loadtestonsoil-160603053255

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Prof. Samirsinh P Parmar
Research Scholar , Civil Engineering Dept., IIT Kanpur, India
Mail: [email protected]
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Load Tests on Soils
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Plate Load Test
• Conducted to determine ultimate bearing pressure (or bearing capacity) of a
soil in-situ, when soil strata is reasonably uniform.
• To determine the modulus of subgrade reaction of a soil strata, used in the
design of raft foundations and pavements
• Conducted at proposed foundation level in a test pit, which is at least 5 times
the plate size
• If the water table is above the test level, it may be lowered down artificially by
adopting pumping
• All the dead loads, viz., ball and socket, loading column, jack, test plate, etc.
should be properly accounted for eccentricity.
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Size of plate
• Circular or square plates of 300-750 mm size (mild steel), thickness not less than
25 mm, or equivalent concrete blocks
• with chequered or grooved bottom for better contact
• Single size of plate may be sufficient for testing in clays
• Three plates of different size are suggested for testing in gravelly and dense sands
(to understand the size effect), and results are extrapolated for real footings.
• Side of the plate should be greater than 4 times the maximum size of particle
present at the location
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Loading
• Stress controlled loading in cumulative equal increments upto 1 kg/sq.cm. (100
kPa) or one-fifth of the estimated ultimate bearing pressure (qu).
•
Ex., Load on the plate should be 100 kPa in the first stage, followed by 200
kPa in the second stage, 300 kPa in the third stage, and so on, till the final load is
reached; or
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1/5qu in the first stage, followed by 2/5qu in the second stage, 3/5qu in the
third stage, 4/5qu in the fourth stage, and qu in the last loading stage.
• Gravity loading
• Reaction loading in the form of
• Kentledge
• Anchored piles
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Hydraulic or mechanical Jack
• A hydraulic jack of required capacity to apply and maintain to
maximum estimated load but not less than 50 tonnes.
4/5qu
3/5qu
2/5qu
1/5qu
120 hours
96 hours
Time
72 hours
48 hours
0
24 hours
7 kPa
Seating
load
Pressure
qu
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Limitations of plate load tests
• Size of the foundation influences the load-settlement response of
shallow foundations. This is very particular in case of cohesionless
soils, wherein the modulus of elasticity increases with depth.
• Bearing capacity increases with increase in size of plate for
cohesionless soils.
• In cohesive soils, particularly those that are saturated, the plate load
test will not be able to capture the ultimate settlements under any
load application, which may require more time than 24 hours, used in
the plate load test.
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Influence zone
• Influence zone in case of plate load test is extended to smaller depths, in
the order of two times the width of the plate. However, in case of real
footings, whose width is many times higher than the plate size, soil at
deeper depths can also significantly contribute to the bearing capacity of
soil.
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Influence zone
100 kPa
N2Bp
Plate width =Bp
N1Bf
N1Bp
100 kPa
N2Bf
Footing width=Bf
Bf>>Bp & N1≈2
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Scale effect
100 kPa
100 kPa
Es
Es
Es=Young’s modulus of soil
For cohesionless soils, Es is a function of overburden pressure
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Scale effect
100 kPa
100 kPa
Loose Sand
Soft clay
Dense Sand
Bed Rock
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Scale effect
100 kPa
100 kPa
Medium to Dense Sand
Geologic surprise
Bed Rock
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Gravity Loading Platform
Source: IS:1888-2002
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Reaction Loading Platform
Source: IS:1888-2002,
pg-6
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Load vs. settlement behavior
Source: IS:1888-2002,
St taken as permissible settlement of footing
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Supports for dial gauges should
not be located in the vicinity of
plate.
Plate load test
Adapted from V.N.S. Murty (2007)
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Interpretation of Bearing Capacity from Load-Settlement Data
From this observation stress value or qu = 380 kpa
Source: Cerato (2005)
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Interpretation of Bearing Capacity from Load-Settlement Data
From this observation stress value
or qu = 275 kpa
Source: Cerato (2005)
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Interpretation of Bearing Capacity from Load-Settlement Data
From this observation stress value
or qu = 460 kpa
Source: Cerato (2005)
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Interpretation of Bearing Capacity from Load-Settlement Data
Source: Cerato (2005)
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Interpretation of Bearing Capacity from Load-Settlement Data
The ultimate load is
defined as the point where
the load displacement
becomes practically linear
Adapted from Brand et al. (1972)
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References
IS 2911 Part 4 (1985): Design and Construction of Pile Foundations;
Load Test on Piles, Reaffirmed 2006, Bureau of Indian Standards.
ASTM D1143-81: Standard Test Method for Piles under Static Axial
Compressive Load.
GEO Publication No.1/2006: Foundation Design and Construction, GEO,
HK.
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Pile load tests
 More reliable method to determine the pile capacity, being an in-situ test
 Test should be conducted at cut-off levels (the level where the installed pile is
cut-off to support pile caps or beams or any other structural component)
 Pile load test are usually carried out to fulfill one or some of the following
objectives :
 To determine ultimate and safe loads on piles
 To countercheck results from static and pile driving formulae
 To determine the load-settlement behavior of a pile, especially in the region
of the anticipated working load that the data can be used in prediction of
group settlement.
 To verify structural soundness of the pile.
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Types of Load Tests
• Vertical load test (compression test)
• Pullout test
• Lateral-load test
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Types of Tests
Initial test and Routine test
• Initial test:
• Conducted to achieve one or more of the following objectives
• to determine the ultimate load capacity and safe capacity
• to provide guidelines for setting up the limits of acceptance for routine tests
• to study the effect of piling on the stability of adjacent structures, and assess
the suitability of type of piles to be used
• to get an idea of suitability of piling system
• to check on the calculated loads by static and dynamic formulae
• Conducted in case of important and major projects
• Number of tests may be one or more depending on the availability of information
on strata and past experience
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Types of Tests
• Routine test:
• Conducted to achieve one or more of the following objectives
• To determine the safe load on the pile
• To check the extent of safety for the specific functional requirement of the pile
at working load
• Detection of any unusual performance contrary to the finding of the initial test
• Number of tests may generally be 0.5% of total number of piles at the site
• May be increased up to 2%, depending on the importance of the project, and
subsoil conditions
• For single piles: carried out for a test load of at least one and half times the
working load; the maximum settlement under the test loading in position not
exceeding 12 mm.
• For group piles: carried out for a test load of at least equal to the working load;
the maximum settlement under the test loading in position not exceeding 25 mm.
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Methods of Application of Loading
• Maintained Load Method (MLT)
• Constant Rate of Penetration Method (CRP)
• Cyclic Load Method
The most common types of test loading procedures are maintained
load test (MLT) and Constant rate of penetration (CRP) test
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Maintained Load Method (MLT)
• Suggested method for determining safe load
• Reaction load should be 25% more than the final test load, proposed to be
applied
• Vertical downward incremental load
• Each increment 20% safe (working) load
• Each stage of loading is maintained till the rate of displacement measured at the
top of the pile is either
 0.1 mm in first 30 minutes
 0.2 mm in first 60 minutes
 2 hours, whichever is earlier.
 Test load (final load) shall be maintained for 24 hours
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Maintained Load Test
Releasing of load for next increment
Elastic rebound after releasing of load
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Safe Load
Compression loading (MLT)
Safe load on single piles from initial test should be lesser of the
following:
• Two-thirds of the final load at which the total settlement attains a value of
12 mm
• 50% of the final load at which the total settlement equals to 10% of the pile
diameter, in case of uniform diameter piles, and 7.5% of the bulb diameter in
case of under-reamed piles.
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Constant Rate of Penetration Test
Elastic rebound after releasing
of load but of small magnitude
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CRP Test
Predominantly friction piles
Maximum skin friction mobilizes at
small penetration of 5 to 10 mm
Ultimate (or peak) load: Load
corresponding to Point A.
Predominantly end bearing piles
Maximum base resistance mobilizes at
relatively large penetration
Ultimate load: Load corresponding to the
penetration equal to 10 percent of the
diameter of the pile base
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CRP (constant rate of penetration)
• Test pile is jacked into the soil, the load being adjusted to give constant rate of
downward movement to the pile. This is maintained until point of failure is
reached.
• Failure of the pile is defined in to two ways that as the load at which the pile
continues to move downward without further increase in load, or
• According to the BS, the load which the penetration reaches a value equal to onetenth of the diameter of the pile at the base.
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PILE LOAD TEST SET-UP
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MLT (Maintained load test)
• Kentledge or adjacent tension piles or soil anchors are used to provide a reaction
for the test load applied by jacking(s) placed over the pile being tested.
• Load is increased in definite steps
• Load is sustained at each level of loading until all settlements has either stop or
does not exceed a specified amount in a certain given period of time.
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Pile load tests (refer figure on the next page)
Load tests are performed to determine or verify the design capacity of piles and the designs
are revised accordingly.
Test piles in clays should not be loaded until some time (at least several weeks) has passed
after the piles were driven.
Test piles in sands may be loaded several days after they were driven.
Load is applied in increments of estimated safe load.
Each stage of loading is maintained till the rate of movement of pile top is not more than 0.1
mm/hr in cohesion-less soil and 0.02 mm/hr in clay for a maximum of 2 hrs whichever is
earlier.
The loading is continued up-to twice the safe load or the load at which the total settlement
of the pile top equals the appropriate value as indicated in the criteria below :
The allowable load on single pile shall be lesser of the following :
1. Two thirds the final load at which the total settlement attains a value of 12 mm.
2. 50 % of the load at which the total settlement equals 10% of pile diameter
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Pile driving equipment
 Piles are commonly driven by means of hammer or occasionally by a vibratory force
generator.
 The hammer operates between a pair of parallel guides suspended on a standard
lifting crane.
Impact hammer
• Steam hammers contain a ram lifted by steam pressure and allowed to fall by gravity
with or without the assistance of steam pressure.
• If the fall is due to gravity alone, the hammer is called single acting as shown in Figure.
1
• If steam pressure adds to the energy, the hammer is called double acting.
• The ram as shown in fig. 1 strikes a hammer cushion position in the base of the
hammer.
• The original purpose of the cushion was to prolong the life of the hammer by reducing
impact stresses.
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SINGLE ACTING HAMMER
Figure. 1
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Pile driving equipment Continue …
 In other hammers the ram strike directly on the base of the anvil.
 The top of the pile itself is protected by a drive head suspended from the base of the
hammer and fitted to the dimensions of the pile.
 Between the drive head and the pile, may also be inserted pile cushion.
 These various elements not only protect the top of the pile from local overstress, but
have a significant influence on the stress waves developed in the pile during driving.
 The selection of suitable cushion affects the driving characteristics of a pile, the depth
to which it can be driven and to some extent, its load carrying capacity
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Diesel hammers
 Are of two types : open ended and close ended as shown in fig. 2
 The ram of either types is lifted by explosion of fuel and compressive gas in a
chamber between the bottom of the ram and an anvil block in the base of the
housing.
 In the open ended hammers, the ram falls by gravity and delivers energy to the
anvil by direct impact.
 As it descends, however, fuel is injected into the space, known as combustion
chamber, between the ram and the anvil.
 At approximately the instant of impact, the fuel ignites and again lifts the ram.
 For a significant time, the pressure of burning gases also acts on the anvil and
increases the magnitude and duration of the driving force.
 In this aspect, the driving characteristics of diesel hammers differ appreciably
from those of steam hammers.
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Continue …
 In the close ended hammers, the housing extends over the cylinder to form a
bounce chamber in which air is compressed by the rising ram.
 The compressed air acts as a spring that limit the rise of the ram and thereby
shortens the stroke.
 It returns its stressed energy to the ram on the down stroke.
 On account of a shorter stroke, the number of blows per minute is increased
with respect to that for open ended hammer.
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Vibratory drivers
 Piles are also driven by force generators consisting of static weight and a pair
of counter rotating eccentric weights as shown in the figure. 3
 The horizontal components of centrifugal forces cancel each other whereas
the vertical components are additive.
 The vibrating part of the machine is positively attached to the head of the pile
to be in driven but the remaining of the machine is isolated from the vibrator
by spring so that it does not participate in the vibratory motion.
 The pulsating force facilitates penetration of the pile under the influence of
the constantly acting downward weight.
 When the frequency of the dynamic force becomes equal to the frequency of
the system consisting of the pile, the driven and the soil, the condition of
resonance development.
 The frequency of the driven must therefore be adjustable.
 The resonant frequency is often within the range of 50-150 Hz.
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Continue …
 Then the system is at resonance, the pile exhibits energetic ups and downs
displacements, limited only by the damping furnished by the surrounding soil.
 Almost the full downward weight of the driver and the pile are then effective
in causing the point of the pile to penetrate the underlying material.
 Penetration may be very rapid unless the resistance to the point penetration
exceeds the weight of the pile driving assembly.
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