ASSESSMENT AND MITIGATION OF LIQUEFACTION RISK FOR EXISTING BUILDING FOUNDATION

Transcription

1 ASSESSMENT AND MITIGATION OF LIQUEFACTION RISK FOR EXISTING BUILDING FOUNDATION Rolando P. Orense 1, Yukio Morita 2 and Masanori Ide 3 ABSTRACT This paper outlines the mitigation measures performed to address the liquefaction concerns of the foundation ground underneath a manufacturing plant site in Nagoya, Japan. Initially, a detailed soil investigation was performed and the liquefaction potential for the specified ground motion was evaluated. Considering the various site restrictions, compaction grouting was chosen as the most practical method to strengthen the foundation ground. Test acceptance criteria based on the relation between target N-values and fines content were established. In addition, dynamic soil-structure interaction analysis was performed to confirm the adequacy of soil improvement. This paper discusses the design and lessons learned from the grouting process and highlights the merits of compaction grouting as a practical method of ground improvement for seismic risk mitigation under existing structures. INTRODUCTION Buildings and other civil engineering structures constructed over loose saturated sandy deposits are highly susceptible to liquefaction-induced damage during major earthquakes. To mitigate such damage, remedial measures should be implemented around or under existing structures without adversely affecting them. In selecting the appropriate remedial measures for constrained and/or developed sites, various factors should be considered. These include effectiveness of improvement, required areas and depth of improvement, effects on surrounding environment, cost and ease of execution, and level of desired improvement, among others. In some instances, two or more remedial measures are combined t o produce the desired results. Because of its versatility and economy in improving ground beneath and around existing facilities, compaction grouting is gaining interest among engineers. Originating in the U.S., compaction grouting technology has been implemented in Japan only in the last ten years. Although initially developed for settle-ment control and re-leveling, the technology has been used to solve a number of geotechnical problems, among them the treatment of liquefiable soils. This paper discusses the application of compaction grouting as a remedial measure against liquefaction, with emphasis on the lessons learned from the grouting process and the merits of compaction grouting as a practical method of ground improvement under existing structures. Note that seismic retrofitting was also performed on the superstructure itself, but the results are not discussed herein. SITE CONDITION AND LIQUEFACTION POTENTIAL EVALUATION The structure concerned is a 3-storey manufacturing plant located in Nagoya City, Japan, referred to as Plant No. 1, and it occupies an area of 33.6mÅ~27.3m. Plant No. 1, which was built in early 196 s, is sandwiched by two other relatively new plants which are comparatively more earthquake resistant. The three plants are contiguous with each other, and are connected by passageways. Plant No. 1 is supported by 6m-long 2mm-diameter friction pile foundations. The ground surface at the site is generally flat. Borings were initially conducted at four sites located just outside the plant, and the results showed that the foundation ground generally consists of 3m-thick backfill material and clayey soil underlain by about 1m thick loose saturated sand and silty sand layer with low s, generally less than 1. 1 University of Tokyo, Hongo, Bunkyo-ku, Tokyo , Japan (formerly, Kiso-jiban Consultants Co. Ltd.) 2 Kiso-jiban Consultants Co. Ltd., Kudan-kita, Chiyoda-ku, Tokyo , Japan 3 Sanshin Corporation, Kouraku, Bunkyo-ku, Tokyo 112-4, Japan

2 A hard deposit containing mixture of sand and gravel (N-values between 3~) lies about 12~13m from the ground surface. The water table is high, close to 2m from the ground surface. No.1 No.3 No.4 No.2 Depth (m) F,k F,k F,k F,k Depth (m) m Figure 1 : Estimated liquefied layer (based on AIJ Code: input PGA=4 ) For the purpose of this study, the peak ground motion at the site for an event with a 1% probability of occurrence in years was set at 4. This ground motion estimate was developed by studying known faults and past earthquake history of the site. Next, liquefaction potential evaluation was performed at the site using the methodology incorporated in the Recommendations for the Design of Building Foundations (AIJ, 1988). The results of the analysis showed that the sand and silty sand layers between GL-m~-12.m have high liquefaction potential for the specified design acceleration, and the estimated liquefiable layer is shown in Figure 1. Since the plant has friction pile foundation which terminates at the mid-depth of the loose sand layer, significant damage to the structure is expected when the foundation ground liquefies during earthquake shaking. Hence, it was necessary for the ground to be strengthened. The selection of ground improvement technique to be applied on the site was constrained by the following: (1) minimum disturbance to plant production and operation; (2) limited space clearance (vertical and lateral) within the plant; (3) some equipment will not be moved during the implementation; and (4) ground heave should be limited to ~mm, depending on the location. Considering these factors, compaction grouting method was chosen as the method to best improve the site. COMPACTION GROUTING Compaction grouting involves the injection of a very stiff grout (soil-cement-water mixture with sufficient silt sizes to provide plasticity, together with sand and gravel sizes to develop internal friction) that does not permeate the native soil, but results in controlled growth of the grout bulb mass that displaces the surrounding soil. The primary purpose of compaction grouting is to increase the density of soft, loose or disturbed soil, typically for settlement control, structural re-leveling, increasing the soil s bearing capacity, and mitigation of liquefaction potential. A grout pipe is advanced into the ground to the Bottom-up Approach Drilling Grout Injection Step-up Injection Finish Figure 2 : Compaction grouting implementation

3 maximum treatment depth and the grout is injected at high pump pressure as the casing is withdrawn incrementally, thus forming a column of interconnected grout bulbs, as shown in Figure 2. At each stage, the soil particles are displaced radially from a growing bulb of grout, into a closer spacing, thus increasing the density of the adjacent soil around the bulb. Note that the strength of the grout is unimportant because the purpose of the technique is to densify the surrounding soil by displacement. Compaction grouting can either be performed top-down, i.e., from the upper to the lower limit of the treatment zone or, more commonly, in a bottom-up process from the lower limit upwards. COMPACTION GROUTING IMPLEMENTATION For the purpose of compaction grouting implementation, the site was divided into 3 zones. A test program was initially conducted on the first zone as part of the initial production grouting program to evaluate the effectiveness of the grouting and to refine the guidelines for production. In the design, a typical soil profile and the relevant soil properties representing the center of the plant were inferred from the four boreholes performed outside the plant. Since the mechanism is similar, the Japanese standard design practice adopted for Sand Compaction Pile method was adopted for the compaction grouting method. First, the minimum and maximum void ratios for each soil layer in the site were estimated based on published ranges for similar soils. Estimates were then made on the in-situ relative density (Dr) based on correlation between Dr and Standard Penetration Test (SPT) N-values. Target s corresponding to a Factor of Safety against Liquefaction, F L >1. were calculated, and the target Dr at various depths were computed. From these values, the replacement ratio, volume of grout, layout and pitch were determined. In general, a square grid pattern of 2.1m pitch was selected, except in areas where the equipment was not moved and a pitch of 1.4m was employed. Table 1 presents the required grout volume at various depths of the deposit (for Type 1), while Figure 3 shows the layout of the grout points within the plant. Other design Depth (m) Table 1 : Design parameters Pitch Area (m) (m 2 ) Rep. Ratio, a s (%) Q* (m 3 /m) ~ ~ ~ ~ *contraction coefficient=.9 N Type 3 Grout Lay- out No.1No.3 Profile Type 2 Type Pri mar y Gr o ut To p - Do wn Seco ndary Grout Botto m - up ƒó 2,k??6 1 1?K (unit :mm) parameters, such as pumping rate and injection pressure, were also evaluated. Figure 3 : Layout of grout points During the compaction grouting implementation, it was necessary to control the operation to ensure that the ground heave associated with the grout injection was within the acceptable limits set for

4 the equipment and other accessories which were not moved during the operation. These limits are as follows: mm in sections with important machinery (Type 2); mm outside the plant (Type 3); and 3mm elsewhere (Type 1). For this purpose, the surface heave of the first floor slab was monitored using automatic leveling survey equipment. Grouting was performed only in existing open space. In case where the grout point was located at a site where an immovable equipment was present, inclined grout holes were used to penetrate under the said equipment with the maximum amount of inclination of about 1 degrees off vertical (see grout profile in Figure 3). Large equipment needed for the operation, including the grout mixing plant and pumps, were placed outside the manufacturing plant; hence, there was minimum disturbance to the structure and surrounding ground during the implementation. Moreover, a top-down, bottom-up approach was employed. This involved injecting primary grouts at the upper 1m-portion of the deposit to be treated, after which secondary grouts were injected from the lower to the upper limit of the treatment zone. This procedure is more effective than the usual bottom-up and top-down techniques mentioned earlier since prior grouting in the upper stages increases the strength of the overlying soils, which increases their restraining capability on the underlying soils. Treatment interval was set at 1m stages, and grouts were allowed to harden after each stage before advancing to the next stage. TEST ACCEPTANCE CRITERIA To evaluate the level of densification at each zone, confirmation borings (post-spt s) were performed in at least two locations within each zone. Considering the spatial variability of soil properties, simply comparing the measured N-values with the target N-values (pre-calculated such that F L >1.) evaluated at the center of the plant may lead to erroneous judgment regarding liquefaction potential. Rather, the target s that should be used must reflect the effect of effective overburden pressure and fines content on the liquefaction potential. For this purpose and with the AIJ Code as basis, plots showing the relation between Target s and fines content (Fc) were formulated for three levels of the deposit, as shown in Figure 4. During each confirmation boring, samples were obtained at each 1m-depth within the improved zone and the fines content (Fc) determined in the laboratory. Then Figure 4 was used for quick evaluation of compaction grouting effectiveness at the site. Any point which plots above a specified line indicates that the for the given Fc is greater than the target N-value for that level of acceleration, and therefore, liquefaction would not occur; conversely, when the point plots below the line, liquefaction potential is high for the specified acceleration level. The following test acceptance criteria were adopted to confirm the effectiveness of the compaction grouting implementation at the site: (1) target s should be those required for 4 ; (2) no below those required for 2 ; and (3) for each boring location, no more than one point should lie below those required for 3. If after compaction grouting implementation, the s were found to be below the specified limits, remedial measures should be performed to meet the above criteria. GL-.m~-7.m GL-7.m~-1.m Figure 4 : Adopted test acceptance criteria showing the relation between target N-values and fines contents GL-1.m~-12.m

5 ASSESSMENT OF EFFECTIVENESS Once 16 grout points have been installed at each zone, confirmation borings were conducted at the center of the pattern where sufficient confinement is provided. Such post-densification tests were performed in at least two points within each zone, at a depth between GL-m~-12.m, where treatment had been applied. First Zone The first zone is located on the northwestern section of the site. Compaction grouting was applied on this zone using the design volume of grout mentioned earlier and summarized in Table 2. Confirmation borings were performed at two locations within the zone and all the data points plot above the 4 line for all depths in the test acceptance criteria shown in Figure 4. Hence, the first zone passed the test acceptance criteria. Second Zone The second zone consists of the eastern half of the site. A design volume of grout similar to Zone 1 was employed. As in the first zone, when 16 grout column grids were installed, check borings were performed at the centers of such grids. Although one of the two sites passed the test acceptance criteria, the other site failed, with two points below the 3 line and one point between 3-4 line. The locations of these points are between GL-7m~-11m. Since this was not acceptable per the test acceptance criteria, remedial measures were performed. For this purpose, additional grout holes, denoted by Type 4 in Table 2, were added in between existing grout columns in the area near the section which failed, resulting in 1.m over-all pitch. In the remaining sections which have not been treated yet prior to the check borings, revised volumes of grout were used such that the replacement ratio at depth GL-1m~-12.m was increased from 3% to 9%. These are summarized as Type 1 and 3 in Table 2. After the whole zone has been completely treated, confirmation borings were again performed at two locations, and both passed the test criteria. Third Zone The third zone is located on the southwestern section of the site. Based on the experience in Zone 2, there was a dilemma whether to adopt the revised design of Zone 2, or the original design of Zone 1. To account for zone variability, a compromise was made wherein the replacement ratio at the depth of GL-1m~12.m was increased. The new design volumes are denoted by Types 1, 3 and 2 in Table 2. Check borings at two sites subsequently confirmed that Zone 3 passed the acceptance criteria. Table 2 : Volume of grouts (m 3 per m depth) used in each zone Zone 1 Zone 2 Zone 3 Depth Type Type Type Type Type Type 2 Type 4 1,3 1,3 1,3 1,3 2 m~7m m~1m m~12. m Over-all Assessment A comparison of the pre- and post- SPT s is shown in Figure. The lone check boring in Zone 2 which did not satisfy the test criteria was excluded. Note that the check borings within the plant all showed increase in N-values beyond those required to prevent soil liquefaction in the site. In addition, the plots showing the relations between target s and Fc are summarized in Figure 6, where it can be seen that, except for one, all the data points plot above the 4 line, indicating that based on the AIJ code, the improved ground would not liquefy for this level of surface acceleration.

6 (m) BHNo.1-1 BHNo.1-2 As mentioned earlier, SPT s were performed at the center of the grid pattern, where the degree of improvement is considered least. Moreover, compaction grouting undoubtedly increased the lateral earth pressure at the site. Hence, considering the target N-values obtained from the AIJ Code, it can be said that the improved ground has adequate factor of safety against liquefaction for the specified level of surface acceleration. In addition, although not quantified in this project, the hardened grout columns have beneficial contributions to the seismic behavior of the site. Note that sand with Fc<2% are the most susceptible t o improvement by compaction grouting. On the other hand, significant improvement in silty soil is not expected due to its 1 BHNo.2-3 BHNo.2-4 BHNo.3-1 BHNo.3-2 Before Target low permeability and partially undrained response during compaction grouting. However, the comparison of plots before and after grouting generally showed an increase in N-value even in the silty sand deposit (GL-7m~-11m), as high as 3~4 in some points. A possible explanation for this is that when grout bulbs were attempted to be expanded in the silty soils, they were more likely to simply be displaced (not densified); the SPT rod hit the displaced grout rather than the densified soil, resulting in GL-.m~-7.m Figure 6 : Plot of target N-values vs. fines content, Fc GL-7.m~-1.m such high N-values. Hence, N-values from SPT s that encountered hardened grout (say, greater than 3) should not be included in assessing the effectiveness of ground improvement. With regards to surface deformation, almost negligible surface heave was measured in the location of important machinery (Type 2), while a maximum heave of 2mm was measured elsewhere (Types 1 and 3). In addition to the controlled grout injection, it is believed that the m-thick untreated surface soil provided an additional overburden cover, and therefore, grouting effects on the surface structures as well as on surface heave, are minimized. Sensitivity check conducted on the unmoved equipment after the whole grouting program showed no effect of the grouting implementation. Over all, a total of 27 grout holes were installed within the site. The total amount of grout injected was m 3. An efficient scheme was formulated through careful scheduling of the various phases of work involved (removal of machines and pipes, compaction grouting implementation, confirmation borings, re-installing of machines, clean-up operations, etc.) to ensure that plant remained in operation during most of the retrofit construction. DYNAMIC SOIL-STRUCTURE INTERACTION ANALYSIS Supplemental dynamic analysis, which incorporates soil-structure interaction, was also performed to check the response of the pile foundation as a result of the soil improvement. For this purpose, the 2- dimensional dynamic total stress analysis program FLUSH (Lysmer et al., 197) was employed. In the absence of site-specific response criteria, the ground is analyzed using the NS-component of the acceleration recorded in Kobe Maritime Observatory during the 199 Hyogoken Nanbu Earthquake BHNo.2-2 Figure : Comparison of SPT N- values GL-1.m~-12.m

7 Because of space limitation, the details of the soil-pile model as well as the results of the dynamic analysis are not presented here. Suffice it is to say that the sectional forces developed in the piles as a result of shaking are well within the corresponding allowable limits. Hence, the piles will not be damaged in the event of a major earthquake with PGA within the design value. CONCLUDING REMARKS To address the liquefaction concerns in the foundation ground under an existing structure, liquefaction potential evaluation of the site was initially carried out and the zone that requires treatment was determined. Considering the various limiting factors, compaction grouting was employed to improve the ground. Subsequent post-grouting SPT s showed a remarkable increase in the N-values in the foundation ground. Moreover, the confirmation borings showed data plots well beyond the Target N-value vs. fines content relations for the specified PGA, indicating the effectiveness of the method as a liquefaction countermeasure. Finally, it is worthy to mention that the plant remained operational most of the time during the grouting program, and therefore, business losses during the compaction grouting implementation were minimized. REFERENCES Architectural Institute Japan (1998). Recommendations for the Design of Building Foundations (in Japanese). Lysmer, J., Udaka, T., Tsai, C.F. and Seed, H.B. (197). FLUSH: A Computer Program for Approximate 3-D Analysis of Soil-Structure Interaction Problems, Report No. EERC 7-3, Earthquake Engineering Research Center, University of California at Berkeley, Berkeley, CA, USA.