Indian Geotechnical Conference – trendz December 16–18, 2010

Indian Geotechnical Conference – 2010, GEOtrendz
December 16–18, 2010
IGS Mumbai Chapter & IIT Bombay
Effect of Sample Preparation on Strength of Sands
Juneja, A.
Raghunandan, M.E.
Assistant Professor
e-mail: [email protected]
Research Scholar
e-mail: [email protected]
Department of Civil Engineering, IIT Bombay, Mumbai
ABSTRACT
Selection of the most suitable method of sand sample preparation becomes difficult because all available methods
affect the fabric and dry density of samples, and none of the available methods are shown to be unique. The
objective of this paper is to address some of these issues. A series of consolidated drained (CD) and consolidated
undrained (CU) triaxial compression tests were conducted on sand samples prepared using pluviation and tamping
techniques, under both dry and moist conditions. The standard triaxial test setup at IITB is described first. Stressstrain behaviour for samples prepared with different sample preparation methods showed considerable difference
in the peak stress and dilation, whilst all sample reached the peak stress at 5 to 10% axial strains. Results showed
that samples prepared using tamping technique usually strain softens, whilst samples prepared by pluviation
technique may harden or soften with strain depending up on the sample relative density and confining pressures
applied during testing. Hence, pluviation technique proves to be the more reliable technique to prepare samples
for triaxial testing.
1. INTRODUCTION
Number of techniques to obtain high quality cohesive
samples for laboratory testing has been developed, whilst
procedure used to obtain undisturbed cohesionless samples
are still very few. The cost to obtain high quality undisturbed
cohesionless samples by ground freezing is prohibitive
(Yoshimi et al., 1994), hence many researchers rely on
preparing remoulded and reconstituted representative
samples of sandy soils by dry or wet pluviation, slurry
deposition, vibrations, or moist-tamping in layers by undercompacting each layer to its succeeding layer (Ladd, 1978,
Amini and Qi, 2000). The structural arrangement of soil
grains remains the most important criteria influencing the
stress-strain behaviour of reconstituted cohesionless soil
samples in the laboratory tests.
Samples prepared using moist tamping (MT)
technique, usually exhibits strain softening behaviour
because of their inherent high void ratios (Vaid et al., 1999).
Some studies on samples prepared using MT seems to
suggest that the sample are not homogeneous and are less
suitable for triaxial testing (Vaid et al., 1999, Frost and
Park, 2003, DeGregorio, 1990, Vaid and Sivathayalan,
2000). Ladd (1974) proposed the method of undercompaction in which includes each layer is undercompacted to its successive layer.
Air pluviation technique is shown to produce
reconstituted sand specimens with least soil degradation
(Cresswell et al., 1999). In this method, a wide range of
initial void ratio can be achieved by controlling the drop
height and pouring rate (Vaid and Negussey, 1988). Air
pluviated samples strain softens to a lesser extent when
compared to moist tamped specimens (DeGregorio, 1990,
Vaid and Sivathayalan, 2000). Amini and Chakravrty
(2004) prepared homogeneous sand samples using dry
pluviation technique. However, when the soil contained
silt in excess of 20 %, air pluviation resulted in soil
segregation because, the fines lagged behind on account of
their lower velocities within the fixed height. Cresswell et
al. (1999) observed that compaction during pluviation
reached peak efficiency when a continuous energetic layer
was formed. Within the energetic layer, the grain
displacement and grain hammering operated to their
greatest effectiveness. Sand samples prepared using wet
pluviation are initially saturated (Chaney and Mulilis, 1978,
Vaid and Negussey, 1988). In wet pluviation, preferred
fabric can develop which behaves similar to that of natural
alluvial deposits (Ghionna and Porcino, 2006, Oda et al.,
1978). Vaid et al. (1999) showed that these samples are
uniform with depth and have small deviation in the relative
density. Terminal velocity was reached at a very small drop
328
A. Juneja and M.E. Raghunandan
height in water, irrespective of total drop height. Pluviation
technique fails when used for sands containing fines because
of particle segregation (Carraro and Prezzi, 2007).
2. EXPERIMENTAL SETUP
All tests were conducted in ADsoil laboratory at Indian
Institute of Technology Bombay using Gujarat sand. Figure
1 shows the particle size distribution curve of the sample,
and the results showed D50 = 0.3 mm, CU = 2.12, CC =
1.47, and specific gravity, GS = 2.63. The maximum and
minimum void ratios of the sand sample were 0.795 and
0.492 respectively. Samples were prepared in 100mm
diameter and 200mm long split mould which had the
facility to attach 80mm long collar at its top. The mould
was clamped to firm base during sample preparation. The
split mould was provided with a vacuum port to stretch the
membrane and prevent necking during sample preparation
(after Wijewickreme & Sanin, 2006).
80
Percentage finer (%)
Table 1: Details of Funnels and Mesh Used
Diffuser
D1
D2
D3
D4
D5
Pore Size
(mm)
2
2
8
9.4
10
Deposition Intensity
x10-3 (g/s/mm2)
2.1
5.1
88.7
121.6
147.8
Note: Diffuser fabricated as per ASTM E 323-80. Center-to-center
distance between the pores was varied to control the deposition
intensity.
100
60
40
20
0
0.01
diffuser during sample preparation. In this second method,
the diffuser was slowly raised concurrently as the sample
was formed. The two methods are referred to as fixed
diffuser (DF), and rising diffuser (DR) in the text. Detailed
procedure and the comparison of both DF and DR are
mentioned in Raghunandan & Juneja (2010). In general,
sand was rained from 280- to 600 mm height above the
base of mould. This height is the usual clearance available
between the cell and the cross head in triaxial shear frame.
0.1
1
Sieve size (mm)
10
Fig. 1: Particle Size Distribution for the Sand Sample
The procedure to prepare sand samples using different
sample preparation technique used in this study are as
mentioned in Raghunandan & Juneja (2010). However, to
brief with tamped samples were prepared using tamping
rod attached to 50 mm diameter circular footing made of
aluminium with total weight of less than 250 g. Air dried
sand was used in dry tamping. Moist tamped samples were
prepared by tamping the sand under submerged conditions.
Dry and wet pluviated samples were prepared using
the same mould as that used in the tamping technique. In
dry pluviation, air dried sand was rained through a diffuser
into the mould. When otherwise was filled with water up
to the brim in wet pluviation. Five diffusers D1 to D5 were
used in the tests. The pore size and deposition intensity of
the diffusers are tabulated in Table 1. In the table, deposition
intensity is equal to the mass flow of the sand per unit area
of the diffuser. Half of the samples were prepared by keeping
the diffuser at fixed height above the base of mould. The
remaining half of the samples was prepared by raising the
Samples prepared using tamping and pluviation
techniques under dry and moist conditions were tested in
standard triaxial compression. Table 2 shows the
experiment program used in this study. The samples were
directly prepared on the triaxial base using split mould.
Initial height and diameter of the samples were measured
at four locations using dial gauge and Pi-tape respectively.
Water was percolated within the sample under a head of
about 10kN/m2 while a small confining pressure of about
15kN/m 2 maintained to hold the sample. The samples
saturated under cell pressure and back pressure increments
Table 2: Sample Preparation and Test Conditions Used in CD
and CU Tests
Sample
Preparation
Description
Initial Void
Ratio
Dry
Tamping
Normal compaction; 3 and
5 layers; 25 blows/layer
0.603 – 0.630
Moist
Tamping
Normal compaction; 3 and
5 layers; 25 blows/layer
0.615 – 0.605
Dry
Pluviation
Fixed diffuser, and rising
diffuser techniques
0.634 – 0.695
Wet
Pluviation
Fixed diffuser, and rising
diffuser techniques
0.681 – 0.687
of not more than 35kN/m2 until B-factor (Skempton, 1954)
of about 0.97 to 0.99 was achieved. The samples were then
consolidated isotropically under effective confining
pressures and sheared to failure with drainage valve open
or closed based on the type of test. The drained shear
strength measured during triaxial shear was corrected for
329
Effect of Sample Preparation on Strength of Sands
the membrane stiffness (ASTM D4767-04). In this study,
0.3mm thick rubber membrane of Young’s modulus equal
to 1780 kN/m2 was used.
1200
(a)
800
-12
600
DP - DF; e = 0.695
400
Dry pluviation (DP); e = 0.634
Wet pluviation (WP); e = 0.687
Dry tamping (DT); e = 0.630
Moist tamping (MT); e = 0.615
200
0
0
10
20
30
AxialAxial
Strainstrain
(%) (%)
DT; e = 0.603
MT; e = 0.605
DP - DR; e = 0.634
-10
Volumetric strains (%)
Deviator stress (kN/m2)
1000
and further followed by shear dilation. The stress-strain
plots of all the samples tends to follow same path at axial
strains (εa) greater than 30%, showing very less volume
change with stress ratio value of about 1.24. Stress ratio is
the term for deviator stress normalised with effective
confining pressure. Hence at this stage the samples are
considered to have reached their critical or steady state.
Figure 3 shows the variation of volumetric strains (εv) with
εa . Similar trend was observed with a small initial
compression followed by shear dilation.
WP - DF; e = 0.681
-8
WP - DR; e = 0.683
-6
-4
-2
500
(b)
Deviator stress (kN/m2)
400
0
2
0
10
20
30
strain
AxialAxial
Strains
(%)(%)
300
Fig. 3: Variations of Volumetric Strains with Axial Strains
DP; Fixed diffuser; e = 0.695
DP; Rising diffuser; e = 0.634
WP; Fixed diffuser; e = 0.681
WP; Rising diffuser; e = 0.683
DT; h = 20mm; e = 0.603
MT; h = 20mm; e = 0.605
200
100
0
0
10
20
30
Axial strain (%)
Axial Strain (%)
Fig. 2a-b: Stress-Strain Behavior of Sand Samples Tested
Under (a) Undrained and (b) Drained Conditions
3. RESULTS AND DISCUSSION
In this paper, the behaviour of sand samples prepared using
different sample preparation techniques, at void ratios 0.603
to 0.687 to consolidated undrained (CU) and drained (CD)
shear is studied. Figure 2a-b show the stress-strain response
of the sand samples to drained and undrained shear
respectively. In general, all specimens showed initial peak
Figure 4 shows the variation in ratio of deviator stress
at end state with consolidated relative density. Ratio of
deviator stress at end state shall be defined as the ratio of
deviator stress at critical to peak state, i.e. qcric/qmax. The
qcric/qmax ratio explains the total energy loss or drop in
deviator stress as a ratio during continued shearing, hence
explains the behaviour of the sample as either strain
softening of strain hardening. For the ratio qcric/qmax less
than 1 implies drop in the deviator stress between peak
and critical states, thus the sample shows strain softening
or over-consolidated (OC) behaviour. Similarly, if ratio qcric/
qmax = 1 implies that the sample shows strain hardening or
normally-consolidated (NC) behaviour, whilst ratio qcric/
qmax can never be greater than 1. The observations from
Figure 4 show that ratio qcric/qmax varies between 0.6 to 1
for all the samples. The samples prepared using dry and
wet tamping techniques have ratio qcric/qmax < 0.85, whilst
for samples prepared using dry and wet pluviation technique
qcric/qmax ratio varied between 0.8 to 1 depending effective
confining pressures applied during testing. Based on the
above discussions it can be concluded that, samples
330
A. Juneja and M.E. Raghunandan
prepared using tamping technique usually strain softens,
whilst samples prepared by pluviation technique may
harden or soften with strain depending up on the sample
relative density and confining pressures applied during
testing.
qcric/qmax
1
0.8
CU - DP
CU - WP
CU - DT
CU - MT
CD; DP-DF
CD; DP-DR
CD; WP-DF
0.6
CD; WP-DR
CD; DT
CD; MT
0.4
0.5
0.6
Initial void ratio
0.7
Fig. 4: Variations in Ratio of Deviator Stress at End State with
Initial Void Ratio
4. CONCLUSION
Discussions in this paper presents data relating to the CU
and CD triaxial compression tests on samples prepared
using pluviation and tamping techniques, under both dry
and moist conditions. All samples showed initial
compression followed by shear dilation when sheared at
σ×3 = 150kN/m2, with peak stress at εa ranging between 5
to 10%. Samples prepared using tamping technique usually
strain softens, whilst samples prepared by pluviation
technique may harden or soften with strain depending up
on the sample relative density and confining pressures.
ACKNOWLEDGMENT
The second author gratefully acknowledges the support of
Indian Institute of Technology Bombay in providing
research scholarship to pursue his PhD program at the
institute.
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