ML20002A254
| ML20002A254 | |
| Person / Time | |
|---|---|
| Site: | Vallecitos File:GEH Hitachi icon.png |
| Issue date: | 10/31/1980 |
| From: | Darmitzel R GENERAL ELECTRIC CO. |
| To: | Clark R Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8011050447 | |
| Download: ML20002A254 (14) | |
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4 GENER AL % ELECTRIC
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ENGINEERING GENERAL ELECTRIC COMPANY, P.O. BOX 460. PLEAsANTON, CALIFORNIA 94566 DIVISION October 31, 1980 i$
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'Y. Robe;-t A. Clark, Chief d.
LN aerating Reactors Branch #3 A
a Division of Licensing I
U.S. Nuclear Regulatory Commission Washington, D.C. 20555
Subject:
Response to Additional Information Request Regarding Bearing Capacity Values for the Soil Beneath the General Electric Test Reactor (GETR) - License TR Docket 50-70
Dear Mr. Clark:
The General Electric Company's (G.E.) responses to the request for information regarding the bearing capacity of the soils beneath the GETR t
are attached.
It has been verified that recent evaluations of the bearing capacity values by Carth Sciences Associates do not change the previously submitted structural evaluations of the GETR, performed by Engineering l
Decision Analysis Company.
It is GE's position that use of bearing capacity values of 20 ksf or less is appropriate for the structural analyses of the most severe load case of combined vibratory motions and surface rup-ture offset.
Very truly yours,
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7 F T fC R. W. Darmitzel, Manager
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i Irradiation Processing Operation
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AFFIRMATION The General Electric Company hereby submits the information pertaining to bearing capacity of the soil beneath the GETR.
To the best of my knowledge and belief, the information contained herein is accurate.
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R. W. Darmitzel, Manager
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n Submitted and sworn before me this 31st day of October,1980, d&&#ew 4f(ss/m
, Notary Public in and for the If I'
County of Alameda, State of California.
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i ATTACHMENT TO RESPONSE TO ADDITIONAL INFORMATION REQUEST REGARDING BEARING CAPACITY VALUES FOR S0IL BENEATH THE GETR Investigations for the combined load case of surface rupture offset and vibratory ground motion were performed to provide a conservative evalua-tion basis for analyses of the concrete core structure of the GETR reactor building.
Combined load cases based on probabilistic considerations and physical considerations (realistic soil pressure capacities) were used in the structural stress analyses and checks against capacities. The investigations recently performed by ESA have further demonstrated that these investigations were conservative. The prior analyses for the combined load case of surface rupture offset and vibratory ground motion encompass the conditions presented by the findings of the ESA investiga-tien.
Thus, as previously concluded, the concrete core structure of the reactor building and systems and components required for safe shutdown are adequate to withstand without damage the combined load case of vibratory ground motion and surface rupture offset due to postulated seisnic events on the hypothetical Verona fault.
31 October 1980
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- 8. t. ww REVIEW OF GETR SOIL PROPERTY EFFECTS i
INTRODUCTION This discussion responds to comments by NRC staff regarding EDAC Figure 6 of EDAC-117-253.01, Revision 1, Supplement 2 in which the bearing capacity of the GETR subgrade is shown to limit the stresses transmitted to the overlying concrete structure.
I General Electric has reported that static and seismic load combinations greater than the top of the shaded zone, EDAC Figure 6, cannot occur because the soil subgrade bearing value would be exceeded and the structure would tilt to the left in such a manner that the unsupported centilever would touch and derive support from the subgrade before excessive load combinations could occur.
In order to define the upper edge of the shaded area on Figure 6, it has been previously assumed that the strength of the subgrade is:
a) adequate to lift the reactor with the cantilever configuration, EDAC Figure 2; b) sufficient to provide up to, but not more than, 20 ksf bearing to the structure for combined static and dynamic loads; and c) fail when subjected to certain combinations of vibratory ground motion and cantilever lengths.
The 20 ksf value has been proposed as a realistic value, and does not incorporate any load or safety factors.
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u CONSERVATISMS INHERENT IN THE " CANTILEVER CASE" 1.
The effect of a soil restraint on the left side of the GETR foundation has not been considered in p;evious submittals. Figure 1 is a non-scale schematic representation of the GETR Foundation. Passive thrust developed on the vertical edge of the mat will be on the order of 75 k/ft (5 ft. thick mat
@ 15 ksf bearing). Frictional resistance against upward movement will then be 75 x tan O, or 54 k/ft for O = 36 I
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54 K]ps.
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Figure 1. Restraint at left edge cf mat The effect of this restraint is to make the structural analysis conservative because:
-it increases the load in the subgrade beyond the gravity loads used in the EDAC analysis, increasing the likelihood for soil bearing failure and tilting, thus decreasing the length or eliminating a potential cantilever.
-it moves the center of gravity of the structure toward the left by roughly 5 ft., increasing the eccentricity of the load, and the tendency for tilting to the lef't.
-it ' acts as a " plastic pin" of 54 k/ft. resistance which supports the
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left end of the mat during seismic shaking, reducing dynamic loads.
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E 2.
No credit has been taken in the structural analysis for any cumulative deformation and redistribution of subgrade soils (dense sand and gravel), due to cyclic loading. Load concentrations would be reduced if this were taken into consideration.
3.
No credit has been taken for the reduction in bearing capacity of the subgrade due to simultaneous horizontal shear stress (load inclination) produced by vibratory ground motion.
4.
It has been assumed that the fault rupture will be a single plane in'which both upper and lower plates are intact. No credit har been taken for a more probable broader zone of deformation, where the upper plate is broken by multiple shears or subject to plastic deformation, enhancing tilting of the structure.
' OIL BEARING CAPACITY Bearing capacity calculations were derived from consolidated undrained (CU) triaxial tests performed by Shannon and Wilson. Tests B-1, S-8 and B-1, S-7, which yielded undrained strengths of about 3300 psf were considered especially significant, since these samples were clearly sand and gravel, which is the material upon which the GETR foundation rests (Shannon and Wilson,1973, Investigation of Foundation Conditions GE Test Reactor: for URS - John A. Blume and Associates).
Using an undrair.cd shear strength value of 3300 pst, bearing values were computed as follows q it = N S = 6 x 3300 = 20,000 psf.
u e u General Electric believes that this value should be used in this analysis because it most accurately represents the strength directly under the GETR soil.
Nevertheless, if three additional triaxial tes.s performed by Shannon and Wilson, which were not performed on solely sand and gravel, ere evaluated, and bearing values computed by use of CU strength envelopes, using equations from'NAVDOCKS 7-11-3,q it is 27,000 psf.
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CORRECTION FACTORS TO BE CONSIDERED i
l 1.
Load Inclination Static bearing capacity values for verticelloading must be corrected downward for the superimposed horizontal seismic load on the 10 ft. wide strip of ground which carries the reactor during the combined load case.
This may be done using NAVDOCKS p 7-11-4 or the more conservative 1
but more precise method of de Beer (Duke Univ. Conference on Bearing Capacity J
and Settlement,1965, p. 25). This yields the following corrected bearing capacities:
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Correcte,d i
Unsupported Vertical Load Shear Load Inclination B.C. Reduction Bearins-Length, ft.
ksf ksf degrees factor Capacity 1
ksf 21 30 0
0 1.0 30 l
18 30.
4.6 8.7
.70 21.
16 30 7.1 13.3
.59 17 13 30 10.6 19.5
.46 13 8
30 15.0 26.6
.29 8
l Bearing capacity reduction reduced presumed 30 ksf bearing values to below 20 ksf for unsupported lengths less than 17 ft.
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c 6 r o.3
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where c
= 1200 psf h
= 240 B
= 10 ft.
L
= 51 f t.
= 73 Pcf sub N
=6 In addition, for the possibl'e alternate condition of low water table, using NAVDOCKS 7-11-2, q it 30,000 psf.
u 9 ult = TN B y
2.
where I
= 135 pef N
= 50 y
B
=9ff c'
=0 0
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= 36 Shear strength parameters C' = 0,f'= 36 were obtained from Earth Sciences Associates' landslide report Figure B-13, strain = 10%, and Lambe and Whitman p.149, ky, sand and gravel. The valuef= 36 is considered appropriate for soils strained by faulting and confined by 20+ ksf effective confining pressure.
The derivation of the foregoing bearing values, using generally conservative assumptions and two independent methods, yields a consistent answer of 30 ksf.
It has been noted.that the geometry of the space between reactor and subgrade, to the left of the fault, does not correspond to the shape of the ground bulge which would occur in a classical bearing capacity failure. It stands to reason that an adjustmer of the bearing capacity factor N would be required. Note that the e
minimum value of 5.14 was not used in the originalsubmittal. We believe that the uncertainty in N, is relatively small, and is masked by the uncertainty in S. Moreover, filling of the space between foundation and subgrade reduces or u
eliminates the unsupported length.
Bearing values must be corrected downward for load inclination. The above table shows that, for all unsupported lengths of 17 ft. or 'ess, the bearing value under combined statie-seismic loading cases is 20 ksf or less. Hence, the curve on ED AC Figure 6 is conservative for all unsupported lengths less than 17 ft, even assuming a nominal bearing value of 30 ksi, instead of the 20 ksf presented in EDAC-117-253-01, Revision 1, Supplement 2.
2.
True Fault Behavior EDAC Figure 6-is idealized insofar as it indicates that a single fault plane is capable of developing and subsequently lifting the reactor with load distributions as shown in EDAC Figure 2. This is an idealized conditi.1n which does not consider true soil behavior under the combined influence of faulting and concentrated surcharge loading.
Simple laboratory model tests show that the pattern of ground failure due to eruption of a thrust fault beneath a surcharged area is likely to be complex.
That is, rupture along a single failure plane is apparently not favored in a soil subgrade. A test conducted in a bench retaining wall model, in which the thrust fault is simulated as a passive earth / pressure failure, consistently caused tilting, rather than cantilevering.
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p Comparison of P and P ' for trial values of l. and m is shown on Figure p
p 6.
The solid curves represent the tendency of the initial fault plane to move toward the right under the initialload distribution with this tendency being most marked when [ = 15-20 ft. This is suggestive of tilting of the structure to the left. However, the curves do not yield a definition of the cantilever length at which tilting toward the left would occur, or of the finalload distribution beneath building.
the reactor
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OCorr CONCLUSIONS 1.
Current calculations yield a conservative bearing value of 30 ksf. This value, when corrected for load inclination, is reduced to below 20 ksf for unsupported cantilever length of less than 17 ft.
e 2.
The addition of 54 k/ft of vertical end restraint on the left side of the slab is justified from a geotechnica1 standpoint. This reduces maximum cantilever
~ '
length.
3.
Simplified wedge analysis of faulting beneath the reactor indicates a tendency
.of faults to steepen in such a manner that they erupt on the near (right) side of load concentrations. This, suggests that faults surfacing 15-20 ft.
from the left side of the reactor foundation evolve into ground deformations which tilt the reactor to the left, rather than lifting it without rotation.'
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