28-09-2016, 12:30 PM
MEASUREMENT OF DYNAMIC PROPERTIES AND EVALUATION OF
LIQUEFACTION POTENTIAL OF A 500MW PROTOTYPE FAST BREEDER
REACTOR SITE LOCATED IN SOUTH OF INDIA
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SUMMARY
A detailed geotechnical investigation has been carried out at a site located on the Bay of Bengal on
the East Coast of Peninsular India at Kalpakkam for the construction of a 500 MW Prototype Fast
Breeder Reactor (PFBR). Insitu penetration tests, Seismic wave velocity measurements, Block
Vibration tests as well as cyclic plate load tests were performed to measure dynamic soil properties
of different soil layers at various strain levels. Based on the field test data, variation of shear wave
velocity with depth, a normalised shear modulus vs. shear strain plots are established for the
selection of design dynamic soil parameters required for the analysis Nuclear Power Plant
foundations / structures subjected to Earthquake, vibratory machine loads, etc.
A level ground liquefaction analysis was performed for the top saturated fine to medium coarse
sand and silty sandy layers of varying density of 8 m thick using field performance data (SPT
tests). The modified penetration resistance is obtained from the field N-value by applying
correction for various equipment and procedural variations in the conduct of test. A design
earthquake (S2 level) of magnitude of 6.0 and peak horizontal acceleration of 0.156g determined
from the site-specific seismic hazard assessment was adopted. The analysis indicates that the
factor of safety against liquefaction for sandy soil layers is much higher than 1.0 and for silty
sandy layer is marginally above 1.0.
INTRODUCTION
A detailed geotechnical investigation with respect to the evaluation of static and dynamic parameters and
liquefaction potential was carried out for a site located on the east coast of peninsular India at Kalpakkam which
is situated about 68 km south of city Chennai in south of India. A 500 MW Prototype Fast Breeder Reactor is
proposed to be constructed at the above site about 250 m from the seacoast.
The geotechnical survey to assess the static and dynamic parameters was based on the field tests and laboratory
tests. The field tests performed at the site includes Geophysical tests using Crosshole technique, in-situ Block
resonance tests, Cyclic plate load tests, 29 boring with SPT tests 1.0 to 1.5 m apart, CPT tests, etc.
In this paper general geological, seismic and geotechnical characteristics of the site are presented. The
measurement of in-situ dynamic properties of soil at various strain levels as well as the evaluation of modulus
reduction curve from the field dynamic test data is also presented. A liquefaction study performed for the
subsurface granular soil layers for a site specific earthquake using field test data is also presented.
GENERAL GEOLOGICAL CHARACTERISTICS
The site falls at the depositional environment by the floods and coastal processes. The ground water table
fluctuates with season and is about 1-2m below the ground level during monsoon.
The alluvium covers the rock masses with few rock outcrops in the surrounding region. The geological setup of
site consists of Charnockite suite of rocks and pyroxene granulates of Archaean age as a basement overlain by
recent coastal alluvium with an unconformity. There are dolerite dyke intrusions in the post charnockitisation
period in the Charnockite rock mass. The bedrock occurs at about 10 -15 m depth in this region. Rock mass
rating is evaluated as 77 and Q- system as 17. This indicates the good quality of rock formations in and around
the site.
Deep borehole investigations at the site shows that there is a North- South trenching major joint with pseudo
tachylites along the fracture of joints. There are weak planes as the major fracture zones encountered at about
100, 200, 400 and 600m depth are traceable in other boreholes. The fractures seen in shallow boreholes upto 60
m depth at the site are of insignificant since the fractures are thin and sealed with pseudotachylites.
EARTHQUAKE CHARACTERISTICS
The largest earthquake in the 300-km radius around site had a magnitude of 5 (VI MM intensity). The largest
earthquake to have occurred in Peninsular India was the Coimbatore earthquake of February 8, 1990, with a
magnitude of 6.0. This epicentre lies about 400 km from the site.
An extract of important faults, their distance from site and maximum Magnitude are given in Table 1 (Ghosh
1992).