ML20138C235

Summary Rept,Adequacy of Backfill Around New Svc Water Buried Headers,Svc Water Improvement Project

ML20138C235
Person / Time
Site:	North Anna
Issue date:	03/11/1986
From:	VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.)
To:
Shared Package
ML20138C230	List: ML20138C224 ML20138C235
References
NUDOCS 8604020410
Download: ML20138C235 (9)

Text

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ATTACIDtENT 2 S1291ARY REPORT ADEQUACY OF BACKFILL AROUND NEW SERVICE WATER BURIED LEADERS SERVICE WATER IMPROVEMENT PROJECT NORTH ANNA POWER STATION

. UNITS 1 AND 2 MARCH 11, 1986 9

e 8604020410 860321 PDR ADOCK 05000330 0 PDR -

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TABLE OF CONTENTS 1.0 Introduction ,

2.0 Soils Testing Subsequent to Backfilling 3.0 Specification NAS-3003, " Excavation, Fill and Backfill - Service Water Buried Piping Installation" 4.0 Missile Protection 5.0 Conclusion '

TABLES 1 Subsequent Soils Testing Results By Law Engineering Testing Company FIGURES 1 Plan and Profile of New Service Water Buried Headers 2 Cross Section Through Pipe Trench and Backfill 3 Comparison of Typical Compaction Tests 4 Gradation Curves of Backfill f

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1.0 Introduction The Service Water Improvement Project at the North Anna Power Station requires the installation of new spray headers in the Service Water Reservoir. These spray headers will be fed through new service water buried headers running from the original buried lines at the new Tie-In Vault to the new Valve House at the edge of the reservoir, as shown in Figure 1.

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The backfill around the new service water buried headers, as shown in Figure 2, is necessary to provide bedding for ' the pipes, develop the appropriate. friction against the pipes, and provide missile protection.

This report describes why the backfill placed to date is adequate for the design and continued construction of the new service water buried headers.

2.0 Soils Testing Subsequent to Backfillina On December 30, 1985, test pits were excavated into the backfill to permit a total of five in place density determinations, four in the select soil fill placed against and around the pipes and one (Sample 2) in the overlying soil fill. The locations of the five test pits are shown in Figure 1. These tests and subsequent laboratory tests were performed by Law Engineering Testing Company in strict accordance with the methods required by Specification NAS-3003, " Excavation, Fill and Backfill - Service Water Buried Piping Installation." The results of the tests are given in Table 1.

Index properties of the select soil fill (Samples 1, 3, 4, and 5) establish that this material meets the requirements of Specification NAS-3003. The average in place density was found to be 94 percent of the maximum density for each sample.

Despite the inadequate documentation of soils testing during backfill operations, the results of the recent testing show that the backfill placed to date is satisfactory. Furthermore, the placement and compaction operations (lift thickness, types of compactors, number of compactor coverages, etc.) are known--if not well documented--to have been in accordance with Specification NAS-3003 and good construction practice.

3.0 Specification NAS-3003, " Excavation, Fill and Backfill - Service Water Buried Pipina Installation" 3.1 Relationship to Piping Design The piping design was based on an average density of the soil over the pipes. The backfill specification was prepared to ensure that these conditions were attained during construction. " Summary Report -

Analysis of Service Water Buried Piping" describes the piping analysis and impact of varying soil densities on the pipe stress. The conclusion of the report indicates that the pipe stress is well within the allowable stress for varying soil densities.

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3.2 Importance of Moisture Content The effect of placement water content on the density of a compacted soil is illustrated in Figure 3. This shows the results of laboratory compaction tests on the same silty clay under two different compactive efforts. Under the lower compactive effort (lower curve), having the placement water content near the optimum water content ensures densities greater than 90 percent of the maximum density. If the placement water content were too low, the target density could still be attained, but a greater compactive effort would be required as shown in the upper curve. The same ~ relationship applies to field compaction.

3.3 Coefficient of Friction The coefficient of friction of soil against a structural material is established by soil type and grain size distribution. The ' material used for backfilling against the pipes is classified as an ML or MH material as shown by the grain size curves in Figure 4.

This type of material has a coefficient of friction 0.4 against the coating placed around che service water pipes. This value is based on NAVFAC DM-7, " Soil Mechanics, Foundations, and Earth Structures," a design manual widely accepted in the nuclear utility industry. Values in this document are independent of soil density.

4.0 Missile Protection The plant missile protection design criteria for buried piping is 6 ft of compacted soil. During original plant construction, backfill for missile protection was placed in accordance with Specification NAS-236.

That specification required the soil to be compacted to a density of 90 percent of the maximum density determined in accordance with ASTM D698, the same requirement as Specification NAS-3003. As described in Section 2.0, the backfill material was placed at the correct density and, therefore, provides adequate missile protection.

5.0 conclusion Based on results of soils testing subsequent to placing backfill, the material placed to date meets the requirements of the specification, satisfies the bases of the piping design, and will provide adequate missile protection when placed to a depth of 6 ft.

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TABLE 1 SUBSEQUENT SOILS TESTING RESULTS BY LAW ENGINEERING TESTING COMPANY IN PLACE LAB Sample STA LL PL PI  %<200 CLASS D MC MAX OMC  % COMPACTION 1 1+00 48 38 10 55.8 ML 87.9 28.4 92.0 27.3 96 2 1+45 25 NP -

27.1 SM 104.8 15.6 112.0 14.9 94 3 2+50 62 38 24 59.2 MH 88.1 31.5 92.2 25.6 96 4 3+00 49 39 10 64.2 ML 83.0 31.8 92.6 26.2 90 5 3+50 60 42 18 66.7 MH 86.8 36.6 91.4 28.0 95 NOTES: 1. In place density determinations were performed in accordance with ASTM D 1556; in-place dry density (D) for each sample was computed using the moisture content (MC) determined by oven drying in the laboratory in accordance with ASTM D 2216.

2. Maximum density (MAX) and optimum moisture content (OMC) for each sample were determined in accordance with ASTM D 698.

3. Two types of backfill were tested, classified as follows:

Select soil fill (Samples 1, 3, 4, and 5): reddish brown sandy clayey silt (ML or Mli).

Soil fill (Sample 2): light brown silty sand (SM).

4. Gradations of samples, shown in Figure 4, were determined in accordance with ASTM D 422.

5. Specfic gravities of the two types of backfill were determined in accordance with ASTM D 854, as follows:

Select soil fill: 2.74 Soil fill: 2.64

6. All tests were conducted in accordance with methods required in Specification NAS-3003 by certified engineering technicians using calibrated traceable equipment.

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