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ANJUMAN COLLEGE OF ENGINEERING & TECHNOLOGY
MANGALWARI BAZAAR ROAD, SADAR, NAGPUR - 440001.
DEPARTMENT OF CIVIL ENGINEERING
Prof. Rashmi G. Bade, Department of Civil Engineering, Geotechnical Engineering – II 1
Geotechnical Engineering – II
B.E. FIFTH SEMESTER
ANJUMAN COLLEGE OF ENGINEERING & TECHNOLOGY
MANGALWARI BAZAAR ROAD, SADAR, NAGPUR - 440001.
DEPARTMENT OF CIVIL ENGINEERING
Prof. Rashmi G. Bade, Department of Civil Engineering, Geotechnical Engineering – II 2
UNIT – V
SHALLOW FOUNDATIONS:
Bearing capacity of soils : Terzagi‟s theory , its validity and limitations , bearing capacity factors ,
types of shear failure in foundation soil , effect of water table on bearing capacity factors , types of
shear failure in foundation soil , effect of water table on bearing capacity , correction factors for
shape and depth of footings. Bearing capacity estimation from N-value , factors affecting bearing
capacity , presumptive bearing capacity.
Settlement of shallow foundation : causes of settlement , elastic and consolidation settlement ,
differential settlement , control of excessive settlement. Proportioning the footing for equal
settlement . Plate load test : Procedure , interpretation for bearing capacity and settlement prediction.
(8)
ANJUMAN COLLEGE OF ENGINEERING & TECHNOLOGY
MANGALWARI BAZAAR ROAD, SADAR, NAGPUR - 440001.
DEPARTMENT OF CIVIL ENGINEERING
Prof. Rashmi G. Bade, Department of Civil Engineering, Geotechnical Engineering – II 3
INRODUCTION
It is the customary practice to regard a foundation as shallow if the depth of the foundation is less
than or equal to the width of the foundation. A foundation is an integral part of a structure. The
stability of a structure depends upon the stability of the supporting soil. Two important factors that
are to be considered are:-
1. The foundation must be stable against shear failure of the supporting soil.
2. The foundation must not settle beyond a tolerable limit to avoid damage to the structure.
Figure.1 Types of shallow foundations: (a) plain concrete foundation, (b) stepped reinforced
concrete foundation, (c) reinforced concrete rectangular foundation, and (d) reinforced concrete wall
foundation.
DEFINITIONS
1) Footing:- A footing is a portion of the foundation of a structure that transmits loads directly
to the soil.
2) Foundation: - A foundation is that part of he structure which is in direct contact with and
transmits loads to the ground.
3) Foundation soil: - It is the upper part of the earth mass carrying the load of the structure.
ANJUMAN COLLEGE OF ENGINEERING & TECHNOLOGY
MANGALWARI BAZAAR ROAD, SADAR, NAGPUR - 440001.
DEPARTMENT OF CIVIL ENGINEERING
Prof. Rashmi G. Bade, Department of Civil Engineering, Geotechnical Engineering – II 4
4) Bearing capacity: - The supporting power of a soil or rock is referred to as its bearing
capacity.
5) Gross pressure intensity (q):- The gross pressure intensity q is the total pressure at the base
of the footing due to the weight of the superstructure, self-weight of the footing and the
weight of the earth fill, if any.
6) Net pressure intensity (qn):- It is defined as the excess pressure, or the difference in
intensities of the gross pressure after the construction of the structure and the original
overburden pressure. Thus, if D is the depth of the footing
7) Ultimate bearing capacity (qf):- The ultimate bearing capacity is defined as the minimum
gross pressure intensity at the base of the foundation at which the soil fails in shear.
8) Net ultimate nearing capacity (qnf):- It is minimum net pressure intensity causing shear
failure of soil. The ultimate bearing capacity qf and the net ultimate capacity are evidently
connected by the following relation:
Where, σ is the effective surcharge at the base level of the foundation.
9) Net safe bearing capacity (qns):- The net safe bearing capacity is the net ultimate bearing
capacity divided by a factor of safety F
10) Safe bearing capacity (qs):- The maximum pressure which the soil can carry safely without
risk of shear failure is called the safe bearing capacity.
11) Allowable bearing capacity or pressure (qa):- It is the net loading intensity at which
neither the soil fails in shear nor there is excessive settlement detrimental to the structure.
BEARING CAPACITY OF SOILS: Terzagi’s theory, its validity and limitations
Terzaghi (1943) used the same form of equation as proposed by Prandtl (1921) and extended
his theory to take into account the weight of soil and the effect of soil above the base of the
foundation on the bearing capacity of soil. Terzaghi made the following assumptions for developing
an equation for determining qu for a c-Ф soil.
(1) The soil is semi-infinite, homogeneous and isotropic,
(2) the problem is two-dimensional,
(3) the base of the footing is rough,
(4) the failure is by general shear,
(5) the load is vertical and symmetrical,
(6) the ground surface is horizontal,
(7) the overburden pressure at foundation level is equivalent to a surcharge load q'0 = γDf where γ is
the effective unit weight of soil, and D the depth of foundation less than the width B of the
foundation,
ANJUMAN COLLEGE OF ENGINEERING & TECHNOLOGY
MANGALWARI BAZAAR ROAD, SADAR, NAGPUR - 440001.
DEPARTMENT OF CIVIL ENGINEERING
Prof. Rashmi G. Bade, Department of Civil Engineering, Geotechnical Engineering – II 5
(8) the principle of superposition is valid, and
(9) Coulomb's law is strictly valid, that is, σ = c + σ tan Ф.
Limitations:-
(1) As the soil compresses, Ф changes, slight downward movement of footing may not develop
the plastic zones.
(2) Error due to separate calculation of three component of Pp, and then their addition, although
their critical surfaces are not identical, is small and on the safe side.
(3) Error due to assumption that failure zones do not extend above horizontal plane through the
base of footing, increase with the depth of foundation, and hence the theory is suitable for
shallow foundation only.
Mechanism of Failure
The shapes of the failure surfaces under ultimate loading conditions are given in Fig. 2. The zones of
plastic equilibrium represented in this figure by the area gedcf may be subdivided into
1. Zone I of elastic equilibrium
2. Zones II of radial shear state
3. Zones III of Rankine passive state
When load qu per unit area acting on the base of the footing of width B with a rough
base is transmitted into the soil, the tendency of the soil located within zone I is to spread but this is
counteracted by friction and adhesion between the soil and the base of the footing. Due to the
existence of this resistance against lateral spreading, the soil located immediately beneath the base
remains permanently in a state of elastic equilibrium, and the soil located within this central Zone I
behaves as if it were a part of the footing and sinks with the footing under the superimposed load.
The depth of this wedge shaped body of soil abc remains practically unchanged, yet the footing
sinks. This process is only conceivable if the soil located just below point c moves vertically
downwards. This type of movement requires that the surface of sliding cd (Fig. 2) through point c
should start from a vertical tangent. The boundary be of the zone of radial shear bed (Zone II) is also
the surface of sliding. As per the theory of plasticity, the potential surfaces of sliding in an ideal
plastic material intersect each other in every point of the zone of plastic equilibrium at an angle (90°
- 0). Therefore the boundary be must rise at an angle Ф to the horizontal provided the friction and
adhesion between the soil and the base of the footing suffice to prevent a sliding motion at the base.
Figure. 2 General shear failure surface as assumed by Terzaghi for a strip footing.
ANJUMAN COLLEGE OF ENGINEERING & TECHNOLOGY
MANGALWARI BAZAAR ROAD, SADAR, NAGPUR - 440001.
DEPARTMENT OF CIVIL ENGINEERING
Prof. Rashmi G. Bade, Department of Civil Engineering, Geotechnical Engineering – II 6
The sinking of Zone I creates two zones of plastic equilibrium, II and III, on either side of the
footing. Zone II is the radial shear zone whose remote boundaries bd and a/meet the horizontal
surface at angles (45° - Ф/2), whereas Zone III is a passive Rankine zone. The boundaries de and fg
of these zones are straight lines and they meet the surface at angles of (45° - Ф/2). The curved parts
cd and cf in Zone II are parts of logarithmic spirals whose centers are located at b and a respectively.
TYPES OF SHEAR FAILURE
1) GENERAL SHEAR FAILURE: -
Fig.a shows a strip footing resting on the surface of a dense sand or a stiff clay. The figure
also shows the load settlement curve for the footing, where ‘q’ is the load per unit area and
‘s’ is the settlement. At a certain load intensity equal to qu, the settlement increases suddenly.
A shear failure occurs in the soil at that load and the failure surfaces extend to the ground
surface. This type of failure is known as general shear failure. A heave on the sides is always
observed in general shear failure.
2) LOCAL SHEAR FAILURE: -
Fig.b shows a strip footing resting on a medium dense sand or on a clay of medium
consistency. The figure also shows the load – settlement curve. When the load is equal to a
certain value . The foundation movement is accompanied by sudden jerks. The failure
surfaces gradually extend outwards from the foundation, as shown. However, a considerable
movement of the foundation is required for the failure surfaces to extend to the ground
surface (shown dotted). The load at which this happens is equal to qu, beyond this point, an
increase of load is accompanied by a large increase in settlement. This type of failure is
known as local shear failure. A heave is observed only when there is substantial vertical
settlement.
3) PUNCHING SHEAR FAILURE: -
Fig.c shows a strip footing resting on a loose sand or soft clay. In this case, the failure
surfaces do not extend up to the ground surface. There are jerks in foundation at a load of
. The footing fails at a load qu at which stage the load – settlement curve becomes steep
and practically linear. This type of failure is called the punching shear failure. No heave is
observed. There is only vertical movement of footing.
ANJUMAN COLLEGE OF ENGINEERING & TECHNOLOGY
MANGALWARI BAZAAR ROAD, SADAR, NAGPUR - 440001.
DEPARTMENT OF CIVIL ENGINEERING
Prof. Rashmi G. Bade, Department of Civil Engineering, Geotechnical Engineering – II 7
Figure. 3 Types of shear failure.
EFFECT OF WATER TABLE ON BEARING CAPACITY
When the water table is above the base of the footing, the
submerged weight γ’ should be used for the soil below the water table for computing the effective
pressure or the surcharge. When the water table is located somewhat below the base of the footing,
the elastic wedge is partly of moist soil and partly of submerged soil and a suitable reduction factor
should be used with the wedge term ½ γBNγ, since it uses effective unit weight.
CASE I : - Water table located above the base of footing
ANJUMAN COLLEGE OF ENGINEERING & TECHNOLOGY
MANGALWARI BAZAAR ROAD, SADAR, NAGPUR - 440001.
DEPARTMENT OF CIVIL ENGINEERING
Prof. Rashmi G. Bade, Department of Civil Engineering, Geotechnical Engineering – II 8
The effective surcharge is reduced as the effective weight below the water table is equal to the
submerged unit.
q = Dw γ + a γ’
where Dw = Depth of water table below the ground surface,
a = height of water table below the base of footing.
= Df – Dw
q = Dw γ + (Df – Dw) γ’
= γ’ Df + (γ – γ’) Dw
Therefore, ultimate bearing capacity is given by
CASE II : - Water table located at a depth ‘b’ below base
If the water table is located at the level of the base of footing or below it, the surcharge term
is not affected. However, the unit weight is modified as,
where b = depth of water table below the base,
B = base width of the footing.
Therefore,
When b = 0, i.e. W/T at the base,
When b = B, i.e. W/T at depth B below the base.
ANJUMAN COLLEGE OF ENGINEERING & TECHNOLOGY
MANGALWARI BAZAAR ROAD, SADAR, NAGPUR - 440001.
DEPARTMENT OF CIVIL ENGINEERING
Prof. Rashmi G. Bade, Department of Civil Engineering, Geotechnical Engineering – II 9
Hence, when the ground water table is located at a depth ‘b’ equal to or greater than B there
is no effect on the ultimate bearing capacity.
For intermediate positions, linear interpolation of reduction be made. For any position of the
water table equations are as below: -
Here, Rw1 and Rw2 are water reduction factors.
When water is much below or at greater depth, then no effect of water table is to be
considered.
SKEMPTON’S ANALYSIS FOR COHESIVE SOILS
Skemton (1951) showed that the bearing capacity factor Nc in Terzaghi’s equation tends to
increase with depth for a cohesive soil (Фu = 0, c = cu). Fig.4 shows the variation of Nc with Df/B
ratio for strip and circular (or square) footings. For a strip footing, the value of Nc is equal to 5.14 for
the surface footing and has a maximum value of 7.50 and Df / B ratio ≥ 4.50.
For square and circular footings, the value of Nc is equal to 6.2 for the surface footing. The
maximum value of about 9.0 is attained for Df / B ratio equal to or greater than 4.50. The curve for
square and circular footings can also be used for rectangular footings using the following relation.
Alternatively, the curve for the strip can be used, making use of the following relation.
The following approximately relations can be used for the determination of Nc for different Df /B
ratios.
(a) Df/B < 2.50
(b) Df/B ≥ 2.50
Ultimate Bearing capacity
For Фu = 0, Nq = 1.0 and Nγ = 0.0
ANJUMAN COLLEGE OF ENGINEERING & TECHNOLOGY
MANGALWARI BAZAAR ROAD, SADAR, NAGPUR - 440001.
DEPARTMENT OF CIVIL ENGINEERING
Prof. Rashmi G. Bade, Department of Civil Engineering, Geotechnical Engineering – II 10
Therefore,
The net ultimate bearing capacity becomes
This is used for the determination of the net ultimate bearing capacity of footings on cohesive
soils, taking Nc value given by Skempton. It may be mentioned that Terzaghi’s value of Nc is
applicable only for shallow footings (Df < B), whereas Skempton’s value can be used for all values
of Df/B ratio.
Figure.4 Skempton’s chart.
SETTLEMENT OF FOUNDATION
(a) Settlement under loads
Foundation settlement under loads can be classified into 3 types.
(1) Immediate or elastic settlement (Si): - Immediate or elastic settlement takes place
during or immediately after the construction of the structure. It is also known as the
distortion settlement as it is due to distortions (and not the volume change) within the
foundation soil. Although the settlement is not truly elastic, it is computed using elastic
theory, especially for cohesive soils.
(2) Consolidation settlement (Sc): - This component of the settlement occurs due to gradual
expulsion of water from the voids of the soil. This component is determined using
Terzaghi’s theory of consolidation.
(3) Secondary Consolidation Settlement (Ss): - This component of the settlement is due to
secondary consolidation. This settlement occurs after completion of the primary
ANJUMAN COLLEGE OF ENGINEERING & TECHNOLOGY
MANGALWARI BAZAAR ROAD, SADAR, NAGPUR - 440001.
DEPARTMENT OF CIVIL ENGINEERING
Prof. Rashmi G. Bade, Department of Civil Engineering, Geotechnical Engineering – II 11
consolidation. It can be determined form the coefficient of secondary consolidation. The
secondary consolidation is not significant for inorganic clays and silty soils.
The total settlement (s) is given by
s = si + sc + sr
(b) Settlement due to other causes
In addition to settlement under loads, the settlement may also occur to a number of other
causes.
1) Underground erosion: - Underground erosion may cause formation of cavities in the
subsoil which when collapse cause settlement.
2) Structural collapse of soil: - Structural collapse of some soils, such as saline, non-
cohesive soils, gypsum, silts and clays and loess, may occur due to dissolution of
materials responsible for intergranular bond of grains.
3) Thermal changes: - Temperature change cause shrinkage in expansive soils due to which
settlement occurs.
4) Frost heave: - Frost heave occurs if the structure is not founded below the depth of frost
penetration.
5) Vibration and Shocks: - Vibrations and shock cause large settlements, especially in
loose, cohesionless soils.
6) Mining subsidence: - Subsidence of ground may occur due to removal of minerals and
other materials from mines below.
7) Land slides: - If land slides occur on unstable slopes, there may be serious settlement
problems.
8) Creep: - The settlement may also occur due to creep on clay slopes.
9) Changes in the vicinity: - If there are changes due to construction of a new building near
the existing foundation, the settlement may occur due to increase in the stresses.
Suitable measures are taken to reduce the settlements due to all above causes.
BEARING CAPACITY OF SQUARE AND CIRCULAR FOOTING
Based on experimental results, Terzaghi gave the following equations for the ultimate bearing
capacity for square and circular shallow footings.
(a) Square Footing: -
where ‘B’ is the dimension of each side of footing.
(b) Circular Footing: -
where ‘B’ is the dimension of each side of footing.
The bearing capacity factors Nc, Nq and Nγ are the same as that for the strip footing.
ANJUMAN COLLEGE OF ENGINEERING & TECHNOLOGY
MANGALWARI BAZAAR ROAD, SADAR, NAGPUR - 440001.
DEPARTMENT OF CIVIL ENGINEERING
Prof. Rashmi G. Bade, Department of Civil Engineering, Geotechnical Engineering – II 12
IMMEDIATE SETTLEMENT OF COHESIONLESS SOILS
As cohesionless soils do not follow Hooke’s law, immediate settlements are computed using
a semi – empirical approach proposed by Schmertmann and Hartman (1978).
where, C1 = correction factor for the depth of foundation embedment =
C2 = correction factor for creep in soils [ = 1 + 0.2 log10 (time in years/0.1].
q = pressure at the level of the foundation, q = surcharge (= γ Df),
Es = modulus of elasticity, Iz = strain influence factor.
The value of the strain-influence factor Iz varies linearly for a square or circular foundation.
The value of Iz at depth z = 0, 0.5 B and 2B are respectively equal to 0.1, 0.5 and 0.0. For rectangular
foundations, with L/B ratio, between 1.0 and 10.0, interpolation can be made.
The value of Es can be determined from the standard penetration number (N) using the
following equations given by Schmertmann (1970).
Es = 766N (kN/m2)
Alternatively, it can be estimated from the static cone penetration resistance (qc) as
Es = 2 qc.
Procedure: - For computation of the immediate settlement, the soil layer is divided into several
layers of thickness Δz, upto a depth z = 2B, in case of square footings and z = 4B, in case of
rectangular footings. The immediate settlement of each layer is computed using equation of si, taking
corresponding values of Es and Iz. The required immediate is equal to the sum of the settlements of
all individual small layers.
ANJUMAN COLLEGE OF ENGINEERING & TECHNOLOGY
MANGALWARI BAZAAR ROAD, SADAR, NAGPUR - 440001.
DEPARTMENT OF CIVIL ENGINEERING
Prof. Rashmi G. Bade, Department of Civil Engineering, Geotechnical Engineering – II 13
ACCURACY OF FOUNDATION SETTLEMENT PREDICTION
The prediction of the foundation settlements prediction:-
1) The soil deposits are sudden isotropic and linearly elastic. The deposits are generally non-
homogeneous.
2) It is not possible to estimate the increase in stresses caused by loads. The Boussinesq solution
gives only approximate results.
3) For estimation of the settlement due to consolidation, it is not possible to locate exactly the
drainage faces.
4) For computation of immediate settlements, it is not possible to estimate the correct value of
the modulus of elasticity.
5) The rigidity of the foundation is usually neglected and the pressure distribution is assumed to
be uniform.
6) It is difficult to obtain undisturbed samples of cohesionless soils. The semi-empirical
methods do not give accurate results.
7) Settlements may occur due to causes other than that due to loads. It is not possible to estimate
these settlements accurately.
Despite all the above reasons, the settlements in most cases can be estimated to an
accuracy of about 25 to 30%, which is good enough seeing the complexity of the problem.
ALLOWABLE SOIL PRESSURE FOR OHESIONLESS SOILS
The allowable soil pressure (qna) of a shallow foundation is limited either by the net safe
bearing capacity (qns) or the safe settlement pressure (qnρ). The design of shallow foundation on
cohesionless soils is generally governed by the safe settlement pressure, as the net safe bearing
capacity for footings of usual size is quite high. However, in the case of narrow footings on water-
logged sands, the net safe bearing capacity may be the controlling criterion for the design.
It is the normal practice for the design of footings of usual size to use empirical methods
based on N-values for the determination of the allowable soil pressure for cohesionless soils. The
plate load tests are also used in the case of soils having small boulders and stones which obstruct the
standard penetration test. The methods using the standard penetration test are preferred to plate load
tests for homogeneous soils, as these are more economical.
Footings on granular soils are generally designed using the following empirical relationships
for the allowable soil pressure.
1) Peck Method
Terzaghi and Peck (1967) gave charts for the safe bearing pressures inducing a total
settlement of 25mm and a differential settlement of 19 mm for different sizes of footing. Peck et al
(1974) revised the Terzaghi and Peck curves to take into consideration the later research, and gave
the following equation for the safe settlement pressure.
where qnρ = safe settlement pressure (kN/m2),
N = average SPT number, corrected for overburden pressure and dilatancy,
s = settlement (mm),
ANJUMAN COLLEGE OF ENGINEERING & TECHNOLOGY
MANGALWARI BAZAAR ROAD, SADAR, NAGPUR - 440001.
DEPARTMENT OF CIVIL ENGINEERING
Prof. Rashmi G. Bade, Department of Civil Engineering, Geotechnical Engineering – II 14
Cw = water table correction factor.
2) Teng’s Equation
Teng (1962) expressed the charts given by Terzaghi and Peck (1948) in the form of
the following formulas. Allowance was made for an increase in pressure with depth by
introducing a depth factor.
For a settlement of 25 mm,
where qnρ = safe settlement pressure (kN/m2), N = SPT number, B = width of footing (m),
Wγ = water table correction factor,
Rd = depth correction factor =
The above equation can be written in general form as
where s = tolerable settlement (mm).
3) Meyerhof’s equation
Meyerhof proposed equations which are slightly different from Teng’s equations.
According to him, for a settlement of 25 mm,
and
where all the terms are the same as in Teng’s equation, except Rd, which is given by
4) Bowle’s equation
Bowles (1977) suggested that the net allowable pressure given by Meyerhof’s equation can
be safely increased by 50%. Thus, for a settlement of 25 mm,
and
5) IS : 6403 – 1971 equation
IS : 6403 – 1971 gives the following equation, which is similar to Teng’s equation. For a
settlement of 40 mm,
ANJUMAN COLLEGE OF ENGINEERING & TECHNOLOGY
MANGALWARI BAZAAR ROAD, SADAR, NAGPUR - 440001.
DEPARTMENT OF CIVIL ENGINEERING
Prof. Rashmi G. Bade, Department of Civil Engineering, Geotechnical Engineering – II 15
The depth factor is not considered.
Fig.5 gives the allowable soil pressure for a settlement of 40 mm.
PLATE LOAD TEST
Plate load test is a field test to determine the ultimate bearing capacity of soil, and the
probable settlement under a given loading. The test essentially consists in loading a rigid plate at the
foundation level, and determining the settlements corresponding to each load increment. The
ultimate bearing capacity is then taken as the load at which the plate starts sinking at a rapid rate. The
method assumes that down to the depth of influence of stresses, the soil strata is reasonably uniform.
The bearing plate is square, of minimum recommended size 30 cm square and maximum size
75 cm square. The plate is machined on sides and edges, and should have a thickness sufficient to
withstand effectively and bending stresses that would be caused by maximum anticipated load. The
thickness of steel plate should not be less than 25 mm.
The test pit width is made five times the width of the plate Bp. At the centre of the pit, a small
square hole is dug whose size is equal to the size of the plate and the bottom level of which
correspond to the level of the actual foundation. The depth Dp of the hole should be such that
The loading to the test plate may be applied with the help of a hydraulic jack. The reaction of
the hydraulic jack may be bores by either of the following two methods:
a) Gravity loading platform method.
b) Reaction truss method.
In the case of gravity loading method, a platform is constructed over a vertical column resting
on the plate, and the loading is done with the help of sand bags, stones or concrete blocks.
When load is applied to the plate, it sinks or settles. The settlement of the plate is measured
with the help of sensitive dial gauges. For square plate, two dial gauges are used. The dial gauges are
ANJUMAN COLLEGE OF ENGINEERING & TECHNOLOGY
MANGALWARI BAZAAR ROAD, SADAR, NAGPUR - 440001.
DEPARTMENT OF CIVIL ENGINEERING
Prof. Rashmi G. Bade, Department of Civil Engineering, Geotechnical Engineering – II 16
mounted on independently supported datum bar. As the plate settles, the ram of the dial guage moves
down and settlement is reconsidered. The load is indicated on the load – guage of the hydraulic jack.
(a) (b)
Figure.5 (a) Trial pit, b) Plate load test: Reaction by Gravity loading.
Test Procedure
1) The plate is firmly seated in the hole, and if the ground is slightly uneven, a thin layer of sand
is spread underneath the plate. Indian Standard (IS : 1888 – 1962) recommends a seating load
of 70 g/cm2 which is related before the actual testis started.
2) The load is applied with the help of a hydraulic jack (preferably with the remote control
pumping unit), in convenient increments, say of about one-fifth of the expected safe bearing
capacity or one-tenth of the ultimate bearing capacity.
ANJUMAN COLLEGE OF ENGINEERING & TECHNOLOGY
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DEPARTMENT OF CIVIL ENGINEERING
Prof. Rashmi G. Bade, Department of Civil Engineering, Geotechnical Engineering – II 17
3) Settlement of the plate is observed by 2 dial gauges fixed at diametrically opposite ends, with
sensitivity of 0.02 mm.
4) Settlement should be observed for each increment of load after an interval of 1, 4, 10, 20, 40
and 60 minutes and thereafter at hourly intervals until the rate of settlement becomes less
than about 0.02 mm per hour. After this, the next load increment is applied. The maximum
load that is to be applied corresponds to 1 ½ times the estimated ultimate load or to 3 times
the proposed allowable bearing pressure.
5) The water table has a marked influence on the bearing capacity of sandy or gravelly soil. If
the water table is already above the level of the footing, it should be lowered by pumping and
the bearing plate seated after the water table has been lowered just below the footing level.
6) Even if the water table is located above 1 m below the base level of the footing the load test
should be made at the level of the water table itself.
7) The load intensity and settlement observations of the plate load test are plotted. Curve I
corresponds to general shear failure, and II corresponds to local shear failure. Curve III is a
typical of dense cohesionless soils which do not show any marked sign of shear failure under
the loading intensities of the test. IS: 1888 – 1962 recommends a log – log plot giving straight
lines the intersection of which may be considered the yield value of the soil. In order to
determine the safe bearing capacity it would be normally sufficient to use a factor of safety of
2 or 2.5 on ultimate bearing capacity.
Figure.6 Load settlement curves.
Limitation of the plate load test
The plate load test has the following limitations:
1) Size effect: - The results of the plate load test reflect the strength and the settlement
characteristics of the soil within the pressure bulbs. As the pressure bulb depends upon the
size of the loaded area, it is much deeper for the actual foundation as compared to that of the
plate. The plate load test does not truly represent the actual conditions if the soil is not
homogeneous and isotropic to a large depth.
2) Scale effect: - The ultimate bearing capacity of saturated clays is independent of the size of
the plate but for cohesionless soils, it increases with the size of the plate. To reduce scale
effect, it is desirable to repeat the plate load test with plates of two or three different sizes and
ANJUMAN COLLEGE OF ENGINEERING & TECHNOLOGY
MANGALWARI BAZAAR ROAD, SADAR, NAGPUR - 440001.
DEPARTMENT OF CIVIL ENGINEERING
Prof. Rashmi G. Bade, Department of Civil Engineering, Geotechnical Engineering – II 18
extrapolate the bearing capacity for the actual foundation and take the average of the values
obtained.
3) Time effect: - A plate load test is essentially a test of short duration. For clayey soils, it does
not give the ultimate settlement. The load-settlement curve is not truly representative.
4) Interpretation of failure load: - The failure load is not well-defined, except in the case of a
general shear failure. An error of personal interpretation may be involved in other types of
failure.
5) Reaction load: - It is not practicable to provide a reaction of more than 250kN. Hence, the
test on a plate of size larger than 0.6 m width is difficult.
6) Water table: - The level of the water table affects the bearing capacity of the sandy soils. If
the water table is above the level of the footing, it has to be lowered by pumping before
placing the plate. The test should be performed at the water table level if it is within about 1m
below the footing.