The Open Civil Engineering Journal

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1 Send Order for Reprint to The Open Civil Engineering Journal, 206, 0, The Open Civil Engineering Journal Content lit available at: DOI: 0.274/ RESEARCH ARTICLE Simulation Study of the Load-ettlement Behavior of a Single ile Uing the Oterberg-cell Tet Baed on the Load Tranfer Theory Lina Xu *,, 2, Xuedong Guo, Lei Nie and Yongmei Qian 2 Jilin Univerity, Changchun, China 2 Jilin Jianzhu Univerity, Changchun, China Received: July 2, 206 Revied: September 9, 206 Accepted: September 27, 206 Abtract: In thi paper, a theoretical relationhip between the load and ettlement of a ingle pile in an Oterberg-Cell tet wa developed, conidering the joint action of pile and oil and a detailed deformation analyi wa conducted baed on the load tranfer theory of pile. The hear tet and the compreion tet were ued to determine the load tranfer parameter for oil layer around a pile at variou depth a well a the parameter for pile-tip oil. Baed on thi method, a imulation analyi program wa applied to determine the location of the balance point in the Oterberg-Cell tet to provide a reference for the tet deign. The analytical method preented in thi paper could be conidered practical becaue the reult from the imulation tet and on-ite meaurement indicate that the theoretically predicted reult i conitent with the meaurement. A reaonable election of the location of the hydraulic jack-like device (O-cell) could maximize the bearing capacity of teting pile to obtain a more accurate ultimate bearing capacity. Thi tudy provide a reference for the deign of the Oterberg-Cell tet a well a pile foundation. Keyword: Load tranfer function, Oterberg-Cell tet, ile, Load-ettlement behavior, Simulation.. INTRODUCTION The Oterberg-Cell tet (O-cell or bi-directional loading tet) i commonly ued to predict the load-ettlement behavior of large-diameter drilled haft. In 984, Oterberg ued thi method in a practical application and made it widely acceible []. The O-cell work in two direction - upward againt the ide hear and downward againt the end bearing, thu eparating the upper and lower reitance component [2]. The Oterberg-Cell tet provide high capacitie at an affordable cot, which ha made it a widely ued alternative method for teting drilled haft. Although the Oterberg-Cell tet till require theoretical improvement, it technological and economic advantage enable it to be widely ued in practical engineering in China and many other countrie [3-6]. reviou tudie on the Oterberg- Cell tet have primarily focued on it engineering application [7-9] and iue during pile tet [0-2], alo an equivalent pile load-head ettlement curve to predict pile capacity [3-7]. Y. Choi et al. [8] had conducted a tudy on the determination of loading capacitie for bi-directional pile load tet baed on actual load tet reult. A modified analytical olution i preented to analyze the axial pile repone of bi- direction O-cell loading by F.S. Niazi and.w. Mayne [9]. In thi paper, a theoretical relationhip between the load and ettlement of a ingle pile in an Oterberg-Cell tet wa developed, conidering the joint action of pile and oil and a detailed deformation analyi wa conducted baed on the load tranfer theory of pile. Additionally, thi paper provide detailed deformation analyi procedure. The hear and the compreion tet were ued to the determine load tranfer parameter for different oil layer around a pile at variou depth a well a the parameter for oil at the tip of a pile. Baed on thi method, imulation can be conducted to determine the location of the hydraulic jack-like device (O-cell) in the Oterberg-Cell tet and determine a * Addre correpondence to thi author at the Jilin Univerity, Changchun, China; Tel: ; xulina0958@26.com /6 206 Bentham Open

2 84 The Open Civil Engineering Journal, 206, Volume 0 Xu et al. reaonable location of the O-cell. A relatively accurate ultimate bearing capacity can be obtained to provide a reference for the deign of the Oterberg-Cell tet and the bearing capacity of the pile foundation. 2. THEORETICAL ANALYSIS OF A SINGLE ILE DEFORMATION USING THE OSTERBERG-CELL TEST 2.. Hyperbolic Model of the Load Tranfer Function The hyperbolic model, which ue in the load tranfer function of the pile haft, ha been propoed by H.B. Seed and L.C. Reee [20], the relationhip between the kin friction along the pile haft and the a well a the relationhip between the end-bearing reitance and tip ettlement wa expreed by the hyperbolic function: On the lateral ide of the pile: = a f b f () At the tip of the pile: b b a b b b b (2) Where, τ i the kin friction along the pile, σ b i the end-bearing reitance, S i the of the pile, S b i the tip ettlement, a f and b f are the load tranfer parameter of oil layer around the pile, and a b and b b are the load tranfer parameter of the pile-tip oil. Fig. () preent the model that ue the hyperbolic load tranfer, where /a and /b are the reciprocal of the load tranfer parameter. τ(σ) /b /a S Fig. (). Relationhip curve between the kin-friction and ettlement (τ-s) Iterative Model of the Load-ettlement Relationhip of the ile Shaft A finite element dz from the pile haft and Eq.(3) can be obtained in accordance with tatic equilibrium condition: d( z) up ( z) dz Where, (z) i the axial force of the pile haft, u p i the perimeter of the ection of the pile, and τ(z) i the kin friction along the pile haft. The elatic compreion modulu produced by the micro-module i expreed a follow: (3) () ds=- z dz EA or ds () z =- dz EA (4)

3 Simulation of Load-ettlement Behavior for a Single ile The Open Civil Engineering Journal, 206, Volume 0 85 Where, S i the of the pile haft, E i the elatic module of the pile, and A i the cro-ectional area of the pile haft. Applying a differential tranformation to Eq. (3) yield Subtituting Eq. (3) and Eq. (4) into Eq. (5) yield d d dz ds dz ds d u pea ds (5) (6) Subtituting the hyperbolic modular form of Eq. () of the load tranfer function into Eq. (6) and etting α = u p EA yield S d ds a ( bs) (7) Expreing Eq. (7) in incremental form yield S S a ( bs) (8) Eq. (8) i the iterative model that wa ued to olve the load-ettlement relationhip of the pile haft [2]. 3. LABORATORY TEST METHOD USED TO DETERMINE THE LOAD TRANSFER ARAMETERS The hear tet and the compreion tet can be ued to determine the load tranfer parameter of the oil, which can then be ued to analyze the load-ettlement relationhip of the pile, if the pile diameter i too large to conduct complete on-ite load tet [2-23]. 3.. Laboratory Shear Tet In thi paper, a laboratory hear tet wa conducted to determine the load tranfer parameter of lateral oil of a pile that are more repreentative of actual condition between the pile and oil. The theoretical background of the laboratory hear tet i dicued by B. Qi et al. [24]. The gravity tre and lateral oil preure can be calculated at variou depth according to the detailed depth of all oil ample in the natural oil layer. Fig. (2) preent chematic profile of the load tranformation. Thee parameter can be ued a the normal tre in the hear tet to determine the relationhip between the hear tre τ and hear S if equal train hear i achieved. A hyperbolic curve wa fit to the tet curve, a hown in Eq. (), which yield the load tranfer parameter a f and b f on the contact urface between the pile and oil. h/2 σ x h Stratum pile Stratum 2 τ σ x pile Soil ample Fig. (2). Schematic profile of the load tranformation.

4 86 The Open Civil Engineering Journal, 206, Volume 0 Xu et al Laboratory Compreion Tet The compreion tet can be ued to determine the compreion modulu E S on the tip of the oil to calculate the modulu of deformation E O. In 979, M.F. Randolph and C.. Wroth [25] uggeted that the equation for the oil at the tip of the pile can be olved by uing the Bouineq formula, yielding d b Sb E 2 ( ) 0 (9) Where, ω wa et a 0.79 for the rigid block. The load i maller; the curve of b and S b i approximately linear. From Eq. (2), the lope can be expreed a /a b, yielding E E a d( ) d( ) b b b (0) The term /b b in Eq. (2) indicate that the maximum force of the reaction can be obtained by performing laboratory triaxial tet. The ample oil on the tip of the pile i collected and placed in a triaxial apparatu. Lateral confining preure i applied, and axial compreion i exerted until the ample i detroyed. A a reult, the compreive trength of the ample i determined, and the maximum force of reaction on the tip of the pile i determined by conidering the ize effect and theory of the equivalent ample cro ection. 4. METHODS OF CALCULATING THE LOAD-SETTLEMENT RELATIONSHIS OF THE ILE SHAFT In thi paper, the body of the pile in the Oterberg-Cell tet i divided into upper and lower part baed on the location of the O-cell. The body i alo ubdivided into everal unit (the interface of the natural oil layer and the location of the O-cell mut be elected a the interface of the ubection), a hown in Fig. (3). Element Element 2 Stratum Dividing line of tratum Upper pile Stratum 2 Element n Element n + O-cell Element n +2 Lower pile Element n +n 2 Stratum n Fig. (3). Geometrical model of the pile.

5 Simulation of Load-ettlement Behavior for a Single ile The Open Civil Engineering Journal, 206, Volume 0 87 The calculation procedure for the upper ection of the pile i decribed a follow: () Firt, the upper ection of the pile i divided into n ubection. (2) Given the on the top of the pile a S 0, the kin friction along the pile of the firt ubection can be calculated uing Eq. (), yielding o = af bfo () (3) The change in the axial force of the pile haft (Δ ) and the axial force of the pile haft ( ) on the top urface of the firt ubection are calculated according to τ, yielding l p (2) = Δ (3) (4) The average axial force of the firt ubection ( ) i calculated according to, yielding 2 2 (5) The elatic compreion of the firt ubection (ΔS ) and the of the pile haft on the underide of the firt ubection (S ) are calculated according to, yielding (4) S l EA S S (5) (6) (6) In the above calculation, the kin friction along the pile of the firt ubection hould be equally ditributed along the pile; however, the of the pile haft varie with depth. A a reult, the lateral friction ditributed along the pile haft alo varie. Therefore, atifactory reult can only be obtained by ubtituting, S, and ΔS into Eq. (2) and performing iterative calculation on the firt ubection, yielding S S ' a ( f bfs) ' ' 2 ' ' S EA ' ' l S S ' ' (7) (8) (9) (20) (2) (7) In general, the value of Δ ', ', ΔS ', and S ' from the calculation at thi time are not equal to the value of Δ,, ΔS, and S and obtained above. So, iterative calculation are required. At each iteration, the value of ', ΔS ', and S ' that were obtained from the calculation are ued for the value of, ΔS, and S and ubtituted into Eq. (7) to calculate new value for ', ΔS ', and S ' until the reult are within the pecified accuracy. In general, the reult are acceptable if the relative error of the load (or ) increment calculated at both time i le than 5%. The value of and S calculated from Eq. (8) and Eq. (2) repreent the axial force and, repectively, of the pile haft on the underide of the firt ubection and on the top urface of the econd ubection. (8) and S are ued a the axial force and repectively, of the pile haft on the top urface of the econd ubection and ubtituted into Eq. () through Eq. (2). 2 and S 2 can be obtained after the ame iterative

6 88 The Open Civil Engineering Journal, 206, Volume 0 Xu et al. calculation are performed on the econd ubection, and o on. The calculation can be performed until the bottom of the upper ection of the pile and n and S n and are obtained. During the calculation, variou value of and are applied to different oil layer (uch a a f, b f, a f2, b f2 ). The calculation procedure for the lower ection of the pile i decribed a follow: (9) Firt, the lower ection of the pile i divided into n 2 ubection. (0) Given the ettlement on the tip of pile a S b. The reaction on the tip of the pile (R b ) i calculated according to Eq. (2) a follow: Ab Rb A b (22) a b b b b () The kin friction along the pile on the n + n 2 ubection i calculated according to Eq. () a follow: b n n = 2 afn n b 2 fnn 2 b (23) (2) The change in the axial force of the pile haft on the top urface of the n + n 2 ubection (Δ n + n 2 ) and the axial force of the pile haft on the top urface of the n + n 2 ubection ( n + n 2 ) are calculated according to τ a l nn2 p nn2 nn2 R nn2 nn2 b (24) (25) (3) The average axial force on the n + n 2 ubection ( n n2 ) i calculated according to n + n 2 a follow: R b nn2 nn2 n n Rb (26) (4) The value of ΔS n + n 2, the elatic compreion on the n + n 2 ubection, and S n + n 2, the ettlement of the pile on the top urface of the n + n 2 ubection, are calculated according to n n2 a follow: S EA n n2 nn l 2 nn2 S S S nn2 b nn2 (27) (28) (5) The iterative calculation tep for the lower part of the pile are the ame a tep (7) and (8). The calculation can be performed until the top of the lower ection of the pile and n + and S n + are obtained. Different value of S 0 and S b can be ued to calculate variou bottom load ( 0) and the bottom (S n ) of the upper ection of the pile a well a variou upper load ( 0) and the upper (S n +) on the top of the pile. 5. DETERMINATION OF THE LOCATION OF THE BALANCE OINT IN THE OSTERBERG-CELL TEST For the Oterberg-Cell tet, the ultimate bearing capacity can only be accurately meaured if the O-cell i located at the balance point. Otherwie, one ection of the pile reache the ultimate bearing capacity and the other ection fail. A a reult, the calculated ultimate bearing capacity will be le than the actual capacity, indicating that the reult tend to be conervative [0, ]. The actual ultimate bearing capacity i not meaured, and unneceary load are produced. An empirical correlation i frequently ued to determine the location of the O-cell. However, thi method ha ome limitation. A a reult, a imulation analyi wa conducted on the relationhip between the load and ettlement of the pile haft on both the upper and lower ection of the pile baed on the load tranfer mechanim to determine the location of the load box in the Oterberg-Cell tet. It preent the calculation procedure that were ued to determine the location of the O-cell in Fig. (4).

7 Simulation of Load-ettlement Behavior for a Single ile The Open Civil Engineering Journal, 206, Volume 0 89 reume O-cell intalling at z m depth from the urface Moving the O-cell downward and calculating again Dividing the pile Determining the load tranfer parameter Moving the O-cell upward and calculating again Simulating upward load- curve: top of O-cell Simulating downward load- curve: bottom of O-cell Obtaining upward ultimate bearing load (Qupper) Obtaining downward ultimate bearing load (Qlower) Comparing Qupper and Qlower Qupper > Qlower Qupper = Qlower Qupper < Qlower The poition of the O-cell i uitable Fig. (4). Calculation tep to determine the location of the O-cell. 6. FIELD OBSERVATION CALCULATIONS AND DISCUSSION By mean of Oterberg-Cell tet, data and laboratory tet information were obtained from tet pile in the Longhua Temple-Songhua River Grand Bridge project in Jilin rovince, China. A computational analyi of the load-ettlement relationhip of the Oterberg-Cell tet wa conducted by uing the numerical imulation method decribed in thi paper. The location of the O-cell wa alo dicued. The diameter of the tet pile wa 2 m, the length of the pile wa 65 m, the elatic modulu of the pile haft wa 3500 Ma, and the O-cell wa intalled at a depth of 50 m. Fig. (5) preent a chematic profile of the oil condition and tet pile. The main geotechnical parameter of the oil around the pile are hown in Table. According to the depth of the oil layer, the normal tre required to be exerted to all oil ample during the hear tet wa calculated and i provided in Table 2. D=2m 0.0m 8.0m Medium-fine 5m Gravel L=65m 65 Clay 4m 5m 48m 50m 55m 6m 65m Medium Gravel Whole Strong Intermediary Fig. (5). Schematic profile of oil ditribution around the pile.

8 820 The Open Civil Engineering Journal, 206, Volume 0 Xu et al. Table. Main geotechnical parameter of the oil. Denity (g/cm 3 ) Water content (%) Liquid limit (%) Coheion (ka) Friction angle ( ) Elatic modulu (Ma) Medium-fine Gravel Clay Medium Gravel Whole Strong Intermediary _ _ _ oion ratio _ Table 2. Normal tre impoed on the oil ample in the hear tet. Soil ample Normal tre (Ma) Medium-fine Gravel Clay Medium Gravel Whole Strong Intermediary The laboratory hear tet can be ued to obtain reult for the hear tre between the contact layer of the oil layer and pile (repreented by τ) and the horizontal hear (repreented by S), a hown in Fig. (6). Shear train (Ma) Diplacement Medium-fine (σ =0.07Ma) Gravel (σ=0.43ma) Clay (σ=0.384ma) Medium (σ=0.44ma) Gravel (σ=0.44ma) Whole (σ =0.480Ma) Strong (σ =0.0.94Ma) Intermediary (σ=0.627ma) Fig. (6). Relationhip between the hear train and horizontal hear in the hear tet. The laboratory compreion tet conducted on the oil at the tip of the pile can be ued to obtain the compreion modulu of the oil at the tip of the pile (E=307 Ma) by applying Eq. (0) and the laboratory triaxial tet a follow: /a b =500 MN/m 3, and /b b =250 Ma. The laboratory tet can be conducted to obtain the load tranfer parameter of the oil on the lateral ide of the pile and at the tip of the oil, a hown in Table 3.

9 Simulation of Load-ettlement Behavior for a Single ile The Open Civil Engineering Journal, 206, Volume 0 82 Table 3. Load tranfer parameter. Stratum Medium-fine Gravel Clay Medium Gravel Whole Strong Intermediary Bottom oil a b The above calculation tep can be ued to obtain the load- curve of the pile haft in the Oterberg-Cell tet, a hown in Table 4. Fig. (7) preent the load- curve from the Oterberg-Cell tet and imulation on the pile. In thi figure, downward i repreented by negative value. Table 4. Meaured and imulated load- data in the bidirectional tet. Load (kn) Meaured upward Meaured downward Load (kn) Simulated upward Load (kn) Simulated downward Diplacement Meaured upward load curve: top of O-cell Simulated upward load curve:top of O-cell Meaured downward load curve: bottom of O-cell Simulated downward load curve: bottom of O-cell -20 Load(kN) Fig. (7). Simulated and meaured load- curve in the O-cell tet. After-tet reidue checking method i ued to check the reliability of the imulated value. The principle of thi method i a follow [26]. () i x() i xˆ () i i,2,,n n (29) Where, ε(i) i the difference in value between the meaured and imulated, x(i) i the meaured, i the imulated, n i the number of data. xi ˆ( ) The mean value of ε(i) can be obtained by Eq.(30).

10 822 The Open Civil Engineering Journal, 206, Volume 0 Xu et al. n = ( i) n i (30) The variance ε(i) of can be obtained by Eq.(3). S n 2 2 = [ ( i) ] n i (3) The mean value of meaured can be obtained by Eq.(32). n x x() i n i (32) The variance of meaured can be obtained by Eq.(33). S x i x n = [ ( ) ] n i (33) C i the ratio of S and S 2, i the mall error probability, and they are the tet indexe. The accuracy level which i divided by thee two indexe i hown in Table 5. Table 5. Accuracy level of After-tet reidue checking method. C= S S ( i) S 2 2 (34) (35) Accuracy level C GOOD >0.95 <0.35 QUALIFIED >0.8 <0.5 JUST MARK >0.7 <0.45 UNQUALIFED In order to check the imulated value, the meaured and imulated under the ame load which are found from Fig. (7) are hown in Table 6. And the difference value between the meaured and imulated are alo hown in Table 6. Table 6. Meaured and imulated. Load (kn) Meaured upward Simulated upward ε(i) Meaured downward Simulated downward ε(i)

11 Simulation of Load-ettlement Behavior for a Single ile The Open Civil Engineering Journal, 206, Volume Calculation reult of each parameter which are obtained by Eq. (29) to Eq. (35) are hown in Table 7. Table 7. Calculation reult. C According to Table 6, C=0.09<0.35,>0.95, o the accuracy level i good, and the reult how that the imulated reult are in good agreement with the meaured reult. In Fig. (7), it illutrate that the load- curve obtained through imulation are in good agreement with the reult obtained from the Oterberg-Cell tet. If the lower ection of the pile doe not reach it ultimate bearing capacity when the upper ection of the pile reache it ultimate bearing capacity, then the O-cell i not in it correct location (but i partially above it). The calculation tep hown in Fig. (4) can be ued to move the O-cell downward to obtain curve that provide the load- curve of the pile haft for different O-cell location, a hown in Fig. (8). If the O-cell move downward, then the length of the pile on the upper ection of the O-cell increae, and the kin friction alo increae and allow for a higher load bearing capacity in the upper ection of the pile. The load bearing capacity of the lower ection of the pile i gradually utilized a the O-cell move downward. If the O-cell i intalled at a depth of 64 m from the top of the pile, then the ultimate bearing capacity of the pile (both on the upper and lower ide of the O-cell) can be maximized. Upw ard load- curve:o-cell i intalled at 64 m depth Diplacement Load(kN) Dow nw ard load- curve:o-cell i intalled at 64 m depth Upw ard load- curve:o-cell i intalled at 6 m depth Dow nw ard load- curve:o-cell i intalled at 6 m depth Upw ard load- curve:o-cell i intalled at 57 m depth Dow nw ard load- curve:o-cell i intalled at 57 m depth Upw ard load- curve:o-cell i intalled at 53 m depth Dow nw ard load- curve:o-cell i intalled at 53 m depth Upw ard load- curve:o-cell i intalled at 50 m depth Dow nw ard load- curve:o-cell i intalled at 50 m depth Fig. (8). Simulated load- curve for different O-cell location. The compreive ultimate bearing capacity of the ingle pile can be calculated according to Eq. (36). Q Q W uu u Qud (36) Where, Q u i the vertical compreive ultimate bearing capacity, Q uu i the limit value of the pile on the upper ide of the O-cell, Q ud i the limit value of the pile on the lower ide of the O-cell, W i the dead weight of the pile on the upper ide of the O-cell, and γ i the correction factor of the lateral reitance of the pile on the upper ide of the O-cell (0.8 i ued for coheive oil and ilt, and 0.7 i ued for y oil). It preent the ultimate bearing capacity of the pile haft if the O-cell i intalled at different depth in Table 8. If the reitance on the tip of the pile i greater than the kin friction or approximately equal to the kin friction, the meaured bearing capacity i cloer to the ultimate bearing capacity when the O-cell wa located on the bottom of pile or near the bottom of the pile.

12 824 The Open Civil Engineering Journal, 206, Volume 0 Xu et al. Table 8. Ultimate bearing capacity for different O-cell location. CONCLUSION oition of O-cell 50 m 53m 57 m 6 m 64 m Ultimate bearing capacity of upper pile Q upper (kn) Ultimate bearing capacity of lower pile Q lower (kn) >34907 >3872 >44782 > Length of upper pile (m) Weight of upper pile (kn) Correction coefficient γ Ultimate bearing capacity Qu (kn) The tudy can be ummarized a follow: >73634 >80686 >9566 > () The load tranfer parameter of variou oil layer at different depth and the load tranfer parameter of oil at the tip of the pile can be determined by the hear tet and the compreion tet. Thi reult can provide parameter that can be ued in the numerical imulation method. (2) According to the after-tet reidue checking method, C=0.09<0.35,>0.95, o the accuracy level i good, and the reult how that the imulated reult are in good agreement with the meaured reult. Moreover, curve obtained by the Oterberg-Cell tet are in good agreement with the curve that demontrate the load- curve determined by uing the numerical imulation method decribed in thi paper. Therefore, thi method can be ued to analyze the deformation law of the pile haft in the Oterberg-Cell tet meaurement and to determine the ultimate bearing capacity of the tet pile, which can reduce or replace a partial pile tet on-ite. Thu, the method decribed in thi tudy ha economic implication. (3) The imulation analyi procedure propoed in thi paper are ued to determine the location of the O-cell in the Oterberg-Cell tet, and the meaured bearing capacity i cloer to the ultimate bearing capacity if the O-cell i in the right poition. A reference for the deign of the Oterberg-Cell tet and pile foundation can be provided. CONFLICT OF INTEREST The author confirm that thi article content ha no conflict of interet. ACKNOWLEDGEMENTS Thi work i financially upported by the National Natural Science Foundation of China ( ). REFERENCES [] J.O. Oterberg, "New device for load teting driven haft", Foundation Drilling., vol. 23, pp. 9-, 984. [2] J.O. Oterberg, "New device for load teting driven pile and drilled haft eparate friction and end bearing", In: roceeding of the International Conference on iling and Deep Foundation, London, 989, pp [3].D. Shi, and Q. Huang, "New device for pile tatic load tet. ", In: ile Engineering Technique, Chinee Building Material Indutry re: Beijing, 996, pp [4] A. into, and X. ita, "The ue of Oterberg cell load tet to predict pile reitance", In: 4 th International Conference on Site Characterization, Brazil, 203, pp [5] H. Seol, and S. Jeong, "Load-ettlement behavior of rock-ocketed drilled haft uing Oterberg-Cell tet", Computer and Geotechnic, vol. 36, pp. 34-4, [ [6] G.L. Dai, W.M. Gong, and Y.S. Jiang, "Engineering application of a new tatic load teting method for pile with large bearing capacity in bridge", Journal of Southeat Univerity, vol. 3, no. 4, pp , 200. [Natural Science Edition]. [7] W.M. Gong, Y.S. Jiang, and J. Zhai, "Self-balanced loading tet for pile bearing capacity", Chinee Journal of Geotechnical Engineering, vol. 22, no. 5, pp , [8] Y.W. Luo, and L. Nie, "Analyi on bearing behaviour of large-cale cat-in-place pile in foundation with elf-balanced method", Global

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