Lecture4_Insitu shear strength testing (1).pdf
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Transcript of Lecture4_Insitu shear strength testing (1).pdf
1
CE 601: In-situ shear strength testing
In-situ shear strength tests
Field Test: In-situ shear strength Testing
g Standard Penetration Test (SPT) Cone Penetration Test (CPT) Dynamic Cone Penetration Test (DCPT) Vane Shear Test (VST) Dilatometer Test (DMT) Pressure meter Test (PMT)
2
Common In Situ Testing Devices
SPT
3In bore holes
DMTVSTCPT
PMTDCPT
Standard Penetration TestIS: 2131-1981
4
3
Components Drilling Equipment
Inner diameter of hole 100 to 150 mm
Standard Penetration Test
Inner diameter of hole 100 to 150 mm Casing may be used in case of soft/non-cohesive soils
Split spoon sampler IS:9640-1980 Drive weight assembly
Falling Weight = 63.5 Kg Fall height = 75 cm
Others Lifting bail, Tongs, ropes, screw jack, etc.
Procedure Procedure The bore hole is advanced to desired depth and bottom is cleaned. Split spoon sampler is attached to a drill rod and rested on bore
hole bottom. Driving mass is dropped onto the drill rod repeatedly and the
sampler is driven into soil for a distance of 450 mm. The number of blow for each 150 mm penetration are recorded.
Procedure (Cont….) N-value
Standard Penetration Test
First 150 mm penetration is considered as seating penetration The number of blows for the last two 150 mm penetration are
added together and reported as N-value for the depth of bore hole.
The split spoon sampler is recovered, and sample is collected from split barrel so as to preserve moisture content and sent to the laboratory for further analysis.SPT is repeated at every 750 mm or 1500 mm interval SPT is repeated at every 750 mm or 1500 mm interval for larger depths.
Under the following conditions the penetration is referred to as refusal and test is halted
a) 50 blows are required for any 150 mm penetrationb)100 blows are required for last 300 mm penetrationc) 10 successive blows produce no advancement
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Precautions during SPT
The ht. of free fall Must be 750 mm The fall of hammer must be free, frictionless and vertical Cutting shoe of the sampler must be free from wear & tear The bottom of the bore hole must be cleaned to collect
undisturbed sample When SPT is done in a sandy soil below water table , the
water level in the bore hole MUST be maintained higher water level in the bore hole MUST be maintained higher than the ground water level. Otherwise: QUICK condition!!Very Low N value
Correction for Overburden Pressure :
' .NN C N
SPT Corrections
N' = Corrected value of observed N
CN = Correction factor for overburden pressure
.NN C N
Peck, Hanson and Thornburn (1974)
p' = Effective overburden pressure at a depth corresponding to N-value measurement
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SPT Corrections
Correction for Overburden Pressure : (Alternative)
Correction for Dilatancy :
If the stratum consists of fine sand and silt below water table, for N' > 15, the dilatancy correction is applied as
[ ]Alternative -
SPT Hammer Energy Correction
Energy is dissipated in some fraction during the impact, and the output energy is usually in the range of 50% to 80% of energy input.
For rope pully system with safety hammer
The N-value is standardized for 60 % energy output. For other hammers, the N-value may be corrected in ratio of their energy input
60%out
in
EE
Although IS 2131-1981 is silent on this issue, the correction may be applied as per the requirement of the project.
60
%.
60out inE E
N N
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SPT Test Data
No. of blows per 0.30m
Data from different bore holes
Interpretation from SPT: Cohesionless Soils
N'' ' D (%) i tN'' ' Dr (%) consistency
0-4 25-30 0-15 very loose
4-10 27-32 15-35 loose
10-30 30-35 35-65 medium
30-50 35-40 65-85 dense
>50 38-43 85-100 very dense
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Interpretation from SPT: Cohesive Soils
not corrected for overburden 6.25. in kPauc N
N cu (kPa) consistency visual identification
0-2 0 - 12 very soft Thumb can penetrate > 25 mm
2-4 12-25 soft Thumb can penetrate 25 mm
4-8 25-50 medium Thumb penetrates with moderate effort
8-15 50-100 stiff Thumb will indent 8 mm
0.689
0.193'
NOCR
p
MN/m2
15-30 100-200 very stiff Can indent with thumb nail; not thumb
>30 >200 hard Cannot indent even with thumb nail
Mayne and Kemper (1988)
Cone Penetration Test (CPT)IS: 4968 (Part –III)
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CPT Procedure Push the sounding rod with cone into the ground for some specified
depth. Then push the cone with friction sleeve for another specified depth (> 35 mm). Repeat the process with/without friction sleeve.
h / Pushing rate = 1 cm/s Mantle tube is push simultaneously such that it is always above the
cone and friction sleeve. Tip Load, Qc = Load from pressure gauge reading + Wt. of cone +
Wt. of connecting sounding rods
Tip resistance
h f l dd lf h ll
cc
c
A
x-sectional area off cone = 10 cm2
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With friction sleeve add its self weight as well Qt = Qc + Qf
Frictional resistance
Friction Ratio
surface area of friction sleevet c
ff
Q Qq
A
fr
c
qf
q Typical range
0%
10%Cohesive
Granular
0
1
2
0 2 4 6 8 10 12 14
CPT Cone Resistance, qc1
(MPa)
0
1
2
0 10 20 30
SPT Blow Count, N1(60)
(Blows/300 mm)
0
1
2
0 20 40 60 80 100
Relative Density, Dr
(%)Interpreted Soil Profile
0
1
2
0 0 .2 0 .4 0 .6 0 .8 1
Fine Sandw/ Shells(SP)
3
4
5
6
3
4
5
6
3
4
5
6
3
4
5
6
Be
low
Ex
cava
ted
Su
rfa
ce (
m)
Interbedded Fine SandandSilty Sand(SP-SM)
Fine Silty Sand
7
8
9
1 0
MeanMean-SDMean+SD
7
8
9
1 0
7
8
9
1 0
From CPT
From SPT
7
8
9
10
De
pth
(SM)
Gray Silty Clay (CL)
Sand (SP)
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Interpreted Soil Profile EQ Drain Test Area 1
0
1Sand
Cone Tip
Resistance, qc
(MPa)
0 2 4 6 8 10 12
Fricton Ratio, Fr
(%)
0 1 2 3 4 5 6
Relative
Density, Dr
0 0.2 0.4 0.6 0.8 1
Pore Pressure, u (kPa)
-100 0 100 200
CPT Profile for Piezocone
2
3
4
5
6
7
8
9
Dep
th (
m)
Silty sand/sand
Silt and Sandy Silt
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10
11
12
13
14
15
Sand to Silty Sand
CPT Results & Soil Classification
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T i l Typical CPT Data
Dynamic Cone Penetration Test (DCPT) Components:
1) Cone (dia = 50 mm)~usually made of steel
IS: 4968 (Part – I, II)
usually made of steel
SPTHollow (split spoon)
2) Driving rods/drill rods
~marked at every 100 mm
SPT
DCPT
Solid (no samples)
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DCPT Procedure Cone – drill rod – driving head assembly is installed
vertically on the ground and hammer is dropped from standard height repeatedlystandard height repeatedly
The blow counts are recorded for every 100 mm penetration. A sum of three consecutive values i.e. 300 mm is noted as the dynamic cone resistance, Ncd at that depth.
The cone is driven up to refusal or the project specified depth.
In the end, the drill rod is withdrawn. The cone is left in the d if th d d d if th d dground if unthreaded or recovered if threaded.
No sample recovered
Fast testing – less project cost / cover large area in due time
Use of bentonite slurry is optional, which is used to reduce friction on the driving rods.
• Modified cone is used in this case: diameter = 62.5 mm
CPT Versus SPT
CPT: Advantages over SPT provides much better resolution, reliability versatility; pore water pressure, dynamic soil
properties
CPT: Disadvantages Does not give a sample Does not give a sample Will not work with soil with gravel Need to mobilize a special rig
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CPTU
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Resistance calculation for CPTU
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13
Typical Measurements with CPTU
25
DOWNHOLE SEISMIC PIEZOCONE PENETRATION TEST (SCPTU)
26
(SC U)
14
Failure Modes around Advancing Cone
27
For clays, and mainly for soft clays. Measure torque required to quickly
h h h d f l
Vane Shear Test (VST)measuring (torque) head
shear the vane pushed into soft clay.
torque undrained shear strength cu
Typical d = 20-100 mm.
undrained
bore hole
vaneh2d y
28vane
soft clay
vane
d
15
Vane Shear Test Interpretation:
Undrained shear strength -
2
2.
. . .
13.
u
Tc
D HD
H
T
For H = 2.D
Test in Progress Failure surface
30.273u
Tc
D
Insert DMT using SPT drilling equipment to the desired depth and pressure
Dilatometer Test (DMT)
desired depth and pressure the cell
Measure pressure when the membrane is flushed with plate and when it enters ground by 1.1 mm.
Decrease the pressure & measure the pressure when
b i i fl h d
30
60 mm dia. Flexible membrane
membrane is again flushed with plate.
Determined: Elastic Modulus Soil Type and state
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Pressure meter Test (PMT)
Determined:Elastic Young Mod, EShear Mod, GUndrained shear strength, Su
Pressure meter Test (PMT)
Measurements:1. Fluid Pressure 2. Fluid volume change
32
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Thank You