub soil consists of two or more distinct layers, some of the primary waves travel down wards to the lower layer and get refracted as the surface. If the underlying layer is denser, the refracted waves travel much faster. As the

distance from the source and the geophone increases, the refracted waves reach the geophone earlier than the direct

waves. Figure below shows the diagrammatic representation of the travel of the primary and the refracted waves. The distance of the point

at which the primary and refracted waves reach the geophone simultaneously is called the ‘critical distance’ which is a

function of the depth and the velocity ratio of the strata.

There are certain significant limitations to the use of the seismic refraction method for

determining the subsurface conditions. These are:

1. The method cannot be used where a hard-layer overlies a soft layer, because there will be no measurable refraction from a deeper soft layer. Test data from such an area would tend to give a single-slope line on the travel-time graph,

indicating a

deep layer of uniform material.

2. The method cannot be used in an area covered by concrete or asphalt pavement, since these materials represent a condition of hard surface over a softer stratum.

3. A frozen surface layer also may give results similar to the situation of a hard layer over a soft layer.

4. Discontinuities such as rock faults or earth cuts, dipping or irregular underground rock surface and the existence of thin layers of varying materials may also cause misinterpretation of test data.

Electrical Resistivity

Resistivity is a property possessed by all materials. The electrical resistivity method is based on the fact that in soil and rock materials the resistivity values differ sufficiently to permit that property to be used for purposes of identification.

Resistivity is usually defined as the resistance between opposite faces of a unit cube of the material. Each soil has its

own resistivity depending upon the water content, compaction and composition; for example, the resistivity is high for

loose dry gravel or solid rock and is low for saturated silt. To determine the resistivity at a site, electrical currents are induced into the ground through the use of electrodes. Soil resistivity can then be measured by determining the change

in electrical potential between known horizontal distances within the electric field created by the current electrodes.

The four electrodes are placed in a straight line at equal distances as shown below.

A direct voltage, causing a current of 50 to 100 milliamperes typically, is applied between the outer electrodes and the

potential drop is measured between the two inner electrodes by a null-point circuit that requires no flow of current at

the instant of measurement. In a semi-infinite homogeneous isotropic materials the electrical resistivity, , is given by:

D E/I

where, D = distance between electrodes (m),

E = potential drop between the inner electrodes (Volts),

I = current flowing between the outer electrodes (Amperes), and

= mean resistivity (ohm/m).

The calculated value is the apparent resistivity, which is a weighted average of all material within the zone created by the electrical field of the electrodes. The depth of material included in the measurement (depth of penetration) is

approximately the same as the spacing between the electrodes. It is necessary to make a preliminary trial on known

formations, in order to be in a

position to interpret the resistivity data for knowing the nature and distribution of soil formations. Average values of

resistivity for various rocks, minerals and soils available. Two different field procedures are used to obtain information on subsurface conditions. One method, known as

“electrical profiling”, is well-suited for establishing boundaries between different underground materials and has

practical application in prospecting for sand and gravel deposits or ore deposits. The second method, called “electrical

sounding’, can provide information on the variation of subsurface conditions with depth and has application in site investigation for major

civil engineering construction. It can also provide information on depth of water-table.

In electrical profiling, an electrode spacing is selected, and this same spacing is used in running different profile lines across an area, as in Fig. (a). In electrical sounding, a centre location for the electrodes is selected and a series of

resistivity readings is obtained by systematically increasing the electrode spacing, as shown in Fig. (b). Thus,

information on layering of materials is obtained as the depth of information recovered is directly related to electrode sparing. This method is capable of indicating

subsurface conditions where a hard-layer underlies a soft layer and also the situation of a soft layer underlying a hard

layer