ML20170A399
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Page 1 of 17 DSAR-5.7 Structures Piling Rev 1 Safety Classification:
Usage Level:
Safety Information Change No.:
EC 69625 Reason for Change:
Section is updated as a result of Design and Licensing Basis Reconstitution.
Preparer:
D. Sojka Fort Calhoun Station
DSAR-5.7 Information Use Page 2 of 17 Piling Rev. 1 Table of Contents 5.7.1 General.......................................................................................................... 5 5.7.1.1 Subsurface Conditions................................................................... 6 5.7.1.2 Pile Installation Procedure............................................................. 7 5.7.1.3 Arrangement at Tops of Piles......................................................... 8 5.7.2 Pile Loading Tests.......................................................................................... 8 5.7.2.1 General.......................................................................................... 8 5.7.2.2 Selection of Pile........................................................................... 10 5.7.2.3 Testing Procedure........................................................................ 11 5.7.2.4 Conclusions................................................................................. 12 5.7.3 Loading and Design Criteria......................................................................... 13 5.7.4 Seismic Considerations................................................................................ 15 5.7.5 Corrosion Protection.................................................................................... 16
DSAR-5.7 Information Use Page 3 of 17 Piling Rev. 1 List of Tables Table 5.7 Secant Coefficients of Horizontal Subgrade Reaction...................................... 12 Table 5.7 Pile Design Loads............................................................................................ 15
DSAR-5.7 Information Use Page 4 of 17 Piling Rev. 1 List of Figures The following figures are controlled drawings and can be viewed and printed from the listed aperture card.
Figure No.
Title Aperture Card 5.7-1 Piling Plan Containment and Auxiliary Building Sections and Details... 16380 5.7-2 Piling Plan Intake Structure................................................................... 16507
DSAR-5.7 Information Use Page 5 of 17 Piling Rev. 1 Piling 5.7.1 General In order to provide information for the design of an appropriate foundation system, several series of subsurface exploratory programs were performed at the plant site.
Initially, a program comprising of 6 exploratory borings dispersed over the site area was performed at the time of acquisition of the property for a general indication of the nature of the overburden and bedrock.
Later upon determination as to where the major plant structures would be generally located within the site, a subsurface exploratory program including 16 borings was performed as discussed in Defueled Safety Analysis Report (DSAR) Appendix C.
Upon establishment of precise location and outline of the plant structures, a program of 12 additional borings within these limits was instituted for final and corroboratory information. During execution of this program, evidence of a cavity was detected beneath the bedrock surface under the southwest corner of the plant.
Additional borings were then performed to delineate the extent of this cavity.
Since all previous explorations at the site revealed sound limestone, it was believed that the detected cavitation as defined by this explained exploration was confined to this limited area. As a result, the plant location was moved upstream a distance of 90 feet in an attempt to avoid the region of observed cavitation, and an intensive program of bedrock investigation was instituted at the revised location.
To ensure maximum probability of interception of any cavities which could be present, borings were performed at randomly distributed locations. A total of 73 borings were made during this final investigation at the adjusted plant location to define the extent and frequency of cavity formation. Additional cavities were encountered and from the information obtained it was possible to establish some conclusions regarding their presence and nature as discussed in DSAR Appendix C.
DSAR-5.7 Information Use Page 6 of 17 Piling Rev. 1 5.7.1.1 Subsurface Conditions Bedrock occurs at depths ranging from 58 to 67 feet below ground surface in the plant area. The upper four to eight feet is a massive, gray thick-bedded medium to fine grained oolitic limestone. Below the oolitic is a light gray, thin to moderately thick bedded, very fine grained aphanitic limestone, which ranges from 19 to 21 feet in thickness.
The cavities were encountered at the base of the oolitic limestone, extending from a few inches to approximately 14 feet into the aphanitic limestone and were believed to have developed by enlargement of major vertical joints by solution. Cavities tend to develop in the aphanitic limestone since the oolitic is more pervious than the aphanitic. The flow concentration is channeled downward and laterally along vertical joints in the less pervious aphanitic. The enlargement probably initiates at the interface of the two limestones, where the flow is first encountered, and progresses along the joints within the aphanitic limestone.
Erosion progresses along the joints and results in long linear shaped cavities. A cavity may expand where a softer or more easily eroded material is encountered within the limestone. With time, weathering and spalling of the oolitic cap rock also takes place, causing enlargement of the cavity into the oolitic limestone, and resulting in a variation in the thickness of cap rock, and simultaneously the downward flow of water also enlarges the vertical joints within the oolitic limestone.
With the disclosure of cavities in the bedrock beneath the plant site, and with the plant arrangement and elevation based on support of the plant on piles to bedrock, it was necessary to select a pile type and method of installation which would permit investigation of the bedrock at each pile and application of corrective measures where necessary, to ensure that each pile was properly founded on sound rock.
To comply with these requirements, open end steel pipe piles were selected, and installed in accordance with the procedure described in Section 5.7.1.2.
DSAR-5.7 Information Use Page 7 of 17 Piling Rev. 1 5.7.1.2 Pile Installation Procedure Piles were driven open end onto bedrock. An exploratory small diameter probe was made within the pile from the bedrock surface extending to a depth of 15 feet to determine if any cavity was present under the pile. The depth of 15 feet was predicated on the basis of the findings from the site bedrock exploration which established that cavities, where present, were detected within the depth of 15 feet below the bedrock surface. If no cavity was encountered, in effect confirming the soundness of the bedrock, the pile installation was considered as complete.
To ensure that the piles were actually seated on rock, a check was made for every pile of its bottom elevation after driving refusal was reached against the elevation of sound rock as indicated by the exploratory probe. Piles that were found not to have reached rock were further driven to the proper seating level on the rock.
Where a cavity was revealed from the results of the exploratory probing, a determination was made of the depth beneath the initially encountered bedrock surface necessary for seating of the pile onto sound rock. The pile was cleaned out and by means of an under reaming expansion rock auger working from within the pile in its initially driven position a hole of slightly larger diameter than the pile was drilled beneath the base of the pile, extending to the predetermined depth to sound rock and providing a flat base for seating of the pile.
Prior to resumption of pile driving, its length was checked against that required to ensure reaching to the level of sound rock at the bottom of the under reamed hole. If it was found to be too short, the pile length was extended by welding on an additional section to the upper end as required.
All welding operations were performed with rigidly established and enforced procedures qualified in accordance with AWS D 2.0, Specification for Welded Highway and Railway Bridges or API No. 1104, Standard for Welding Pipe Lines and Related Facilities. Welds were of the full penetration type, using the manual metal arc process. A base metal preheat temperature of 250°F was applied.
DSAR-5.7 Information Use Page 8 of 17 Piling Rev. 1 Finally, upon assurance of the sufficiency of the pile length, pile driving was resumed for proper seating onto sound rock. Piles were considered as seated when movement under ten blows each equal to at least 55,000 foot pounds of energy (minimum required for Class A piles), was no more than one quarter inch. In actual construction, the piles were driven with a pile driver rated at approximately 57,000 foot-pounds of energy per blow. Refusal criteria for the piles was retained at ten blows per one-quarter inch.
By means of this pile installation procedure it was possible to investigate the condition of the bedrock at each individual pile and to apply appropriate corrective measures where necessary to ensure that each pile was properly seated on sound rock.
Plans and elevations of Class "A" piling installations are shown on Figure 5.7-1 and Figure 5.7-2.
5.7.1.3 Arrangement at Tops of Piles After completion of seating of the piles onto sound bedrock, the tops of piles were cut off to the proper elevation and ground as required to provide a true plane surface for seating of cap plates.
The cap plates are of suitable size for transfer of loading from the base mat concrete to the pile. The piles are embedded into the foundation mat for a length of three feet.
Detail of piling-to-base mat connection is shown on Figure 5.7-1.
5.7.2 Pile Loading Tests 5.7.2.1 General To confirm the appropriateness of the pile selected for supporting the containment and auxiliary building structures from the aspects of both feasibility of installation and load capacity, actual tests were performed on the various piles considered at the design stage.
After it became evident, by the encountering of cavities within the limestone bedrock, that conventional steel H piles were not appropriate, tests were conducted on the feasibility of utilizing steel pipe piles, driven open end and concrete filled. To this end, a number of such piles of potentially appropriate size and capacity were installed and investigated, under a pile testing program [See USAR Appendix D (historical)].
DSAR-5.7 Information Use Page 9 of 17 Piling Rev. 1 It became obvious during the course of this program that no dependence could be placed on the concrete fill within the pipe pile for any contribution to the pile capacity, because of the inability to satisfactorily remove the interior soils without affecting the surrounding soils and to adequately install the concrete for direct positive bearing on the bedrock surface.
As a result, it was necessary to select a pipe pile size of adequate capacity based upon steel cross sectional area only, without reliance on a concrete core.
As a final check, after completion of the subsequent soils densification operation, all piles were retapped to the refusal criteria of ten blows per one-quarter inch to ensure proper seating.
To eliminate any possibility of liquefaction occurring under Class I structures during the maximum hypothetical accident, the soil beneath these structures was densified to a relative density that will preclude this possibility.
After installation of the piling for the reactor building, auxiliary building and intake structure the in-situ sands between the piles were densified by vibroflotation. The pattern of vibroflot insertions was coordinated with the piling, the maximum spacing between insertions being on the order of six feet with the average somewhat less. The densification was performed from the level of the underside of the foundation mat to the top of rock and covered the entire area of the reactor building, auxiliary building and intake structure. The criterion used was that average relative density should be not less than 85% and the minimum not less than 70%.
After densification, a total of 83 borings were drilled into the compacted material to evaluate the vibroflotation results.
Standard penetration tests were performed at three feet vertical intervals in each boring and the relative density of the sand was determined in accordance with Gibbs and Holtz's correlation between relative density and spoon penetration resistance (Reference 5-13). If an individual boring indicated unsatisfactory results the extent of the unsatisfactory material was determined by drilling additional borings. All soils of unsatisfactory density were recompacted and additional borings were drilled to certify that adequate compaction was achieved. A statistical analysis based on 696 standard penetration test results indicates an overall confidence level of 96.6% that the average relative density for the entire area is not less than 85%.
DSAR-5.7 Information Use Page 10 of 17 Piling Rev. 1 5.7.2.2 Selection of Pile The pile size thus selected, investigated, and ultimately utilized for the foundation piling was 20 in. O.D. with 1.031 in. wall thickness as manufactured under the requirements of the American Petroleum Institute Specifications for Line Pipe, designated API Std. 5L Grade B (35,000 psi minimum yield strength).
DSAR-5.7 Information Use Page 11 of 17 Piling Rev. 1 5.7.2.3 Testing Procedure Two pile groups, I and IV, were installed at the bottom of a cofferdam constructed to the anticipated construction grade. The piles were then tested to evaluate their compressional, uplift and lateral capacities. Test piles 15 and 16 from Group IV were subjected to a compression loading of 650 tons representing approximately twice the maximum vertical load design capacity of the pile section in accordance with the requirements of the AISI Pile Foundation, Fourth Edition, 1963.
One pile experienced a total vertical deformation at the top of the pile in the order of 3/4 inch and a net settlement after removal of the load of 1/4 inch. The other pile indicated a total gross deformation of slightly over an inch and a net settlement of less than 1/4 inch. One of the piles was instrumented with strain measuring apparatus (tell tales), results from which indicated that less than 10 percent of the compressive load was taken in skin friction and the remainder in end bearing on the limestone bedrock.
After completion of compressive loading tests, an uplift test was performed on each pile. The first pile experienced a yield resistance to pull out of approximately 55 tons. The uplift test on the second pile revealed a total resistance to upward force on the order of 65 tons. These capacities were consistent with results obtained from uplift tests performed earlier on smaller concrete filled pipe piles.
Lateral load tests were performed by development and application of horizontal load to each of the piles by hydraulic jacking between the two test piles. This test did not attempt to duplicate the situation of the piles in actual construction, in which the embedment of the piles in the foundation mat creates a degree of fixity at the top of piles. It was recognized that this test simulating free head individual pile behavior would result in larger deflections per unit amount of applied load than for fixed headed piles.
However, the data derived therefrom were considered valid in confirming soil parameters developed during tests of piles performed earlier during the initial phase to the program, and pile displacements could be converted from free-ended to fixed-ended conditions.
The test of the free ended piles indicated lateral deflections at the
DSAR-5.7 Information Use Page 12 of 17 Piling Rev. 1 tops of the piles of from four to six inches at a horizontal load of 120 tons.
5.7.2.4 Conclusions The following pile design capacities and criteria were established on the basis of the data obtained from the pile loading tests:
- a.
Compression: Design capacity: 325 tons per pile.
Corresponding maximum pile vertical deformation:
one-quarter to one-half inch.
- b.
Uplift: Maximum ultimate uplift capacity has been assessed at 40 tons per pile. For design use this value was modified by a factor of safety appropriate to the nature of the application.
- c.
Lateral Load: Pile behavior was determined to be in accord with conventional lateral pile capacity theories up to the elastic limit of the pile-soil system. Beyond that point predictions regarding pile behavior were based on the data developed in the load test program.
Secant coefficients of horizontal subgrade reaction for use in foundation design are presented in Table 5.7-1. These coefficients were conservative in that they reflect data realized in the test program for subsurface conditions as then existing, whereas subsequently the entire soil block beneath the structure foundation was densified.
Table 5.7 Secant Coefficients of Horizontal Subgrade Reaction Allowable Deflection Coefficient nh*
(inches)
(lb/in3) 1/4 40 1/2 33 3/4 27
- See USAR Appendix D (historical)
DSAR-5.7 Information Use Page 13 of 17 Piling Rev. 1 5.7.3 Loading and Design Criteria The piles for the foundation under the containment and auxiliary building structures were designed for the loading conditions and combinations previously outlined in Section 5.5.2 (historical) for the containment structure concrete shell. The determination of pile size, number and arrangement was made on the basis of the most conservative requirements obtained by comparison of the results of two independent methods of design. The following criteria and methods were utilized:
- a. For the loading combinations given for working stress design, Section 5.5.2.2, the piles were designed in accordance with the basic formula, fa + fb 1 Fa Fb where: fa = computed axial stress fb = computed bending stress Fa = axial stress that would be permitted if axial stress alone existed, in accordance with AISI, Pile Foundations, Fourth Edition, 1963.
Fb = 60% of the specified minimum yield strength of the steel.
When wind or design earthquake loadings (vertical and lateral) are included, the formula was modified to the following:
fa + fb 1.33 Fa Fb
- b. For the factored load equations given for modified ultimate strength design, Section 5.5.2.3 (historical), and no loss of function design, Section 5.5.2.4 (historical) the maximum stresses permitted for the piles was the guaranteed minimum yield strength of the steel. The factor is 0.90.
DSAR-5.7 Information Use Page 14 of 17 Piling Rev. 1
- c. The soil reaction modulus (sometimes referred to as the coefficient of lateral subgrade reaction) was assumed to vary linearly with depth:
K = K0 + K1 X where: K = soil reaction modulus, psi X = depth The following values of K0 and K1 have been used for design:
K0 = 0; K1 = nh = 35 lb/in3 for design earthquake; K1 = nh = 17.5 lb/in3 for maximum hypothetical earthquake
- d. The difference-equation method for elastic pile theory was used for the determination of pile stresses.
No reduction in vertical pile load capacity due to group action was considered since all piles were driven to essential refusal on bedrock.
All lateral loads were assumed as resisted directly by the piles, and then transmitted to the soil block through the piles.
The tops of the piles were assumed to be restrained against rotation for design and analysis purposes by their embedment into the foundation mat.
The pile section design properties were based on the assumption of a 1/16 inch reduction in wall thickness as an allowance for corrosion. This has been introduced as an additional conservatism since the piles are protected against corrosion by a cathodic protection system (see Section 5.7.5).
DSAR-5.7 Information Use Page 15 of 17 Piling Rev. 1 Pile design loadings are shown in Table 5.7-2.
Table 5.7 Pile Design Loads*
Maximum Load per Pile (Kips)
Vertical Loading Summaries for the Following Combinations of Concurrent Design Conditions:
I.
Dead Load + Live Load + Post-Tensioning
+ Operating Temperature 360 II.
I + Accident Design Pressure + Design Earthquake 580 III.
I + Accident Design Pressure + Maximum Hypothetical Earthquake 610 Horizontal Loads Due to Earthquake:
Design Earthquake 44 Maximum Hypothetical Earthquake 68
- Loads for Post-Tensioning, Operating Temperature, and Accident Design Pressure are no longer applicable to the post defueled condition.
5.7.4 Seismic Considerations A related phase of work concerned investigation and application of appropriate measures to the soil beneath the plant foundations to ensure stability against liquefaction when subjected to seismic disturbance.
Preliminary studies of the soil in its initial undisturbed state indicated that there was a potential tendency for liquefaction to occur, and established the need for further investigation and development of appropriate criteria as guidelines. The criteria subsequently established dictated that to ensure against liquefaction of the soils for the seismic intensities postulated relative densities of 85 percent average with a 70 percent minimum were required.
Measurement of soil densities was made by means of standard penetration tests, and evaluation of observed blow counts were determined in accordance with data presented by Gibbs and Holtz (Reference 13).
Upon completion of the foundation piling installation, a check of the soil densities indicated that additional densification was necessary to meet the specified criteria. The Vibroflotation system was subsequently utilized to provide the necessary densification of the soil from the top of the bedrock to the underside of foundation to the specified values of relative densities.
DSAR-5.7 Information Use Page 16 of 17 Piling Rev. 1 5.7.5 Corrosion Protection Although preliminary chemical analysis performed on the soils and ground water at the site indicated that the sub-surface material is only slightly basic and its effect on embedded steel material would be insignificant, subsequent soil-resistivity investigation revealed that the underground environment could be mildly corrosive to buried, unprotected steel. If no precautions were taken it is possible that some metal loss could occur. Therefore, to ensure the integrity of the piles, a system of active, electrolytic corrosion protection was provided, and as an additional precaution a 1/16 inch corrosion allowance was included in the pile wall thickness.
Cathodic Protection Service of Houston, Texas, was engaged as consultants to review accumulated data, make necessary further tests, and design a comprehensive impressed-current system for protection of all steel, but with particular emphasis on retaining the full, structural integrity of the pile system.
The recommendations of that organization were followed in the design of plant and substation grounding systems to ensure compatibility with the corrosion protection system.
The number, size, and distribution of impressed-current anodes ensure the capability of supplying one milliampere, dc, to each square foot of surface of steel to be protected with no more than 50 percent anode weight loss in 40 years. To meet this requirement, a total of 416, 2-inch by 60-inch, high-silicon, cast-iron anodes were installed. Impressed-current anodes are buried in a surround of coke breeze.
Twenty-six zinc reference anodes were installed to permit periodic checks for system polarization and re-adjustment of anode-group currents to maintain proper operation. The containment liner, reinforcement, and tendon sheath steel are electrically interconnected to each other and to the piles.
The containment liner plate was coated on the exposed face with an application of 4 dry mils of Carboline Phenoline 305 over a 3 mil base coat of Carbozinc 11. The rear face of the liner plate is unpainted; concrete of the containment shell was poured directly against it and protects it against corrosion.
The tendon system was protected against corrosion after installation and stressing of the tendons by filling the tendon sheaths and caps with a corrosion preventative grease. The caps enclosing the end anchorage of the dome and wall tendons at the ring girder were protected by a corrugated aluminum siding enclosure.
DSAR-5.7 Information Use Page 17 of 17 Piling Rev. 1 Reinforcing steel of the reactor containment building, the reactor auxiliary building, and the mat were connected to the plant grounding system, the steel piles, and thus, if exposed to ground water are afforded the same cathodic protection as the piles.