ENIGINII!I!IIIINIG GEOLOGY ELSEVIER
EngineeringGeology46 (1997) 331-347
Soil classifications for estimating site-dependent response spectra and seismic coefficients for building code provisions in western Kentucky R. Street a.,, E. Woolery b, Z. Wang b, I.E. Harik c a Department of Geological Sciences~Kentucky Geological Survey, University of Kentucky, Lexington, KY40506-0059, USA b Department of Geological Sciences, University of Kentucky, Lexington, KY40506-0059, USA c Department of Civil Engineering, and Head, Structures Section, Kentucky, Transportation Center, University of Kentucky, Lexington, KY40506-0281, USA Received 27 May 1996; accepted 10 November 1996
Abstract
One hundred thirty-four horizontally polarized shear-wave (SH) seismic soundings have been performed in the Jackson Purchase Region of western Kentucky. These data have been used to derive the thicknesses and shear-wave velocities for the various impedance horizons in the upper 30 m of the soil column at each site for the purposes of general land-use management and loss estimation in the event of an earthquake. A generalized seismic susceptibility soil classification scheme adopted by the Borcherdt (1994a, b) was fit to the results. Soils across western Kentucky were found to be either SC-II or SC-III soils, correlating to gravelly soils having soft to firm consistency and stiff clays to loose/dense sands, respectively.
Keywords: Soil classification; Site-dependent response spectra; Seismic coefficients; Building code provisions; Western Kentucky
1. lntrodm~i~
1.1. Problem statement Holzer (1994; p. 299) concluded "that approximately 98% of the $5.9 billion in property damage from the Loma Prieta earthquake was caused directly by ground shaking; enhanced ground shaking was directly responsible for approximately * Corresponding author. Tel.: + 1 606 2573758; fax: + 1 606 2581938; e-mail:
[email protected] 0013-7952/97/$17.00© 1997ElsevierScienceB.V. All fights reserved PH S0013-7952 (97)00010-0
two-thirds, $4.1 billion, of the total property loss". He also noted that "although locally devastating, only 2%, $131 million, of the property damage was attributed to liquefaction, landslides, and tectonic ground rupture". These findings have important implications with respect to seismic hazard maps and building codes in the central United States. Catastrophic ground failure, such as liquefaction, landslides, and settlement, is dramatic and very damaging, but enhanced ground shaking, not catastrophic ground failure, is the primary cause of damages resulting from most earthquakes in
332
R. Street eta/.
Eng#u,ering (ieol¢)gy 46 / 1997) 331 347
I
\
New Madrid Seismic Zone
Thickness
fJ }
of S e d i m e n t s
(
(in feet) 0
100
C
..J
MISSOURI
38 °
I
/
/
ILLINOIS f-..,
/
r ~
\ 3
t
\
~?f/,/I [ "-" T'~N-'NES'SEE "-~-
ARKANSAS
36 °
",.p f
S
4"" h2 92 °
~ I 90 °
% I
34 ° 88 °
Fig. 1. Isopach map of the Upper Mississippi Embayment showing thickness of the clastic sediments from the surface to the top of the Paleozoic bedrock (from Dorman and Smalley, 1994).
the central United States. Structures and lifeline facilities in the northern Mississippi Embayment are of necessity built on a thick layer of loose to semiconsolidated soils (Fig. 1). It is well known
that these soils will selectively amplify the ground motions resulting from an earthquake in the New Madrid Seismic Zone ( N M S Z ) , just as ground motions were amplified in similar soils in the
K Streetet al. / EngineeringGeology'46 (1997) 331-347 Northridge, California, earthquake of 1994 (Chang et al., 1996) and the Hyogo-Ken Nanbu, Japan, earthquake of January 17, 1995. Most researchers in the central United States agree that a major earthquake in the NMSZ (i.e., an earthquake of mb >6.6) is unlikely in the foreseeable future. Johnston and Nava (1985) offered the following probability estimates for the New Madrid Seismic Zone: Average repeat Probabilityby time (years) Year 2000 rnb_>6.0
rob>6.6 rob>7.0
70 (___15) 254 (+60) 550 (+125)
Probabilityby Year 2000
A.D. (%)
A.D. (%)
40-63 5.4--8.7 0.3--1.0
86-97 19--29 2.7--4.0
Because of the low probability of a major earthquake, and the lack of a major urban area (with the exception of Memphis, Tennessee) or critical facilities (with the exception of the Gaseous Diffusion Plant at Paducah, Kentucky) such as nuclear power plants in the northern Mississippi Embayment, the ground-motion parameters needed for the next generation of national groundmotion maps in the area are different than those needed for an area such as southern California. In practice, what are needed are parameters that can be used to build schools and hospitals, highways, bridges, pipelines, etc., in a manner that will lead to realistic, cost-effective, earthquake-resistant designs for a 6-6.5 mb earthquake. Fig. 2 shows the location of the Jackson Purchase area of western Kentucky with respect to the contemporary earthquake activity in the NMSZ, southern Illinois, and southeastern Missouri. Because of its proximity to the seismicity, and the thick layer of sediments (Fig. 1), enhanced ground shaking is expected to be the primary source of damages in the Jackson Purchase area in the event of a significant earthquake in the region. Borcherdt (1994a,b) presented a comprehensive technique for calculating free-field, site-dependent, response spectra that utilizes, as one of its parameters, the average shear-wave velocity of the uppermost 30 m of soils at sites underlain by soils. The method, derived from observations in California, provides alternative procedures for estimating both
333
input ground-motion spectral levels and amplification factors, depending upon available information. Borcherdt (1994a) suggested that the technique provides a general framework for design, as well as site-dependent building-code provisions and predictive maps of strong ground motion for purposes of earthquake hazards mitigation. Specifically, the technique is proposed for inclusion in section 4.2.1 of the N E H R P Recommended Provisions for Development of Seismic Regulations for New Building. The Federal Emergency Management Agency (FEMA) and the National Institute of Building Sciences (NIBS) are currently using a site classification that is slightly different than Borcherdt's (Borcherdt, 1994a,b), in their Loss Estimation Methodology software (developed by Risk Management Solutions, Inc.), to promote earthquake awareness, the need for mitigation, and to estimate losses. Fig. 3 summarizes the two site classification schemes. As a consequence of the concern about enhanced ground shaking in the Jackson Purchase and the need for information to classify sites in accordance with Borcherdt's site classification scheme (Borcherdt, 1994a), a study has been undertaken to map the shear-wave velocities and thicknesses of the soils throughout the Jackson Purchase area. In this paper, we present the results of our shearwave studies and their use in the Borcherdt technique. The part of the study aimed at determining the depths to bedrock in the area has not been completed. 1.2. Geological setting The Jackson Purchase of Kentucky is located in the northeastern part of the Coastal Plain physiographic province of extreme western Kentucky. The study area (Fig. 1) is an eight-county, approximately 6102-km 2 region that is bound by Kentucky Lake, the Mississippi River, the Ohio River, and the Tennessee border on the east, west, north, and south, respectively (Schwalb, 1969). The area is located in the extreme northeastern part of the Mississippi Embayment, which was described by Mooney et al. (1983) as a broad, elongated, southwest-plunging re-entrant of the Gulf Coastal Plain that extends northward into
1~ Street et al. / Engineering Geology 46 (1997) 331-347
334
N E W M A D R I D S E I S M I C Z O N E : 1974-1989 91° i
ill e ~ o o ° i
""
90° -t.~
TM
"".+:"
..° °.
.-\
a
o •
89° i ~
Q
'
"i
,:,.
°"° ~_,II ° )\ . '•l ~.° ~°~
°
I
o
°
•
IL
I
%,*°
o
m
88 °
711 380
+¢,.,
~. ~ .
[
+-J
,~°
•° A
I~ o
o***
,$°
o
,O •
"0~
o
i
*,
O !
•O
o°
-'I
j
0
,---/
o
'd~
~°
,,--.
IR.' ' °
°
° •
°
o o
AR
,
o 0
)o
O•
0
0 0
36 °
0
oe"
¢b
o
°
a
TN
o
D
o 0
• o •
2.0_~U200 gravd).
Site Class
Vs" (m/s)
Criteria
Name
General Description
V,"(m/s)
A
Hard Rock
> 1500
1050
B
Rock
76050 or s,> 100kPa.
3605 blows/fl)). Minimum thicknessof 5 m.
290
Soft Soils
150
SC-IV SC-IVa
NEHRP's Method
Criteria
Name
335
D
Non-Special-Study Soft Soils (e.g. loose sulmaerged flUs and very soft to soft 04
