ML19319D704

App 2H to Crystal River 3 & 4 PSAR, Bedrock Solution Studies.

ML19319D704
Person / Time
Site:	Crystal River, 05000303
Issue date:	08/10/1967
From:	FLORIDA POWER CORP.
To:
References
NUDOCS 8003240696
Download: ML19319D704 (5)

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l I APPENDIX 2H '

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! BEDROCK SOLUTION STUDIES

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APPEUDIX 2H BEDROCK SOLUTION STUDIES 1 INTRODUCTION The near surface marine carbonate rocks constituting the germane portion of the stratigraphic column of the Crystal River area belong to the Inglis limestone me=ter of the Moodys Branch Formation of Jackson Age and the Avon Park limestone of Claiborne Age. These two lithostromes are separated by a well-marked unconformity. These licy sediments are quite porous and have high interstitial permeabilities which have been secondari)y sugmented by fracturing in response to the minor diastrophism responsible for the Ocala uplift. In the vicinity of the Plant Site, the regional dip of these strata is southwesterly at approximately five feet per mile off of the axis of the Ocala uplift. Since these rocks are inherently pervious and are broken by high angle fractures, infiltration and recharge to the groundwater table is rapid. Fresh water entering the underground moves rapidly down gradient (toward the Gulf of Mexico) and attacks limy sediments according to the following chemistry:

2H2 O ^ 2C02 % 2H CO3 2

2H2 003 % 2H+ + 2 (HCO3 )

2H+ + 2(HCO3 ) + CACO 3 HO+CO2 2 + (Ca " + 2(HC0 in solu ion The resultant of this natural process is the destructive alteration of the carbonate rock leaving a labyrinth cf channels throughout the rock mass.

The purpose of this study is to determine the rate at which this solutioning process takes place and to establish the effect such a deleterious process will have upon the foundation of the proposed generating station during its life.

1.1 METHOD #1 If the total volume of secondary voids (solution channels) existing in the subsurface is determined, and the time required for these voids to have occurred is known, it is possible to estimate the additional volume of voids to be formed in any given time.

Well documented subsurface data obtained from exploration and grouting of the foundation for Crystal River Unit 2 shows that the solutioning process has been most intense in the first one hundred feet of section below existing ground surface. Curtain and consolidation grouting carried out to closure on final order holes spaced on a maximum of 5 foot centers successfully 1 injected 7 per cent of grout over the total volume of rock. This figure l represents the volume of voids existing in the subsurface, but does not equal the total volume of solution channels because certain solution channels O

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have been infilled by secondary depositional processes. However, the results of the exploratory drilling and the grout hole drilling indicate that the volume of solution channels probably does not exceed 15 per cent of rock h

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Based upon accepted geochronology of the area, the age of the solutioned sediments is considered to be h0 million years. Carbon 1h dating of organic material developed on and included in the unconformity between the Inglis and the Avon Park members of the Moodys Branch Formation revealed the rock to be older than the limits of the dating process (h0,000 years). It is therefore certain that the age of the rock is more than 40,000 years, and it is accepted as being h0 million years old.

Assuming the law of uniformitatianism to be true, the statement can be made that 15 per cent of the rock mass has been dissolved in a period of 40 million years and definitely in more than h0,000 years. On this basis therefore, the solution rate of the limestone is 15 per cent per h0,000,000 years or approximately 3.75 x 10-7 per cent per year. In the life of a k0 year plant, an additional 1.5 x 10-5 per cent could be expected to be dissolved.

If we consider the most extreme case under this line of reasoning-that all of the solutioning has occurred during the Recent Epoch (the last 10,000 years) or since the Pleistocene Ice Age, when base level was established as it essentially is today-our calculations give us the maximum solution rate.

We then assume that only 10,000 years have been required for 15 per cent of the rock mass to dissolve. Therefore, the solution rate is 1.5 x 10-3 per cent per year. In a forty year life of the plant only 6.0 x 10-2 per cent of the total volume would be dissolved. Such a small percentage g

of solutioning is insignificant to the stability of the rock mass.

1.2 METHOD #2 Based upon verbal communication with R. D. Cherry (U. S. Geol. Survey),

recent studies indicate that within an area of infiltration of 720 sq mi including the area of the plant site, a total of 2h3 tons per day of solids are being dissolved by the solutioning effect of groundwater.

This represents a total of 764 pounds per day per sq mi. If we consider that the area of the proposed generating facilities vill cover an area of approximately 230,h00 sq ft or 0.0082 sq miles, the expected quantity of dirsolved solids removed from beneath the proposed plant area daily would be about 6.3 pour.ds. If we then assume that all of the solutioning vill occur in the first 100 ft of depth beneath ground surface and that the unit weight of the limestone in 100 lbs per cu ft, we find that .063 cu ft per day or 2."1 cu ft per year are dissolved from 23,0h0,000 cu ft of rock.

This figure represrnts 1 x 10-h per cent per year.

In the life of a h0 year plant an additional h x 10 -3 per cent could be expected to be dissolved. 'If one considers the volumes involved, b x 10-3 per cent represents only 920 cu ft of material dissolved during the entire life of the plant.

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2 CUEMRY AND CONCLUSION r (

Comparing the figures obtained by

• he two methods, we see by Method No.1, Case II,1.5 x 10-5 per cent of tne rock is dissolved over the life of the . .

plant, and by Method No. 2, 1 x 10 h per cent of the total volume 'is dissolved.

' , Dissolved volumes calculated by either metnod represent. insignificant amounts.. ,

of deleterious action. Further. the grouting process used in'the foun'dation of the fossil unit new under Scnct r'.etien red.:es tha termenbi tity of the car-bonate rocks from a figure in excess .'f 65,500 feet per year to less than 2,000 feet per year. With the permeabilt ty decreased'by more than 30 times, exposure of the limestene to solvent potential groundwater v112 te effectively reduced by the same factor.

It is concluded that the 9stural sciati:n crecess vili in no way affect the structural integrity of the reandation of the tropo. sed power plent ,

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