My sincere thanks to other faculty members of the Department of Geotechnical Engineering, IIT Guwahati for their cooperation whenever required. In addition, I would like to thank the official staff of the Department of Civil Engineering for their support in administrative work.
A BSTRACT
To evaluate the dynamic properties of the soil, load- and stress-controlled cyclic triaxial tests, with regular excitations, were carried out at different shear strains (γ), Dr and σ′c. The differences in the results highlighted the importance and necessity of considering the experimentally evaluated dynamic soil properties of the regional soils to obtain accurate site-specific GRA results.
L IST OF F IGURES
List of Figures xiii Fig. 4.22 Variation in excess PWP in SBS at different loading rates and Dr.. 4.23 Variation in voltage path of SBS at different loading rates and Dr.. h) Poisson's ratio at different Dr and σ′c.
L IST OF T ABLES
List of tables xx Table 6.6 Summary of results subjected to reduced ground motions with the same PGA. 205 Table 7.2 Comparison of percentage differences in PGA, PSA and FAR obtained using VD-SI and experimental data for GRA of Guwahati city using Bhuj motion.
N OTATIONS AND A BBREVIATIONS
I NTRODUCTION
GENERAL
Therefore, the choice of test method should be based on the conditions of the site and problem of concern (Jafarzadeh and Sadeghi, 2012). Although there are several literatures available on the assessment of dynamic properties of soils from different parts of the world, it is extremely necessary to find the same for local regional soils for prudent applications.
AIM OF THE STUDY
Further, the results of this investigation will be used in field response analysis to ascertain the behavior of the region under strong motions.
ORGANIZATION OF THESIS
Chapter 2 presents a review of literature relevant to the present research work. The chapter concludes with critical appraisal of the literature and detailed scope of the work
Chapter 4 presents results of the monotonic compression shear tests conducted on cohesionless and cohesive soils along with the interpretation of the outcomes
L ITERATURE R EVIEW
INTRODUCTION
MONOTONIC RESPONSE OF SOILS
It can be seen that increasing the degree of saturation leads to a decrease in deviatoric stress resistance (Fig. 2.4a) and an. The stress path graph in Fig. 2.4c also implies the role of the degree of saturation in reducing the mean effective stress and the maximum deviatoric stress.
CYCLIC RESPONSE OF SOILS
Studies based on the regular (sinusoidal) loading
It was reported that the shear modulus and damping ratio are significantly affected by the initial void ratio shown in Fig. However, Wong et al., (1975) and Wang (1972) reported that the lower loading frequency gives slightly higher strength, which is in contrast to the above findings.
Studies based on the irregular (earthquake) loading
Literature review 34 Table 2.3 Summary of literature review on the variation of liquefaction potential with different parameters. CO2 Cyclic Triaxial Test Stress Control. a) constant relative density (b) constant sand skeleton void ratio (c). constant overall void ratio Dash and Sitharam.
LOCAL STRAIN MEASUREMENTS IN STATIC AND CYCLIC LAODING
The measuring capacity of LDT in triaxial tests has been performed with gravel, cemented sand, soft rock and it has been reported that the LDT measures strain levels ranging from 10-4 % to 1%, subjected to both monotonic and cyclic loading. The SDT has been reported to measure the local deformations of soil samples with high accuracy.
APPLICATION OF DYNAMIC PROPERTIES OF SOILS
Ranjan (2005) has conducted seismic GRA for Dehradun city and reported that the spectral acceleration varies in the range of 0.06g-0.37g in the frequency range of 1-10 Hz. Shukla and Choudhury (2012) have conducted one-dimensional GRA for Gujarat and reported that the soil amplification factor was in the range of 1.3–2.0 for the frequency range 1.0–2.7 Hz.
CRITICAL APPRAISAL OF LITERATURE REVIEW
In the absence of a proper site-specific soil model, the use of existing soil model may lead to inaccurate estimation of the ground motion parameters involved in earthquake resistant design. In the absence of a proper site-specific soil model, the use of existing soil model may lead to inaccurate estimation of the ground motion parameters involved in earthquake resistant design.
OBJECTIVES AND SCOPE OF THE STUDY
Since the soils of each region/continent are different in terms of particle size characterization, the evaluation of dynamic soil characterizations of each region, especially the seismically active region, is of significant importance for the accurate estimation of ground motion parameters. To see the importance of regional dynamic soil properties in site-specific GRA, the obtained results will be compared with those estimated from the existing material models.
SUMMARY
Furthermore, cyclic triaxial tests were carried out to evaluate the liquefaction behavior of Assam soil at various test conditions such as confining stress, relative density, test methodology and stress amplitude on cohesionless soil subjected to the irregular seismic excitations (i.e. true earthquake motion) and regular earthquake motions. (sinusoidal) excitations. One-dimensional equivalent linear GRA will be carried out for Guwahati city to utilize the estimated dynamic soil properties for application purposes and for accurate measurement of seismic design parameters.
M ATERIALS AND M ETHODOLOGY
MATERIAL DESCRIPTIONS
Cohesionless soil
Cohesive soil
The maximum dry unit weight of the soil was 17.5 kN/m3, and the optimum moisture content (OMC) was 19.3%.
TESTING APPARATUS AND COMPONENTS
Triaxial frame
The loading frame is a system for applying the load to the mounted soil specimens, where various types of monotonic and cyclic loading can be applied as a stress-controlled or a stress-controlled mechanism. Different types of cyclic/dynamic loads (harmonic, triangular, sinusoidal, haversine and user-defined random seismic movements) at different strain rates (10-5 to 10 mm/min) can be applied.
Triaxial cell
At the top of the triaxial cell, an air vent valve is provided to apply vacuum or suction in order to maintain a proper connection between the load cell and the top cover. The load cell ram moves up and down through a seal during monotonic or cyclic loading.
Dynamic actuator
The submersible load cell is connected to the top plate of the triaxial cell through a frame called load cell stop and measures the load applied to the sample. During cyclic loading, the actuator position is maintained at the center of the operating displacement capacity (±15 mm) so that it can subject the specimens to both loading and unloading in one of the load or displacement controlled modes.
Submersible load cell
Air water cylinder
The targeted cell pressure and counterpressure are applied to and in the specimen, respectively, via AWC. Two AWCs are required, one for the cell pressure channel and one for the back pressure channel.
Automatic volume change device
Pressure transducers
Dry air receiver unit
Compact dynamic controller
This CDC unit is connected to the computer and transfers the transducer data to the computer, and vice versa, via a data logger (Fig. 3.13b). The arrows entering and exiting the CDC unit, as shown in the schematic diagram, reflect the importance of the CDC unit in channeling information from DYNATRIAX software to hardware components and vice versa.
TESTING METHODOLOGY
Sample preparation
A vacuum pressure of 15–20 kPa was applied before removing the mold to maintain the verticality of the sample (Fig. 3.15c). A vacuum pressure of 15–20 kPa was applied prior to mold removal to establish proper contact between the rubber membrane and the surrounding boundary of the soil sample.
Saturation
The sample was considered fully saturated when the B value was obtained as greater than 0.95. The sample was considered fully saturated when the B value was obtained as greater than 0.96.
Triaxial shear tests
To evaluate the dynamic properties and liquefaction potential of the sandy soil, strain and stress controlled cyclic triaxial tests were performed using sinusoidal wave (regular excitation) under different test conditions. For the stress-controlled cyclic load test, the magnitude of the cyclic load (shown in Table 3.6) was first estimated, based on the desired cyclic stress ratio (desired CSR).
Strain-controlled loading
In this study, the load-controlled and stress-controlled cyclic tests have been performed at a frequency range of 0.1–4 Hz, described in Chapter 5. Materials and Methods 72 reported that the predominant frequency range for earthquakes is 0.5–10 Hz. The different effective confining load reflects that the soil samples are located at different confining depths below the ground surface.
Measurement of local strains
The following sections deal with the assembly of the on-sample LVDTs to the sample and also calibration. The responses of the on-sample LVDTs during saturation and consolidation were found to be insignificant.
M ONOTONIC S HEAR T ESTS
TESTING PROGRAM
TESTS ON DRY COHESIONLESS SOIL
The response of the soil under these test conditions is reported in terms of stress-strain variations, stress path, excess pore-water pressure ratio and stiffness variation as recorded from the tests.
TESTS ON SATURATED COHESIONLESS SOIL
The contractive behavior of the soil together with the excess development of PWP is more pronounced at high confining pressure. A further increase in the static load results in an increase in effective stress due to a decrease in PWP (Figure 4.7), indicating dilatational behavior of the soil with a gradual redistribution of excess PWP in the soil sample.
Effect of confining pressure
Thus, based on the observation by Ishihara (1993) and the results obtained for soil specimens at Dr = 30% from the present study, it can be said that even loose sand (as in its initial stress state) can behave like a dense sand, depending of the high confining voltage. 4.12, it can be observed that the A parameter depends on the confining stress, and initially varies up to ~1% of the axial strain from +0.5 to +0.7, showing the normally consolidated behavior of the soil; while its variation up to -0.35 reflects a change in soil behavior from an overconsolidated to a highly overconsolidated state.
Effect of relative density
It can be observed that the rate of increase of the excess PWP up to an axial strain of 0.5% is almost the same for all Drs. However, the further increase in load reflects the slower rate of reduction of excess PWP at Dr = 10%.
Effect of loading rate
It can be found that with the increase in Dr (from loose to very dense) the ductility of the soil is partially compensated, as the higher interlocking of the particles at high Dr generates higher internal friction angle (ϕ) and strength. 4.20, it can be observed that the secant modulus of saturated sand decreases with the increase of loading rates up to approx. 1% strain levels and becomes asymptotically constant thereafter.
Local strain measurements
Monotonic Shear Tests 96 a comprehensive response of the BS soil based on the LVDT measurements on the specimen. This is attributed to the fact that the external transducer measures an average global displacement of the sample.
COMPARATIVE BEHAVIOUR OF DRY AND SATURATED COHESIONLESS SOIL
Since DBS represents the dry state of the specimens, no excess PWP/PWP can be generated. However, the SBS representing the saturated state of the specimens, the excess PWP increases during loading up to 0.6% axial strain.
TESTS ON COHESIVE SOIL
Monotonic compression shear tests were conducted on BS (dry and saturated conditions) as well as RS (saturated) soil samples to observe soil behavior under different test conditions such as confining stress, relative density, displacement rate. It was found that the behavior of BS soils is significantly influenced by saturation, confining stress, relative density and degree of displacement.
C YCLIC S HEAR T ESTS : D YNAMIC
P ROPERTIES
DBS 60
TESTS ON SATURATED COHESIONLESS SOIL Strain-controlled cyclic loading
Cyclic shear tests: dynamic properties 124 of those observed in the damping ratio curves of previous studies (Seed and Idriss, 1970;). The effect of Dr on the damping ratio obtained from SHL and ASHL is shown in Fig.
Dynamic properties of saturated cohesionless soil using on-sample LVDTs
Cyclic shear tests: dynamic properties 132 variation and accumulation of local strains, when attached to different parts of the specimen. These findings therefore highlight the importance of using on-sample LVDT for the measurement of local strains which can be used to obtain the dynamic response of cohesionless soil over a wide range of strain.
Stress-controlled cyclic loading
The effects of loading frequency on the variations of shear modulus and damping ratio have. To observe the effect of excess pore water pressure ratio and shear stress developed on shear modulus and damping ratio at the first loading cycle for different loading frequencies, plots are shown in Figs.
Comparison of strain-controlled and stress-controlled tests results
Cyclic shear tests: Dynamic properties of 144 from load-controlled tests performed at different γ (considering only the first cycle, N = 1) and from stress-controlled tests performed for different CSRs (considering all hysteresis cycles, N = 1-40). Under such conditions, the strain-controlled approach can provide accurate dynamic properties for all constant shear strain amplitudes.
COMPARISON OF STRAIN-CONTROLLED TESTS RESULTS OF DRY AND SATURATED COHESIONLESS SOIL
The changes in the damping ratio with the number of cycles for the DBS and SBS samples are presented in the figure. Stress-controlled cyclic loading tests were performed to evaluate the shear modulus and damping ratio of the cohesive soil.
C YCLIC S HEAR T ESTS : L IQUEFACTION E VALUATION
STRAIN-CONTROLLED TESTS WITH REGULAR EXCITATIONS
Cyclic Shear Tests: Liquefaction Evaluation 164 Variations in crude with number of cycles at different γ, σʹc and Dr. It can be observed that the probability of soil liquefaction increases with the increase of γ and N.
