ML071710073

8.7.1 Aquifer Tests for Construction Dewatering

ML071710073
Person / Time
Site:	Vogtle, 05200011
Issue date:	05/03/1972
From:	Bechtel Corp
To:	US Atomic Energy Commission (AEC)
References
+reviewedcja, AR-07-0924, Job No. 9510-001
Download: ML071710073 (21)

Text

May s, C. R *.

Vogtle Nuclear Plant Excavation Proposal for dewatering test well program Test wells 80 feet deep, 10" f/J hole, 4" dia. casing, (2) 10 feet of well screen, gravel pa~k.

Obs .Pts. two 80 feet deep; and two 65 feet deePJ4" (4) dia. with 2" casing, 10 feet of well screen.

1. Drilling, setting casing and gravel pack of test wells; est. 160 feet - cost per linear foot

2. Drilling, setting casing and gravel pack of obs

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est. 290 feet - cost per linear foot

3. Cleaning and develo~ment of test wells est. 40 hrs. (20 each) - cost per hour

4. Test Pumping of wells est. 144 hrs. (72 each) - cost per hour

5. Move in, set up, and clean ul linear sum cost TOTAL COST ESTIMATE Pay Unit of Cost per Estimated Total Item Measure Unit Total Units Cost 1 linear foot $25.00 160 $4000.00 2 linear foot $ 6.00 290 1740.00 3 hour $30.00 40 1200.00 4 hour $30.00 144 4320.00 5 lump sum $3,000.00 1 3000.00 TOTAL COST $14,260.00

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I Bechtel Corporation Inter-office Memorandum To Files Date May 10 t 1972 Subject Investigation for Dewatering From C. R. Farrell of Plant Excavation, Vogtle Nuclear Plant Job No. 9510-001 Of Geology Copies to w. Holland At E & I Division A. Luft C. McClure

w. Ferris On FridaYt April 28, R. Bush, consulant to the project t attended a meeting in our offices to discuss dewatering problems we might expect in the excavation for the plant site, and at the water intake structure near the river. I briefly attended the meeting to provide clarification of our intrepretation of ground water conditions at the site.

Site Excavation Dewatering Mr. Bush is concerned that wells might not be an effective means for dewatering the area. He is basing this concern on the information collected to date; pump-in tests within the shallow sands and the description of materials in the shell zone overlying the marl (bearing unit). Although the experience of drilling and knowledge of the materials suggests that the shell zone is relatively high in permeability, it is not certain that it would act as an effective underdrain for dewatering the overlying sands. Should the proposed plan for well points in the shell zone not adequately drain the sands, serious delay in construction scheduling t as much as 2 or 3 months, could occur. I agreed with Bush that our knowledge of the permeabilities was not firm enough to preclude this possibility. It was decided that a test-ing program be conducted.

Test wells selected at two sites, representing the most favorable conditions and the least favorable conditions, as evid-enced from our exploration of the site for Units I and 2 t will provide data 'to evaluate a well system. After selecting the sites t and preparing a tentative construction plan, I contacted Layne-Atlantic of Savannah, Georgia t concering their availability to do the work. After verifying their willingness t I contacted R. Bush by telephone t Thursday, May 3, to review the details of test well construction.

FiJes -2.,. May 10, 1972 There was apparently some misunderstanding as to Bush's primary objective for the test wells; I had thought it was to determine the permeability of the shell zone. Although this will be desireab1e, Bush is first concerned about the maximum yield of wells. Construction wise, this does not make a large difference (primarily it will call for 15 to 20 feet of per-forations opposite the upper sands also, in order to intercept all inflows of water available to the well.

With these additional factors in mind, the test wells and observation points to be constructed will consist of the following:

Test Wells (2)

Depth: 80 feet (+/- 5 feet)

Diameter of bore: 12-inch Casing diameter: 6-inch Well screen: length; 15 feet diameter; 4-inch slot opening; 1/8-inch Observation Points Quantity: 3 points for each well Depth: 80 feet (+ 5 feet)

Diameter of bore:- 4-inch Casing diameter: 2-inch Screen: length; 15 feet diameter; 2-inch slot opening; 1/8-inch After placing the screen and casing in the bore, the annular space in the wells and the observation points will be filled with clean, fine-gravel up to height of 15 feet above the screened intervals. During placement of the gravel, clean water will be pumped through the casing to clean the hole of drilling fluid.

The observation points will then be "pumped" by air injection to confirm hydraulic continuity with the aquifer zone.

The wells will be developed by pumping, possibly preceded by air injection. It is anticipated that 8 to 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> of development will be sufficient before commencing a testing of the well. The pumping tests will be conducted at a constant discharge rate for a continuous period of 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> (3 days).

I have asked Terry Scafidi of Layne-Atlantic to submit an estimate of cost for the work as a lump sum to be added to the present contract. He will submit an estimate by the end of this week.

They would be able to conduct the work following completion of the test well construction and testing.

Files May 10, 19:72 Intake Structure Invert elevation of the intake structure adjacent to the Savannah River will be at elevation 54 feet, or approximately 10 feet below the base of the marl. Piezometric levels measured at various depths below the marl in the vicinity of the plant site indicate the level below the marl is at elevation 110 feet.

However, where the confining marl is breached, as in the river channel, the upward flow reduces the point hydraulic head, and it is believed that piezometric levels adjacent to the river will not be as high as 110 feet. This will be significant both for dewatering at the intake structure and in considering possible uplift pressures. It is therefore, recommended that an obser-vation point be placed at the intake structure, to a depth corresponding to elevation 45. The point should be isolated by grouting the annular space above elevation 65. This could possibly be done by a Law Engineering drilling rig presently at the site conducting soils exploration for Units 3 and 4. Following completion of that work, a piezometer could be easily constructed by them, as they are familiar with the site and have placed similar ones in the vicinity. It is my understanding that data for de-watering conditions are not needed for the PSAR, so that construction of the piezometers can be planned on the availability of a drilling rig. If it is not convenient for Law Engineering to do it, we can arrange for placement of the point by Layne-Atlantic.

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C. R. Farrell

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~.i'he purpose of this report is to pJ>::~sent the results of our s ttldy of the de.. .v atering problem ant.Lc Lpa t.ed in connec*tioh '.vi th the co~struction of the suhject project.

DcscriptiOl":-. of Study

~rhis investigation _p-onsist.ed of a r-ev.i.ev of.1preliminary con-

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stT~~i:.J.on ~);a~vi!lg:s~;.;:*studies of geolo/:3ica1 informa~io.n \\!h~ch included borings logs l .. _:~a draft of a ground "later report by

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Hr. C. Farrell* of Becht:l, aTf.,:1)variou;:; map s of geological conditions at the s.i.te;::?studies o f rai.n f a Ll, Lrrt.errs Lt.y as related "to possible flood damace in the exc av at.Lon a.rea; (-:analyses 6f"'-

pump test data 0v~ained by your personnel; and t.he prel~rnin~~y design of a combination d ewe t.er i.nq and s t o rrn i,;,ater purap i.nq

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an opportunity to personally visit the project site.

Groundwater Conditions

'J~he report draft on grO\.m~1\'7a.ter conditions "by Mr. Fdrrell provided valuahle information. Significant items 60ntained in this report

1. liThE: impervious marl. 0;: bea:c inc; un i, t, acts as an aquiclude

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(impervious barrier) to grouY1,:lw.:. t.e r . "

2. 'Tni; only source of TE';d12.rge 1:,:; th",~ uric o nf ined g!_-0und~~*ater G .. JJ v t:..., tho

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\:;*l~:.? r cpo r t; describes *the outflow from Hathes Pond as an e s t ima t.ed 300 gpm \.. . h i.ch is considered to be the amount corresponding to a final equilibrium condition during dewatering. It is pointed out t.h a t; initial pumping for d ewa't.e r Lnq would be considerably greacer than this amount. An excellent check on the 300 gpm was obtained by a planimeter measurement of' the tributary area to the site, which appears to be about 367 acres. For this area, a rate of

,300 gp~) would correspond to 50 inches per year with 30% infiltration, bo.t.h r.easonablevalues.

Data obtained from two pump tests were analyzed. Descriptions of the test wells follow:

1. Well #l-total depth 94'; white sand with shells 72'-80'; marl below 80'; coordinates NI, 142,660 and E623,570.

2. Well #2-total depth 87'; white sand with shells 52'-61';

shell, hard, lime.stone 61'-85'; marl below 85'; coordinates NI, 14.3; 225 and*E623, 075. In addition to the pumped i.velis I 8 observation wells, 4 per test well, were installed to permit themeasnrement of water levels during pumping.

Well #1 was pumped for approximately four days at rates .of generally in the range of 30 to 38 gpm. Well #2 was pumped for about 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br /> at rates of 10 to 15 gpm. Pumping on well #2 was discontinued due to the lack of response of the water levels in the observation wells. Additional "pump in" tests were performed on \vell #2 observation we LLs . .Due to the relatively small rate of pumping from well #2 and the correspondingly small amount of lowering of water, a quantitative evaluation of pe~rneability waS.not possible in this case. This test did indicate that transmissibility at this location is very small.

l):xt:a obtained from test well #1 was analysed on the bas i s of ncnequ i r Lbr Lnm methods, using data obtained du r i.riq both d r ax.. .d own a rid r ebou nd periods. ].I,.ttac11cd plots indicate fair agreement b0t},'7(~On t.he various ob s e rva t.ion Hells "Ii th the c~xce:pti.on oE l-c.

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'r:ne erratic behavior was due to interruptions in the rate of punqJing and to a lesser extent due to variations in barometric pressure d~ring a storm period.

Based on our analysis, the transmissibility of the unconfined ag]~1.j.fer is es*timated to be in the range of 0.7 tol.8fli'm5.n.

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(7r~60 to 19 ,440 gal/day ft.). Corresponding permeability values, bas ed on an a.quifer thickness of 10.' would be 0.07 to 0.18 f't./min.

l\verage permeabili ties in the a r e a are probably less than this due to the fact *that well 11 was probably-located in a relatively high permeabili ty a:cea.* Considering the variable nature of the shell zone, a wide range of local permeabil::ty should be anticipat~d.

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Cj Al though the apparent permeability of the shell zone is relati";.Tely hi~h, bec~use of its limited thickness, the transmissibility of

( the aqu Lf e r is qui te 10v1.. Due to this condition, wh i ch results

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in 10\'; individual vrell capacity, ,the application of predra.ining me t.hod.s employing deepwells or: ....l ellpoint.s is not considered practical or economi~allyfeasible.

The volume of water to be removed during the initial dewatering pe rLod until the. fi.na.l "equilibrium condition II is' reached l i s estimated at abou_t 1'+/-QL2..00.!,g_~9._JL'::::,~.:*~: j:\n average rate of 1, 000 gpm vzouLd therefore require about 100 days which should coincide"!

reasonably well your ~nticipated excavation rate. An initial

"..+ -,500g,E,ID i s Lee orr.8ended . Tl'le rate of pumping T,,7C?ulcl gre.tduztlly c1qcL~ease.. \-111:11 ti.me until _fin~JJ~.Y,_~tt!S~_...§:g'§'~~2:i.q.~d"~._c9.Q~.iE.;L9]'ll e~iiA§~~g st-J00 gp~, is reached.

A system of ditches 0.110. rsump s ,' c

.1.~ reCOTr..m,8nc1l~d to per f o rrn t.h i.s dewatering. ~he basic scheme is illustrated on figures 3,4,5,6.

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-c.t-at var Lou s cons t ruc t Lon con s i deca t.i or.s unknown to the V!riter

( could ne c e s s.i.t.atie the extensive revision of the layouts as proposed.

Tli8 h~sic dewatering scheme proposed consists tif the following:

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Preliminary excavation is made to an elevation slightly above the initial water table.

(2~..... Ditches are excavated across the excav~tion area to allow the

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we t; materials *to drain by. gravity f Low through the d i.t che s tosu..rnps from wh i.ch the wa t e r is pumped , It should be noted that the spacing of the di*tches is indicated as a_~qQ:I.:-.}p,?c~imJd~

This is to insure that dewatering of the materials bet~een ditches occurs in a reasonably short period of time.

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Excavation continues to the surface of the mar-L, the bearing.*

material, a;': wb i ch time the rate 0:C pumping should havo .

diminisheo. to a relatively small' rate, approaching the sus-tained rat.e .

.At this time ditches are excavated in the marl to provide drainage during periods. of hig-h ir.~c.en$ity ra.infall. Thisi tem

.C is discussed in greater detail sub~equently.

Prior to ba..ckfill, a perimeter porous dra ;.:1 pipe is insta.lled to allow dewa t.er Lnq during "'ehe backfill period. This drain leads to vertical pump wells fron which the wat.e r can be pumped during the backfilling operation. 'l'his pumping on the per.iInet:er drain wou Ld continue untiJ. backfill has reached a sufficiently high elevation, and the weight of the concrete placed is su,fficicntlyheavy' so that no further control of hydxo st.ac.i c up Li.f t; is regrdred.

'Thr:~ Tr';ajor po::r,pi:lg .r equ i.r-emerrt; ~<7ill be to rerncve storrm.*rater from the e.y.cavation cb.::.::ing per Lod s of high .i nt.errs Lt.y rai:-:,fall wh i.ch must be a.nt.icipated in this area. The combined. effects of this high intensity rainfalJ. with the extremely large area of the excavation YE:;sul i:s in ext r emc Ly high rates of pump Ln q r e qu i r-ed dUI:ing" storms to };:cep t:j~C! cxcava t i on free o f wa t er . Figure 7 illustrates a

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re?rcsented by Units 1 and 2. It is assumed th~t the top of the e~c~vation slope is provided with proper d~ainage ditches and

( th~t therefor only rainfall falling on the actual excavation area would be pumped from it. Consideration should be given to the use of appropriate stabilizing materi~ls to the slope to minimize erosion.

D"uring normal conditions, only a portion. of the pumping equipment wou Ld be required to operate for de1'iateringi that is, handling r.he groundwater* entering the excav~tion. The desiqn of the dis-charere

~ .... ,. .J piping for t.he combined svstem 'would obviously be Q9.sed 11l8'M~":--"'''''¥;I~~;'~*:-_~-'-''''';.ii>-''''P>'--'-''''''''''''--- " .

on the pumping rate during.. the

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period.

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• The actual *size of the system mu.st be based on a careful consideration of the financial consequences of a heavy rGin~torm due to d~~age c~used to conc re t.e and other operations 1 weighed against the probability of extreme sto'rms occurring say of' th~' 50 to 100 year variety.

We will not attempt to evaluate this complex problem since we are not sufficiently acquainted vlith the various cost and cons t.r-uc cLon r: cC12siderations involved on this project. It wou Ld appeal; that a

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pumping plant to provide reasonable protection against s t.ozrn damage should have__a capacity

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in the range of from 5,000 to 10,000

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Conclusions and Recommendations

1. Although the permeability of the shell zone immediately ahove the mar.L appears to be-quite h Lqh , due to its limited thickness, the 'cransmissibili ty in this a~ea is quite IGw, in the range of 2 -

0.7 to 1.8 ft.  ! min.

2~ Due to the li.rnited thickness of the pervious zone directly above the ma r L, along Hi t.h -Q.ther considerations such as the difficulty and high expense of drilling, the application of p2*ec1:co.ining me.thods employing we Ll.pod.n t s or deepwells is considered impractical and economically not feasible.

3. A me chod o f d i, t.che s and sumps should be used to perform the dewatering cf the excavation.

4~ The size 0:1.' the pumping plant p!."ovidec1 should be based on a consider-at .i.o n of h and LLl10 s t.o rrnwa t.er s .i.uc e t.h i s pumping rate will greatly exceed the anticipated rate of dewatering.

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5. A perimeter drain should be installed to allow dewatering and nydros 1: a t i c uplift control during *backfill operations.

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R. Y. Bush

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