Presentation on theme: "Impacts of Seismic Stress on Pore Water Pressure in Clayey Soil By: Qazi Umar Farooq Lecturer Civil Engineering Dept Univ of Engg & Tech Taxila."— Presentation transcript:
Impacts of Seismic Stress on Pore Water Pressure in Clayey Soil By: Qazi Umar Farooq Lecturer Civil Engineering Dept Univ of Engg & Tech Taxila
Contents Introduction Objective Motivation Test Material Equipment and Methodology Experimental Results Conclusions and Recommendations
Introduction Most of the Geo-Hydraulic structures are made of Clay and subjected to cyclic loading Earthen dams, Transportation embankments in earthquake suspected areas and geotechnical structures on sea shores. Rise in pore water pressures due to cyclic loading is common and may lead to the failure of structures
Introduction PWP measurement is a direct mean to assess about dam integrity It can be increased at any time during the life of a dam due to any natural phenomena like flood, earthquake etc. Excess PWP may cause: Internal Erosion (Piping & Breach) Failure (In-Stability)
Objective The objective of this research is to investigate the effect on pore water pressure in soil due to dynamic loading, by cyclic triaxial tests, using Fujinomori Clay Mainly focus on Dam core or small dams (height<= 20 m) with uniform fill. Also applicable to Shore /River dykes and Rail/Road Embankments in contact with water. obliged in Risk Assessment and Design
Test Material Fujinomori-clay is selected as a testing material due to its good compressibility, Low hydraulic conductivity, High Shear Strength, and easy availability.
In the light of GSD and consistency Tests as per Unified Soil Classification system Fujinomori Clay is classified as CH (inorganic clay of High Plasticity)
Triaxial Machine The measuring devices used are: 5kN load cell, 30 mm displacement transducers, 1MPa pressure transducers, 100 ml Difference pressure transducer.
Methodology Element of clay under ~ 200kpa Pressure, Saturated & Farthest form filter drain (Un-drained) 15m U/S Dia=7.5cm H=15cm Dia=6cm H=2cm Material Preparation (Clay-water Slurry) K 1 -Consolidation compression confining pressure K 0 -Consolidation Tri-axial confining pressure Cyclic loading C-U Permeability Test (k i ) (Self Developed Apparatus) Without Loading Permeability Test (k f ) After Cyclic Loading Comparison ki k f Material Preparation (Clay-water Slurry) K 1 -Consolidation compression confining pressure K 0 -Consolidation Tri-axial confining pressure Cyclic loading C-U Failure not Allowed Permeability Test (k i ) (Self Developed Apparatus) Without Cyclic Loading Permeability Test (k f ) After Cyclic Loading Comparison k i k f
F Direction of Major Principal stress 1 Along seepage path in Dam Vertical in Sample An Element of Soil subjected to simple Shear in Dam Core Cracks and Permeability in vertical direction (k v ) are more crucial then horizontal ones Sample Direction
Constant Parameters Material : Fujinomori Clay 0 = 150 kPa, b = 1.79 g/cc, d = 1.38 g/cc, S r = 96% No of cycles = 10, q rate = 25kPa/min Test Results Varying Parameter: Cyclic Stress Amplitude & TEST Code. FM-B-02 Stress Amplitude (q max = 50 kPa, q min = -50 kPa)
Test Results Permeability (k initial = 27.32x10 -7 cm/s, k final = 33.95x10 -7 cm/s) Increased Axial Strain ( max =0.147%, min = % ) Pore Pressure (u in =27kPa, u f =53kPa), Residual Axial Strain (Compression) TEST Code FM-B-02 Stress Amplitude (q max = 50 kPa, q min = -50 kPa)
Test Results TEST Code. FM-O-03 Stress Amplitude q max = (50+25)kPa, q min = (50-25)kPa Permeability (k i = 9.59x10 -7 cm/s, k f = 10.74x10 -7 cm/s) Slight Increased Axial Strain ( max =0.24%, min = 0.00% ) Pore Pressure (u in =27.50kPa, u f =53.75kPa), Residual Axial Strain (Compression) Element at Shallow Depth h 0) Let h = 200kPa, v =250 kPa
Discussions Values of PWP are Taken at the end of final cycle (at compression) Above Tests were started from Isotropic stress Condition
Discussions 1.Micro cracking at compressive Residual Strain 2.Closure of Pores at extensive Residual Strain. 1 Micro cracked Sample 2 Sample with closed pores 1 2
Field and Lab Section Volume ~ m 3 Section for RW
Conclusions Pore pressure Increases almost exponentially by increase in cyclic stress amplitude which may cause instability at high stresses. In actual dam problem seepage other than by Pore pressure has been observed which may be due to micro or Macro cracking. Cyclic stress with combination of Pore pressure causes damage to sample and may induce Micro cracking or closure of Pores. In case of Fujinomori Clay, when flow net analysis is done the change in permeability results in very small change in seepage as compared to the seepage increment observed at real dam.
Recommendations Due to exponential increase in pore water pressure with seismic intensity it is recommended that a performance based drainage system for a dam must be designed. Relatively better drainage in upper portions of the dam is recommended. It is very crucial to properly maintain the drainage system of pre existing Dams in earthquake expected areas. For future researchers Shaking table model tests may be a good option to study the actual behavior of pore pressure and seepage, but special care should be required in scaling. Numerical modeling for simulation of actual stress generated in dam core by an earthquake. A simple shear apparatus can be modified and provided with cyclic loading and drainage facilities. This will give better idea of seepage variation even during cyclic loading. Behavior of different materials can also be checked.