ML20112D378
| ML20112D378 | |
| Person / Time | |
|---|---|
| Site: | Shoreham File:Long Island Lighting Company icon.png |
| Issue date: | 08/07/1984 |
| From: | LONG ISLAND LIGHTING CO. |
| To: | |
| References | |
| I-SC-LP-051, I-SC-LP-51, OL-4, NUDOCS 8501140253 | |
| Download: ML20112D378 (6) | |
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DGCM.T !' '; ?, SE t 1:.I, i !!J 04 ATTACHMENT 4
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8501140253 840807 PDR ADOCK 05000322 o
o SNPS-1 FSAR Dewatering during con. m tion had no adverse effects on the structures of the plant.
'Blere is no plan for dewatering during the life of the plant, other than the removal of water from wells for potable water supplies.
The effects of these wells are discussed in Section 2.4.13.
2.5.4.7 Response of Soil and Rock to Dynamic Loading A detailed discussion of the response of granular soils to earthquake induced shear stress is presented in Section 2.5.4.8 l
(Liquefaction Potential) and Appendixes 2L and 2M.
Detailed dynamic analyses are presented in Section 3.7.1.6 (Soil-Structure Interaction),
Section 3.7.2.1 (Seismic Analysis Methods), and Section 3.7.2.5 (Methods used to Couple Soil with Seismic-System Structures).
2.5.4.8 Liquefaction Potential Granular soils, when subjected to cyclic shearing stresses or to vibration, tend to reduce in volume.
The magnitude and rate of this reduction in volume is dependent upon the initial looseness of the deposit (its density) and the magnitude of the cyclic shearing forces.
The reduction in volume can occur only as fast as fluids contained in the pore spaces between particles can be expelled from the soil mass.
If the soil is totally saturated with an incompressible fluid such as
- water, there will be a
temporary increase in pressure within the pore water and a decrease in the load carried by the soil.
If a number of cycles of loading are applied quickly relative to the time required for drainage to occur, the increase in pore water pressure may become significant and there will be a corresponding decrease in the effective stress.
The shear strength of granular soils is proportional to the effective stress; thus the decrease in effective stress accompanying such a l
phenomenon results in a decrease in shear strength of the soil mass.
The number of cycles of load required before there is a
j significant decrease in shear strength of a
given soil is i
dependent upon the magnitude of the shear stress in relation to the initial ef fective stress, and upon the initial density of the soil.
In very loose, granular soil, only a few cycles of loading
- ray be required to cause a complete transfer of external loads from the soil structure to increased pressure in the pore water.
In such very loose
- soils, heavy vibration or repeated cyclic shear loading may cause the individual soil grains to become completely separated from each other by films of water.
- Further, as such soils distort in shear, there is a tendency to
- contract, thus pore water pressures are maintained over a vide range of distortion, even though vibratory motion ceases.
The soil mass b2 haves almost as a
dense liquid.,, This phenomenon is termed liquefaction or flow f ailure.
ti5 In cense
- soils, continued cyclic shear loading will, in time, result in an increase in pore water pressure and a
decrease in shear strength.
However, many cycles of loading may be required 2.5-50 Revision 7 - August 1977
SNPS-1 FSAR before the pore water pressure increases to equal the external loads upon the soil mass.
Initial liquefaction occurs when the pore water pressure first reaches equality with the external loads.
In dense
- soils, however, pore water pressures do not remain constant throughout each cycle of
- loading, but reach a
- peak, and during the remainder of the
- cycle, reduce to substantially lower values.
Thus, in dense soils, a significant loss in shear strength occurs only momentarily during each loading cycle.
Many additional cycles of loading are required beyond the point of initial liquefaction before significant deformations develop.
Further, as the soil distorts in shear it
- dilates, reducing pore pressures.
Shear strength is immediately reestablished.
Thus, flow failures cannot occur in dense soils.
- This plant is founded upon granular soils at or near the groundwater level.
Properties of the underlying materials, including relative densities and gradation characteristics, are discussed in Section 2.5.4.2.
A discussion of groundwater at the site is included in Sections 2.4.13 and 2.5.4.6.
Procedures for liquefaction analyses, developed by Seed et al. ( 66 )( 6 7)( a s )
require the following quantitative evaluations:
1.
The resistance of the sands to liquefaction, which may be expressed as the cyclic shear stress necessary to cause
" initial liquefaction" in the number of cycles estimated to occur in an earthquake of the intensity selected.
2.
The magnitude of shear stresses which may occur at varying depths in the underlying sand due to earthquakes.
I Since submittal of the PSAR, Seed and IdrissC67) have published a procedure for evaluating soil liquefaction potential which is l
very similar to the procedure used in the PSAR except for a recommendation that the shear strength data from cyclic triaxial tests be reduced to reflect field conditions more closely.
Using this method, there is still an adequate f actor of sab ty against liquefaction.
In the method proposed by Seed and Idriss, the average shear stresses caused by earthquakes are calculated by:
. 5 m a x
@ SSI #d I
=
av9 def where 0.65 = coefficient determined analytically to produce the came increase in pore pres-sure in a specified number of cycles of loading using equal cyclic stresses as would be developed by the irregular motions,of actual earthquakes
= maximum ground surface acceleration = 0.2 g a
2.5-51
SNPS-1 FSAR
.An additional method of assessing liquefaction potential.can be developed by comparing standard penetration resistance data from the vicinity of the Seismic Category I structures with standard penetration resistance data from sites which have been subjected to earthquakes.
This method also indicates there is no danger of liquefaction.
A number of field reports of the observed behavior of sand deposits subjected to earthquakes have been summarized by Seed and Idrisc.C67) The cases reported by Seed and Idriss are presented in Fig. 2.5.4-10 as a plot of the corrected mean blow
- count, N', of the sand at the depth of interest versus a
stress ratio representing the equivalent of about ten cycles of the earthquake induced shear stresses divided by the effective overburden pressure.
Similar plots have been presented by Whitman.cas) Since for a constant relative
- density, Gibbs and Holtzt2*)
data show increasing number of blows with increasing effective stress, a corrected value of the standard penetration resistance, N',
has been used based on the expression proposed by Teng(78) as follows:
SON y1
=
5 +10 v
v = effective vertical overburden pressure (psi) where a
N
= blows per foot from standard penetration test N'
N corrected to B
= 40 psi
=
y The earthquake induced cyclic shear stresses were calculated as follows:
0.65 a bK^b$} #
=
T,y max d
where 0.65 = coefficient determined analytically to produce the same increase in pore pressure in a specified number of cycles of loading using equal cyclic stresses as would be developed by the irregular motions of actual earthquakes
= maximum gr und surfaceacceleration = 0.2 g amax MASS
= total overburden pressure divided by the acceleration of gravity = a /g v
and r
= reduction factor to account for the deform-d ability or soil Two boundary lines have ' been drawn on this figure to separate zones of ground failure and no ground failure.
The mean N' value 2.5-53 1
/
SNPS-1 FSAR earthquake by 25 percent.
- However, the analyses have oemonstrated that even with 0.2 g maximum ground acceleration, the factors of safety against liquefaction are sufficiently large to give assurance that liquefaction will not occur either under the plant structures or in the yard area.
Therefore, there is no danger of liquefaction.
2.5.4.9 Euthquake Design basis
'Ibe earthquake on which the analyses are based is the Design oasis Earthquake (DBI:,) described in Section 3.7.1.
The maximum borizontal acceleration of the DBE is 0.2 g, and the maximum vertical acceleration is 0.133 g.
2.5.4.10 Static Analyses All structures are soil supported.
Crystalline bedrock is estimated to be at a depth of 1000 ft.
Settlements due to design loans were calculated using elastic theory and moduli developed at the Brookhaven National Laboratory (BNL),
shown on Jig. 2.5.4-11.
The outwash deposits present at the Shoreham site extend southward, across the Brookhaven National Laboratory (BNL)
- property, to the northerly flanks of the.Ronkonkoma Moraine.
Early in the present investigations, it became apparent that the soils underlying the proposed site were similar in mode of deposition and character to those underlying BNL.
Design of the Alternating Gradient Synchrotron at BNL involved unusual and extremely demanding settlement tolerances, and therefore, a very detailed foundation study was made.C76) Following completion of the synchrotron, precise records of the relations between settlement and soil loading were obtained, and revised moduli of deformation of the soils beneath the synchrotron, were l
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2.5-54a/b Revision 7 - August 1977
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