Shear resistance degradation of lime –cement stabilized soil during cyclic loading
Alex Gezahegn Gebretsadik
Master of Science Thesis 14/01 Division of Soil- and Rock Mechanics
Department of Civil, Architectural and the Built Environment
Stockholm 2014
© Alex Gezahegn Gebretsadik Master of Science Thesis 14/01 Division of Soil and Rock Mechanics Royal Institute of Technology
ISSN 1652-599X
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ii ABSTRACT: This thesis presents the results of a series of undrained cyclic triaxial tests carried out on four lime-cement stabilized specimens and clay specimen. The shear resistance
degradation rate of lime-cement column subjected to cyclic loading simulated from heavy truck was investigated based on stress-controlled test. The influence of lime and cement on the degradation rate was investigated by comparing the behavior of stabilized kaolin and
unstabilized kaolin with similar initial condition. The results indicate an increase in degree of degradation as the number of loading cycles and cyclic strain increase. It is observed that the degradation index has approximately a parabolic relationship with the number of cycles.
Generally adding lime and cement to the clay will increase the degradation index which means lower degree of degradation. The degradation parameter, t has a hyperbolic relationship with shear strain, but it loses its hyperbolic shape as the soil getting stronger. On the other hand, for unstabilized clay an approximate linear relationship between degradation index and number of cycles was observed and the degradation parameter has a hyperbolic shape with the increase number of cycles. It was also observed that the stronger the material was, the lesser pore pressure developed in the lime-cement stabilized clay.
Keywords: undrained cyclic triaxial test ;lime-cement stabilized column; shear resistance; shear strain; degradation index; degradation parameter; pore pressure
iii SAMMANFATTNING : I detta examensarbete presenteras resultat från en serie odränerade cykliska triaxialtest som utfördes på fyra kalk- och cementstabiliserade prov och ett ostabiliserat lerprov. Nedbrytningen av skjuvmotståndet hos kalkcementpelare vid cyklisk belastning
undersöktes med hjälp av spänningskontrollerade triaxialförsök. Inverkan av inblandning med kalk och cement på nedbrytningen av skjuvmotståndet undersöktes genom att jämföra beteendet hos stabiliserad och ostabiliserat kaolin med liknande initiala förhållanden. Resultaten visar på en ökad grad av nedbrytningen allteftersom lastcykler och cyklisk töjning ökar. Det framgår att nedbrytningsindex har ungefär ett paraboliskt förhållade till antalet cykler. Att tillsätta kalk och cement till leran ökar i allmänhet nedbrytningsindex vilket innebär en lägre grad av nedbrytning.
Nedbrytningsparametern t har ett hyperbolisk förhållande med skjuvtöjningen, men den förlorar sin hyperboliska form när jorden blir starkare. Å andra sidan observerades för ostabiliserad lera ett ungefärligt linjärt samband mellan nedbrytningsindex och antalet belastningscykler och nedbrytningsparametern har en hyperbolisk form med ökande antalet cykler. Det framgår också att ju starkare material, desto mindre utvecklades porvattentrycket i kalk- och cementstabiliserad lera.
Nyckelord: odränerade cykliska triaxialtest, kalkcementpelare, skjuvmotstånd,
spänningskontrollerade triaxialförsök, skjuvtöjning, nedbrytningsindex, nedbrytningsparameter, porvattentryck
iv Acknowledgement
I would like to express the deepest appreciation to Almir Draganovic for tremendous support and help throughout the process of this master thesis. Without his guidance and persistent help this dissertation would not have been possible. I would like to express my gratitude to my supervisor Stefan Larsson for introducing me to the topic as well for the useful comments and remarks on the process of this master thesis. Furthermore I would also like to thank Stefan Lagerquist from IMCD Group and Håkan Wernersson, plant manager of Nordkalk Corporation for their support in delivering necessary materials for the laboratory test. I would like to thank my friends, family and colleagues who have supported me throughout entire process.
Alex G.Gebretsadik Stockholm, February 2014
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In memory of my dad, Rest In Peace
vi TABLE OF CONTENTS
1 INTRODUCTION ... 1
1.1 Aim and objective ... 2
1.2 Limitations ... 3
2 LITERATURE REVIEW ... 4
2.1 General ... 4
2.2 Post-cyclic response of soils ... 4
2.3 Factors affecting the degradation of soils ... 8
2.4 Soil stabilization ... 10
2.4.1 Mass Stabilization ... 10
2.4.2 Deep soil mixing (DSM) ... 10
2.5 Stabilization effect on degradation of soils ... 11
2.6 Cyclic triaxial shear test ... 14
2.7 Summary ... 14
3 METHOD AND MATERIALS... 16
3.1 Degradation model ... 16
3.2 Column stress, σcol and confining pressure, σ3 ... 17
3.3 Experimental Procedure ... 19
3.4 Unconfined Compression (UC) test ... 22
3.5 Cyclic triaxial tests - test set-up and test procedure ... 23
4 TEST RESULTS AND ANALYSIS ... 28
4.1 Uniaxial compression test result ... 28
4.2 Cyclic Triaxial test results and discussion ... 29
vii 5 CONCLUSIONS AND COMMENTS ... 36 6 REFERENCES ... 37
viii List of figures
Figure 1: A plot of degradation index versus number if cycles in log-log scale (Basack and
Purkayastha, 2009) ... 6
Figure 2: Variation of degradation parameter with cyclic shear strain amplitude for different marine clays (Basack and Purkayastha, 2009) ... 6
Figure 3: The effect of frequency, f on cyclic degradation ( Mortezaie, A. and Vucetic, M. ,2013) ... 9
Figure 4: Shear strength of different soils mixed with two quantities of lime and cement at three curing times (Hartlen and Holm, 1995) ... 11
Figure 5: Variation of degradation index with N/Nf, (a) uncemented sample, (b) 1.5 % cemented sample and (c) 3 % cemented sample (Haeri et al., 2002) ... 13
Figure 6: Layout of lime-cement column under the embankment ... 18
Figure 7: Position of point A of in lime-cement column under moving vehicle load ... 18
Figure 8: Stress distribution at point A due to a track passing the road ... 19
Figure 9: GDS triaxial testing system ... 20
Figure 10: The four components used to prepare the specimens; (a) cement (b) lime (c) clay (d) water ... 21
Figure 11: Unconfined uniaxial compression test ... 22
Figure 12: Sample is covered with a rubber membrane and sealed before putting the chamber. 24 Figure 13: Typical example of test plan during testing a sample ... 26
Figure 14 : Stress-strain during uniaxial compression test. ... 28
Figure 15: Variation of axial strain with number of cycles (a) for samples cured for 7 days and (b) for samples cured for 28 days ... 31
Figure 16: Variation of shear strain with number of cycles; (a) for samples cured for 7 days and (b) for samples cured for 28 days ... 31
Figure 17: Degradation index plotted against number of cycles in log-log scale; (a) for samples cured for 7 days and (b) for samples cured for 28 days ... 32
ix Figure 18: Degradation parameter plotted against cyclic shear strain; (a) for samples cured for 7
days and (b) for samples cured for 28 days ... 32 Figure 19: Pore pressure variation with number of cycles; (a) for samples cured for 7 days and
(b) for samples cured for 28 days ... 33 Figure 20: Plot results for a clay soil sample for 28 days curing time ... 34 Figure 21: (a) Degradation index plotted against number of cycles and (b) degradation parameter
plotted against cyclic shear strain for unstabilized clay sample for 28 days ... 35
x List of tables
Table
1. Stabilizer combination scheme for stabilized soils ... 20 2. General input data summary for each specimen ... 24 3. Test conditions ... 25
xi List of symbols and abbreviations
δ Degradation index
G Shear modulus
Gmax maximum shear modulus
Secant shear modulus at cycle N Secant shear modulus at 1st cycle Cyclic shear stress at cycle N Cyclic shear stress at 1st cycle Cyclic shear strain
Degradation parameter
Cyclic axial strain
Parameter used to determine degradation parameter, t Number of cycles
Nf Number of cycles at failure Plastic index
ESAL Equivalent single axle load GDS Global Digital Systems
Major vertical stress Confining pressure Effective major stress Effective confining pressure
xii Column stress
A Point of investigation in the lime-cement column
r Horizontal distance of the moving vehicle from the point above the column point, A
R Radius of the vehicle from point, A Z Depth of column point A
Q Vertical point load Specific gravity
f frequency
OCR Overconsolidation ratio
NGI Norwegian Geotechnical institute Effective consolidation stress DSM Deep soil mixing
CD Consolidated- Drained CU Consolidated-Undrained UU Unconsolidated-Undrained
1 1 INTRODUCTION
Improvement of soft ground using lime-cement columns has widely been used as a construction technique since 1970´s for railway and highway embankments over organic and soft soils. Many studies are performed on stability and settlement of lime-cement column under the embankment due to static loading but very limited attention has been given to understanding their shear resistance behavior under cyclic loading of moving vehicles (Thach et al., 2013). Studying the shear resistance degradation is important to know how the structure is endangered in the long- term serviceability after large number of cycles. In this study, how the cyclic shear resistance behaves in lime-cement columns under repetitive heavy vehicles are investigated through laboratory tests.
Different researchers have different ideas concerning post cyclic strength of soils. Seed et al.
(1971) concluded that the response to cyclic loading can be either complete loss of strength in sands to an increase or decrease of strength in clays. The decrease in stiffness and strength with the number of load cycles is called degradation. Cyclic degradation can be investigated based on the results of either strain-controlled or stress-controlled cyclic tests. During cyclic stress-
controlled test, the cyclic stress amplitude is set to constant, where as in the cyclic strain- controlled test; the cyclic strain amplitude is kept constant. Cyclic degradation changes with, N number of cycles and it is the most important factors influencing the degradation. Ishihara (1996) stated the fundamental aspects of the cyclic degradation of clays in cyclic strain-controlled tests, and also the cyclic stress-controlled tests.
Cyclic laboratory studies performed using the cyclic strain-controlled undrained direct simple shear tests have shown that the loading parameters which govern the cyclic response of saturated soils are those that govern the deformation of soil skeleton (Vucetic, 1992). The main reason for such distortion is relative displacement between soil particles, which can be expressed in terms of the shear strain, γc. Such displacements are directly responsible for the breakage of particle
2 bonds, slippage at the particle contacts, corresponding change of microstructural repulsion forces and the tendency towards volume change which causes pore pressure variation. The most
important cyclic loading parameters are therefore the shear amplitude γc, (measures the relative magnitude of displacements between soil particles in a single loading cycle), and the number of cycles N (related to the cumulative distortion of the soil skeleton).
Poulos, H.G. (1980) pointed out main reasons behind the degradation in a single pile due to cyclic load. Partial to zero dissipation of excess pore water pressure which is generated during cyclic loading process, destruction of inter-particle bond with particle realignment &
rearrangement and gradual accumulation of irreversible plastic strains are the reason behind the degradation.
1.1 Aim and objective
The primary purpose of this investigation is to increase the knowledge about the extent of the cyclic shear resistance reduction in lime-cement column in soft clay when subjected to different degrees of disturbance. This will help to be able to monitor the lime-cement column that would be subjected to cyclic loading in the long term and to get a better idea of how they react to the real repeated load.
From the laboratory test program the following were analyzed:
• How the degree of shear resistance degradation of the stabilized soil behaves with a number of load cycles
• The effect of the stabilizing agent and curing time on the degree of degradation of a soil
• Pore pressure behavior with a number of load cycles
3 The test results are presented in different plots to reveal the behavior of the shear strain, axial and radial strain, degradation index, degradation parameter and pore pressure due to a cyclic load applied on the specimen.
1.2 Limitations
The main limitation is the number of test performed. Some of the other limitations in this laboratory study include:
• The soil type
• Binder content
• Type of binder
• Water content
4 2 LITERATURE REVIEW
2.1 General
There are different situations in which soils are subjected to cyclic loading such as sea wave loading, cyclic loads on permanent way support systems (roads, airfields, railways) subject to moving load, blasting, piling, sheet piling, installation of lime-cement columns, heavy transports, etc.
Infrastructure renewal projects often require placement of roadway embankments on soft, compressible ground. One of the options available for controlling stability and mitigating settlement problems is to provide columnar support (deep soil mixing) through the soft ground.
And these columns are repeatedly exposed to cyclic loads from heavy vehicles running on the embankment.
2.2 Post-cyclic response of soils
Many studies have been performed on the investigation of post-cyclic behavior different types of soils. These studies include the loss of static undrained shear strength and strain softening of soils under cyclic loads. However, different researchers have different opinions regarding; for example the strength of clay soils after cyclic load. While researchers like Thiers and Seed and Yasuhara et al. (1992) claim there is a considerable reduction in the undrained strength of clay after cyclic loading whereas Moses et al. (2003) observed an increase of undrained strength after cyclic loading for cemented marine clay under higher effective confining pressure. Pillai et al.
(2013) also observed a higher undrained strength during post-cyclic monotonic test compared to samples that was not subjected to prior cyclic loading.
5 According to Carter et al. (1982), when a soil which is saturated is exposed to cyclic loading under undrained condition an accumulated pore pressure which is not released completely during unloading phase is observed. And this residual pore pressure resultants in decrease in the mean effective stress that governs the behavior of the soil mass in an undrained condition, causing the yield stress to decrease.
The behavior of soils under cyclic loading is different from soils under monotonic loading.
Under cyclic loading there is excessive settlement and the progressive generation of pore pressure causes the reduction in the effective normal stress which is the reason behind in reduction in strength (Koutsoftas, 1978).
Soltani-Jigheh and Soroush (2006) investigated on the post-cyclic behavior of compacted clay- sand mixtures using monotonic and post-cyclic triaxial tests. It is observed that the undrained cyclic loading reduces the effective stresses by generating excess pore pressures and induces apparent overconsolidation in the specimens. The results of the test show that cyclic loading degrades undrained shear strength and secant deformation modulus of the mixed specimens.
Different constitutive models have been developed to understand the behavior of soft clays.
Idriss et al. (1978) have introduced the index δ and parameter t in the context of the evaluation of the cyclic degradation of marine clay deposits underlying offshore structures for oil explorations.
According to the findings of Idriss et al. (1978) for many types of clay the relationship δ versus N, in a log-log format is approximately a straight line. For overconsolidated clays, δ versus N in a log-log format is also approximately a straight line (Vucetic and Dobry, 1988), while for sands it is typically curved (Dobry et al., 1982 and Mortezaie, 2012). Bahr (1991) and Matsui et al.
(1999) found that the parameter t depended on plasticity index of the soil. Lower values of δ mean the higher degrees of degradation.
Basack and Purkayastha (2009) carried out an investigation on cyclic characteristics of the marine clay from the eastern coast of India. A series of cyclic tests were performed in
6 unconsolidated and undrained state under strain controlled mode and put the results in the
following diagrams.
Figure 1: A plot of degradation index versus number if cycles in log-log scale (Basack and Purkayastha, 2009)
Figure 2: Variation of degradation parameter with cyclic shear strain amplitude for different marine clays (Basack and Purkayastha, 2009)
7 .
Bahr (1991) has also suggested an equation to predict the degradation parameter t for clay as follows:
= ( ) . Equation 3.1
= 0,52 − 0,0170 − − − −( < 23) Equation 3.2
= 0.16 − 0.0014 − − − − − ( > 23) Equation 3.3
Where εc is the cyclic axial strain and a the parameter which depends upon the degree of clay plasticity. Tan and Vucetic (1989) and Bahr (1991) have observed that clays become less susceptible to the degradation in the course of cyclic load application because of high plasticity.
Yasuhara et al. (1997) used a similar approach to that of Idriss et al. (1978) to find out the shape of the graph for plastic silt, but a more linear relationship is obtained when δd is plotted against N on a semi-log scale, and proposed the following relationship:
δ* = 1 − dlgN Equation 3.4
Where δ*is degradation index and dis degradation parameter.
Diaz-Rodríguez (1989) described a series of cyclic triaxial tests on undisturbed soil samples of Mexico City soil. Based on tests results, a procedure to determine a stress threshold is proposed from the reduction of post-cyclic strength after 100 cycles of loading. Below the stress threshold, repeated loading has a negligible effect on the post-cyclic undrained shear strength. Over this, a remarkable reduction of shear strength is observed.
8 Depending upon the cyclic stress level the soil either fails during cyclic loading itself or reaches a non-failure, stable state (Seed and Chan, 1966, Castro and Christian, 1976, Vucetic and Dobry, 1988 and Yasuhara and Hyde, 1997).
2.3 Factors affecting the degradation of soils
Strain rate and load cycles effect on undrained shear strength is investigated on sensitive Canadian clay by Lefebvre and LeBoeuf (1987) using monotonic and cyclic triaxial tests. The test results show that the cause of the reduction in undrained shear strength with lower strain rate appears to be different for structured (naturally overconsolidated) and destructured (normally consolidated) clays. Decreasing the rate of loading reduces effective stresses and, as a result of this the undrained shear strength is reduced. Reducing the strain rate or cycling the load results in weakening the resistance of the clay skeleton due to a fatigue phenomenon.
Type of consolidation method (anisotropic or isotropic) is found to affect the degradation rate of soft clay under the same cyclic loading according to test done by (Li and Huang, 2010) on offshore soft clay. The experimental results show that the strength and stiffness degradation of anisotropically consolidated soft clay is lower than that of isotropically consolidated soft clay.
This means anisotropic consolidation decelerates the degradation of stiffness of the clay soil. It is also observed that under the same consolidated condition there is a decrease of undrained
strength and stiffness of soft clay with the increase of the cyclic stress ratio and number of cycles.
The effect of overconsolidation ratio (OCR) on the cyclic shear modulus degradation of clay is presented by Vucetic and Dobry (1986). The research is based on a series of strain-controlled test on offshore Venezuelan clay by consolidating to ORC= 1, 2 and 4. The results show that the rate of cyclic modulus degradation decreases with increased OCR and increases with cyclic strain, .
9 The effects of frequency, f of cyclic shearing on cyclic degradation were investigated by
Mortezaie and Vucetic using NGI (Norwegian Geotechnical Institute) simple test device. The test was performed on normally-consolidated kaolinite clay (PI =28) with the help of the cyclic strain-controlled simple shear test. The testing program made up of three cyclic strains, γc=0.1, 0.25, and 0.5%, two vertical effective consolidation stresses, σ’vc =220 and 680 kPa, and three frequencies, f =0:001, 0.01, and 0.1 Hz. According to the findings the cyclic degradation parameter, t increases with f and decreases with σ’vc. It is also shown in figure 3 that the degradation will increase with increasing of cyclic strain, γc.
Figure 3: The effect of frequency, f on cyclic degradation ( Mortezaie, A. and Vucetic, M. ,2013)
An investigation was done by Soltani-jigheh, H. and Soroush, A. using cyclic triaxial test on the degradation effect when a granular material is mixed with clay. According to the result obtained, the inclusion of sand and gravel material into clay leads to increasing of degradation and pore water pressure build up during cyclic loading. And the degradation index decreases as the number of loading cycles and cyclic strain increase.
10 2.4 Soil stabilization
Stabilization is the process of mixing stabilizing agents with a soil to improve geotechnical properties of the soil material such as compressibility, strength, durability and permeability. It includes the blending of soils to get a desired gradation or the mixing of additives that may alter the gradation, texture or plasticity of the soil.
Stabilization of soft soils with binders is the most popular method of ground improvement technique in Sweden, and is increasingly being used internationally. The most common binders used today are cement and lime. Lime modification has been traditionally utilized as a
construction expedience for highway project with clayey sub-grade. These highways are likely exposed to heavy vehicles loads which induce cyclic load effect in a soil mass.
2.4.1 Mass Stabilization
Stabilization is done by mixing an appropriate amount of dry or wet binder throughout the volume of the treated soil layer. The binder can be a single substance or a mixture of various substances like cement, lime, fly ash or furnace slag. This system is used to stabilize soils to a maximum of 5m depth.
2.4.2 Deep soil mixing (DSM)
This method is used to improve soil strength and stabilities for foundations, deep excavations, highways and other engineering projects in soft soils to a max depth of 40m. During DSM installation, either cement slurry (wet mixing) or cement powder (dry mixing) is injected into the soil ground under pressure. This method is expensive but very fast and effective deep down the soft soil.
11 Most of current studies of DSM are focused on the soil strength improvement and soil treatment effectiveness. Cement treatment leads to significant increase in unconfined compressive strength and modulus of elasticity of the soils.
Laboratory unconfined shear strength test results on different soil types treated with 25% lime and 75% cement is reported by Hartlen and Holm (1995). The results are shown on Figure 11.
2.5 Stabilization effect on degradation of soils
Many researches have been done concerning the benefit of stabilizing a soil with lime and cement with respect to increase strength and resistance to permanent deformation but limited experimental data and constitutive models are, however, available on the resistance of cyclic strength for cement-lime improved soft clays.
Figure 4: Shear strength of different soils mixed with two quantities of lime and cement at three curing times (Hartlen and Holm, 1995)
12 Marshll et al. (2007) have tried to improve the durability of cement stabilized expansive
subgrade by pretreatment with lime. It is found that lime treatment increases resistance to degradation of strength upon saturation and cyclical wet/dry strength testing and maintaining plasticity reduction after a year of exposure to in place conditions.
Sharma and Fahey (2003) studied cemented sand under cyclic load and found that the deviator stress and deviatoric strain at yield reduced with increasing number of cycles. This is due to the continuous degradation of bond, which results in a very significant decrease in stiffness.
Haeri et al. (2002) conducted an experiment on mechanical behavior of cemented gravely sand under cyclic load. The samples range from uncemented to 3% cemented. The degradation index is plotted against normalized number of cycle, N/Nf, Nf is the number of cycles associated with the failure. Here, the degradation index is the normalized shear modulus or G/Gmax, Gmax is the maximum shear modulus.
,
,
13 Figure 5: Variation of degradation index with N/Nf, (a) uncemented sample, (b) 1.5 %
cemented sample and (c) 3 % cemented sample (Haeri et al., 2002)
Yasuhara et al. (1997) suggested a linear relationship between degradation index and the number of cycles in logarithmic scale. However, the test results in this study show that almost a nonlinear relation for all samples.
(b)
(c)
14 2.6 Cyclic triaxial shear test
A cyclic loading can be defined as a periodic action that when applied to a material body tends to change its stress and strain state. In Peralta (2010) a definition of cyclic loading is given as a load frequency between 0 and 1 Hz. Inertia forces can be neglected due to the low frequency, and the accumulated strain is mainly plastic (Shajarati et al., 2012).This test is similar to conventional triaxial shear strength test except the load is applied in a cyclic way with certain limited frequency range.
The triaxial shear test is one of the most reliable methods available for determining shear strength parameters. It is mostly used for research and conventional testing.
Generally there are three standard types of triaxial tests:- 1. Consolidated-drained test or drained test (CD test) 2. Consolidated-undrained test (CU test)
3. Unconsolidated-undrained test or undrained test (UU test)
Consolidated-undrained (CU) test method is preferred for this project. In this test, drainage from the soil specimen is not permitted during the application of chamber pressure. The test specimen is sheared for couple of hours by the application of deviator stress.
2.7 Summary
Different results were obtained on cyclic response of soils under cyclic loading. The results mainly depend on the type of soil investigated, method of testing, test condition, etc. However, the majority of the investigations show the reduction of strength on post-cyclic response. The introduction of index δ and parameter t by Idriss et al. (1978) has helped to evaluate the cyclic degradation of soils by many researchers. And similar results were obtained with Idriss et.al (1978) on investigation of many types of clays.
15 The post-cyclic degradation of soils is dependent on different factors such as strain rate, vertical effective stress, consolidation method, frequency of loading and soil type. It can also be observed that adding stabilizing agents such as lime and cement will increase the resistance to the
degradation of a soil. Both linear and non-linear relationship is observed on degradation index relationship with the number of cycles on log-log scale depending on the type of soil being investigated.
16 3 METHOD AND MATERIALS
In this study, a series of cyclic undrained triaxial tests were conducted on both stabilized and unstabilized soil specimens, by using a triaxial testing apparatus, type Geotechnical Digital System (GDS). The rate of cyclic shear resistance degradation is calculated from the laboratory test data results obtained using a degradation model according to Idriss et al. (1978) as described below. Stress- controlled cyclic tests were performed under different curing time and additive amount. Different combinations of cement and lime were used for stabilization of the clay soils.
3.1 Degradation model
According to Idris et al. (1978) the cyclic degradation in the cyclic stress-controlled mode is expressed with a degradation index, δ which describes the reduction of the secant shear modulus,
with number of cycles, N as follows:
δ = = /0
/01
= Equation 3.1
And the degradation parameter, t
= −log δ
log 34 5 = 67
Equation 3.2
Here, cyclic shear stress, is constant while the cyclic shear strain amplitude,γ9:, varies with N. is the shear strain registered at first cycle and γ9: is the final shear strain at which the test is stopped. The index δ decreases with N because γ9: increases with N. Lower values of δ mean the higher degrees of degradation.
17 Idriss et al. (1978) found that for many types of clay soils the relationship δ versus N, in
a log-log graph is approximately a straight line. The slope obtained from this line is the degradation parameter, t which for a given γc describes the rate of cyclic degradation with N.
3.2 Column stress, σcol and confining pressure, σ3
In this experiment, it is assumed that the stress on a column comes from a heavy truck on a stabilized soil column underneath the embankment. The column stress level at a point A, 2 meters below the embankment surface, is calculated from a truck axle load. The concept of equivalent single axle load (ESAL) which is equivalent to 80 kN is used as a unit to measure the effect of the truck load on the column. The embankment has 8 meter width and 1 meter height .The type of the stabilization is assumed to be deep mixing stabilization with cement and lime forming a column type.
The type of vehicle considered which causes cyclic load is heavy vehicles such as heavy truck with six axle loads ignoring light traffic e.g. cars. For this specific scenario an average of 40 ton of traffic load (maximum of 60 ton of truck load is permitted in Sweden and Finland) is
considered. And its equivalent ESAL will be approximately 2 (Truck Size and Weight, North Dakota department of transportation).
18 Assuming the load from the truck is applied approximately as a point load Q on the pavement from the single axle load of 2 ESALs, equivalent to 2*80 kN=160 kN. The stress level from the heavy truck at a desired depth Z can be calculated using Boussinesq (1883) formula for a point load, Q and presented on Figure 7.
Moving vehicle
Desired column Point, A
Figure 6: Layout of lime-cement column under the embankment Soft Clay A
Z
r, horizontal distance
Figure 7: Position of point A of in lime-cement column under moving vehicle load
19 0
5 10 15 20 25
0 5 10 15 20
vertical Stress, kPa
Horizontal distance from point A ,m Stress induced at A as a function
of horizonatal distance, r
Figure 8: Stress distribution at point A due to a track passing the road
A vertical stress 20 kN is considered as a vertical sinusoidal load at a point A on the column at a depth 2 m depth from surface. At the same point A, the confining pressure = 20 ; and water pressure of 5 kPa is approximately assumed for the entire tests performed.
3.3 Experimental Procedure
The soil used to be stabilized is kaolin type ASP 400 with a specific gravity Gs =2580 kg/m3.The type of binders used are Portland cement and burnt lime (quick lime).
The cyclic tests were conducted using GDS Triaxial Automated System. The equipment unit consists of a triaxial cell, a load frame, three computer controlled flow pumps for delivering the cell, back pressure and load, electro actuator for applying loading, data acquisition unit and a PC for controlling the resulting data. 3 transducers are mounted in the system for confining pressure, back pressure and load chamber. Water is used as a chamber medium.
20 Table 3-1: Stabilizer combination scheme for stabilized soils
Sample mixture proportion
Sample no. Binder dosage Water cont. lime:cement ratio Curing time (Kg/m3) (%) (%) (days)
1 150 50 70:30 7 2 150 50 70:30 7
3 150 50 70:30 28
4 200 50 70:30 28 5 - 35 - 28
Four kinds of materials; tap water, dry kaolin, quicklime and cement were used to prepare the specimen. A total of 5 samples were prepared for the tests, 4 samples with binder and one sample without binder. All the samples had 2 to 2.5 heights to diameter ration. A cylindrical tube of diameter 50 mm was used for all tests performed. Most specimens had approximately 2:1 height to diameter ratio.
Figure 9: GDS triaxial testing system
21 It is assumed that the typical lime-cement stabilized column has a density of 1.5 t/m3.Binder contents of 150kg/m3 and 200 kg/m3 was used during sample preparation. The binder
percentage 30% cement to 70% lime was used for all samples prepared. A water content of 50%
was used for all specimens except for the clay sample where the water content was 35%. Two samples, only clay with no binders was prepared and tested to observe the effect of adding the binders.
The specimen mix was prepared using hand mix. The dry clay powder and lime was first blended thoroughly for 5 min. Then cement was added and mixed. Water was added step by step and mixing was continued until a uniform mix was obtained. To compact the sample in the cylinder a wooden block weighing 260 gm was used. 10 blows of wooden block were applied for every 2 cm thickness during the compaction process.
(a) (b) (c) (d)
Figure 10: The four components used to prepare the specimens; (a) cement (b) lime (c) clay (d) water
22 Curing period and temperature
The curing time of the samples was 7 and 21 days. To keep the humidity 100%, the specimens were sealed with plastic bags and submerged in a water bath and placed at room temperature.
3.4 Unconfined Compression (UC) test
The primary purpose of this test was to determine the unconfined compressive strength of the specimen. 60 percent of the maximum strength value was then taken as the initial vertical principal stress on the specimens as an input in the later cyclic triaxial tests. The specimen had a diameter of 50 mm and height of 80 mm.
A cylindrical specimen was subjected to a steadily increasing axial load until failure occurs. The sample is loaded at strain rate of 0.3% per min during the test.
Figure 11: Unconfined uniaxial compression test
23 3.5 Cyclic triaxial tests - test set-up and test procedure
The purpose of this test was to determine the behavior of axial strain, radial strain and shear strain and also to investigate the behavior of cyclic shear degradation after some repetitive loads.
There are two kinds of cyclic triaxial test namely stress-controlled and strain-controlled cyclic traixial test. Stress-controlled test was performed in this study. An important requirement of the cyclic axial load testing was to achieve load-controlled repeated load tests which consist of applying predetermined controlled load intensity to the specimen in a specific wave shape at a specific frequency.
Consolidated undrained (CU) triaxial tests with pore pressure measurement were performed in this project because it is required to deal with long term stability problems in the embankment requiring effective stress analysis. The tests were performed according to the following procedure:
• The test specimen was extruded from the tube carefully, measured and cut as required. Then the sample weight was recorded. Disturbance to the specimen was kept to minimum during the preparation.
• Following placement of the specimen in the apparatus, the triaxial cell components and system were assembled.
• Then the cell was filled with water. Cell pressure and back pressure controllers and transducers readings set as required before creating the test stages.
24 Table 3-2: General input data summary for each specimen
Test sample
no.
Height, mm
Diameter, mm
Binder, kg/m3
Curing days
Vertical cyclic load
(kPa)
Effective vertical stress
,σ1 ( kPa)
Average verical load
(kN)
1 103 50 200 7 20 57 67
2 92 50 150 7 20 57 67
3 110 50 150 28 20 57 67
4 117 50 200 28 20 57 67
5 100 50 - 28 20 57 67
Figure 12: Sample is covered with a rubber membrane and sealed before putting the chamber.
Probable failure plane
25 The following four basic stages were performed for each sample in the test plan:-
a. Saturation: This process is designed to ensure all voids within the test specimen are filled with water, and that the pore pressure transducer and drainage lines are properly de-aired. In this case due to the cement in the specimen it is difficult to get the desired saturation stage unlike unstabilized clay soil. The back pressure is increased to ensure a good saturation and so the cell pressure by the same amount to maintain the same effective consolidation stress.
b. B-check: To check the degree of specimen saturation is sufficiently high before moving to the consolidation stage, a short test was performed to determine Skepton’s B-value called B- check. It is recommended to obtain B-check ≥0.95 but it was obtained 0.8 in average in most of this test trials.
c. Consolidation: This stage is used to bring the specimen to the effective stress state required for shearing. It is typically conducted by increasing the cell pressure whilst maintaining a constant back pressure. An isotropic consolidation type was performed in this stage.
d. Shearing or cycling stage: This is the last stage which lasts for longer period of time.
Necessary input date such as time for one cycle and datum for pressure and amplitude was set. Confining pressure was set constant and variable repeated deviator stress was applied by the axial loading device in the vertical direction in the form of sign wave. The frequency of cyclic loading was set to 1 cycle per minute.
Table 3-3: Test conditions
sample
Additives (kg/m3)
curing time (days)
cyclic loading frequency, f (Hz)
loading cycles, (N)
1 200 7 0.02 1200
2 150 7 0.02 1200
3 150 28 0.02 1373
4 200 28 0.02 1309
5 NA 28 0.02 457
26
Various testing and material factors that may affect cyclic shear resistance results are:
• Membrane stiffness which restrains the specimen. The stiffness of the membrane can resist freedom of the specimens to displace horizontally. The specimen become stiff and this affect the results on axial and radial strain.
• Contact between the specimen end and porous stone. If the porous stone is not placed correctly on the ends of the samples the loads will not be transferred fully to the sample.
• Lateral motion or tilting of the specimen during cyclic loading process. If the sample is tilting the vertical load will not be transferred to the sample vertically and this will give different result
• Presence of foreign matters or impurities. Foreign matters in the soil will alter the
behaviour of the soil and beside it will affect the hydration of cement in the stabilized soil.
Figure 13: Typical example of test plan during testing a sample
27
• Water to cement - lime ratio. An optimum amount of water to cement-lime ratio is used to obtain sufficient compaction which is desirable for the test. The amount of water ratio has also an effect on the hydration of cement in the stabilized soil.
28 4 TEST RESULTS AND ANALYSIS
This section presents the test results obtained in the laboratory. One uniaxial compression test and four triaxial tests were performed on the stabilized soils in the laboratory. One specimen which was unstabilized clay was tested for comparison with the properties of the treated samples.
The results are shown in a number of different plots.
4.1 Uniaxial compression test result
In this type of test failure is defined as the peak stress, which typically occurred at 2 to 8 percent strain (Jacobson and George, 2002).The specimen had 150 kg/m3 of binders and was cured for 21 days. During the test the specimen was unable to take anymore load after 4% of strain level.
The failure of specimen was observed at 95 kPa and elastic strain is about 2.5% as shown on Figure 13.
Figure 14 : Stress-strain during uniaxial compression test.
0 20 40 60 80 100
0 1 2 3 4 5
Unconfined Compressive Strength (kPa)
Axial strain, ε (% )
29 4.2 Cyclic Triaxial test results and discussion
During the laboratory test process the Skepton’s B-Value was obtained as 0.8 which shows that the sample was not saturated fully. The binders used might have an effect on the degree of saturation and it is also difficult to get full saturation in soils in reality.
The test results revealed that there is a sudden increase of axial and shear strain in the first 100 cycles then continues with almost constant value. The amount of the binders had a significant effect on the strain magnitude with time as shown in Figure 15-16. It means that curing time has a major effect on the magnitude of the plastic strain in the column. As shown in Figure 16(a), the amount of binder is insignificant on the shear strain for the first 7 days. From the Figure 15-16, it can be observed that the axial strain and shear strain values were similar in every cyclic test performed in the case of treated soils.
For the stabilized kaolin the degradation index, 5 had a half parabolic relationship with number of cycles on log-log scale graph as shown in Figure 17(a). In figure 17(b), it can be observed that when the specimen getting stronger after 28 days, the relationship becomes approximately straight line. The figures also show that most of the degradation has taken place during the first 100 cycles. As shown in Figure 17(b), the amount of binder is almost insignificant on the value of degradation index at 28 days. Generally, the degradation index decreases as the amount binder increases and the specimen getting stronger. The decrease in degradation index is probably due to granular effect of lime and cement on kaolin. Adding lime and cement to kaolin will change the mechanical behavior and the mixture turns into composite kaolin with a granular texture.
The degradation parameter, t versus cyclic shear strain, plot maintains its hyperbolic relationship for the 7 days old stabilized soil which is in a good agreement with the results presented by Idriss et al. (1980) for clays. However, in the case of treated soil which is cured for 28 days as shown in Figure 18(b), the degradation parameter t is non-hyperbolic. The parameter, t is highly affected by the shear strain which is the main parameter responsible for the breakage
30 of interparticle bonds. Generally the parameter, t increases with increased amount of binder in the first 7 days of curing time. However, for the 28 days old sample the parameter, t is not affecting by the binder content. Comparing the evaluated t of stabilized soil with unstabilized clays, in the case of stabilized soil the parameter starts to decrease after reaching some maximum value because of the small strain value. But for the unstabilized clay it increases with even higher rates which lead to breakage after certain value of strain.
As shown in Figure 18, the strain threshold where material degradation starts to occur is different from one sample to another. The samples cured for 7 days have higher values of strain threshold at the beginning of cyclic loading because they already had higher strain values before they were sheared during cyclic stage as shown in Figure 18(a). By increasing the binder content from 150 kg/m3 to 200 kg/m3 at 28 days the strain threshold decreased from 0.323% to 0.065% as shown in Figure 18(b).
Generally, the test results show that adding the binder had an effect on decreasing the
degradation of a soil. However, increasing the amount of binder was not as significant as thought in decreasing degradation for higher curing time, 28 days. Therefore it is economical to use 150 kg/m3 of binder content for 28 days instead of 200 kg/m3. The test results were based on frequency value of 0.02 Hz; however, increasing and decreasing the frequency could affect the degradation. The pore water pressure was not built throughout the test as expected on both stabilized and unstabilized soils. This might be due to the low frequency value used during the test process. Using low frequency would allow the specimen to have enough time to relief the pore pressure instead of building up during the cyclic process.
31 0,4
0,6 0,8 1 1,2 1,4 1,6
0 500 1000
Axial strain, ε (%)
Number of cycles, N 150 kg/m3 200 kg/m3
0 0,2 0,4 0,6 0,8
0 500 1000
Axial strain, ε(%)
Number of cycles, N 150 kg/m3 200 kg/m3
0,4 0,6 0,8 1 1,2 1,4 1,6
0 500 1000
Shear strain, γ (%)
Number of cycles, N 150 kg/m3 200 kg/m3
0 0,2 0,4 0,6 0,8
0 500 1000
Shear strain, γ(%)
Number of cycles, N 150 kg/m3 200 kg/m3
(a) (b)
(a) (b)
Figure 15: Variation of axial strain with number of cycles (a) for samples cured for 7 days and (b) for samples cured for 28 days
Figure 16: Variation of shear strain with number of cycles; (a) for samples cured for 7 days and (b) for samples cured for 28 days
32 0,1
1
1 10 100 1000
Degradation index, log δ
Number of cycles, logN 200 kg/m3
150 kg/m3
0 0,1 0,2 0,3 0,4
0 0,2 0,4 0,6 0,8
degradation parameter, t
Cyclic shear strain, γc (%)
150 kg/m3 200 kg/m3
0 0,1 0,2 0,3
0,4 0,9 1,4
Degradation parameter, t
cyclic shear strain, γc(%) 150 kg/m3 200 kg/m3 0,1
1
1 10 100 1000
Degradation index, log δ
Number of cycles, log N 150 kg/m3
200 kg/m3
(a) (b)
(a) (b)
Referring to Figure 19(a) and 13(b), the pore pressure increases significantly at the beginning of the test at the first load cycles. Then it decreases slowly before it continues as a constant
Figure 18: Degradation parameter plotted against cyclic shear strain; (a) for samples cured for 7 days and (b) for samples cured for 28 days
Figure 17: Degradation index plotted against number of cycles in log-log scale; (a) for samples cured for 7 days and (b) for samples cured for 28 days
33 8
9 10 11 12 13 14
0 500 1000
pore pressure , kPa
Number of cycles, N 200 kg/m3 150 kg/m3
0 5 10 15 20 25
0 500 1000
Pore pressure, kPa
Number of cycles, N
150 kg/m3 200 kg/m3
magnitude after about 1200 cycles. The stronger the material, the lower pore pressure at the beginning of the test. Eventually the two stabilized soil samples with different binder contents tend to have a closer constant magnitude of pore pressure.
For samples treated with 150 kg/m3, the pore pressure is built up in the first few cycles then it decreases. However, for the sample with a binder content of 200 kg/m3, the pore pressure is built up through the entire test as shown in Figure 19(a). From the laboratory results, failure was not observed in any of the stabilized soil samples. Looking on the results in Figure 19(a) and 19(b), it can be deduced that the stronger the material the less pore pressure developed in the sample.
(a) (b)
Below are the results from the unstabilized clay sample. The clay sample was loaded by a limited number of cycles compared to the stabilized soil samples due to its soft behavior. From the test result, a horizontal crack and a small diameter increase at the middle of the sample was observed.
This phenomenon occurred at around 300 cycles which can be observed from the discontinuity of the graphs in the following Figures.
Figure 19: Pore pressure variation with number of cycles; (a) for samples cured for 7 days and (b) for samples cured for 28 days
34 0
2 4 6 8 10
0 200 400 600
Axial strain, ε(%)
Number of cycles, N Clay
0 1 2 3 4 5 6
0 200 400 600
Radial strain (%)
Cyclic Number, N clay
0 2 4 6 8 10 12
0 100 200 300 400 500
Shear strain,γ, (%)
Number of cycles, N Clay
0 5 10 15 20 25
0 200 400 600
pore pressure ,kPa
Number of cycle, N Clay
The result shown in figure 20(d) shows that the pore pressure decreased as the number of cycles increased. This could be because the sample has enough time to relieve pore water pressure due to small frequency used during test as a result it didn’t develop pore pressure. It can be
concluded that the small crack observed during the test was due to the gradual development of shear strain rather that gradual development of pore pressure.
(a) (b)
(c) (d)
For the clay sample which is shown in Figure 21(a), the degradation index decreases linearly with the number of loading in a log-log scale and matches reasonably well with the previous
Figure 20: Plot results for a clay soil sample for 28 days curing time
35 0
0,2 0,4 0,6 0,8 1 1,2
1 10 100 1000
Degradation index,log δ
Number of cycles, N
0 0,1 0,2 0,3 0,4 0,5 0,6
0 5 10
degradation parameter, t
cyclic shea r strain, γ c(%)
studies. The degradation parameter, t versus shear strain plot maintains its hyperbolic
relationship for the clay soil which has a good agreement with the result presented by Idriss et al.
(1980).
(b) (b)
Figure 21: (a) Degradation index plotted against number of cycles and (b) degradation parameter plotted against cyclic shear strain for unstabilized clay sample for 28 days
36 5 CONCLUSIONS AND COMMENTS
In order to analyze the cyclic shear resistance degradation of stabilized kaolin, cyclic tiaxial tests were performed on specimens prepared in the laboratory. The following conclusions can be drawn from the test results:
1. For the stabilized specimen, the shear strain and axial strain increased rapidly during initial cycles then it continued with a constant but small rate of deformation.
2. The influence of the binder content on the magnitude of the shear strain was insignificant for the first 7 days of curing time.
3. The stronger the material, the quicker the strain rate stops within few numbers of cycles.
4. The degradation index of stabilized kaolin has a parabolic relationship with the number of cycles on log-log scale graph.
5. Degradation index decreased as the number of loading cycles and cyclic strain increased. The major parts of degradations have taken place during the first 100 cycles. Increasing amount of binder decreased the degradation index.
6. This study shows that above certain cyclic shear strain amplitudes, the cyclic degradation rate change practically do not take place for stabilized soils.
7. There was an accumulation of pore pressure during the test for the samples with binder content of 200 kg/m3 .However, for the samples with binder content of 150 kg/m3, the pore pressure decreased.
8. Unlike many studies, the results on the clay didn’t show gradual development of excessive pore pressure which is mostly governing the failure in sand and clay.
9. Regarding the clay test result, at 300 cycles there was a sudden change in the shear strain vs number of cycles graph shape due to failure in the specimen. The plane of failure was not diagonal but horizontal due to soft material property of the clay sample.
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