LIQUEFACTION BEHAVIOUR OF SAND DURING VIBRATIONS

Transcription

1 13 th World Conference on arthquake ngineering Vancouver, B.C., Canada August 1-6, 24 Paper No LIQUFACTION BHAVIOUR OF SAND DURING VIBRATIONS Ravi Kant MITTAL 1, M.K. GUPTA 2 and Sarbjeet SINGH 3 SUMMARY For prediction of liquefaction behavior, test data from dynamic triaxial and simple shear tests on small samples have been widely used. Castro [1] has concluded that these tests don t represent the field conditions. Gupta [2] have also indicated that studies on small sample tests predict liquefaction of dense sand deposits with relative densities of 7-8% to a depth of more than 3 m which is anomalous to the known behavior of dense sands. Hence triaxial test data seems to be in error. For study of liquefaction behavior of dense sand under surcharge pressure large sand samples of 1.5 m x.6 m x.32 m were prepared in shake table. Steady state vibrations of desired accelerations and frequency can be imparted to the sand sample prepared in shake table. To simulate field conditions at depth different dead weight surcharge was applied. Overall forty tests were conducted under varying accelerations, relative densities and surcharge pressures. Dense sand samples have shown dynamic stability at low accelerations and dilation at high accelerations. This study suggests that compact surcharge fill can be used as antiliquefaction measure. Key Words: Liquefaction, shake table, surcharge pressure, relative density. INTRODUCTION Liquefaction of saturated sands has often been one of the causes of earthquake damage to structures resulting in loss of life and property. The additional safety in the design of superstructure is not of any help in the event of liquefaction of foundation soil during an earthquake. It is therefore very necessary that possibility of liquefaction is examined before hand and necessary remedial measures are adopted against damages due to liquefaction of foundation soil. 1 Lecturer, Civil ngineering Group, Birla Institute of Technology & Science, Pilani, India. mail: rmittal@bits-pilani.ac.in, ravikant_mittal@yahoo.co.in 2 Professor, Department of arthquake ngineering, Indian Institute of Technology, Roorkee, India. 3 ngineer, Weidlinger Associates, Inc. Consulting ngineers, Cambridge, MA mail: ssingh@ma.wai.com

2 For determining the possibility of liquefaction of a site, two type of laboratory tests are available i) small sample tests under dynamic triaxial or simple shear conditions and ii) Large sample test on vibration table. It has been well reported that no uniform agreement to date has been achieved from different tests performed by different investigators. The tests results both quantitatively as well as qualitatively are affected by the method of tests and test equipment used ( Peacock [3], Gupta, [4], Castro [1] ). The method of determining possibility of liquefaction from laboratory triaxial test on small soil sample is not free from errors (Castro [1], Gupta [2]). A method for determining the possibility of liquefaction has also been developed which uses the laboratory test data on large soil samples (Gupta, [4], Gupta [5], Gupta [6]). The approach road embankment of Tezpur Bridge in Assam was designed using the vibration test results. During Assam earthquake of August 1988 widespread damage of embankments bridges and foundation soil due to liquefaction was observed. The high embankment designed using the vibration table test data, performed satisfactorily (Gupta [7]). This provided confidence in vibration table test on large soil samples. Liquefaction behavior of saturated sand is largely affected by its relative density and surcharge on it. A comprehensive vibration table test data on soil sample with surcharge is reported. This paper also, describes the effect of surcharge on dense sample of sand regarding the liquefaction characteristic. XPRIMNTAL STUP Tests were performed on horizontal vibration table on a saturated soil sample. A water tight tank one meter long,.6 m wide and.4 m high is mounted on a vibration table in which the soil sample is prepared. A known quantity of water was taken in the tank and a known quantity of air dried sand was poured in the tank through a constant height to obtain a uniform deposit. The overlying water was removed and weighed to compute the initial relative density. The desired relative density sand samples were prepared in test tank and subjected to steady state vibrations of desired accelerations. To simulate field conditions dead weight surcharge pressures varying from.76 kg/cm 2 to.263 kg/cm 2 were applied on the top of soil by placing concrete blocks of desired weight. All tests were carried out at a frequency of 5cps. Pour pressures were measured by simple glass tube piezometer. RSULTS AND DISCUSSIONS ffect of relative density of sample on rise in pore pressure under different accelerations and surcharge pressure is shown in FIG 1,2,3,4. Pore pressure equal to over burden pressure (u/σ=1) required for complete liquefaction is indicated by line AB. It was observed that rise in pore pressure decreases with increase in relative density. In FIG 1 no pore pressure rise is observed beyond 71.5, 76.5 and 79.5% relative density at 2%, 3%and 4%of g accelerations respectively under zero surcharge indicating that the dense sand remains stable. However relative density beyond which no pressure rise observed is different for different acceleration & surcharge pressure. ffect of surcharge on rise in pore pressure for varying relative density of sample under different accelerations 2%g and 4%g are shown in FIG 5 & 6 respectively. From FIG 5 & 6, it is observed that with increase in overburden pressure, initially pore pressure increases, after a certain threshold value, it started decreasing. Hence for a sand deposit of particular relative density & expected maxi ground acceleration there is a value of surcharge pressure beyond which rise in pore pressure decreases sharply. However it may be noted that difference between pore pressure rise and line of complete liquefaction keep on increasing as surcharge pressure increases which shows increase in liquefaction resistance of soil with

3 every increase of surcharge pressure. Which suggest compacted surcharge fill can be used as antiliquefaction measure. FIG 1 R Vs RLATIV DNSITY FOR ZRO SURCHARG R R RLATIV DNSITY (%) 2% g 3% g 4% g LIN AB FIG 2 R Vs RLATIV DNSITY FOR SURCHARG.76 Kg/sq. cm R RLATIV DNSITY % 2 %g 4 % g

4 FIG 3 R Vs RLATIV DNSITY FOR SURCHARG.17 Kg/sq cm R RLATIV DNSITY (%) 2 %g 4 % g FIG 4 R Vs RLATIV DNSITY FOR SURCHARG.263 Kg/sq cm R RLATIV DNSITY (%) 4 %g 7 % g

5 FIG 5 R Vs OVR BURDN R R OVR BURDN R (gm/cm2) 8% 6% Dri = 4% LIN AB FIG 6 R Vs OVR BURDN R FOR 2% g R OVR BURDN R 8% 6% RD = 4 % LIN AB

6 Figure 7 shows pore pressure v/s acceleration at a relative density of 8% under different surcharge pressure. In there dense sample it is observed that negative pore pressure developed at high acceleration of 7%or more. Dilation of samples was observed visually in test tank. These results indicate dense sands are not prone to liquefaction and show greater stability under vibration. Higher surcharge pressure doesn t allow dilation of sample and negative pore pressure decreases. Thus under surcharge pressure conditions also dense sands do not liquefy. FIG 7 R VS ACCLRATION AT RLATIV DNSIT Y OF 8% R ACCLRATION % g kg/cm2.123 kg/cm2.29 kg/cm2 ZRO CONCLUSIONS Possibility of liquefaction decreases with increase in relative density of sand. However, desired relative density of sand deposit for no liquefaction depends on surcharge pressure and expected ground acceleration. Dense sand shows dynamic stability at low acceleration and at high acceleration dilation i.e. negative pore observed under different surcharge pressure. Thus dense sand may not liquefy under both with and without surcharge condition. Vibration table tests appears to be more close to field conditions compare to small sample tests under triaxial test. Surcharge pressure effects the liquefaction behavior of sand in a characteristics manner and suggests that compacted surcharge can be used as anti-liquefaction measure. ACKNOWLDGMNTS Author s thankfully acknowledge, experimental facilities provided by Department of arthquake ngineering, I.I.T., Roorkee. RFRNCS 1. Castro G, Poulos SJ. Factors affecting liquefaction and cyclic mobility Journal of Geotechnical ngineering Division, ASC, 1977;Vol.13,No.GT6:

7 2. Gupta MK, Sharma HM. Possibility of liquefaction during an earthquake Bulletin Indian Society of arthquake Technology, 1977; vol. 14, no. 3: Peacock WH, Seed HB. Sand Liquefaction under cyclic loading simple shear conditions Journal of Soil Mechanic and Foundation ngineering Division, ASC, 1968; Vol.94, No.SM3: Gupta MK. Liquefaction of sands during earthquakes Ph.D. Thesis, University Of Roorkee, Roorkee, Gupta, M.K, Prakash S. A new realistic approach for liquefaction analysis, Bulletin Indian Society of arthquake Technology 1986; Vol. 23, No Gupta, MK, Agrawal RC. Seismotectonic and liquefaction studies of an industrial site in northern India, Journal of Soil Dynamics and arthquake ngineering1998; 17: Gupta M K. Liquefaction during 1988 earthquakes and a case study, Proceedings Third International Conference On Case Histories In Geotechnical ngineering, University Of Missouri, Rolla, USA, 1994.