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Small-strain Stiffness and Damping of Soils in a Direct Simple Shear Device

B. D'Elia, G. Lanzo and A. Pagliaroli

Small strain cyclic behaviour characteristics of soils are of major concern in many geotechnical engineering problems, such as the response of ground during earthquake, machine foundation design, pile driving and effects of explosions.

In this study the small strain cyclic properties of reconstituted Santa Barbara clay and Toyoura sand are presented and discussed. The tests were performed with a special simple shear device for small-strain testing called the Double Specimen Direct Simple Shear (DSDSS) device, originally developed at the University of Los Angeles (UCLA) and recently constructed, with few modifications, at the Department of Structural and Geotechnical Engineering of the University of Rome “La Sapienza”.

The testing program encompassed wide range of vertical consolidation stresses (óvc), cyclic shear strain amplitudes (ãc), between 0.0004% and 1.0%, and frequency of cyclic loading (f). The tests were cyclic strain-controlled, i.e., with a constant ãc, having on average 10 cycles. Secant shear moduli, Gs, were measured at ãc as small as 0.0004% enabling a reliable extrapolation of the maximum shear modulus, G0, at ãc =0.0001%, while clearly-defined loops were generated at ãc as small as 0.0008% enabling the calculation of the equivalent viscous damping ratio, D.

The laboratory tests results from testing similar sand and clay specimens with the original and the modified DSDSS device were compared in terms of the maximum shear modulus G0, secant shear modulus Gs, and the damping ratio D. Further, the effects of the cyclic shear strain amplitude ãc, effective consolidation stress and the frequency of cyclic loading, f, on G0, Gs and D are presented, evaluated and discussed.

Paper 111: [Read] [Print]

Keywords: Small-strain stiffness, damping, simple shear, sand, clay

Embankment Dam Deformations Caused by Earthquakes

J.R. Swaisgood

An extensive review of case histories of embankment dam behavior during earthquake was undertaken after several major embankment dams were severely shaken by the 1990 Philippines earthquake. The objectives of the study, which continues to date, were to determine if there is a “normal” trend of seismic deformation that can be predicted and if there are certain factors that consistently have an effect on the amount of damage and deformation incurred during earthquakes. Nearly 70 case histories have been reviewed, compared and statistically analyzed in this effort. The results of this empirical study have shown that the most important factors that appear to affect dam behavior during earthquake include the peak ground acceleration at the site and the earthquake magnitude. A chart has been prepared to summarize the relationship between the amount of measured settlement and the peak ground accelerations experienced in the incidents that were studied. In addition, an empirical equation was formulated and a graph developed as an aid in estimating of the amount of deformation to be expected.

Paper 014: [Read] [Print]

Keywords: embankment, dam, earthquake, settlement, damage

Deterministic Seismic Hazard Analysis in Northwest Oregon, U.S.A.

A.G. Hull, A. Augello and R.S. Yeats

We identified and evaluated 54 possible earthquake sources to determine controlling maximum credible earthquakes within 200 km of 12 dam sites in northwest Oregon. Seismic hazard at the sites comes from earthquakes that could be potentially generated by 14 active crustal faults within 50 km of the dam sites; large, deep earthquakes generated within the subducted Juan de Fuca plate; great earthquakes occurring on the Juan de Fuca-North American plate boundary; and a Mw 6 background earthquake 15 km from each dam site. Our hazard analysis showed that three crustal faults and the interplate subduction earthquake provide the controlling maximum credible earthquakes for all 12 dam sites.

Estimated peak horizontal ground accelerations (PGA) values range from 0.57g to 0.16g at the dam sites. Recommended PGA values are similar to those estimated by the U.S. Geological Survey to have a 2% probability of exceedence in 50 years (2,475-year return period). Acceleration response spectra (5% damping) were developed for each site based on recommended PGA values. We developed four spectrally matched acceleration time histories from instrumental records of strike-slip, normal and subduction zone earthquakes.

Paper 157: [Read] [Print]

Keywords: seismic hazard, deterministic, MCE, fault slip rate, Oregon, dam

Reconstuctional Survey of 3 Villages - Selari, Kalyanpar, Sangramsar (district Rapar, Gujarat)

A. Pranab and S. Dhanda

The Gujarat Earthquake, India (26th January, 2001) illustrated major deficiencies throughout Indian building construction, which led to massive destruction of both life and property. Preliminary estimates suggest damages to the extent of Rs.20875 Crores (US $4.5 Billion) to private and public properties.

Delhi University shared its deep concern with the fate of the people in various parts of Gujarat. It came up with project UDAI – with the aim of providing technical support for rehabilitation and reconstruction of three quake affected villages.

This paper is an in depth study of the prevailing conditions in three villages, namely Selari, Kalyanpar and Sangramsar in Rapar district of Gujarat, as surveyed during the UDAI project. The paper gives a brief account of the prevailing social and economic conditions and disaster management before and after the quake.

It analyses the various aspects of reconstruction as undertaken by the villagers. It outlines certain areas of defective reconstruction and also gives note of few innovative methods of reconstruction used by villagers.

In the end, it summarizes the various activities of the Delhi University students for rehabilitation and reconstruction.

Paper 061: [Read] [Print]

Keywords: Gujarat Earthquake, Bhuj, UDAI, rehabilitation, reconstruction

Seismic Safety Retrofit of a Major Bridge – A Canadian Study

U.D. Atukorala and H. Puebla

Key aspects of a displacement-based design that was adopted to retrofit seismic deficiencies of a major bridge in Vancouver, Canada, are presented. The displacements anticipated as a result of the 475-year event seismic loading were computed using a state-of-the art stress-strain model developed at the University of British Columbia (Canada) that models the observed post-liquefaction behaviour of soil. This stress-strain model was incorporated into the public domain finite difference program FLAC2D for the analysis of deformations. The results were used to arrive at an optimum ground improvement scheme at selected critical locations along the bridge profile rather than at each and every pier location. Where ground improvement using conventional methods such as vibro-compaction and gravel compaction piles was not feasible due to access and headroom constraints, pore pressure dissipation measures were considered using seismic drains. The paper presents some of the results of the ground response analyses and discusses the ground improvement program.

Paper 098: [Read] [Print]

Keywords: seismic retrofit, bridge, ground response analysis

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