Date of Award

8-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil Engineering

Committee Chair/Advisor

Ronald D. Andrus

Committee Member

Glen Rix

Committee Member

Nadarajah Ravichandran

Committee Member

Weichiang Pang

Abstract

This dissertation presents the development of base case small-strain shear-wave velocity ( ) profiles and non-linear materials property models of local site conditions in South Carolina for seismic site response analyses. The need to address South Carolina's seismic hazards is partly motivated by the 1886 Charleston earthquake (moment magnitude, Mw= 6.7 to 7.5). Required inputs for seismic site response analysis include profile down to a specified reference outcrop condition and non-linear material properties for each layer in the profile. Although firm rock (i.e., ≥ 2,500 ft/s) or hard rock (i.e., ≥ 9,850 ft/s) is commonly assumed for the reference outcrop condition, reaching those conditions is often not economically feasible in South Carolina because of their significant depths. For this reason, new seismic hazard maps of the State of South Carolina have been developed for use by the South Carolina Department of Transportation (SCDOT) assuming the top of partially weathered rock or the top of Tertiary or Cretaceous sediments as the reference outcrop conditions. The base case profiles and non-linear models of the local site conditions, above the reference outcrop conditions, are needed for developing site adjustment factors that are compatible with the new seismic hazard maps.

A total of 211 profiles compiled from various project reports are used to characterize the local site conditions for the South Carolina Blue Ridge and Piedmont (SCBRP) and South Carolina Coastal Plain (SCCP). Local site conditions in the SCBRP and SCCP are primarily the residual soil and saprolite, and the Quaternary sediment and fill, respectively. For the SCBRP, the median local site profile is characterized by a time-averaged shear-wave velocity in the top 100 ft ( ) of 1,091 ft/s. For the SCCP, the median local site profile is characterized by a of 642 ft/s. Also considered in the modeling are the transitions from the reference outcrop to local site conditions and the variations in depth to the reference outcrop conditions. Epistemic uncertainty is modeled using lower and upper estimates of the profiles and various depths to the top of reference outcrop site conditions.

New normalized shear-modulus reduction and damping models for southeastern U.S. residual soil and saprolite are used. The models, based on resonant column (RC) and torsional shear (TS) measurements, are expressed as functions of small-strain shear modulus, confining stress, and plasticity index. At shear strain levels greater than 0.2%, beyond the RC/TS test data range, there is considerable uncertainty in their predictions. A procedure for validating/correcting the implied dynamic peak shear strength of residual soil and saprolite modulus reduction curves at shear strain levels greater than 0.2% is described. Engineering properties needed to complete the evaluation (e.g., , total unit weight, plasticity index, dynamic peak shear strength) and their uncertainties are discussed.

Because the residual soil and saprolite shear-modulus database is limited to measurements with ≤ 1,500 ft/s, a proposed shear-modulus reduction model for rock and cemented sand is selected to model the saprolite-to-rock transition. The rock and cemented sand model is a function of , instead of confining stress and plasticity. A procedure for evaluating the implied shear strength of the shear-modulus reduction curves for rock and cemented sand at shear strain levels around 1% is described; however, the procedure is not recommended due to the large uncertainty in the estimation of implied shear strength.

The shear-modulus reduction model for Quaternary sediment and fill is selected based on comparing the results of 20 RC tests by other investigators with five proposed models. Interestingly, the residual soil and saprolite model provides the highest coefficient of determination and Nash-Sutcliffe model efficiency coefficient compared to the other four models. A procedure for validating/correcting the implied peak shear strength of the shear-modulus reduction curves at shear strain levels beyond the RC test data range (greater than 0.2%) is described. Engineering properties needed to complete the evaluation and their uncertainties are presented. New relationships for modeling the small-strain linear damping and the medium- to large-strain nonlinear damping are developed using the compiled RC/TS tests.

To illustrate the significance of various aspects of the developed models, site response analyses are conducted on threeprofiles that are characteristic of the local site conditions in the SCBRP and SCCP using random vibration theory and the equivalent linear method. The threeprofiles are formed from the median estimates assuming three different depths to the reference outcrop condition. The results show that site adjustment factors can vary significantly with input earthquake intensity, depth to reference outcrop condition, local-site-to-reference-outcrop transition, and non-linear material properties. The results also show that computed maximum shear strains are less than 0.1% for an input peak outcrop acceleration of 0.75 g in the SCBRP transition from local site to reference outcrop conditions. On the other hand, the maximum shear strains reach around 1% at shallow depths for an input peak outcrop acceleration of 0.75 g in the SCCP transition.

The results of this study are being used by others to develop a more complete set of seismic adjustment factors for the local-site-to-reference outcrop conditions that are compatible with the new seismic hazard maps.

Author ORCID Identifier

https://orcid.org/0009-0003-9092-7378

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