Numerical Analysis of the Settlement of a Large Scale Nuclear Power Plant for Difficult Subsurface Conditions

Transcription

1 International Conference on Geotechnical Engineering Numerical Analysis of the Settlement of a Large Scale Nuclear Power Plant for Difficult Subsurface Conditions Erdem Onur Tastan, Ph.D.; Antonio Fernandez-Ares, P.E., Ph.D.; Diego Rivera Benard, M.S. Paul C. Rizzo Associates, Inc. ABSTRACT: The settlement analysis for the Calvert Cliffs Nuclear Power Project (CCNPP Unit 3) is a case history of particular interest due to the nature of the foundation soils and the challenges involved for the estimation of settlement and building tilt. The uneven topography on site results in uneven overburden stress distribution at a given elevation. The stress-strain behavior of the foundation soils during excavation and construction is expected to be impacted by the lack of symmetry, which, in turn, influences the structural tilt especially in the Nuclear Island (NI) common basemat. The subsurface layers, comprised of soils of different engineering properties, present variable thickness beneath the Powerblock Area. Assuming a one-dimensional horizontally layered model for the computation of settlement is unrealistic, since such model would neglect additional tilt introduced by variable thicknesses and soil properties underneath the foundation. Each building load is expected to influence the stress distribution and the settlement underneath neighbor buildings. Total settlement and tilt may be underestimated if the effect from neighbor buildings is not incorporated. The building loads are uneven as well and assigning a uniform pressure distribution throughout the Nuclear Island common basemat is not realistic. Furthermore, settlement will occur as construction progresses and the sequence of construction will play a major role on the settlement under each building. Finally, analyzing the behavior of soil and foundation displacements during excavation (heave), dewatering, and reloading is imperative. The settlement analysis of the CCNPP Powerblock Area was designed to provide an estimate of the time dependant settlement and heave distribution throughout the footprint, including maximum tilt for each building. The authors have developed a three-dimensional model capable of capturing irregular subsurface conditions, realistic foundation footprint shapes, and uneven building loads. The settlement simulation is time dependant incorporating a staged construction load sequence. 1 INTRODUCTION The CCNPP Unit 3 buildings in the Powerblock Area include the Reactor Building (RB), the Fuel Building (FB), Safeguard Buildings (SGB 1, SGB23, and SGB4), Essential Service Water Buildings (ESWB), and Emergency Power Generation Buildings (EPGB). Other important buildings in the Powerblock Area are the Nuclear Auxiliary Building (NAB), the Radioactive Waste Processing Building (RWPB), the Access Building (AB), and the Turbine Building (TB). In the Powerblock Area, ground surface elevations at the time of the exploration ranged from approximately El. 15 m to El. 37 m, with an average of El. 26 m. The planned elevation (rough grade) in the Powerblock Area ranges from about El. 23 m to El. 26 m, with the centerline of Unit 3 at El m, or approximately El. 26 m (Figure 1). Figure 1 shows a three-zone subdivision of the site that follows

2 different surface topography elevation levels. This paper summarizes the settlement analysis conducted for the Powerblock Area. B A BUILDINGS A B Figure 1. Site layout and elevation contours 2 SITE LAYOUT The area considered in the computer model is 762 m by 762 m. The area occupied by the buildings is 335 m by 335 m. A schematic plan view of the site geometry is shown in Figure 1. 3 SITE GEOLOGY/SOIL PROFILE The site geology is comprised of deep Coastal Plain sediments underlain by bedrock, which is about 762 m below the ground surface (BGE, 1982). The site soils consist of marine and fluvial deposits. The upper 122 m of the site soils were the subject of the CCNPP Unit 3 subsurface investigation. In general, the soils at the site can be divided into the following stratigraphic units: Stratum I: Terrace Sand light brown to brown sand with varying amounts of silt, clay, and/or gravel, sometimes with silt or clay interbedded layers. Stratum IIa: Chesapeake Clay/Silt light to dark gray clay and/or silt, predominantly clay, with varying amounts of sand. Stratum IIb: Chesapeake Cemented Sand interbedded layers of light to dark gray silty/clayey sands, sandy silts, and low to high plasticity clays, with varying amounts of shell fragments and with varying degrees of cementation. For the purposes of settlement analysis, Stratum IIb was further divided into three sub-layers. The investigation encountered variation of SPT values both in depth and horizontal distribution. The position of the sub layers beneath the Powerblock Area footprint is variable and this condition needs to be accounted for in a detailed three dimensional settlement analysis. Stratum IIc: Chesapeake Clay/Silt gray to greenish gray clay/silt soils, with interbedded layers of sandy silt, silty sand, and cemented sands with varying amount of shell fragments. Stratum III: Nanjemoy Sand primarily dark greenish-gray glauconitic sand with interbedded layers of silt, clay, and cemented sands with varying amounts of shell fragments and varying degrees of cementation. Information from 20 boring logs were used to input the soil profile into the computer model. Cross sections AA and BB (Figure 1) going through the nuclear island are shown in Figure 2.

3 4 METHODOLOGY/COMPUTER MODELS Plaxis 3D Foundation v2 (3D load-deformaton analysis software utilizing finite element method) was used to estimate the settlements and tilt on site. Two separate sets of models were used: 1. An excavation and dewatering model (ED Model) to estimate heave due to excavation and dewatering; and 2. Construction and post-construction models (PC Models) to compute settlement during and after building construction. 15-noded wedge elements were used in the analysis. The model depth is approximately twice the size of the foundation width. The exploration depth of the borehole program extended down to El. -95 m. However, given the dimensions of the NI common base mat, the model depth was extended to El m to accomodate footprint dimensions. It is necessary to assume that the deepest Nanjemoy sand (the continuous soil layer deeper than -63 m elevation) extends down to the bottom end of the model based on the best information available. 4.1 Excavation and Dewatering (ED) Model The ED model was designed to evaluate heave and settlement in the Powerblock Area between the end of excavation and beginning of construction. For the ED model, the CCNPP Unit 3 area was subdivided in three zones considering different average ground elevations for each zone. The subdivision was performed based on site topography, as shown in Figure 1. The average ground elevations for these zones are 18 m for Zone 1, 24 m for Zone II, and 32 m for Zone III. Since the model does not consist of only the Powerblock Area, the surrounding area of the Powerblock Area was also divided into three separate zones. For Zone III, the ground surface is modeled at El. 24 m, and the overburden effect between El. 24 m and El. 32 m is modeled with a loading of 142 kpa. This stress was calculated based on the average moist unit weight (18.6 kn/m 3 for CCNPP Unit 3 area) for Terrace Sand and Chesapeake Clay IIa layers. Based on available information, the ground water level in CCNPP Unit 3 Powerblock and construction laydown areas (CLA) ranges from approximately El. 18 m to El. 26 m with an average of El. 32 m. For this analysis, the groundwater table was considered to be at El. 18 m in Zone I, and at elevatation El. 32 m in Zones II and III. This groundwater table is associated with the surficial aquifier. The surficial aquifer rests on the the upper Chesapeake aquitard, which separates it from the lower Chesapeake aquifer. Two different aquifers are present on site. Therefore, two separate groundwater tables exist, one associated with the surficial aquifer, and another one associated with the Chesapeake aquifer. The deeper groundwater table for the Chesapeake aquifer varies between El. 10 m and El m. The groundwater table for the Chesapeake aquifer was assumed to be at El m. The ED model simulated the excavation process down to the El m, the foundation elevation of the Nuclear Island Common mat. Therefore, a total of 5.6 m, 11.7 m, and 19.4 m of soil will be excavated in zones I, II, and III, respectively, for the total area of 335 m by 335 m resulting in the removal of approximately 1.5x10 6 m 3 of material. 4.2 Post-Construction (PC) Models The PC model was designed to evaluate the settlements during and after construction of buildings. The excavation and dewatering stages included in ED model were assumed to be completed. Excess pore pressure generated due to excavation and dewatering is fully dissipated. Also, the ground surface was assumed to be re-leveled after the immediate settlement and heave. The initial stage for PC model had the ground surface at El m. Initial effective stress conditions for the PC model is consistent with the post-excavation overburden geometry. The PC and ED models were independant, and the initial stress configuration of PC model did not use the results from ED model.

4 Figure 2. Cross sections A-A and B-B including the foundation profiles (see Table 1 for building names) The building loads were applied in a sequential manner (8 steps) as indicated by Figure 3. During the application of each load, consolidation analysis was conducted simultaneously with the estimation of immediate settlements. Backfill was placed between El m and El m during Step 1, between El m and El m during Step 2, between El and El m during Step 3, between El m and El m during Step 4, and between El m and El m during Step 5. The developed PC model is shown in Figure 4. During construction, groundwater elevation in the Powerblock Area is El. 116 m. Outside of the Powerblock Area, the assigned average groundwater elevation is El. 21 m. For post-construction conditions, groundwater elevation in the Powerblock Area was considered to increase up to El m in 20 years and remains constant at that level, while the groundwater elevation outside the Powerblock Area remains at El. 21 m. 4.3 Properties of Soil and Structural Elements Retaining walls were placed in the model around the buildings with the sole purpose of eliminating undesirable stresses and failure due to slope stability. The effect of having these walls on the vertical settlement around the site is negligible. The vertical displacement pattern observed underneath the buildings heavily depended on the stiffness of the foundation. As the construction proceeds, the displacement pattern of the foundations is expected to be closer to the rigid body motion due to the incorporation of structural walls and beams above. Therefore, in this analysis, the foundation stiffness was increased as a function of time in correspondence with the loading sequence (reflecting the construction sequence). Three-step increase in stiffness was incorporated into the analysis (some building foundations have only two-step stiffness increase): Configuration A: Thickness, d, = 1.8 m, E = 27.8 GPa, ν = 0.15, no reinforcing walls, applied until the end of 2nd loading step.

5 Configuration B: d = 3.1m, E = 957 GPa, ν = 0.15, no reinforcing walls, applied until the end of 3rd loading step. Configuration C: d = 3.1 m, E = 957 GPa, ν = 0.15, with reinforcing walls, applied from the beginning of the 4th loading step until the end of last loading step. Reinforcing walls were considered only for the nuclear island common mat, and consist of four walls: two extending in north-south direction and two extending in east-west direction as shown in Figure 4. Foundation Pressure (kpa) 1, Reactor Building, RB Fuel Building, FB Safeguard Building 1, SGB1 Safeguard Buildings 2&3, SGB23 Safeguard Building 4, SGB4 Nuclear Auxiliary Building, NAB Access Building, AB Radioactive Waste Processing Building, RWPB EPGB ESWB Turbine Building, TB Time (days) Figure 3. Building loads as a function of time The soil properties are provided in Table 1. The hydraulic conductivity for various layers were determined based on field, laboratory, and analytical hydrogeologic investigations. The analysis was conducted based on effective stress failure parameters. Mohr Coulomb failure model was adopted for all soils. Consolidation analysis and Mohr Coulomb failure were simultaneously adapted (or coupled) in the model. The modulus E was the average of modulus obtained from shear wave velocity measurements, pressuremeter measurements ( initial modulus according to Menard method), SPT measurements, and undrained shear strength measurements. Unloading and reloading deformation moduli of soils, E r, were implemented for loads that are above the initial overburden load (before excavation). The E r was determined based on E r / E obtained from pressuremeter tests and E determined using the aforementioned methods. Table 1. Soil properties LAYER Unit Weight kn/m 3 K x or z cm/s (1) K y cm/s (1) c' kpa φ' o Figure 4. PC model E r MPa E MPa ν' Loading Condition (2) Backfill (6) E E Drained IIb - Chesapeake Cemented Sand (1) E E Drained IIb - Chesapeake Cemented Sand (2) E E Drained IIb - Chesapeake Cemented Sand (3) E E Drained IIc - Chesapeake Clay/Silt E E Undrained IIc - Chesapeake Clay/Silt - Sand E E Drained II - Nanjemoy Sand E E (3 ) Drained (1) K x or z = hydraulic conductivity in horizontal planes, K y = vertical hydraulic conducitivity. (2) Conditions indicating pore pressure evaluation during analysis. (3) Estimated based on SPT blow counts.

6 For the ED model, E r was incorporated during the complete excavation simulation. For the PC model, the E r modulus was used only up to a given loading step while foundation imposed stresses are still below the initial stress before excavation. After the initial conditions were reached due to the application of building loads,the settlement simulation continued with the use of the primary modulus E. Preexcavation topography on the Powerblock Area suggests that the overburden stress is lower on the North-East. At this location, in Fuel Building and Nuclear Auxiliary Building, loading will reach the initial overburden stress condition sooner in the construction process. Other portions of the Powerblock Area will reach initial stress conditions at later stages. E r should be used up to the end of the reloading, and, E should be used after the end of the reloading. The initial ground surface elevations vary between El. 15 m and El. 37 m in the Powerblock Area. Therefore, the reloading may last until the end of 2 nd loading step or the end of 5 th loading step depending on the location of interest in the Powerblock Area. To adequately simulate the effect of initial topography four different cases were considered. Case 1: E r was used until the end of the second loading step (low topography model, LT model). Case 2: E r was used until the end of the third loading step (medium topography model 1, MT1). Case 3:E r was used until the end of the fourth loading step (medium topography model 2, MT2). Case 4: E r was used until the end of the fifth loading step (high topography model, HT). By performing the settlement analysis under the previous scenarios, it is possible to assign the most representative case for each point throughout the foundation footprint, and to obtain a reliable estimate of the increase of tilt for each structure, specifically the NI. The stiffness of the deepest Nanjemoy sand layer was increased linearly. The stiffness of dense sands is expected to increase by depth. Determination of such a rate was based on soil-hardening model, which describes the stiffness change as a function of the confining stress. 5 RESULTS 5.1 ED Model: Figure 5 presents the sum of immediate heave and heave due to de-consolidation or dissipation of negative excess pore pressures when the soils beneath the Powerblock Area are excavated down to El m. The maximum immediate heave due to excavation is 14 cm. Total heave is 17 cm. Figure 5 shows the uneven heave pattern around the safety related buildings due to irregular initial surface topography. The maximum de-consolidation related heave is 3 cm as shown in Figure 5. Additional deconsolidation analysis indicated that more than 99 % of the expected total heave takes place during a 6 month period that starts after the end of the excavation. 5.2 PC Models: The cross sections analyzed in terms of settlement and tilts are shown in Figure 6. In total 15 sections and 88 discrete points are monitored during the simulation. Settlements at these locations were used to compute tilts. Estimated settlement at the center of each building as a function of time is shown in Figure 7 for the MT2 model. Even though the LT model results in larger settlements than those shown in Figure 7, the MT2 model is considered to be a better approximation of the existing site surface topography. Settlements at the end of the 8 th loading step are obtained from all models and combined results are shown in Figure 8. As expected, MT2 results provide an average settlement for most buildings. According to MT2 model, the highest settlements (31 to 33 cm) take place underneath the NI, FB, SGB4, and NAB. The most significant settlement occurs beneath the FB. The lowest settlements occur beneath ESWBs. Uniform heave (rebound) of 3 cm is expected due to groundwater recharge in the powerblock area from

7 Figure 5. Total heave after excavation Figure 6. Sections analyzed for tilt SETTLEMENT [ cm ] BUILDING RB FB SGB1 SGB23 SGB4 NAB AB RWPB ESWB1 ESWB2 ESWB3 ESWB4 EPBG1 EPBG2 RB FB SGB1 SGB23 SGB4 NAB AB RWPB ESWB1 ESWB2 ESWB3 ESWB4 EPBG1 EPBG Settlement (cm) LT MT1 MT2 HT Combined Step 8 Step 7 Step 6 Step 5 Step 4 Step 3 Step 2 Step 1 50 REACTOR FB SGB1 SGB23 SGB4 NAB AB RWPB ESWB1 ESWB2 ESWB3 EPGB1 ESWB4 EPGB2 50 Figure 7. Settlements for each building according to MT2 model Figure 8. Settlements for each building at the end of 8th loading step according to all four PC models and also combined results of all models El m to El m. Since rewatering takes place during a lapse of 20 years, no more additional consolidation is expected. The creep, or the secondary compression, is expected to be negligable based on the overconsolidated nature of the clay layer and the magnitude of loads applied. The amount of settlement at the center of nuclear island is shown in Figure 9 for all four models. The final settlements range between 25 cm and 42 cm. The MT2 model (average or the best estimate model) predicts 32 cm of settlement under the center of NI at the end of load step 8. Figure 10 shows the progression of settlement throughout the 8-step loading and rewatering stages. Up to the end of fourth stage, the effect of backfill placement is distinctly reflected and settlements are more uniform throughout the Powerblock Area. Applied loads do not induce a distinguishable settlement pattern up to the end of Step 4 (500 days). However, after Step 5 (800 days), the effect of building loads on settlement pattern becomes more pronounced compared to the effect of backfill load. Tilt becomes evident in the direction of SGB23 and SGB1. After Step 7 (1400 days), tilt pattern changes, and the nuclear island common mat starts tilting towards NAB. After Step 8, tilt is still towards NAB, but the settlement throughout the nuclear island common mat becomes more uniform. Also seen in Figure 10, is the more significant effect of nuclear island loading on the 1EPGB compared to 2EPGB, and on the 2ESWB compared to 3ESWB.

8 DAYS HT MT1 MT2 LT LONG TERM SETTLEMENT [ cm ] CONSTRUCTION BEGINS Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Step 7 Step 8 Figure 9. Settlement at the center of NI according to four models Figure 10. Progressive settlements according to MT2 model When analyzed separately, each of the four models considered is capable of capturing tilt due to nonuniform load distribution and non-uniform subsurface conditions. However, each model alone is not sufficient to address the effect that initial surface topography has on the amount tilt. The followed approach to overcome this difficulty was to determine which model would best describe the stiffness variation over time for a point underneath each one of the 88 locations at El m. This elevation was chosen as a reference elevation, since it corresponds to the bottom of excavation and base of foundation on the Powerblock Area. To match each location with a particular model, the results from the four models were combined as follows: 1) Surface elevations for the 88 locations analyzed were determined from initial site topography. 2) Initial overburden stresses were determined for 88 locations at El m using an average overburden unit weight of 18.5 kn/m 3 and groundwater table elevation of 24 m if the point of

9 interest has an elevation between 18 m and 32 m, and 18 m if the point of interest has an elevation less than 18 m. 3) Using the results of the LT model, the step for which the applied structural induced stress reaches the initial overburden stress before excavation was determined for each point. 4) Each point is matched with one of the models based on the load step determined in Item 3. The four models represent the deformation modulus reduction at the end of 2 nd, 3 rd, 4 th, and 5 th loading steps. This approach results in conservative estimates of tilt, since the foundation elements may seem to deform unrealistically. Unrealistic deformation pattern may emerge since two or more points across the same foundation element are analyzed using different models. In reality, the foundation stiffness is expected to compensate some of this additional tilt due to initial surface topography effect. However, the approach does provide a reliable upper bound threshold of expected building tilt. An important aspect in tilt computation is the construction sequence. The construction of some buildings such as ESWBs do not start until some deformation already takes place on site. Ideally, the foundation base for these buildings is releveled before the construction begins. In other words, tilt computations for these buildings should not consider the deformations observed before the foundation works started. An example of this correction is for ESWB, where the tilt observed at the end of the fourth stage (just before the foundation construction for ESWBs starts) was subtracted from the later stages (fifth, sixth, etc.), since the computer model does not reset the displacements under ESWB foundation to zero at the moment building foundation construction begins. Maximum corrected (for construction sequence) and uncorrected tilts are shown in Table 2 for all buildings as determined by combining the results from the four models. The combination of results represents the effect of surface topography. Sections AA and BB are influenced by the surface topography effect significantly, which have maximum tilts of 1/1017 and 1/531, respectively. As mentioned before, these numbers were obtained using the results from different models, and the effect of foundation stiffness is expected to lower this tilt, i.e., these numbers represent the upper bound for the tilt. Section CC has a tilt towards the SGB23 according to MT2 results (Figure 10). When the surface topography effect is considered, this tilt direction is reversed through NAB. In other words, the loads and subsurface profile induce a tilt in the opposite direction compared to the tilt induced by the surface topography effect for section CC. For both sections CC and DD, the estimated tilt is below 1/3000. Section CC in Figure 6 represents the combination of NI and NAB, while Section DD represents the combination of NI and AB. If Sections CC and DD are considered for only NAB and AB buildings, then, the tilt is well above 1/600 independent from the model used, even after the construction sequence correction is applied. Table 2. Tilts determined for all buildings across the sections shown in Figure 6 Building Name Section Label Tilt Uncorrected (1) Corrected (1) Tilt Direction AA 1/1017 1/1017 North Nuclear Island BB 1/531 1/531 East CC 1/3529 1/3529 South West DD 1/5000 1/5000 North West NAB CC 1/435 1/423 South West AB DD 1/339 1/351 South East ESWB1 EE 1/923 1/1364 North West FF 1/1071 1/1053 North East ESWB2 GG 1/2400 1/1935 North East HH 1/682 1/833 North West ESWB3 II 1/ /3529 South West

10 ESWB4 EPGB1 EPGB2 JJ 1/2143 1/4615 South East KK 1/4286 1/2143 North East LL 1/488 1/1304 South East MM 1/1463 1/3750 South West NN 1/779 1/1224 South East OO 1/1091 1/1429 North East PP 1/2143 1/4286 North West (1) Correction to subtract the observed tilt before the construction of the building. 6 CONCLUSIONS A detailed and realistic FEM analysis has been implemented to simulate settlement of a complex power generation station founded on compressible soils underneath an irregular surface topography site. The simulation was performed with PLAXIS3D Foundation software, which proved to be adequate for the problem at hand. The total average settlement at the end of construction beneath the centerline of the footprint is estimated at 32 cm. Settlement at center point is estimated at 25 cm and maximum settlement will occur beneath the Fuel building and is estimated at 38 cm. Maximum tilt for each building is estimated by incorporating the three dimensional nature of the problem, with an accurate representation of the subsurface conditions and the building foundation geometry. The settlement simulation adequately captures the interaction between buildings, especially the effect that the high loaded buildings have on adjacent, smaller facilities. Differential settlement or tilt depends on the uneven nature of loads, the irregular thickness of the subsurface strata, and on the uneven nature of surface topography. The first two aspects, load asymmetry and irregular subsurface conditions, are naturally captured by the FEM simulation. The third aspect, influence of uneven topography, is captured by means of a sensitivity analysis that involves four cases with different ground surface elevation. The analysis indicates that surface topography has potential of altering the estimate of differential settlements across the footprint. This study indicates that tilt may be increased as much as 100%. The limitations of the applied methodology suggest that such increase is conservative, but a good indicator of an upper bound threshold. To overcome this limitation it is necessary to elaborate a slightly more realistic representation of the initial surface topography by adjusting the initial stress conditions and allowing for an automatic assignation of unload/reload conditions. This work is currently in progress. REFERENCES BGE Updated Final Safety Analysis Report, Calvert Cliffs Nuclear Power Plant (Units 1 and 2), Docket and , Calvert County, Maryland, Baltimore Gas and Electric Company, Baltimore, Maryland [Report] Brinkgreve, R.B.J., Swolfs, W.M PLAXIS 3D Manual Version 2. Plaxis BV, Netherlands.