A data base of geotechnical properties of surface deposits of Los Angeles has been developed at UCLA for seismic studies of the Los Angeles Basin. The data base contains more than 1000 digitized geotechnical boring logs which are integrated into SCEC Geographic Information System (GIS), using the GIS software called "Techbase" by Minesoft (1994). The 1997 research had the following objectives: (1) to further densify the existing SCEC Geotechnical Data Base, and (2) to integrate the data base and a 1-D nonlinear site-response model into a unique GIS for evaluation of the nonlinear site-response effects over a selected area. The objective (1) has been achieved partially by including into the data base a certain number of new boring logs. This number was limited due to the difficulties associated with obtaining the data from private firms. Therefore, the densifying of the database will continue irrespective of funding.
The objective (2) of integrating the digitized data base and 1-D nonlinear site-response model into a unique GIS has been achieved almost completely. This was the primary task of the 1997 project, as well as a long-term task of several projects funded through SCEC in previous years. It involved the computer automatization of: (i) the procedure for configuring geometry of soil profile suitable for selected 1-D nonlinear analysis, (ii) the derivation of soil nonlinear input data from the geotechnical data base, and (iii) the calculation of the response of a number of soil profiles representing the boring logs within a chosen geographic area. This was achieved by complex computer programming in GIS environment. The final product is a GIS written in "Techbase" which enables automatic calculation of ground response features over a selected area, and the comparison between these features, soil conditions and actual shaking recorded during past earthquakes. This system was named SCEC Site-effects GIS, and it is described fully in Doroudian and Vucetic (1997).
Particular steps taken to achieve the above objective (2) were:
(1) A nonlinear 1-D site response model called DESRAMOD2, which is a modification of the original model DESRA-2 (Lee and Finn, 1978), was selected and integrated with the geotechnical database into a GIS. DESRA-2 and DESRAMOD2 employ a horizontally-layered soil profile consisting of a number of layers of different soils excited at the base of the profile by an acceleration-time history. The soils in DESRAMOD2 are characterized by the cyclic stress-strain curves shown in Fig. 1.
(2) A subroutine for division of the geotechnical boring logs into a certain number of layers suitable for selected 1-D analysis was written in the "Techbase" format.
(3) A set of correlations between the parameters describing the nonlinear cyclic stress-strain properties (required as the input for DESRAMOD2) and the soil classification characteristics contained in the geotechnical database was developed. Some of them are shown in Fig. 1.
(4) Based on the correlations in (3) above, a subroutine for the transfer of the data base information into the DESRAMOD2 input parameters was developed in "Techbase" format. This interface is the most critical element of the new GIS, as it provides a link between the database and DESRAMOD2.
The operation of the new GIS system was demonstrated and its usefulness critically evaluated in Doroudian and Vucetic (1997) in the following way:
(1) The DESRAMOD2 analytical soil profiles were created for 32 boring logs in the selected part of Downtown Los Angeles shown in Fig. 2a.
(2) The DESRAMOD2 analyses were conducted for all 32 boring logs using base rock input acceleration-time history with the peak value of 0.28g and the predominant period of 0.38 seconds. For each single layer of each profile the following outputs were calculated with time: accelerations, velocities, displacements and cyclic pore water pressures.
(3) The output information obtained in (2) above were used to plot the maps of the maximum ground surface accelerations, amplification of vertically propagating seismic motion, maximum strains (deformations) and stresses in the soil mass, predominant periods of the ground surface motions, and maximum seismic pore water pressures. As an example, the map of the amplification of the maximum ground surface accelerations is presented in Fig. 2b, while the map of the predominant periods of soil deposits is presented for comparison in Fig. 2c. It can be seen that significant amplifications are calculated for the areas where the predominant period of soil deposit is similar to the predominant period of the base rock excitation of 0.38 seconds. Such results are encouraging. Furthermore, two maps of seismic pore water pressures caused by strong ground shaking are presented in Fig. 3. All of these four GIS maps demonstrate the capability of the new SCEC Site-effects GIS for advanced microzoning.
References:
Doroudian, M. and Vucetic, M. (1997) "Development of Geotechnical Data Base and its Application to the Evaluation of Nonlinear Site Effects and Seismic Zonation", Res. Report Eng-97-186, Civil and Env. Eng. Dept., UCLA, 347 pages, September, 1997.
Lee, K.W. and Finn, W.D.L. (1978). "DESRA-2: Dynamic Effective Stress Response Analysis of Soil Deposits with Energy Transmitting Boundary including Assessment of Liquefaction Potential", Ressearch Report, The University of British Columbia. Minesoft (1994). "Techbase Reference Manual", by Minesoft, Ltd., Lakewood, Colorado.
List of publications and other products prepared based on the 1997 SCEC funding:
Doroudian, M. and Vucetic, M. (1997) "Development of Geotechnical Data Base and its Application to the Evaluation of Nonlinear Site Effects and Seismic Zonation", Research Report Eng-97-186, Civil and Env. Eng. Dept., UCLA, 347 pages, Seprtember, 1997.
Vucetic, M., Lanzo, G. and Doroudian, M. "Damping at Small Strains in Cyclic Simple Shear Test," ASCE Journal of Geotechnical and Geoenvironmental Eng. (accepted)
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