ML13330A915

Soil Backfill Conditions

ML13330A915
Person / Time
Site:	San Onofre
Issue date:	08/12/1982
From:	Southern California Edison Co
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ML13317A288	List: ML13317A286 ML13330A915
References
TASK-03-06, TASK-3-6, TASK-RR NUDOCS 8208190208
Download: ML13330A915 (14)

Text

SOIL BACKFILL CONDITIONS SAN ONOFRE NUCLEAR GENERATING STATION UNIT 1 AUGUST 12, 1982

[GULATORY DOCKET FILE COPY 8208190208 620817 PDR ADOCK 05000206 P

PDR

TABLE OF CONTENTS

1.0 INTRODUCTION

2.0 SOIL BACKFILL CHARACTERIZATION 3.0 RESPONSE OF BACKFILL SOILS TO SEISMIC LOADING 4.0 RECONCILIATION OF SOIL CONDITIONS ON SAFETY RELATED STRUCTURES 5.0 RECONCILIATION OF SOIL CONDITIONS ON SAFETY RELATED SYSTEMS AND COMPONENTS

6.0 CONCLUSION

S

7.0 REFERENCES

8.0 APPENDICES A:

SUMMARY

OF FIELD OBSERVATIONS AND BACKFILL CHARACTERIZATION FOR NORTH, NORTHWEST, AND SOUTHWEST PROJECT AREAS TURBINE BUILDING FOOTING MODIFICATIONS B:

LIQUEFACTION EVALUATION

1.0 INTRODUCTION

1.1 Background

The overall site soil conditions present at the San Onofre site are reported in Reference 1. The results and the soil parameters described therein are applicable to the native San Mateo formation.

At the beginning of the Systematic Evaluation Program (SEP) Seismic Reevaluation, the backfill at the San Onofre Unit 1 site was assessed to be San Mateo sand having a minimum relative compaction of 95 percent. Therefore, the soil parameters developed for the backfill and used in the SEP analyses were based on this assessment (References 2 and 3).

In a letter to the USNRC dated April 30, 1982, SCE indicated that as a result of soil testing conducted during the construction of the seismic upgrade modifications for the turbine building during the current outage, it was discovered that fill soil with relative compaction less than 95 percent was present. In the local areas where this was encountered, remedies were implemented. In addition, SCE committed to investigate the potential for similar conditions in other areas of the site and to resolve the potential impact of such conditions on the seismic analyses.

1.2 Purpose and Organization The purpose of this report is to provide a characterization of the fill soils at San Onofre Unit 1. Additionally, where differences are identified between this characterization and the corresponding basis for the seismic reevaluation of the various structures, systems and components, the effects of the differences are assessed and resolved.

This report is comprised of seven sections including this introduc tion. Specifically, in Section 2, the site backfill conditions are characterized based on a thorough review of site grading drawings, construction photographs, the documentation of San Onofre Unit 1 compaction testing during the original construction, and more recent observations and testing performed in conjunction with various plant modifications including the foundation modifications of the north extension and the west heater platform of the turbine building. The turbine building modifications were performed during the period March-June 1982.

Section 3 provides a detailed description of the backfill behavior during a 0.67g Housner Design Basis Earthquake (DBE) event. The methodology which is used to determine the effect of the backfill on the SEP seismic reevaluation analysis parameters is also discussed.

1-1

Section 4 addresses the specific analysis effects of the soil fills for each of the structures. The significance of the soil fill analysis effects on the previously completed seismic analysis of the structures is evaluated and described.

Similiary, Section 5 addresses the specific backfill soil behavioral effects which are pertinent to the affected safety related systems and components. These effects and results are then evaluated for each individual component. If further resolution is necessary, conceptual modifications are identified which, when implemented, will either preclude the cause of the effects, or adequately mitigate the effects on the component.

A summary of the report conclusions is provided in Section 6, and the report references are listed in Section 7.

1-2

2.0 SOIL BACKFILL CHARACTERIZATIONS This section describes the characterization of the backfill areas at the site.

The development of the characterization involved:

defining the backfill areas; evaluating the available information on the relative compaction of the fill in the various areas to identify the amount of compaction; indexing the fills in accordance with the degree of compaction; and assigning an appropriate category to the backfill in each area.

The sections that follow describe these steps.

2.1 Areal Extent of Backfill The first step in defining the backfill areas was the examination of the original site grading plan and the available construction photo graphs.

The plan dimensions and locations of structures, as shown on the original excavation plan, were determined to be correct. However, the excavation slopes (shown as 1:1 on the plan) were not in agreement with the construction photographs.

Based on an interpretation of numerous construction photographs, discussions with construction personnel, and field observations made during subsequent plant excava tion (See Section 2.2), it was determined that the actual slope of the construction cuts were 1/2:1.

In addition, a working space of about two to three feet between a structure and the base of the excavation slope was considered to be consistent with the apparent construction procedures used and with photographic evidence. An excavation plan was drawn depicting the tops and bottoms of the excavations using two to three feet of working space around structures and 1/2:1 cut slopes except where photographic evidence indicated otherwise.

Areas between the excavation slopes and structural walls were designated as backfill areas.

In addition, areas above the anchor blocks where the finished grade was higher than the elevation at the top of the anchor blocks, were designated as backfill areas.

Based on these considerations a site plan showing the areal extent of the backfill was prepared and is presented in Figure 2-1. A water table elevation of +5 feet is used to distinguish between fills which are above and below the water table.

2.2 Characterization of Backfill Compaction After defining the backfill areas at the site, as discussed in Sec tion 2.1, the compaction of backfill was characterized based on all available information. This information consisted of:

results of field tests made during the original construction; observations and tests made in utility trench excavations constructed subsequent to initial construction; observations made during the construction of the sphere enclosure building and the diesel generator building; and observations and tests made during the recent turbine building north extension and west heater platform foundation modifications.

2-1

During the initial construction of the plant, field density tests were made by Twining Laboratories. Tests made in the power block area are summarized in Table 2-1.

It is noted that the degree of compaction, as reported by Twining Laboratories, was based on a laboratory maximum dry density of 121 pcf. More recent tests made over the past 8 years with the San Mateo sand from San Onofre Units 1, 2 & 3 following the ASTM D 1557-A procedure indicate that a more representative laboratory maximum dry density is 120 pcf.

Therefore, the percent relative compactions shown in Table 2-1 are based on a maximum dry density of 120 pcf. The use of a maximum dry density of 120 pcf for San Mateo sand provides a consistent basis for comparisons of relative compaction in the backfills evaluated here and at other locations at the site.

Figure 2-2 is a location map for Figures 2-3 to 2-6 which summarize, in plan view, the approximate locations of these tests and their results.

In addition to the above, and subsequent to the original construction, backfill observations were made for plant modifications including utility excavations, construction of foundations for the sphere enclosure building, the diesel generator building, and the recent turbine building footing modifications. These backfill observations consist of field tests and/or probing (with a 3/8-inch diameter, 3-ft long steel probe) as excavations progressed. The location of the tests, results and observations were carefully documented. The approximate locations and results of these tests are summarized in Figures 2-3 to 2-7.

Summaries of the observations made in utility trench excavations are presented in Table 2-2. At the time these observations were made, it was concluded that they were the result of placement of uncontrolled utility trench backfills. Therefore, these observations were concluded to be localized conditions and not representative of the generic condition of backfills. Field density tests and observations made on soil exposed in excavations for foundations for the sphere enclosure building are summarized in Table 2-3.

Woodward-Clyde Consultants 1982 report summarizing the results of field tests and observations made during the turbine building footing modifications, is included in Appendix A.

The observed conditions at the bases of various excavations made for the turbine building footing modifications are shown in Figure 2-8.

The legend notes in Figure 2-8 describe the observations made during the excavations. The "daylight" lines between backfill and native soil which were observed during the footing excavations were checked against the areal distribution of fill as delineated in Figure 2-1 and were determined to be in good agreement.

In addition to the plan views showing the locations of the observations described above, the test results were also plotted on cross sections to aid in characterizing the backfill in the various locations.

Ten cross sections showing the configuration of the backfill at various locations and the results of these field tests are presented in Figures 2-9 through 2-12.

2-2

2.3 Characterization of Backfills To characterize backfills at various locations, the configuration of the backfill in the excavations was also considered with regard to the amount of working space and the type of compaction equipment observed in construction photographs. Based on this information and the information presented in Sections 2.1 and 2.2, the backfills delineated in Figure 2-1 were characterized into four general cate gories as described below and as shown in Figure 2-13. Figure 2-14 reflects the characterization of Figure 2-13 as well as the remedial measures which were undertaken during the construction of the sphere enclosure building and the recent turbine building footing modifications.

Category A -

This characterization represents well compacted backfill, with a minimum relative compaction of 95 percent. As shown in Fig ure 2-14, the area with this type backfill is located mainly over the discharge culvert. Cross-sections presented in Figures 2-9 to 2-12 show that the backfill in this area is wide and placed over a rela tively flat base.

In addition, construction photographs show the presence of compaction equipment in this area. Further, tests made in the area of the turbine building southwest footing modification which are summarized in Figure 2-4 (see Appendix A), indicated high levels of relative compaction.

Category B -

This characterization represents moderate to well com pacted backfills, with relative compaction of 90 to 95 percent.

Backfills of this category are located adjacent to the intake struc ture, between the intake-culverts and in the narrow sloping surfaces shown in Figure 2-14. This is based on the available information which includes:

the Twining test data summarized in Tables 2-1 and 2-3 and Figures 2-3 to 2-7; the utility trench observations summarized in Table 2-2; tests and observations made along the southern portion of the west footing and outrigger footing, summarized in Figures 2-4 and 2-6 and cross sections presented in Figures 2-9 to 2-12.

In addition, deep narrow fills in these areas, with widths of less than 6 to 10 ft, are assumed to have a lower degree of compaction, which is estimated to be about 85 percent, because of the difficulty of access and maneuvering of compaction equipment.

Category C - This characterization represents near surface moderate to well compacted backfills, with relative compactions of 90 to 95 per cent, extending to depths of 2 to 6 feet. At greater depths, the backfill may have a lower degree of compaction, estimated to be about 85 percent. Based on the limited available data (specifically the utility trench observations summarized in Table 2-2), these fills are in areas around the screen well and tsunami gates and other narrow fills shown in Figure 2-14.

2-3

Category D -

This characterization represents backfills with an esti mated 85 percent relative compaction. This is based on observations made during construction of a portion of the north extension footing during the recent turbine building footing modifications (Appendix A),

as shown in Figures 2-3, 2-8 and 2-9, and on observations made for mis cellaneous pipe support foundations, which are summarized in Table 2-2.

These fills have been defined to include narrow, long areas around structures, where it was difficult to maneuver compaction equipment and where a high degree of compaction may not have been considered essential at the time of construction. As shown in Figure 2-14, the fills in this category include the areas around the reactor auxiliary building, the fuel storage building, narrow fills around the turbine mat, between the west anchor block and the discharge culvert, and shallow, narrow fills around the control building.

It should be noted that the degree of compaction which is shown to be 85 percent relative compaction for categories B, C, and D fills repre sents an average value based on the results of probings and field density tests discussed in Section 2.2.

The backfill conditions summarized on Figure 2-14 are considered conservative since in those areas where limited or no data were available, the worst conditions were assigned from areas with available data.

2.4 Remedial Measures Implemented During Footing Construction Some of the new footings for the sphere enclosure building and for the recent turbine building modifications are located within the backfills placed during the original plant construction.

A summary of the specific remedial measures which were implemented to accommodate backfill conditions which were encountered during the construction of the turbine building foundation modifications is presented in Table 2-4.

In general, if the fill exhibited a density beneath a new footing of less than 95 percent relative compaction, the soil was overexcavated and the footing base extended to native soil.

The overexcavated area was backfilled with lean concrete or the soil was compacted to 95 percent relative compaction. When backfill adja cent to a foundation was found to exhibit a density below 95 percent relative compaction the backfill was generally left in place and the foundation stiffness parameters were modified to reflect this condi tion as discussed in Section 3. Further detail is provided in Table 4-1.

The resulting structural evaluations using the revised design param eters for the footings are discussed in Section 4.0. The final foot ing configurations reflected by these changes are shown on the cross sections shown in Figures 2-15 through 2-18.

Table 2-4 also includes a description of the overexcavation remedial measure which was undertaken during the construction of the sphere enclosure building foundation. As shown in Figure 2-7 only a very minor portion of the foundation was affected.

2-4

0 1

TABLE 2-1

SUMMARY

OF FIELD TEST RESULTS BY TWINING LABORATORIES IN POWER BLOCK AREA Approximate Elevation Moisture Relative Test of d Field Content Compaction*

No.

Date Location Test (ft)

(pcf)

Field (%)

(%)

Comments (from Twining Reports) 27 16 Dec 64 Pump Well Area East

-10 114.8 8.7 95.7 of Intake Structure 28 16 Dec 64 Pump Well Area East

-10 107.4 6.4 89.5 Retested, new designation as test No. 33 of Intake Structure 29 16 Dec 64 North of Intake

-10 115.1 8.7 95.9 Structure 30 12 Dec 64 North of Intake

-10 111.6 7.5 93.0 Does not meet the required 95% compaction Structure 31 12 Dec 64 South Side of Field

+12 108.6 3.6 90.5 To be retested Storage Building 32 12 Dec 64 South Side Field

+12 106.4 3.6 88.7 To be retested Storage Building 33 18 Dec 64 Pump Well Area East

-10 115.1 8.1 95.9 of Intake Structure 34 18 Dec 64 North Side of Field

+13 115.1 5.3 95.9 Storage Building 35 18 Dec 64 North Side of Field

+13 109.2 8.1 91.0 Storage Building 36 12 Jan 65 South of Turb-Ped

+14 115.2 4.7 96.0 Mat.

37 12 Jan 65 South of Turb-Ped

+14 115.4 5.8 96.2 Mat.

38 12 Feb 65 Between Intake Culverts 2.0 119.9 9.9 99.9 39 1.2 Feb 65 Between Intake Culverts 2.0 119.2 8.3 99.3 40 12 Feb 65 Top of Discharge 8.0 119.0 7.5 99.2 Culvert -

East End 41 12 Feb 65 Top of Discharge 8.0 119.8 8.3 99.8 Culvert -

East End 42 24 Feb 65 South of Screen Well

-1 103.6 5.3 86.3 43 24 Feb 65 South of Pump Well

+4 112.1 7.5 93.4 44 24 Feb 65 South of Pump Well

+4 113.5 8.1 94.6 45 24 Feb 65 North of Intake Culverts

-6 113.8 6.4 94.8 46 24 Feb 65 West Side Screen Well

-1 114.5 8.7 95.4 47 24 Feb 65 North of Screen Well

-1 11.7.2 9.9 97.7 48 24 Feb 65 North Side Pump Well

-1 117.1 9.9 97.6 55 23 Mar 65 South of Screen Well 8.0 119.5 8.1 99.6 56 23 Mar 65 South of Screen Well 8.0 118.8 12.4 99.0 57 23 Mar 65 South of Pump Well 13.0 117.8 7.5 98.2 58 23 Mar 65 Area 12 Over 13.0 111.4 5.6 92.8 Discharge Culvert 59 23 Mar 65 Area 13 13.0 120.5 8.1 100.4 60 23 Mar 65 Area 11 Over 18.0 118.0 7.0 98.3 Discharge Culvert 61 23 Mar 65 South of Intake 13.0 107.4 3.6 89.5 Structure Relative compaction based on a laboratory maximum dry density of 120 pcf.

TABLE 2

SUMMARY

OF OBSERVATIONS MADE IN VARIOUS EXCAVATIONS Excavation Approximate Date Description Depth (ft)

Location Observation*

Mar 76 Manhole Excavation 10-20 Electrical manhole structure Backfill with relative compaction 710A and 711A of 87 percent to an unknown depth.

Apr 76 Sphere Enclosure 7

Southwest of column no. C-2 Backfill with estimated relative Foundation near the Fuel Storage Building compaction of less than 95 percent to about el. +7 was removed and replaced with 95 percent compacted backfill.

Oct 76 UPS Trench Backfill 8

West and south side of trench Backfill with relative compaction next to manhole nos. 743 & 744 of about 85 percent to a depth of at least 8 ft (el. +6).

Sep 77 Catch Basin #5 12 South of screen well Backfill with relative compaction of about 85 percent from surface (el. +14) to at least el. +2.

Oct 77 Utility Trench 6

South of pump well between Backfill with relative compaction column lines K & L and west of about 85 percent from surface of column line 13 (el. +14) to at least el. +8.

Oct-Nov 77 Trenches for Misc.

11 West of pump well near Backfill with relative compaction Piping screen well of about 85 percent from surface (el. +14) to at least el. +3.

Feb 78 Chlorination Tank Pad 3

South of intake structure Backfill with relative compaction and Yard Sump near west wall of pump well of about 85 percent from surface (el. +14) to at least el. +11.

Jun 78 Cathodic Protection 9-10 Between pump well and screen Backfill with relative compaction Boring well of 90 to 95 percent from surface (el. +14) to a depth of 4 to 6 ft and with relative compaction of about 85 percent below el. +15.

May 80 Miscellaneous Footings 5

East of anchor block, north Backfill with relative compaction of column line 1 of about 85 percent from surface (el. +14) to at least el. +9.

Dec 80 -

Miscellaneous Pipe 4

Against north and west walls Backfill with relative compaction Jan 81 Support Footings of Fuel Storage Building of about 85 percent from surface to bottom of excavation.

Probing indicated loose soil to additional depth of at least 3 ft (el. +7).

Jan 82 Miscellaneous Footings 5

Against north wall of Fuel Backfill with relative compaction Pool near northeast corner of about 85 percent from surface of Fuel Storage Building (el. +19) to at least el. +14.

BObservation interpreted from field notes and on discussions with field personnel. Approximaterelative compaction estimated by using a 3/8-inch diameter, 3-ft long steel probe.

TABLE 2-3 mmary of Field Density Tests in 0

oundation Excavations, Sphere Field Data Sheet Enclosure Building Sheet No:

1 Job Name:

SONGS Unit 1 Sphere Enclosure Building Job Number:

B675F Field Dry Max.. Rel. Spec.

Drawing 1.976

'fest Retest Retest Grid Density Moist.

Lab.

Comp. Reg.

No.

Quality.

[ate Number by of Number Location of Test Elev. (pd)

Mos(pt)

I Spec..

class.

S9+5 SErg Apr 07 1

S 9

Blow Down Header 15 124 6

SC 120 103 95 S.

SR

• Apr 08 2

S9+58 of of 17 118 9

99 f

Apr 13 3

S9+43 Column C-7 13 123 10 102 4

W4+87 Column C-8 13 121 7

100

^Pr 14 5

S9+30 Column C-9 13 121 8

101 f

S9+578 6

WS+50 Blow Down Header 19 120 8

100 Apr 19 7

Column C-7 15 119 6

100 W59+310 8

W4+90 Column C-8 15 120 6

100

+319

+

Column C-9 15 121 6

100 n

S 9+44 0 10 W4+38 Column C-10 13 121 5

101 S9+4

___of Apr 20 11 W1t3 Column C-10 15 122 8

102 S 9+62 f

1 tt AjIA 12 Column C-6 13 118 9

98

" A Apr 26 13 S9+59 Column C-6 15 114 5

I" 95 I"

"s s

S59+68 pr 29 14

_68_

Column C-11 13 118 7

98 15 S9+90 15 W4+6 Column C-12 13 121 7

101 Apr 0

16S9+60 Apr 30 16 W4+15 Column C-11 15 118 7

98 "f

17 S9+95 Column C-12 15 122 8

102 Ray 10 18 W1l5 Column C-2 9

118 10 98 May 11 19 W5+20 Column C-2 11 121 9

S0 May 2

20S10+40 May 12 20 j1474 Stack footing 9

121 8

101 21 S10+58 11 122 9

101 itemaIks:

SC -

Sand Cone Density Test (ASTM D1556-64)

• = Test requested by Bechtel SR - Safety Related Class 1 4 2 Reviewed By:

('nce T F A

iaview 11'rv

TABLE 2-3 (continued)

Field Data Sheet Sheet No:

2 Job Name:

SONGS Unit 1 Sphere Enclosure Building Job Number:

B675F Field Dry Max.- Rel.

Spec. Orawing 1976 Test Retest Retest Grid Density Moist..

Lab.

Comp.

Reg.

No.

Quality.

Date Number by of Number Location of Test Elev* (pcf)

(pcf) t Spe..

Class.

S10+40 sEPan1 May 12 22 5+141 Stack footing 9

116 9

SC 120 96 95 IS SR

• May 13 23 SI1-0+38 H

9

_y_13_23_W5+39 11 121 9

101 24 W4+05 Column C-13 12 118 8

98 S10+44 o

May_ 14 25 145+30 Stack footing 10 123 7

102 Sep 24 41 S9+60 Seal Water Filter

.15 123 8

103 42 S9+61 Enclosure Structur1 W5+52 17 122 7

__102_"

Remarks:

SC Sand Cone Density Test (ASTM D1556-64)

Test requested by Bechtel SR -

Safety Related Class 1 2 Reviewed B1

TABLE 2-4

SUMMARY

OF REMEDIAL MEASURES FOR SOIL BACKFILL CONDITIONS ENCOUNTERED (sheet 1 of 2)

Foundation Soil Condition Encountered Remedial Measures Implemented A. Turbine Building

1. North Turbine Footing Most of the footing is founded on Backfill on western end of native soil or 95% compacted back-excavation was removed and fill. During the excavation of the replaced with concrete to western portion of the footing el. +1 ft.

Backfill below 80-93% compacted backfill was el. +1 ft. was compacted in encountered.

place by vibration.

2. Northwest Turbine Footings

• Footing E-ll Most of the footing is founded on The stiffness parameters were native soil except for a small modified.

portion along the east wall.

Backfill against the side of the footing is dense except for small portions of the east and south walls where it is 80-82%

relative compaction.

• Footing C-9 Most of the footing except a small The stiffness parameters were width near the north side is modified.

founded on native soils. Backfill against the side of the footing is 85-87% relative compaction.

TABLE 2-4 (continued)

SUMMARY

OF REMEDIAL MEASURES FOR SOIL CONDITIONS ENCOUNTERED (Sheet 2 of 2)

Foundation Soil Condition Encountered Remedial Measures Implemented

3. West Turbine Footing The footing is founded on native soil The stiffness parameters were or backfill with 90-95% relative modified.

compaction. Backfill against the side of the footing has 90% relative compaction.

4. Southwest Turbine The northern and western portions of The excavation was deepened to Footing the footing are founded on backfill about elevation +3 ft., and the with relative compaction of 95-100%.

overexcavated area was filled The remaining footing had backfill with concrete.

with 83-85% relative compaction at elevation +7 ft.

Backfill against the side of the footing varied between 90 and 95% relative compaction.

5. Outrigger Turbine Tests in the excavation showed back-The footing was modified to be Footing fill at a relative compaction of 87 supported by the intake culverts, to 93% to elevation +3 ft.

and the overexcavation below the footing base at elevation +5 ft.

was backfilled with concrete.

B. Sphere Enclosure Building During the construction of the sphere The excavation for the sphere enclosure building, the footing enclosure building's foundation excavation at the southwest end of at this location was extended to the building indicated some backfill elevation +7 ft. and replaced at a relative compaction of less with fill compacted to 95%

than 95%.

relative compaction.