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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.
Field monitoring on piled raft foundation subjected to unsymmetrical earth pressure Quan trắc trường móng bè cọc chịu áp lực đất khơng đối xứng J Hamada(1), K Yamashita(2) Tóm tắt Bài báo giới thiệu nghiên cứu cụ thể cơng trình nhà bảy tầng với ba tầng hầm, chịu áp lực đất không đối xứng Để giảm thiểu độ lún lớn lớp đất sét bè móng để giảm lực cắt phát sinh lớn tác động lên cọc lực ngang từ áp lực đất, phương án móng bè cọc sử dụng Để khẳng định tính đắn phương án thiết kế móng này, cơng tác quan trắc dài hạn phân chia tải trọng bè cọc tiến hành phép đo khoảng thời gian 13 năm Dựa kết đo tòa nhà cho thấy móng bè cọc khơng chịu tốt tải trọng đứng cơng trình mà chịu tải trọng ngang áp lực đất khơng đối xứng Từ khóa: móng bè cọc, đo đạc 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 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] 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 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 Instrumentation To confirm the validity of the foundation design, field measurements were performed on the load sharing between the raft and the piles Figures and 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 S¬ 28 - 2017 79 KHOA HC & CôNG NGHê Figure Schematic view of monitored building and foundation with soil profile Figure Foundation profile with locations of monitoring devices (a) Pile_5G Photo Inclined setting earth pressure cells (D4, D5 and D6) (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 Long-term measurements Figure 3: Locations of strain gauges on monitored piles 80 Figure shows the time hisories of axial loads on Pile_5D and Pile_5G Figure 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 shows time histories of bending moments on Pile_5D and Pile_5G, respectively These values are T„P CH KHOA HC KIƯN TRC - XY DẳNG (a) Pile_5D (b) Pile_5G Figure Time histories of axial load on piles (a) Pile_5D (b) Pile_5G Figure Relationship of axial load between head load and intermediate depth (a) Pile_5D Figure Time histories of bending moment on piles Figure Time histories of earth pressures and water pressure around pile_5D (b) Pile_5G Figure Time histories of earth pressures acting on side walls Table 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 1.44 x 10-6 kPa Figure Time histories of load sharing between raft and pile S¬ 28 - 2017 81 KHOA HC & CôNG NGHê negligibly small Figure 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 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)) Tohoku Earthquake which a seismic intensity at the observed building site was little less than 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./ 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 Tài liệu tham khảo Poulos H.G., Piled raft foundations: design and applications, Geotechnique 51(2), pp.95-113, 2001 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.323392, 2000 Figure 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 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.513532, 2011.6 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 Nghiên cứu thực nghiệm phá hoại biến dạng (tiếp theo trang 61) Kết luận kiến nghị Trên sở phân tích phá hoại mẫu thí nghiệm biến dạng cắt nút khung rút kết luận sau: -Cách thức thiết kế khác dẫn tới cách ứng xử khác mẫu thí nghiệm Phá hoại mẫu NK1 dạng phá hoại dẻo với khớp dẻo uốn xuất dầm sát mặt cột Phá hoại vùng nút khunglà phá hoại dẻo có biến dạng tương đối toàn vùng nút Phá hoại mẫu NK2 NK3 thuộc dạng phá hoại giòn Vùng nút khung ởhai mẫu bị ép vỡ dưới tác động nén cục chuyển vị xoay đầu mút cột dầm Các dầm cột quanh nút khung không phát triển biến dạng dẻo đầy đủ khơng hồn tồn biến dạng uốn Nguy phá hoại (uốn cắt) dầm, cột nút khung gần ngang Tài liệu tham khảo 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 Nguyễn Lê Ninh: Động đất thiết kế cơng trình chịu động đất, nhà xuất Xây dựng – 2007 Paulay T., Priestley M.J.N “Seismic design of reinforced concrete and masonry buildings”, John Wiley – 1992 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 SP 14.13330.2011 -СТРОИТЕЛЬСТВО В СЕЙСМИЧЕСКИХ РАЙОНАХ TCVN 9386:2012(2012),”Thiết kế cơng trình chịu động đất”, Nhà Xuất Xây Dựng, Hà Nội 82 - Dưới tác động cắt uốn dầm cột truyền vào, nút khung bị biến dạng cắt đáng kể, thiết kế theo quy định tiêu chuẩn thiết kế kháng chấn đại TCVN 9386:2012 Do đó, việc xét tới biến dạng nút khung tính toán hệ kết cấu khung BTCT chịu động đất cần thiết - Hiệu ứng bó bê tơng vùng nút khung ảnh hưởng định tới ứng xử nút khung Để tạo hiệu ứng bó bê tơng vai trò cốt đai cốt thép cột trung gian vùng nút khung quan trọng -Thí nghiệm cho thấy, hệ kết cấu khung thiết kế theo tiêu chuẩn Nga SP 14.13330.2014 Việt Nam TCVN 5574:2012 không phù hợp để phát triển cấu phá hoại dẻo hệ kết cấu khung BTCTchịu động đất mạnh./ TCVN 5574:2012(2012), “Kết cấu bê tông bê tông cốt thép”, Nhà Xuất Xây Dựng, Hà Nội 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 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 T„P CHŠ KHOA H“C KIƯN TRC - XY DẳNG ... Schematic view of monitored building and foundation with soil profile Figure Foundation profile with locations of monitoring devices (a) Pile_5G Photo Inclined setting earth pressure cells (D4,... 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... 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./ The axial