Quan trắc hiện trường móng bè cọc chịu áp lực đất không đối xứng

79 S¬ 28 - 2017 Field monitoring on piled raft foundation subjected to unsymmetrical earth pressure Quan trắc hiện trường móng bè cọc chịu áp lực đất không đối xứng J. Hamada(1), K. Yamashita(2) Tóm tắt Bài báo này giới thiệu nghiên cứu cụ thể về công trình nhà bảy tầng với ba tầng hầm, chịu áp lực đất không đối xứng. Để giảm thiểu độ lún quá lớn do các lớp đất sét dưới bè móng và để giảm lực cắt phát sinh lớn tác động lên cọc do lực ngang từ áp lực đất, phương án móng bè cọc đã được sử dụng. Để khẳng định tính đúng đắn của phương án thiết kế móng này, công tác quan trắc dài hạn sự phân chia tải trọng giữa bè và cọc đã được tiến hành bằng các phép đo trong khoảng thời gian 13 năm. Dựa trên các kết quả đo được của tòa nhà cho thấy rằng móng bè cọc không những chịu tốt tải trọng đứng của công trình mà còn có thể chịu được tải trọng ngang do áp lực đất không đối xứng. Từ khóa: móng bè cọc, đo đạc hiện trường, áp lực đất không đối xứng, phân chia tải trọng Abstract This paper offers a case history of a seven-story building with three basement floors, subjected to unsymmetrical earth pressure. To reduce excessive settlements due to clay layers below the raft, and to reduce excessive shear force acting on piles due to lateral load from earth pressure, a piled raft foundation was employed. To confirm the validity of the foundation design, long-term field measurements on the foundation have been conducted on the load sharing between the raft and the piles during about thirteen years. Based on the measurement results of the building, it is confirmed that a piled raft foundation works well not only for vertical structure load but also for lateral load due to unsymmetrical earth pressure. Keywords: piled raft foundation, field measurements, unsymmetrical earth pressure, load sharing (1) Dr, Group Leader, Research & Development Institute, Takenaka Corporation, hamada.junji@takenaka.co.jp (2) Dr, Executive Manager, Research & Development Institute, Takenaka Corporation, yamashita.kiyoshi@takenaka.co.jp 1. Introduction Piled raft foundations are recognised as one of the most economical foundation system, and have been applied for a lot of building in many countries such as Germany and Japan. Several case histories have been reported about piled rafts [1, 2, 3]. However, only a few case histories exist on the monitoring of the soil-pile-structure interaction behavior for lateral load. This paper offers a case history of a piled raft foundation focusing on pile bending moments in addition to vertical load sharing. During the monitoring period, the 2011 off the Pacific coast of Tohoku Earthquake struck the site. Subsequently, the monitoring was frequently conducted. In addition, the seismic observation records on the foundation have been reported by Hamada et al. (2015) [4]. 2. Monitored building and soil conditions The monitored building, which is a seven-story residential building with three basement floors, is located in Tokyo, Japan. The building subjected to unsymmetrical earth pressure is a reinforced concrete structure, 29.3 m high, with a 71.4 m by 36.0 m footprint. Figure 1 shows a schematic view of the building and its foundation with a typical soil profile. The soil profile consists of fine sand layer just below the raft with SPT N-values from 10 to 20 and clay strata including humus between depths of 17 m and 24 m from the ground surface with unconfined compressive strength of about 140 kPa. Below the depth of 24 m, there lies a Pleistocene fine sand layer with SPT N-values of 40 or higher. The shear wave velocities derived from a P-S logging system were about 200 m/s between the depths of 17 m and 24 m, and 480 to 570 m/s in the sand layers below the depth of 24 m. The ground water table appears at a depth approximately equal to the basement level. The average contact pressure over the raft was 159 kPa. If a conventional pile foundation were used for the building foundation subjected to unsymmetrical earth pressure, the piles should carry large lateral load not only for seismic condition but also for ordinary condition, where a design horizontal seismic coefficient (lateral load over building dead load) was 0.15 for ordinary condition and 0.34 for severe seismic condition. On the other hand, if a raft foundation were used, the clay layer between depths of 17 and 23 m has a potential of excessive settlement while the sand layer just below the raft has enough bearing capacity for the dead load of the building and lateral frictional resistance between the raft and the subsoil can be reliable. Consequently, a piled raft foundation consiting of cast-in-place concrete piles with 1.2 m in diameter and 12.2 m in length was employed, where the lateral load can be resisted by both the piles and the frictional resistance beneath the raft. 3. Instrumentation To confirm the validity of the foundation design, field measurements were performed on the load sharing between the raft and the piles. Figures 2 and 3 show the layout of the piles with locations of monitoring decices. Axial forces and bending moments of the piles were measured by a couple of LVDT-type strain gauges on Pile_2D (2-D street), Pile_5G (5-G street) and Pile_5D (5-D street). Eight earth pressure cells and a pore-water pressure cell were installed beneath the raft around the instrumented piles. Three sections of Pile_5D at depths of 1.0 m, 2.0 m and 9.14 m below the pile head and those of Pile_5G at depths of 1.0 m, 1.7 m and 8.19 m were measured. Earth pressure cells of D4 and D6 were set obliquely on the soil around Pile_5D, as shown in Photo 1, in order to evaluate a frictional resistance beneath the raft by the difference of the earth pressure from the two earth pressure cells. Earth pressure cells of D8-1, D8-2 and D9 were set on the embedded side wall in order to evaluate a lateral force acting on the side wall of the building. 80 T„P CHŠ KHOA H“C KI¦N TR”C - XŸY D¼NG KHOA H“C & C«NG NGHª Figure 1. Schematic view of monitored building and foundation with soil profile Figure 2. Foundation profile with locations of monitoring devices Photo 1. Inclined setting earth pressure cells (D4, D5 and D6) Figure 3: Locations of strain gauges on monitored piles (a) Pile_5G (b) Pile_5D The axial forces and the bending moments of two piles, the contact earth pressures between the raft and the soil as well as the pore-water pressure beneath the raft were also measured. The resolutions of strain and earth pressure are about 1.0×10-4μ and about 5.0×10-6 kPa, respectively as shown in table 1. 4. Long-term measurements Figure 4 shows the time hisories of axial loads on Pile_5D and Pile_5G. Figure 5 shows the relation of the axial load at the pile head with those at the intermediate depth and near the pile toe. Axial loads are about 4500-4000 kN on pile head at Pile_5D, while about 600 kN near pile toe. This means relatively large pile skin friction, ((4500-600)kN / (1.2π m x 7.14 m)=145 kPa). And axial load on pile head are gradually increasing after the end of construction with seasonal variation. Figure 6 shows time histories of bending moments on Pile_5D and Pile_5G, respectively. These values are 81 S¬ 28 - 2017 Figure 4. Time histories of axial load on piles Figure 5. Relationship of axial load between head load and intermediate depth Figure 6. Time histories of bending moment on piles Figure 7. Time histories of earth pressures and water pressure around pile_5D Figure 8. Time histories of earth pressures acting on side walls Figure 9. Time histories of load sharing between raft and pile (a) Pile_5D (b) Pile_5G (a) Pile_5D (b) Pile_5G (a) Pile_5D (b) Pile_5G Table 1. Measuring devices Istrument Number Resolution Strain gauge 26 0.99 ~ 1.06 x 10-4 μ Earth pressure cell 10 3.71 ~ 5.68 x 10-6 kPa Piezometer 1 1.44 x 10-6 kPa 82 T„P CHŠ KHOA H“C KI¦N TR”C - XŸY D¼NG KHOA H“C & C«NG NGHª negligibly small. Figure 7 shows the time hisories of contact earthpressures and water pressure beneath the raft around Pile_5D. Measured values are relatively stable comparing to axial load on pile. Figure 8 shows the time-dependent earth presure acting on the embedded side walls. The earth pressure was stable after the earthquake. Judging from 55 kPa at D8-2, a coefficient of earth pressure K was approximately evaluated as 0.3 (55 kPa / unit weight (17 kN/ m3) / depth (11.2 m)). The axial load of the pile and the earth pressures acting on side wall fluctuate according to a season due to temperature. The seasonal variation of the incremental earth pressures of D8-2, D9 shows opposite relation, that is positive and negative. Figure 9 shows the time-dependent load sharing among the pile load (kPa), the earth pressure and the water pressure in the tributary area of Pile_5D. The earth pressure is an average of the measured values from D7 and D5. The pile load (kPa) is estimated by the axial force of the pile divided by the tributary area of 39 m2. The ratio of the load carried by the pile to the total load is 40% (42%) at the end of the construction and 43% (44%) about eleven years after that time. Here, the values in parentheses are the ratios of the load carried by the pile to the effective load. The ratios were almost same before and after the 2011 off the Pacific coast of Tohoku Earthquake which a seismic intensity at the observed building site was little less than 5. 5. Conclusions Based on the long-term monitoring, no significant changes in load sharing between the piles and the raft or earth pressures acting on the side wall were observed after the 2011 Tohoku Earthquake. Consequently, the foundation design was found to be appropriate./. Tài liệu tham khảo 1. Poulos H.G., Piled raft foundations: design and applications, Geotechnique 51(2), pp.95-113, 2001. 2. Katzenbach R., Arslan U. and Moormann C., Piled raft foundation projects in Germany, Design applications of raft foundations, Hemsley J.A. Editor, Thomas Telford, pp.323- 392, 2000. 3. Yamashita, K., Yamada, T. and Hamada, J., Investigation of settlement and load shearing on piled rafts by monitoring full-scale structures, Soil and Foundations, Vol.51, pp.513- 532, 2011.6. 4. Hamada, J., Aso, N., Hanai, A. and Yamashita, K., Seismic performance of piled raft subjected to unsymmetrical earth pressure based on seismic observation records, 6ICEGE, 2015. Tài liệu tham khảo 1. Beckingsale C.W. Post-Elastic Behavior of Reinforced Concrete Beam-Column Joints, Research Report 80-20, Department of Civil Engineering, University of Canterbury, Christchurch, New Zealand, August 1980. 2. Nguyễn Lê Ninh: Động đất và thiết kế công trình chịu động đất, nhà xuất bản Xây dựng – 2007 3. Paulay T., Priestley M.J.N. “Seismic design of reinforced concrete and masonry buildings”, John Wiley – 1992. 4. Sangjoon Park, Khalid M. Mosalam, Experimental and Analytical Studies on Reinforced Concrete Buildings with Seismically Vulnerable Beam- Column Joints, Pacific Earthquake Engineering Research Center (PEER), 2012. 5. SP 14.13330.2011 -СТРОИТЕЛЬСТВО В СЕЙСМИЧЕСКИХ РАЙОНАХ. 6. TCVN 9386:2012(2012),”Thiết kế công trình chịu động đất”, Nhà Xuất bản Xây Dựng, Hà Nội. 7. TCVN 5574:2012(2012), “Kết cấu bê tông và bê tông cốt thép”, Nhà Xuất bản Xây Dựng, Hà Nội. 8. A.K. Kaliluthin, S. Kothandaraman, T.S. Suhail Ahamed, A Review on behavior of reinforced concrete beam-column joint, International Journal of Innovative Research in Science, Engineering and Technology, 2014. 9. Jaehong Kim, James M. LaFave. Joint Shear Behavior of Reinforced Concrete Beam-Column Connections subjected to Seismic Lateral Loading, Department of Civil and Environmental Engineering University of Illinois, 2009. 10. Sangjoon Park, Khalid M. Mosalam, Shear strength models of exterior Beam-column joints without transverse reinforcement. Pacific Earthquake Engineering Research Center (PEER), 2009. 11. Nilanjan Mitra. An analytical study of reinforced concrete beam-column joint behavior under seismic loading. University of Washington, USA, 2007. 6. Kết luận và kiến nghị Trên cơ sở phân tích sự phá hoại của các mẫu thí nghiệm và biến dạng cắt của các nút khung có thể rút ra các kết luận sau: -Cách thức thiết kế khác nhau dẫn tới cách ứng xử khác nhau của các mẫu thí nghiệm. Phá hoại mẫu NK1 là dạng phá hoại dẻo với các khớp dẻo uốn xuất hiện ở các dầm sát mặt cột. Phá hoại vùng nút khunglà phá hoại dẻo và có biến dạng tương đối đều trên toàn bộ vùng nút. Phá hoại các mẫu NK2 và NK3 thuộc dạng phá hoại giòn. Vùng nút khung ởhai mẫu này bị ép vỡ dưới dưới tác động nén cục bộ của chuyển vị xoay đầu mút cột và dầm. Các dầm và cột quanh nút khung không phát triển được biến dạng dẻo đầy đủ và không hoàn toàn là biến dạng uốn. Nguy cơ phá hoại (uốn và cắt) giữa dầm, cột và nút khung gần ngang nhau. - Dưới tác động cắt và uốn của dầm và cột truyền vào, các nút khung sẽ bị biến dạng cắt đáng kể, ngay cả khi được thiết kế theo các quy định của tiêu chuẩn thiết kế kháng chấn hiện đại TCVN 9386:2012. Do đó, việc xét tới biến dạng của nút khung trong tính toán hệ kết cấu khung BTCT chịu động đất là hết sức cần thiết. - Hiệu ứng bó bê tông trong vùng nút khung ảnh hưởng quyết định tới ứng xử của nút khung. Để tạo được hiệu ứng bó bê tông này vai trò của cốt đai và cốt thép cột trung gian trong vùng nút khung hết sức quan trọng. -Thí nghiệm cho thấy, các hệ kết cấu khung được thiết kế theo tiêu chuẩn của Nga SP 14.13330.2014 và của Việt Nam TCVN 5574:2012 không phù hợp để phát triển cơ cấu phá hoại dẻo ở hệ kết cấu khung BTCTchịu động đất mạnh./. Nghiên cứu thực nghiệm sự phá hoại và biến dạng... (tiếp theo trang 61)