Analysis of the effect of stopping dewatering on group piles foundation

Transcription

1 Advanced Materials Research Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Analysis of the effect of stopping dewatering on group piles foundation Junfa Zhang 1,a, Lei Tao 2,b, Jianjun Wen 3,c and Wenxiang Liu 1,d 1 Department of Civil Engineering, Xi an University of Technology, Xi an, China 2 Department of Hydraulic Structure Engineering, Xi an University of Technology, Xi an, China 3 Shanghai Geotechnical Engineering Reconnaissance Design Academe Limited Company, Shanghai, China a zhangjf@xaut.edu.cn, b taolei0100@126.com, c kevinwen229@126.com, d miki @yahoo.com.cn Keywords: stopping dewatering; group piles; equivalent temperature-changed simulation; In-Situ Testing Abstract. After the construction of building foundation with high groundwater table was finished, stopping dewatering of pit could create some effect on the force of foundation. In this paper, equivalent temperature-changed method was brought forward to simulate the influence of stopping dewatering which could conveniently use current structural analysis program to simulate the change of pore water pressure in soil. And based on ANSYS program, this method was applied in the project of Telecom Network Management Center in Shaanxi Province. Numerical results indicate that the effect of stopping dewatering on pile-raft system is significant, and some abnormal phenomenon in In-Situ Testing is reasonably explained. Introduction High-rise building structure owns the characterizes of complex systems, huge bulk and immense weight. And if this kind of building is built on the foundation of soft soil or deep soil layer and rigid bases hypothesis continue to be used, this is not reasonable since there will be a large discrepancy with the actual situation. In this case, considering the interaction among foundation, base and structure is quite necessary. Implementing the real project with In-Situ Testing is one of the most important ways to consider interaction [1, 3]. The author implemented In-Situ Testing on the project of Telecom Network Management Center in Shaanxi Province in order to realize the working behavior of super long, large group piles base of high-rise building and the mechanical interaction between upper and lower structure in Xi an area [1]. In testing with reinforcing steel bar buried at the top of pile, the following phenomenon emerged. Up to April 16th 2000, the 10th reading was obtained after the completion of raft construction and conservation, and followed by underground construction of first story and assembling of steel reinforcement of the two-story underground. On May 25th 2000, the 11th reading was acquired. Compared with the previous data, it was found that tension of reinforcing steel bar laid at the top of the pile came forth widely, and also there was prominent reading change of reinforcing steel bar which was installed in border pile, corner pile and the pile near it while it was little in internal piles. Generally speaking, only the upper layer of underground was done and the load imposed on it was little, so the internal forces of reinforcing steel bar would not change much. It is commonly believed that tensile phenomena existing in the pile-heads is abnormal. Hence it was difficult to explain it temporarily which influenced the analysis of In-Situ testing results. Further investigation, however, revealed that wells around the pit stopped dewatering and water table rose about 10 m which was still about 1m away from the raft bottom during two reading periods. So it suggested that stop dewatering affected the force of group pile-raft foundation. The paper analyzed the influence of stopping dewatering because it was often necessary to make a rational explanation on first-hand test results. During the period of foundation construction, in the site of higher groundwater table, most required dewatering and stop dewatering after the completion of foundation construction. Therefore this paper is universal. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-11/05/16,05:51:07)

2 Advanced Materials Research Vols Equivalent disposal of pore water pressure effect In the effective stress analysis, the total soil stressσ can be divided into effective stress σ ' and pore water pressure u [4, 5]: σ = σ ' uδ Where δ is Kroneckerδ. The positive or negative sign convention of normal stress σ ( i j) is the same with elastic mechanics, which provided that tensile stress was positive, compressive stress was negative. To facilitate the analysis, the following two assumptions were made in this paper: 1: Soil in the area of changing water table was at saturated point 2: Soil structure couldn t be influenced by rebound of water table, that is the slaking action of soil is neglected, suggesting that elastic constant of soil remained unchanged. Equilibrium equation represented by total stress σ, j + f i = 0 (2) Deformation value of soil depends on effective stress; therefore the strain can be evaluated as: ε = C klσ ' kl WhereC kl, is the flexibility coefficient tensor and kl = (1) (3) C was only related with two independent elastic constants namely the elastic moduluse and Poisson's ratio µ for the isotropic soil. To denote the effective stress of (3) with total stress, the effective stress can be given by 1 2µ ε = Cklσkl + uδ (4) E Where the second item of right hand reflects the impact of pore water pressure and can be directly equal to the impact of temperature changes. The strain caused by the equivalent temperature changes can be evaluated as T 1 2µ ε = αtδ = uδ (5) E Where T represents the equivalent temperature, α is the soil linear expansion coefficient. In specific calculation, soil coefficient of linear expansion can take an arbitrary value and then deduce equivalent temperature which is given by 1 2µ T = u (6) αe The question discussed in this paper is that incremental form should be adopted to simulate the variation( u ) of pore water pressure in soil when comparing the situation between water table fully restored after stopping dewatering and the dewatering process. Physics equation: 1 2µ ε = Ckl σkl + uδ E Equivalent temperature changes: (7) T 1 2µ = u αe (8)

3 1350 Advanced Building Materials In analysis of raft-pile-soil system, the function of pore water pressure variation in each point of soil rock can be converted into equivalent temperature changes around stopping time of dewatering. Equivalent temperature is a scalar so that a variety of general-purpose finite element analysis software can be utilized for the analysis of such problems and the impact on foundation from changes of water table can also be inspected. Real measured data from Telecom Network Management Center in Shaanxi Province This project is a super high-rise building, of which the main structure contains two-storey underground, 39-floor on ground with the height of m. Pile-raft foundation with the raft of 2.5m thickness was adopted in this project and the depth of raft bottom is m (corresponding altitude is m). A total of 271 bored cast-in-situ piles were arranged to compose the pile group in the plane of m 2.Diameter of signal pile is 800 mm and the length of which is 60 m. The first half from the top pile to 30 m down is located steel reinforcement while the remain is located halving In-situ testing, 53 piles were selected from 271 piles and 55 reinforcing steel bar was located in the top pile. A lot of data was obtained from a total of 30 tests implemented in various construction stages of this project. When locating reinforcing steel bar at each pile tip, arbitrary one of the 16 longitudinal steel reinforcement surrounding the pile section was truncated and then was connected with the reinforcing steel bar. The majority reinforcing steel bar in the pile section should be located at the side pointing to the center of pile group (Figure 1 # of a pile) in order to connect wore conveniently. Unfortunately, there is no detail record of located orientation of reinforcing steel bar in each pile. To facilitate analysis and consider the foundation layout as biaxial symmetric one, a quarter part of group piles in northwest was selected emphatically, as illustrated in Fig. 1. A total of 15 piles force condition among the East-West axis, North-South axis and three lines from the base center to the north-west corner was studied mainly in the analysis. The related piles were also individually remarked by (1#-15#) Fig. 1 A quarter part of group piles in northwest

4 Advanced Materials Research Vols The internal force changes of each reinforcing steel bar between the two readings in pile tip was given in table 1 and from which it can be seen that internal force changes of reinforcing steel bar, located in the angle and surrounding parts of pile,is great, while changes of which in each pile is little. In addition, the discrepancy of the two readings from two reinforcing steel bar G40, G40, located at the center of group piles is not great. While there is an obvious discrepancy between the two readings of internal force from reinforcing steel bar G11, G11 which was buried in the angle pile 15 #. Table 1 The internal force change of each reinforcing steel bar in pile tip between two readings (The eleventh data-the tenth data) Reinforced Difference Reinforced Difference Pile Pile instrument of internal instrument of internal number number serial number force[kn] serial number force[kn] G1 6# G24 3# 1.44 G9 14# G29 2# G10 13# G40 1# 2.70 G11 15# G40 1# G11 15# 4.18 G44 8# 2.30 G14 12# 0.97 G45 9# 5.64 G18 4# G46 10# Model of calculation Finite element discrete model. In this paper, ANSYS finite element analysis software was used to make a simulation analysis on the impact of stopping dewatering. Fig. 2 (a) FEM model of foundation Fig. 2 (b) FEM discrete model of pile-raft From the whole finite element discrete model as shown in Fig 4.6, discrete block element was used for soil, discrete beam elements was used for the piles, and discrete shell element was used for raft block elements, beam elements and 415 shell elements were utilized in the whole model and each piles along the length was divided into 39 beam elements. The confirmation of pore water pressure difference value changes caused by stopping dewatering. A dewatering funnel was formed just because of dewatering wells around the pit, water table, actually measured within the group pile, was m and the one of the external remote was

5 1352 Advanced Building Materials m. After the cessation of dewatering, the water table rose from to m and the difference of pore water pressure caused by the dewatering [6] can be deduced from this as shown in Fig 3. Three-dimensional model could be adopted in calculation and the space contour diagram of this Fig can be based on. In addition, the contour diagram was formed by the rotation of drainage wells as vertical axis near the corner of the pit. Therefore equivalent temperature change T of different points can be deduced according to equation (8) based upon these. Fig. 3 The isoclines of pore water pressure difference value caused by stopping dewatering Fig. 4 An overview of the displacement contour plot Simulation results and analysis of finite element Displacement results and analysis. Fig 4 is the top surface contour of displacement.from this, it is obvious to see that the largest upward displacement of outer pit could reach 1.4 ~1.6mm and the one of four corners could reach 1.6mm.When the displacement of raft center raise 0.12 mm, the one of the four corners will reach about 0.55mm. It can be seen that the upward displacement of raft is obvious less than the one surrounding the soil ground, for group piles and raft was connected as a single entity which limits the rise of raft. The deformation of each pile in west and east axes was individually presented in Fig 5. Horizontal displacement, which renders the pile to bend, came into being to the lateral foundation of each pile and the maximum one of side pile is approximately 1.1 mm which came forth in the largest depth of pore water pressure changes of soil ground. From the side pile to the center pile, lateral level displacement reduced gradually. Fig. 5 The deformation of each pile in west and east axes Fig. 6 The moment of pile 6(unit: N m)

6 Advanced Materials Research Vols Internal force results of pile. The moment map of pile 6# was shown in Fig 6 and from which it can be seen that there is a big moment at the pile tip. The moments and axial forces on the top section of 1 #-15 # pile were given at table2. From the table, it can be seen that larger moments engender in side piles, head of corner piles and the largest moment can reach kn m. Also it can be concluded that internal pile-head was commonly pressed while piles near the corner of side pile was widely tensile and the largest tension value can reach kn. Table 2 The moment and axes force of each head pile caused by stopping dewatering Moment Total Moment Mz Pile serial number Mx moment[kn.m] [kn.m] 2 2 [kn.m] M = M X + M Z West and east axes South and north axes From center to northwest corner Axial force[kn] 1(center) (pressure) (pressure) (pressure) (pressure) (pressure) 6(east) (tension) 1(center) (pressure) (pressure) (tension) (tension) 10(north) (tension) 1(center) (pressure) (tension) (tension) (tension) 14(northwest corner) (tension) 15(northwest corner) (tension) The axial force of each pile along west and east axes, from center to northwest corner, changes with the depth was shown in Fig 7. Also from the Fig it can be seen that axial force (tension) of pile 6 # would reach 606kN near the place of elevation m where the largest changes of pore water pressure exists. As mentioned above, in order to connect wore conveniently, most reinforcing steel bar in piles, which is close to edge foundation, corner and the nearby piles was located on the section pointed to the one side center of group pile. Besides, the steel reinforcement with the raft can be viewed as rigid connections because the one in head pile was anchored in the raft. And a greater moment was created and steel reinforcement, pointed to one side center of group piles, was tensile because of water table rebound caused by stopping dewatering. In other words, most reinforcing steel bars, located at these parts illustrated above, happen to be in the tensile side. To sum up, the occurring phenomenon described formerly in the test is consistent with the consequence resulted from reasons mentioned above. In September 1995, the upper 20-storey structure of construction bank office building, located in Zhuque Gate in Xi'an Shaanxi Province, the roof of which was sealed. On October 20 th, 1995 the dewatering completely stopped and the ground water table gradually rebounded. From March 5 th to July 2nd in1996, rebound phenomenon appeared in the overall building and larger rebound volume occurred in the annex department [7]. In addition, High-Tech square block A, located in the intersected place between South Second Ring and Gao Xin Road Four, was a high-rise building with 25-storey aboveground, two storey underground and the height of which was 99.8m. The

7 1354 Advanced Building Materials construction was completed up to 11 layers at the end of January 2002 and it came to halt during Spring Festival and the dewatering began to stop on 31 st January. From the three subsequent settlement observations, it is discovered that each observation point constantly rebound and the average cumulative rebound amount was 1.63 mm [6] [8]. These examples illuminate that there is greater impact, which is caused by stopping dewatering, on foundation in Xi'an area and it should be paid attention to. 0 pile axile force/kn depth calculated from raft bottom/m pile one pile two pile three pile four pile five pile six 60 Fig. 7 (a) The axes force of each pile in west and east axes Fig. 7 (b) The axes force of each pile from the center of foundation to northwest corner Conclusions 1. The method of equivalent temperature changes to analyze the impact of stopping dewatering was proposed in this paper which was carried out based on the combination of ANSYS software and project of Telecom Network Management Center in Shaanxi Province.It has been substantiated by settlement observation of some projects that the ground rebound, caused by stopping dewatering, came forth in the project in Xi'an area. Hence, it is proposed that the related organizations or departments should bring forward requests on settlement observation of some major projects that settlement observation should be carried out before stopping dewatering of pit in order to obtain engineering design, instituting relevant norms, statutes and other valuable information. 2. Each pile of foundation pile system should be rigidly connected with raft in project of Telecom Network Management Center in Shaanxi Province. Under the impact of stopping dewatering, the moment, which generated at each pile tip, caused each pile section pointing to one side of group pile center to be tensile and the farther distance from group pile center the greater impact there will be. Moreover, side piles and corner piles would be seriously affected. These conclusions have important value and inspirations to deeply understand the force and deformation mechanism and to further organize In-Situ Testing observation.

8 Advanced Materials Research Vols References [1] Super-high In-Situ Testing discussion group: Building Structure. Vol. 30 (2002), p (In Chinese) [2] Jinmin ZAI, Jinzhang ZAI: High-rise building foundation analysis and design (soil-structure interaction theory and application) China Architecture & Building Press, Being, (1993), in press (In Chinese) [3] Jianguo DONG, Xihong ZHAO: High-rise building foundation-together theory and practice, edited by Tongji University Press, Shanghai, (1997), in press (In Chinese) [4] K. Terzaghi(Zhiying XU translation): theory of soil mechanics (Geological Press, Being) (In Chinese) [5] Jiahuan QIAN, Zongze YIN: theory and computation of geotechnique (2nd edition), edited by China WaterPower Press, Being,(1996), in press (In Chinese) [6] Junfa ZHANG. High-rise building considering the soil-pile-structure interaction of static and dynamic research [D]. Xian University of Architecture and Technology doctoral thesis, 2005 (In Chinese) [7] Yangping YAO, Baoyin ZHANG and Guoqiang ZHANG: Journal of Geotechnical Engineering, Vol. 23 (2001), p (In Chinese) [8] Xian University of Architecture and Technology Building Survey Institute. Xian High-Tech Square Block A settlement observation bands of the report (9-14) [R], (In Chinese)

9 Advanced Building Materials / Analysis of the Effect of Stopping Dewatering on Group Piles Foundation /