ML071710071
| ML071710071 | |
| Person / Time | |
|---|---|
| Site: | Vogtle, 05200011  |
| Issue date: | 05/13/1974 |
| From: | Bush R - No Known Affiliation |
| To: | Duffin J Bechtel Power Corp, US Atomic Energy Commission (AEC) |
| References | |
| +reviewedcja, AR-07-0924 | |
| Download: ML071710071 (2) | |
Text
Attention:
Mr. J. D. Duffin, Project Engineer 1><
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~-.I Bechtel Power Cor-poz-abi.on P. O. Box 60860 Terminal Annex Los Angeles, California 90060 ENGiNEERiNG RECORDS SECTiON May 13, 1974
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Genflem.er>>.
Subject:
Alvin W. Vogt1' Plant - Excavation Dewatering k1.l3Go~
This letter presents observations made during my recent visit (May 7, 1974). to the site of the subject project.
I was accompanied by your Mr.
Alberto Tejidor and Dr. H. A. Balakrishna Rao; Mr. William Conn, Con-sulting Geologist to Georgia Power Company; representatives of the Georgia Power Company; and representatives of the contractor, Harbert Construc-tion Corporation.
Conditions observed confirm, generally, the r ecommendattons previously made in my report of January 12, 1973.
The general soil profile definitely appears to be well suited to the recommended method of dewatering employing parallel drainage ditches, and not to predraining methods utilizing well points or wells.
The application of wells is particularly questionable con-sidering the apparently highly variable water carrying capacity of the shell zone.
It appears to range from very low, where the material is quite dense and' solid to extremely high where it contains voids, fissures or solution channels.
The possibility of intersecting those openings with vertical wells would ob-viously be quite unpredictable.
Dr. Rao indicated that based on his obser-vat'ions, the majority of exploratory holes drilled into the shell zone en-counte.red large losses of drilling water (300 gpm+) which was probably due to solution channels or similar cavities.
Mr, Farrell's report indicates similar, but apparently less frequent conditions.
He states Ira few of the exp'loratory holes encountered voids in the shell zone. indicating solution-ing has occurred.
Utley's Cave, at the upper end of Mathes Pond, is further evidence of solutioning in the shell zone. II The material above the shell zone appears to have a generally low perme-ability.
In spite of this, it does appear that satisfactory dewatering of
. these materials can be accomplished due to the presence of relatively thin strata of fine, uniformly graded sand.
The process probably would involve the vertical drainage of low permeability rnabe r-ia.Ls to the sand strata, thence horizontally to the drainage ditches.
Since this process will probably be quite slow in the upper zone, a closer spacing of the ditches would be advisable.
Presumably two levels of 'ditches will be required:
initially those in the upper zone directly below the water table where the stratified sand exists, and later the lower ones going into the shell zone.
Considering
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Bechtel Power Corporation May 13, 1974 this, I suggest that the top level be excavated at approximately half the spacing indicated on your Drawing AX2D46T008, with the lower Leve'l ditches "rerna.tntng as shown on the drawing.
This will increase the drainage rate thereby reducing the pos sibility of a r-equir ed excavation rate slower than that scheduled.
A definite prediction of this drainage rate is not possible due to the uncertainty regarding the effective perme-ability of the upper soils.
The rate of water lowering (drainage) between ditches is directly dependent upon permeability, and inversely propor-tional to the square of the spacing between ditches.
During my visit to the site, the rate of flow from Utley's Cave at Mathes Pond was e strirnated by rough.measurements of cross sectional area and velocity at about 900 gpm, about three times that estimated in my January 12, 1973 report.
This indicates that-the assumed product of the drainage times the runoff factor of 0.35 indicated in my report, is probably too low.
A ssum.ing that the contributary area to the cave is basically the same as the drainage area around. the excavation, which should be reasonable,* the estimated rate of ultimate pumping should probably be correspondingly in-creased to about 900 gpm.
This should not cause any particular problem since the discharge piping was designed to carry considerably larger flows during the pumping of storm water.
The 8" perimeter drain should be ade-quate to carry the larger rate of seepage.
As of the date of this letter, I have not had an opportunity to review those portions of the PSAR pertaining to dewatering considerations.
Mr. Tejidor indicated that I could study these items at his,office in the near future.
Also.
I would appreciate receiving the final drawings pertaining to subsurface geology and geologic sections.
In response to your Mr. Duffin's request made in his letter of May 2, 1974, I am enclosing herewith marked up copies of drawings and the specifications, indicating suggested revisions.
Please contact me if you have any questions or if you wish to discuss Cl;ny items in g'r eat.er detail.
RYB:nt cc:
Mr. Mike Thakar Bechtel Power Company Mr. Walt Ferris Bechtel, Inc.
P. O. Box 3965 San Francisco, CA 94119