ML20127C060
| ML20127C060 | |
| Person / Time | |
|---|---|
| Site: | Waterford  |
| Issue date: | 04/30/1977 |
| From: | Ehasz J, Ehosz J, Pavone M EBASCO SERVICES, INC. |
| To: | |
| Shared Package | |
| ML19263A633 | List:
|
| References | |
| FOIA-84-455, FOIA-84-A-56 NUDOCS 8409180210 | |
| Download: ML20127C060 (10) | |
Text
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. : LOUISIANA POWER AND LIGHT CO.
WATERFORD SES UNIT NO. 3 /s ALIDWABLE MAT BEARING PRESSURE -
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Prepazed By: M. Pavone '89- *(* v Q .
Reviewed By: J.L. Ehasz 4 /
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l Ebasco Services, Incorporated Fo i A-84-@
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- 1. PURPOSE
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The purpose of this report is to'present the rationale for EBASCO's -
- reconnendation that the maximum effective mat bearing pressure be increased
. from 4000 psf to 4500 psf during construction. The original bearing pres-sure of 4000 psf presented in the Waterford SES PSAR will be accordingly up-dated in the FSAR to 4500 psf. , ,
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SCOPE . . .
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This repor't first provides background on the foundation design prin- .
ciple utilized at Waterford. It then' presents predicte'd as well as measured foundation response resulting from construction' and accounts for any dis-
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crepaneles. The effect of the proposed pressure increase relative to mini-
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sizing the . effects of these, discrepancies is, presented. Finally, the report
' discusses the effects of the increase fro ~m 4000 psf to 4500 psf maximum allow-
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able bearing pressure. - .
! 3. . DISCUSSION ,
l . 3.1 Foundation Desien Backcround Generally the foundation soils below E1.-40 at th'e Waterford site are .
overconsolidated. The existence of the only slightly overconsolidated Pleistocene clays at E1.-92 f t, indicated' that significant long term and differential settlements could be expected for structures founded on indi-vidual spread footings. To eliminste differential and long term settlement D
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considerations, all Class I structures were located on a cocmon mat founda- ,
tion. The . floating foundation principle was utilized with the combined structure foundation applying an effective load to the bearing stratum -
clays equal to the existing overburden pressure. .
The soil conditions at the site were eval,uated in terms of vertical ,
effective stresses. These original stresses were initially on the order of 3300 lbs per sq foot at the mat bearing level. The effective stress is defined as the total weight of the existing overburden.solls minus the uplift due po groundwater presstire. The effective stress is considered 1 .
with the groundwater table at E1.+8 ,
ft. - .
Upon completion of Excavation Phases I through IV, the effective stresses at the mat bearing level were reduced to 0 psf. During the period of excavation and up to concrete placement the foundation ' soils re- .
bounded or heaved in response to the relief of overburden stresses. 3 e.
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Presently,' concrete placement has been in progress for core than a
,. . year and backfilling is active. With this increasing load on the founda-tion soils the rebound previously experienced is being compressed. The schedule as presented in the PSAR allows for this effective loading to -
reach 4000 psf or a 700 psf overload beyond ,the initial 3300 psf loading.
The objective of the overload is to accomplish the total recompression
.' during;heconstructionperiodandminimizeoreiiminateanypostcon-atruetion,settiements. .
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To maintain this maximum allowabic. cffective bearing pressure a .
procedure of throttling down' the " dewatering system 7nd possible pumping ,
into_ recharge wells has been planned and provided for. This release ~
of dewatering will result in increased buoyant or uplift forces acting on the foundation soils as well as' the concrete structure and backfill materia 1. ,
The introduction of buoyant forces will be controlled so that additional construction and. backfill. loads, are equally balanced with up-
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lift. Thus the maximum allowable bearing pressure can be maintained duri,ng _
the construction period.
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At.a construction stage, approximately 6 months prior'to termination'
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of active construction and L :kfill imposed . loads, the entire dewatering ..
system will be. released. This will result in a final effective bearing pressure of 3100 psf, slightly less than th:: initial 33'00 pof. This 200 psf
,-jeduction of effective pressure will preclude the possibility of having any ;
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settlement considerations when the plant becomes operable. , ,
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l 3.2 Foundation Response . .
3.2.1 Predicted Behavior The foundation rebound or heave calculated during the PSAR prepara-tion, to occur between the time of excavation and start.of concrete place .
ment.was approximately 2 inches. This rebound was' anticipated to be nearly recompressed at the time the 4000 psf maximum allowable load was reached.
Any further recocpressio'n would be controlled through manipulation of the dewatering system and recharge wells.
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increasing the uplift on the foundation soils and caused r. ore heave than calculated for the dewatered excavation procedure. The original scheme envisioned a balanced effective pressure system for.the Phase I excava-tion, which would have resulted' in no' heave and potentially a settle- .
ment under the dewatered condition. This balance would have come about by the increased weight of dewatered soil and the decreased pressure due to Phas'e I excavation. When the dewatering was started the foundations
. responded by recompressing; however[the shor't duration of the dewatering prior to project shutdown was not effective in recovering the heave. This .
long shutdown period simply allowed cot:p1,ete relaxation of soils .to the stress relief. .
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I The differential heave from 1.5" along the soutti to 3.5" along the .
north during the Phase I excavation is attributed to the excavation pro-cedure which essentially handled the material from north.to south as well
'as the fact that the ' grade along the south, east and west sides of the excavation was raised four to five, feet for construction facilities. The-i \
north side was not surcharged with the additional fill which in essence allowed more relaxation along.the north. Additionally, the piezometric pressures along the north are always somewhat higher due to the recharge l- from the Mississippi River. All of the above factors tend to increase the .
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heave potential of the north side of the excavation, as is the case seen '
- on Nigure 1 for heave point Hl.
1he heave experienced, in excess of the 2 inches predicted, is felt to be attributed to more rapid rebounding of the foundation clays than
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became nr. ore effective consequently lowering the piezometric levels 15 ' feet below their lowest 1972 position. This additional, piezometric drop ini-tiated further recompression of the foundation material.
With the exception of the north end of the site, the Brade around the excavation was raised approximately four feet which apparently in- j creased the compression *of' the dev'a'tered site in all areas except on the north end.
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. In January,1975, the remainder of the excavation was started. -As -
a result, foundation heave readings increased. to values between 4 inches ,
and 9 inches. The heave rate leveled with comencement of concrete place-
.' - ment, reversed, and has been recompressing since. . Presently, the heave' remaining is between 1 inch and 6 inches. ,
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3.,2.3 Discrepancies ,
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..- It* is evident in a comparison of predicted rebound verses actual --
measured rebound that there is a 2 to 4 fol'd difference. These differ- ~
ences have been continuously monitored and evaluated during the construc-
! tion and do not seriously affect the design of the plant. -Only the re- , ,
compression phases of the foundation-soil system are affected and are ,
l . presently being addressed. -
As described above, the initial excavation of 20 feet of soils with ,
out lowering of piezometric icvels decreased the effective pressures by l -
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At the c mpletien of construction with t,ha grcundwstar back to its initial position the- foundation material will experience an effective stress 200 psf less than the ' initial effective stress experienced prior to construc-tion. This siight and apparent " net unloading'" was considered due to' the
. slight uncertainties in total loading during the early design PSAR stage.
3.2.2 Mensured Behavior -
Refer to Figure No.1 for an, extended time plo't of the foundation re-sponse. -
With Ehe initial removal of 20 feet of material in 1972 (Phase I.ex .
cavation), the site experienced between 1.5 and 3.5 inches of heave. This initial excavation was done without the benefit of the dewatering due to l' .
f scheduling difficulties. This resulted in more heave than would have occ~urred
, if the de6~ater*ng were operative. As shown on Figure No. 1, the effective
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str' esses during this Phase I excavation reduced to 1200 psf very rapidly and initiated the rapid rebound of the foundation-clays. With increase in ,
effective stress due to installation and operation of the dewatering system approximately 1 inch of this heave was recompressed. However, the dewater-ing was not in operation long enough to balance the Phase I excavation and the release of the dewatering system due to the job shutdown caused elastic .
foundation rebound to its pre-dewatered position maintnining this position for two subsequent years. .
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4 The dewatering was reinstated in November 1974. Due to the complete on-off-on operation of the system the wells essentially were purged and I
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I anticipated. Early calculations, formulated during the PSAR stage, con-sidered that approximately 20% of the rebound would be realized during a 10 to 12 month excavation phasc. The actual measurements indicated that
- a more rapid rebound has been experienced, perhaps on the order of 70 to
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80% of full rebound, under the relexed stresses of full excavation.
In order to ensure the full compression of this rebound, the founda-tion must be overloaded and controlled in order to minimize post construc-
' tion setelemerts. ,
v 3.3 Justifien' tion of Increased Pressure- -
," The intent of increaseing'the allowable bearing pressure is. twofold. - -
It allows us to maintain a fully operative dewatering system while the tur- -
, bine building backfilling continues and it furtiher recompresses, at a fast-
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, er rate, the soil, heave incurred during,and subsequent to Phase I through -
Phase IV excavation. The increase pressure,will still a,dequately maintain a factor of safety, against a bearing capacity failure 'in excess of 3.
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Presently, the turbine building backfill is only at about E1.-25 (MSL). To start throttling down the dewatering system and recharging through the wells would cause groundwater difficulties with backfill construction and possibly additional heave of the insitu soil and backfill material in this area. -
,- To ensure the un, interrupted backfilling of'the turbine biulding exca-vation and still maintain the original 4000 psf allowable bearing pressure, would restrict increases in mat pressures. This would result in serious and unnecessary curtailment of concrete placer.ents in the combined structure.
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Th2 previcusly specifisd 4000 psf c11c.able bearing pressure was realized during the last week in March,1977, however, a significant portion of the highers than anticipated heave remains. Conveniently the
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area of the largest heave is along the northern portion of the excava-tion and is coincident with the area of anticipated bearing pressures
- l. above 4000 psf. Thus, the additional loading resulting from a 4500 psf limit will not only increase the rate of recompression but also has .the potential of reducing the differential heave experienced from north to ,
, - south along the excavation. -
Thus,'an , increase in the allowable bearing prUsure to 4500 psf i is justified in that it allows construction to proceed uninterrupted 4
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- on both the main plant island and the turbine building; fit also affords
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. , - the ophrtunity of more efficiently recompressing the foundation heave
, experienced, and the,reby ensures the design intent.
4., RECOMIENDATION .
Based upon the actual foundation response and the above rationale and discussion, it is recommended that the allowable effective'bcaring pressure be increased to 4500' psf. Presently it is anticipated that this pressure will be adequate to recompress the foundation to its original ~
position or lower, as anticipated in the design; however, the effective bearing pressures will be closely monitored and adjusted as necessary to fulfill the design intent. . .
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