Research on the Uplift Bearing Capacity of Suction Caisson Foundation under Local Tensile Failure

Available online at www.sciencedirect.com

ScienceDirect Procedia Engineering 166 (2016) 61 – 68

2nd International Symposium on Submerged Floating Tunnels and Underwater Tunnel Structures

Research on the Uplift Bearing Capacity of Suction Caisson Foundation under Local Tensile Failure Xu Fenga, Xiaojian Pib, Shilun Fenga, *,Chen Biana b

a Institute of Civil Engineering, Tianjin University, Tianjin 300072, China China Southwest Architecture Design and Research Institute Corporation, Chengdu 610084, China

Abstract The research on the uplift bearing capacity is one of the important issues worth concerning and solving. For this reason a lot of studies about this aspect have been done by many scholars, but there are still no wide spread engineering specifications on design and calculation of uplift bearing capacity for suction caisson foundation. The quantitative calculation method on the uplift bearing capacity of suction caisson foundation under local tensile failure is proposed by the author. And the indoor uplift tests are carried out on the single-bucket suction caisson foundation models with different diameters and length-diameter ratios in the wet sand, by which the feasibility of this method is initially proved. © Published by Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license © 2016 2016The TheAuthors. Authors. Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of SUFTUS-2016. Peer-review under responsibility of the organizing committee of SUFTUS-2016 Keywords: suction caisson foundation:uplift bearing capacity:the local tensile failure

1. Introduction Submerged floating tunnel is a new mode of transportation across bay, strait, lake or other waters, which has many advantages such as short passage time, little impact on the surrounding environment, the choice freedom of construction sites and convenient wiring. It is generally composed of tube structures suspended in water, located in design position by buoyancy, and anchored in the water bed with reasonable anchoring systems in order to maintain stability. The environment of submerged floating tunnel is the ocean, causing that the main load is from ocean currents and waves[1,2]. The traditional alternative foundation forms of submerged floating tunnel are pile foundation, gravity foundation, holding power anchor foundation etc[3]. Pile foundation has complex construction process, expensive cost and many quality problems, which could cause noise and other environmental problems; the capacity of gravity foundation is * Corresponding author.Tel.: +086-139-0216-4874; fax: +086-022-2740-5816. E-mail address: [email protected](Shilun Feng)

1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of SUFTUS-2016

doi:10.1016/j.proeng.2016.11.563

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relied on anchorage gravity, whose design and installation is simple while the efficiency is low and cost is high; for holding power anchor foundation, it has poor control of the buried depth, serious seabed erosion and large environmental disturbance. Suction caisson foundation is a new foundation form of submerged floating tunnel, with such advantages as expense saving, convenient transportation and installation, high bearing capacity etc, whose application prospect is considerable[4,5]. The stability of anchor base is important for the performance of submerged tunnel under various loads, and the uplift bearing capacity of submerged floating tunnel foundation is one of the keys that researchers must focus on. Therefore, a quantitative calculation method on the uplift bearing capacity of suction caisson foundation under local tensile failure is proposed by the author from the perspective of force. Current researches about suction caisson foundation are mostly in soft clay and dry sand[6,7], our indoor uplift tests are carried out on the single-bucket suction caisson foundation models with different diameters and long-diameter ratios in a wet sand, by which the feasibility of this method is initially proved and the basis for the design of uplift bearing capacity is provided. 2. The uplift capacity formula of suction caisson foundation Local tensile failure is one of the common failure modes of suction caisson foundation under uplift load[8]: the bucket together with its internal sand are pulled off the ground. For this kind of failure mode, the uplift bearing capacity of suction caisson foundation is made up of its self gravity, internal sand gravity, the frictional force of outer wall, the tensile force of the bottom sand. The uplift bearing proportion coefficient of suction caisson foundation ȟp is defined to present the different contributions on bearing capacity between the frictional force of outer wall and the tensile force of the bottom sand. The force analysis result is as follow:

Vult

Ws  Wa  f out  [ p Fp

(1)

Vultüthe uplift bearing capacity of suction caisson foundation in unit of N; Wa- bucket weight in unit of N; Ws üinternal sand gravity, Ws=ȡAlg, where ȡ is the density of the sand in unit of kg/m3, A is the area of the bucket bottom in unit of m2, l is the calculation height of suction caisson foundation in unit of m. foutüthe frictional force of outer wall;ȟpüThe uplift bearing proportion coefficient of suction caisson foundation, which is obtained by numerical fitting; Fp- the tensile force of the bottom sand. fout (Fp) is related to the bucket wall area (the bucket bottom area) and the sand shear strength, but the sand shear strength is not readily available because of the difficulty to obtain undisturbed sand. The existing uplift bearing capacity formulas of suction caisson foundation are represented with sand shear strength, given that the correlation between standard penetration degree and sand shear strength[9,10], and the standard penetration degree is readily available, so the bucket wall area (the bucket bottom area) and the standard penetration degree can be used to calculate fout (Fp). While the standard penetration degree is just the average characterization of sand shear strength, by which the depth change of sand shear strength is not reflected, so the depth correction factor of suction caisson foundation ȟd can be combined with the standard penetration degree N as the equivalent standard penetration degree N' to consider the depth change of sand shear strength, then the result is as:

f out Fp

SDl f N ' [ p A f N ' [ p

(2)

SD 2 4



f N'

(3)

Düdiameter of the suction caisson foundation in unit of m; f(N')üthe correlation between the equivalent standard penetration degree and sand shear strength, the specific expression should be determined by numerical fitting of the relevant test.

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N'

(4)

[d N

ȟdüthe depth correction factor of suction caisson foundation, ȟd=1+0.4tan-1(d/D), d is the burial depth of suction caisson foundation in unit of m; N üthe standard penetration degree. The uplift bearing capacity formula of suction caisson foundation is as follow:

· § 4l Vult ȡAlg  Wa  A¨  [ p ¸ f [ d N ©D ¹

(5)

Next, f(N') is determined according to the test result, making it easy to use the formula. 3. The indoor uplift tests of suction caisson foundations For the tether-typed submerged floating tunnel, its tunnel tube is straight and the cross section is circular, there are two single pile foundations arranged side by side laterally at regular distances along the direction of tunnel length, whose tether of each foundation is arranged in a single vertical style.The length of tether-typed submerged floating tunnel L is taken as 1000m, the wall thickness of tether-typed submerged floating tunnel t is taken as 1m, the ratio of buoyancy and gravity ȝ is 1.1-1.5, the diameter of tether-typed submerged floating tunnel D is 10m-40m, the corresponding layout plans of the tether-typed submerged floating tunnel with 1#-3# pile foundations are as follows and a comparative analysis is made, then these plans’ costs are accessed from the perspective of the steel amount, filtering out the optimal plan. 3.1. The test sand The test sand is a wet sand, whose relevant physical and mechanical parameters are shown in Table 3.1 and grading curve is as Fig. 1. it is concluded that the test sand is medium sand with poor grading, which is deposited in the 2.5m×2.5m×2.5m test pool. The standard penetration test is finished to get N = 5. Table3.1

Relevant physical and mechanical parameters Uniformity coefficient Cu 1.94

Fig. 1. Grading curve

Curvature coefficient Cc 1.43

Water Content w/% 2.83

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3.2. Experimental apparatus and method Six single-bucket suction caisson foundation models with different diameters and long-diameter ratios are as Fig.2. their specific dimensions are shown in Table 3.2.

(a)

(b) Fig. 2. (a) 1#—3# single-bucket models;(b) 4#—6# single-bucket models Table3.2 The size of single-bucket models Model Number Model 1 Model 2 Model 3 Model 4 Model 5 Model 6

Material

Iron bucket

mass/kg 2.36 3.77 6.55 0.4 0.66 1.19

Wall thickness/mm 3 3 3 1.5 1.5 1.5

feature diameter/mm 220 220 220 48 48 48

length-diameter ratio 0.5 1.0 2.0 3.0 5.0 10.0

The main used test instruments are counter-force planes, a cylinder, a pump, a tension sensor, two rope-type fiber displacement meters, a data acquisition system, a chain and wires etc, the load schematic diagram is as Fig. 3. The indoor uplift tests of single-bucket suction caisson foundation models are conducted half an hour after the completion of sinking tests. The initial uplift state is that the bucket top is flush with the sand surface. Since there is no loading specification for suction caisson foundation, the uplift specification for pile foundation (JGJ94-2008) is referred and the step-wise loading method is used, whose uplift load of each level is 0.2kg ~ 1kg directly on top of each single-bucket suction caisson foundation model. The next load is applied after last level load is relatively stable, 8cm-10cm uplift displacement can be used as the test terminated mark.

Xu Feng et al. / Procedia Engineering 166 (2016) 61 – 68

Fig. 3. Test apparatus program

3.3. Test Results The uplift load-displacement curves of six single-bucket suction caisson foundation models are shown as Fig.4.(a), and the peak of the curve is defined as its uplift failure criterion, the corresponding peak load is defined as the uplift bearing capacity of suction caisson foundation[11,12]. It can be seen from Fig. 4. (a) that the size of suction caisson foundation has an effect on the uplift bearing capacity and peak displacement(the corresponding displacement to the uplift bearing capacity). The peak displacements of six single-bucket suction caisson foundation models are listed in Table3.3, and a numerical fitting is made between peak displacements and corresponding length-diameter ratios of six models as Fig. 4. (b), R2=0.818, which shows the good correlation between them, it is indicated that the more length-diameter ratio is, the more peak displacement.

(a)

(b)

Fig. 4. (a)Uplift load-displacement of 1#-6# model;(b)Peak displacement—length-diameter ratio of 1#—6# model

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Xu Feng et al. / Procedia Engineering 166 (2016) 61 – 68 Table3.3 Peak displacement of 1#-6# models model Peak displacement/mm

1# model 1.57

2#model 2.75

3#model 2.65

4#model 2.98

5#model 3.32

6#model 3.69

4. The Fitting Results of uplift capacity formula for suction caisson foundation The test uplift capacity Vult of 1#-6# six single-bucket suction caisson foundation models in wet sand are as Table3.4: Table3.4

The test uplift capacity Vult

Model The uplift capacity Vult /N

1#model 288.51

2#model 641.23

3#model 1065.41

4#model 78.25

5#model 141.27

6#model 250.12

In order to obtain the expression of f (N '), N' is required as independent variable to fit the data, so the equivalent uplift capacity V'ult is defined to process the data:

Vult'

Vult  Wa  U Alg 4l A(  [ p ) D

(6)

The equivalent uplift capacity V'ult and the equivalent standard penetration degree N' of six single-bucket suction caisson foundation models are as follows in Table3.5: Table3.5

V'ult and N'

Model Equivalent uplift capacity V'ult/N equivalent standard penetration degree N'

1#model

2#model

3#model

4#model

5#model

6#model

1098.37

1731.22

1816.65

2682.69

2985.98

3058.40

5.93

6.57

7.21

7.49

7.76

7.94

The relevant data of 1#-6# six single-bucket suction caisson foundation models are drawn to make a numerical fitting with the equivalent standard penetration degree N' as the horizontal and the equivalent uplift bearing capacity V'ult as the ordinate. The result is that V'ult and N' have best fitting when ȟp=3, the fitting curve is shown in Fig. 5.

Fig. 5. The fitting curve

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The calculation formula of uplift bearing capacity V'ult for single-bucket suction caisson foundation model is as follow:

§ 4l · Vult Ws Wa  A¨  3¸ 56.31e0.51[d N ©D ¹





(7)

R2=0.938, it shows good fitting and can be used in engineering. 5. Conclusion Under the special soil condition of the south China sea region, the correlation of main design parameters (submerged floating tunnel length and diameter, the ratio of buoyancy and gravity, the space and length of single pile foundation) is researched based on the straight line tether-typed submerged floating tunnel with double-row single pile, single vertical tether, round section, and a series of different layout plans for tether-typed submerged floating tunnel are proposed and their economy is compared, which provides a reference for optimizing the design of tether-typed submerged floating tunnel. The main research results are as follows: the uplift bearing capacity of suction caisson foundation under local tensile failure in a wet sand is researched and a new method to calculate the uplift bearing capacity of suction caisson foundation is proposed, the results are as follows: 1) The peak displacement us(the corresponding displacement to the uplift bearing capacity) of suction caisson foundation is related to the length-diameter ratio l/D of it, the relationship of them according to the experimental data is for: us=2.22(l/D)0.25,R2=0.818. 2) The failure mode of iron single-bucket suction caisson foundation in a wet sand is local tensile failure, the bucket together with the internal sand are pulled off the ground. The power function is chosen to make a data fitting to get the uplift bearing capacity formula:

§ 4l · Vult Ws Wa  A¨  3¸ 56.31e0.51[d N ©D ¹





(8)

R2=0.938, which shows good fitting, and the uplift bearing capacity formula can be used for engineering reference.

Acknowledgments The research was financially supported by Ministry of Transport of the People's Republic of China (20130318740050), National Science Foundation of China(51379145), National Key Basic Research Program of China (2014CB046800) and National Science Foundation of China(51239008). References [1] Du Feng. Research on dynamic demonstration for submerged floating tunnel under water working environment [M]. Chengdu: Southwest Jiaotong University, 2008. [2] Shengnan Sun. Dynamic response analysis of submerged floating tunnel [D]. Dalian: Dalian University of Technology, 2008. [3] Keqian Zhang. Structural design and analysis of submerged floating tunnel and its health monitoring [M]. Hangzhou: Zhejiang University, 2011 [4] Li Wang,Xiaobing Lu,etc. Experimental Study of bucket foundations’ response in calcareous sand under dynamic load [J]. Engineering Mechanics, 2010, 27 (2): 193-203. [5] Zhonghuang Hu. The researeh on horizontal bearing capacity of bucket foundation [M]. Dalian: Dalian University of Technology, 2007.

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