This paper aims to study the design process and service performance of a deep excavation supported by tieback anchored pile walls. The design procedure and design approaches for deep excavation in China are described. Based on the excavation case history for Shenyang, China, design results obtained using the elastic method and the finite element method (FEM) are compared and analyzed. Special emphasis is given to the analysis of horizontal wall deformations, internal forces in the wall, earth pressures on the wall, ground surface settlements, and stabilities of the excavation. The similarities and differences between the Chinese code (JGJ 120-2012) and the European code (EN 1997-1) for the design of geotechnical structures are presented based on a design example. Through the comparison, it is indicated that the Chinese code focuses on the design result, while the European code focuses on the design process. The crucial construction methods for reducing construction risk based on the excavation case history are described. The mechanical behaviors of the excavation retained by an anchored pile wall were investigated by analyzing observed field cases. The results provide good, practical guidelines for the design and construction of a tieback anchored pile wall retained excavation in sandy soil.
As the number of deeper and wider excavations in urban districts increases, the use of retaining structures to overcome problems related to settlement and bearing capacity is becoming more common. The use of tieback anchored retaining walls has been a widely adopted practice in geotechnical engineering, especially for deep excavations [
Our understanding of the design of tieback anchored walls has improved since the emergence of geotechnical engineering design standards in different countries. An increasing number of researchers and engineers have contributed to the development of design methods over the last three decades to provide helpful, practical guidance (e.g., Briaud and Lim [
Since the publication of Peck [
This study focuses on the design, construction, testing, and monitoring of a deep excavation retained by a tieback anchored pile wall in Shenyang, China. A detailed analysis of the design of tieback walls, including the design procedure, design approaches, and design theory, is conducted. The design results are compared with calculations made using the finite element method (FEM) and observations based on an excavation case. The similarities and differences between the Chinese and European codes for the design of geotechnical structures are analyzed using a design example. The observed performance of a tieback anchored pile wall in sand, which serves as an excellent guide for structural design, is presented by analyzing the test results and in situ measurements. This study provides information that is useful for the design and construction of deep excavations supported by anchored pile walls.
The design method of tieback anchored wall or cantilever pile wall for deep excavation has been studied for about 60 years. In 1943, Terzaghi [
Recommendations of total safety factors.
Item | Safety factor |
---|---|
Earthworks | 1.3–1.5 |
Retaining structures, excavations, offshore foundations | 1.5–2 |
Foundations on land | 2-3 |
Pile load tests | 1.5–2 |
Dynamic formulas | 3 |
After the 1950s, with the development of the theory of soil mechanics, it was believed that total safety factors cannot satisfy the requirements of geotechnical design based on the limit states. Taylor [
For the comparison between European code and Chinese code for geotechnical design, Li et al. [
The performances of tieback anchors used to reinforce the ground such as pullout capacity, load changes during excavation, prestress loss, and antifatigue performance have been studied using different techniques, including physical models, finite element models, and field investigations. Kim [
The deep excavation studied in this paper was located in one of the central districts of Shenyang, China, with busy roads and buildings existing around it. The excavation was divided into two regions, that is, Region A and Region B (see Figure
Site layout and monitoring plan.
The retaining pile walls used for the deep excavation were all supported by prestressed tieback anchors. The soils between the piles were reinforced using mesh reinforcement and shotcrete. The pile walls used to support Region A and Region B, with a diameter of 0.6 m and a pile spacing of 1.1 m, were reinforced by seven and three levels of prestressed tieback anchors, respectively, with a 3 m vertical spacing on average.
Cross-section profile of retaining structure.
A geotechnical investigation was carried out through the boreholes at the excavation construction site, which showed that the in situ soils included a Quaternary Holocene artificial filling soil layer, Quaternary Holocene Hun River floodplains and paleocurrent shock layer, and Quaternary Hun River rushed diluvium. The in situ soils, including medium-coarse sand and gravelly sand with a dense or medium dense state, show the mechanical behavior of coarse-grained sand [
Mechanical parameters of soils.
Number | Soil | Depth/m | Unit weight/kN·m−3 |
|
|
|
fak (kPa) |
---|---|---|---|---|---|---|---|
|
Filled soil | 4.6 | 16.66 | - | 15 | 10 | - |
|
Medium-coarse sand | 0.5 | 19.11 | 1 | 18 | 30 | 200 |
|
Gravelly sand | 16.9 | 19.60 | 1 | 30 | 36.4 | 500 |
|
Medium-coarse sand | 3.1 | 19.11 | 1 | 27 | 33.4 | 480 |
|
Rounded gravel | 20.9 | 20.58 | 1 | 40 | 37.8 | 650 |
The groundwater at the site consists of pore phreatic water. The stable groundwater level is approximately 8 m, and the permeability coefficient of the soils ranges from 40 m/d to 80 m/d. The groundwater level inside the excavation was required to be below the bottom of the excavation, which was achieved through dewatering, for normal construction.
Currently, various retaining technologies are employed in the construction of deep excavations in China, such as cast-in place piles, precast piles, diaphragm walls, and sheet pile walls. It is quite complex to optimize a retaining plan for deep excavation. Hence, design standards are applied with no alternative for construction works to ensure the security and low cost of retaining structures. In China, the recommendations, design approaches, and requirements for the design of deep excavations are presented in one national standard, that is, the Technical Specification for Retaining and Protection of Building Foundation Excavations (JGJ 120-2012) [
Design procedure for deep excavation in China.
According to JGJ 120-2012, a design should demonstrate that the retaining structures, ground, and surrounding buildings do not exceed the limit states, ultimate limit states (ULS), and serviceability limit states (SLS). When considering the ultimate limit states, two conditions should be met. The first condition requires that internal failure or excessive deformation of the retaining structure does not occur; that is, the following inequality should be satisfied:
The second condition requires that loss of stability of the structure or the ground, due to, for example, ground sliding, excavation basal heave, and retaining structure overturning, does not occur; that is, the following inequality should be satisfied:
When considering the serviceability limit states, the deformation of structures and surrounding buildings and the ground surface settlement can be verified as follows:
For this case study, the retaining structures were designed by following the design procedures mentioned in Figure
After the retaining structures were determined, the design parameters were selected via calculations related to the mechanical characteristics and stability of excavation. In China, two geotechnical design software programs, that is, Leading Software-Deep Excavation and Tongji Qim Star Software-FRWS, are commonly used for the calculation and design of deep excavations [
Primary characteristics of classical method and elastic method.
Calculation method | Assumption of strut/anchor | Passive earth pressure | Pile stiffness | Calculation theory |
---|---|---|---|---|
Classical method | Hinged support | — | Ignored | Static equilibrium method, equivalent beam method, etc. |
Elasticity method | Spring | Winkler elastic foundation | Considered | Beam on Winkler elastic foundation |
For the classical method, the active and passive pressures are determined according to Rankine theory. The classical method includes the static equilibrium method and equivalent beam method, which are adequate for the design of simple retaining structures, such as cantilever walls and pile walls anchored by a tieback. Compared with the classical method, the elastic method performs the calculation based on the theory of a beam on a Winkler elastic foundation, in which the deformation of the retaining structure is considered. A more realistic earth pressure is obtained using the elastic method, but more accurate parameters are required to ensure a reliable result. Particularly, the stiffness parameters, such as the stiffness coefficient of tiebacks/struts and Young’s modulus, greatly influence the calculation results. Thus, if the in situ geotechnical data are accurate, a more reasonable result will be obtained using the elastic method. If not, the classical method is the better choice for the primary design [
The calculation models of section AA′ (see Figure
The mechanical parameters shown in Table
The design parameters of the soil bodies.
Number | Soil |
|
|
---|---|---|---|
|
Filled soil | 20 | 10.0 |
|
Medium-coarse sand | 90 | 15.1 |
|
Gravelly sand | 190 | 23.0 |
|
Medium-coarse sand | 130 | 19.1 |
|
Rounded gravel | 140 | 24.9 |
For the elastic method, the stiffness parameters of the soils and structures are crucial parameters for the prediction of the wall deformation. The horizontal stiffness coefficients of the soils in front of the wall were obtained by using the following formula:
For the second model, two-dimensional plane stain triangular mesh elements with fifteen displacement nodes were used in the numerical analysis by using Plaxis, an FEM program. Considering the symmetry in deformation, only half of the excavation geometry was modeled (see Figure
Model geometry of the deep excavation.
The soil body was divided into five sublayers according to the geotechnical investigation. The mechanical parameters of each soil sublayer are given in Table
Mechanical parameters of retaining structures.
Retaining structure |
|
|
|
---|---|---|---|
Pile wall | 25 | 30 | 0.2 |
Tieback | 78 | 195 | 0.3 |
To verify the accuracy of the FEM models in simulating the excavation and to examine the design results of the elastic method, the variation curves of the maximum horizontal wall deformation,
Variation of the maximum wall deformation and the pile head deformation with the excavation depth. (a) The maximum horizontal wall deformation. (b) The horizontal deformation of the pile head.
Figure
Comparison of the observed and calculated results using various methods for the deep excavation. (a) Horizontal wall deformation. (b) Bending moment of pile wall. (c) Shear force of pile wall. (d) Earth pressure. (e) Settlement of ground surface.
For the bending moments and shear forces of the pile wall, the calculated results of the FEM using the HS model were in good agreement with those using the MC model, as shown in Figures
As shown in Figures
To evaluate the accuracy of the earth pressures on the wall predicted by the elastic method, the normal stresses on the wall obtained using the elastic method and the two FEM models are shown in Figure
Figure
For the elastic method, three ground settlement calculation methods were employed in the current calculation program: triangle method, exponent method, and parabolic method, which are distinguished based on the predicted settlement profile. The parabolic method was selected for this excavation case because the retaining structure was a tieback anchored pile wall [
Ground surface settlement for the elastic method.
According to the comparison of the results of the elastic method, the FEM models, and the observed data, the elastic method overestimated the primary design parameters, such as the deformations and internal forces of the retaining structure and the settlement of the ground surface. This mainly occurred because of the limitation of the calculation method and the influence of the partial factors. The overestimation of the results can ensure the security of design in practice. However, an overly high cost and an unnecessary waste of materials may occur.
In the design process of a deep excavation, verifications of not only the resistance from the retaining structure but also the stabilities of the excavation and retaining structures are necessary. The limit equilibrium methods are frequently used by geotechnical engineers to study the stability problems of both slopes and deep excavations [
For the FEM model, the strength reduction method is commonly applied in stability analysis to calculate the safety coefficient. In the Plaxis program, the strength parameters,
Slip surfaces of the excavation based on various calculation methods.
In addition to the stability analysis of excavation, the verifications of resistance to overturning and basal stability are also important for an excavation design. For a multirow tieback anchored pile wall, the prestressed tieback anchors can effectively restrain the overturning failure, causing the rotational failure and the basal stability failure to occur with higher probability compared to the overturning failure. Due to this fact, the verification of resistance to overturning is not required according to JGJ 120-2012.
Several limit equilibrium methods are available for evaluating the basal stability of excavations. The calculated safety factors vary with the different methods [
In Europe, the Structural Eurocode program comprises ten standards (EN1990 Eurocode-EN1999 Eurocode 9), of which Eurocode 7, Geotechnical Design, provides guidance and rules for the geotechnical design of civil engineering works [
Regarding the geotechnical designs for deep excavation, the Chinese standard and European standard have many rules in common. For both the Chinese standard and European standard, verification is a critical procedure during the design process and includes the verification of resistance to internal failure for structural elements, such as verification of the bending resistance and shear resistance, the verification of resistance to overturning, and the verification of the overall stability of excavation. In addition, the design theories based on the limit states, including the ULS and SLS, are described in two standards, where a slight difference exists regarding the definition of the SLS. The classical soil mechanics theory is applied to evaluate the geotechnical actions and the resistance of soils for both the Chinese and European standards.
There are lots of differences between the Chinese and European design standards in terms of the design focus, partial factors, design approaches, and so on. For the European code EN 1997-1, the primary design theory is the reliability theory based on the limit states, the requirements of which are defined to satisfy the following inequality:
Compared with inequality (
In terms of the partial factors, the design standards of China and Europe for deep excavations have significant differences. For the European code, the partial factors are assigned with different values for the actions, soil parameters (material properties), and resistances. For example, the partial factors of actions are distinguished according to the various action conditions, that is, permanent actions or variable actions and favorable actions or unfavorable actions (see Table
The partial factors of actions and effects of actions.
Action | Set | ||
---|---|---|---|
A1 | A2 | ||
Permanent | Unfavourable | 1.35 | 1.0 |
Favourable | 1.0 | 1.0 | |
Variable | Unfavourable | 1.5 | 1.3 |
Favourable | 0 | 0 |
The concept of the partial factor was developed from the safety factor in stability estimates which was proposed by Coulomb [
Various partial factors for actions, material properties, and resistance. DS 415: Code of Practice for Foundation Engineering; CFEM: Canadian Foundation Engineering Manual; NBCC: National Building Code of Canada; ANSI: American National Standard.
Compared with the partial factors in European and Chinese standards, the partial factors used in the European design standard are more detailed than those used in the Chinese design standard. Moreover, the reliability theories of the actions, materials, and resistances are relatively clear for the European design standard.
In terms of the design approaches, when considering the limit states of damage or excessive deformations of structures or sections of the ground (STR and GEO) in persistent and transient situations, three design approaches can be selected from the European design standard, which serve as the critical part of EN 1997-1 and constitute the primary difference with respect to the Chinese design standard. The three design approaches, DA1, DA2, and DA3, differ in the way in which they distribute partial factors among actions, material properties, and resistances.
In Table
The partial factor combinations for three design approaches.
Design approach | Combination | |
---|---|---|
DA1 | Except for the piles and anchors | Combination 1: A1“+”M1“+”R1 |
Combination 2: A2“+”M2“+”R1 | ||
For the piles and anchors | Combination 1: A1“+”M1“+”R1 | |
Combination 2: A2“+”(M1 or M2)“+”R4 | ||
|
||
DA2 | Combination: A1“+”M1“+”R2 | |
|
||
DA3 | Combination: (A1 or A2) “+”M2“+”R3 |
A sheet pile wall design example was considered using the European and Chinese design standards to compare the design results of the two codes [
Excavation retained by an anchored sheet pile wall.
The three design approaches are used in the design, the partial factors used for which are listed in Table
The partial factor used in the European code.
Parameter | Symbol | DA1-1 | DA1-2 | DA2 | DA3 | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
A1 | M1 | R1 | A2 | M2 | R1 | A1 | M1 | R2 | A1 | A2 | M2 | R3 | |||
Permanent action | Unfavourable |
|
1.35 | 1.0 | 1.35 | 1.35 | 1.0 | ||||||||
Favourable |
|
1.0 | 1.0 | ||||||||||||
Variable action | Unfavourable |
|
1.5 | 1.3 | 1.5 | 1.5 | 1.3 | ||||||||
Favourable | - | 0 | 0 | 0 | 0 | ||||||||||
Shearing resistance ( |
|
1.0 | 1.25 | 1.0 | 1.25 | ||||||||||
Unit weight ( |
|
1.0 | 1.0 | ||||||||||||
Bearing resistance ( |
|
1.0 | 1.0 | 1.4 | 1.0 | ||||||||||
Sliding resistance ( |
|
1.1 | |||||||||||||
Earth resistance ( |
|
1.4 |
The design results for the sheet pile wall using European code EN 1997-1.
Symbol | DA1 | DA2 | DA3 | ||
---|---|---|---|---|---|
DA1-1 | DA1-2 | ||||
Soil parameters |
|
38 | 32 | 38 | 32 |
|
30 | 30 | 30 | ||
|
20 | 20 | 20 | ||
|
0.21 | 0.26 | 0.21 | 0.26 | |
|
7.39 | 5.18 | 7.39 | 5.18 | |
|
|||||
Actions |
|
1.11 | 2.01 | 2.05 | 2.05 |
|
1790 | 2040 | 2180 | 2035 | |
|
1789 | 2061 | 2188 | 2148 | |
|
272 | 291 | 310 | 289 | |
|
190 | 209 | 222 | 218 | |
|
81.9 | 81.7 | 88.2 | 71 | |
|
94.6 | 101.9 | 82.0 | ||
|
5.42 | 5.57 | 5.63 | 5.23 | |
|
296 | 303 | 331 | 247 | |
|
81.9 | 88.2 | 71 | ||
|
|||||
Resistance to overturning | Overdesign factor | 1 | 1.01 | 1 | 1.06 |
|
|||||
Bending resistance |
|
497 | 355 | 497 | |
Overdesign factor | 1.64 | 1.07 | 2.01 | ||
|
|||||
Shear resistance |
|
763.2 | 545.1 | 763.2 | |
Overdesign factor | 9.32 | 6.18 | 10.75 | ||
|
|||||
Anchor pullout capacity | Overdesign factor | 1.37 | 1.16 | 1.59 |
The design results for the sheet pile wall using Chinese code JGJ 120-2012.
Symbol | Classical method | Leading Software |
---|---|---|
|
0.24 | 0.24 |
|
4.20 | 4.20 |
|
2.4 | 2.4 |
|
249.4 | 281.9 |
Bending resistance overdesign factor | 1.99 | 1.76 |
|
80.2 | 127.2 |
Shear resistance overdesign factor | 9.52 | 6.00 |
|
92.8 | 100.5 |
Anchor pullout capacity overdesign factor | 1.40 | 1.29 |
The comparison between the Chinese and European design approaches showed that the design theories used in the two codes are similar, with the exception of a major difference in terms of the definitions of partial factors. In the European code, the partial factors for actions, materials, and resistances are defined separately. However, in the Chinese code, the comprehensive partial factor
The design results obtained using the various design approaches are illustrated in the column graph of Figure
Design results obtained using various design approaches.
As shown in Figure
The overdesign factors represent the gap between the actions calculated and the resistance of structures. The larger the overdesign factors are, the more insecure the design results are. For the two design approaches of the Chinese code, the Deep Excavation software was more conservative than the classical method, especially for the shear resistance. For the three design approaches of the European code, DA2 was the most conservative one because the partial factors of the resistance were used in DA2, while DA3 was the most insecure among the three approaches.
Actually, a total safety factor can reflect the security of an excavation or a slope to some extent. Therefore, the strength reduction method was selected using the Plaxis program to obtain the safety factors of the design results. Seven finite element models were established based on the design results. Figure
Safety factors of various design results.
In construction, uncertainties, such as uncertain geological conditions, uncertainties in laboratory and in situ tests, and uncertain construction quality, introduce several risks to a deep excavation. Thus, knowing how to reduce the risk during excavation construction is critical to ensure the safety of the excavation and its surroundings. Routine programs of inspection and monitoring can provide early warnings regarding the need for precautionary or remedial measures to safeguard the stability of an excavation.
The deep excavation case history was located in the center district of the city, where a busy street and dense constructions exist in close proximity (see Figure
Planar graph of the excavation during different construction periods. (a) 5th excavation (−15.3 m). (b) 8th excavation (−22 m).
For a deep excavation, a temporary road used for construction is necessary to transport soils or construction machines between the ground surface and the bottom of the excavation. Figure
The construction of the underground passage connecting the north site and Region A was a critical part of this case history (see Figure
The underground passage not only will serve as a passage connecting two business zones for people after construction is completed but also is the passage used to transport soils and machines during construction. Two passages, Passage 1 and Passage 2, used during construction are shown in Figure
The calculated results for three types of columns.
Column type | Vertical deformation (mm) | Horizontal deformation (mm) | Shear force (kN) | Axial force (kN) |
---|---|---|---|---|
Column A | −9.57 | 2.19 | 63.8 | 13173.6 |
Column B | −13.27 | 2.13 | 353.5 | 14031.7 |
Column C | −12.51 | 2.65 | 162.1 | 13599.6 |
Underground passage (under construction).
Anchor tests aim to obtain the pullout capacity and the performances of tieback anchors. In China, the anchors used in projects need to be tested based on the requirement of CECS 22:2005 [
Load-displacement response of the acceptance test.
Load-displacement response under cyclic loading.
For the acceptance test, the tieback anchor was test-loaded in accordance with the requirements of CECS 22:2005. This involved the application of a load from 0.
For the basic test, the tieback anchor was tested using 6 loading cycles. The load-deformation curves of the basic test for the selected anchor are presented in Figure
In addition to the acceptance test and the basic test, the creep test is another important anchor test that aims to verify the long-term performance of an anchor based on the measured creep rate. For anchors in sandy soils in Shenyang, prestress loss caused by anchor creep is not significant. Due to this, the creep test is not often carried out for the construction of a ground anchor.
Anchor tests are also required according to the European codes EN 1537: 2013 and ISO 22477-5: 2009 and the American code FHWA-IF-99-015. For the European code, the basic test is called a suitability test. For the American code, the acceptance test and basic test are called the proof test and performance test, respectively. The processes of the tests for the three codes are similar, except for some detailed differences, such as the peak load, the start and end time of each cycle, the capacity criterion, and the test quality of anchors. The Chinese code CECS 22:2005 is much more similar to the American code, which is simpler than the European code.
For a tieback anchored pile wall, the quality of the tieback anchors can influence the performance of the whole supporting system. Thus, the steel wales and their weld seams need to be constructed in accordance with the design documents to keep the fixing of anchor heads reliable. The holding-load tension method and overload tension method should be adopted during the process of anchor installation. The tension load should be set to 105% to 110% of
At the beginning of 2013, part of the weld seams connecting steel wales cracked due to the inferior construction quality (see Figure
Weld cracking in steel wales.
Prestress of the anchor cable of M12.
The anchors initially showed different degrees of prestress loss after they were installed (Figure
Change in the prestress rate of the anchor cable with time.
The in situ measured data were compared with the fitting curves using the Origin program, and
The horizontal deformations of the five pile heads on the east side of the deep excavation are shown in Figure
Horizontal deformations of pile heads.
The processes of soil excavation and tieback installation were mainly carried out from early December 2012 to late January 2013. After that time, the temperature dropped, causing the construction to stop. The horizontal deformations of the pile heads increased rapidly during the soil excavation (see Figure
In late December 2012, a large number of rebar and concrete formworks were placed on the ground surface behind the pile wall, with a distance of 3 m to 10 m from the edge of the excavation and just behind the monitoring point, ZD2. The overweight loads caused significant deformation, and the deformation rate of ZD2 increased to 4.3 mm/d, which exceeded the critical value set by GB50497-2009 [
Artificial dewatering methods, such as the tube well method, vacuum well method, and ejector well method, are recommended in China. The application scope of each method is different. The vacuum well method and ejector well method are suitable for clay, silt, and sand, of which the permeability coefficients are less than 20 m/d, and the dewatering depth is less than 20 m. The tube well method is suitable for soils of which the permeability coefficients are less than 200 m/d. The permeability coefficients of in situ soils in this excavation case range from 40 m/d to 80 m/d, measured via pumping tests. As a result, the tube well method was selected for this project.
In this deep excavation case, the dewatering wells were set around three sides of the excavation, except the north side (see Figure
The total length of the drainage line was 765 m, including twenty-nine dewatering wells and three water collecting basins with a size of 1 m × 1.5 m × 2 m. The drainage line used steel tube drainage pipes with a diameter of 0.426 m. The water pumped out of the ground was drained to the city’s drainage system through the drainage line.
After construction dewatering, the groundwater level declined, resulting in the pore water pressure in the ground reducing and the effective stress of the soil skeleton increasing. As a result, the soil consolidation caused the ground surface to settle. Because the permeability coefficient of the sandy soils in Shenyang is large, the influence scope of dewatering is wide. The surrounding buildings and underground pipeline were affected by dewatering to some extent. According to the code JGJ 120-2012, the influence radius of construction dewatering in an unconfined aquifer is
In this excavation case,
The groundwater level was measured throughout the excavation. Figure
Groundwater level measured at JS23.
The design, construction, testing, and monitoring of a tieback anchored pile wall were described. By back-analyzing Young’s modulus, FEM models using the MC model and the HS model were established to compare with the design results calculated using a design program. The similarities and differences between the Chinese and European design standards were described, and the design results were analyzed through a design case. The crucial construction method and the monitoring results of the case history were presented. The performance of the tieback anchored pile wall was studied based on the test results and measurements. The following conclusions were drawn: Based on the back analysis of the deformation parameters of in situ soils, the horizontal wall deformations and ground settlements calculated using the HS model and the MC model agreed well with the observations. However, the MC model resulted in slightly conservative ground settlements. Thus, for the FEM analysis of a deep excavation, the MC model is not appropriate. The design estimates of deformation and resistance obtained using the elastic method were larger than the FEM results and the observations. The overestimation of the results can make the design more secure but less economical in practice. For the excavation supported by a tieback anchored pile wall, the ground behind the wall was reinforced by prestressed tieback anchors, resulting in the sliding scope extending further behind the wall. The potential slip surface predicted by the limit equilibrium method showed a significant deviation with respect to the results of the strength reduction method. The strength reduction method is recommended to predict the potential slip surface of a tieback anchored pile wall supporting excavation. Compared with the Chinese design standards, the employment of the reliability theory based on the limit states and the division of partial factors in the European design standards are clearer. In the European design standard, three design approaches, DA1, DA2, and DA3, are recommended for conducting verifications, which distinguish the distribution of partial factors in terms of actions, material properties, and resistances. However, the Chinese design standard tends to use the comprehensive partial factor According to the design case of a tieback anchored pile wall, the Deep Excavation software (the elastic method) is more conservative than the classical method for the Chinese code. For the European code, DA2 is the most conservative design approach because of its use of partial factors of resistance. DA3 is the most insecure of the three approaches. Comparing the Chinese and European codes, the deviation between the design results is small. The Chinese code focuses on the design result, while the European code focuses on the design process. The test results showed that the tieback anchors in gravelly sand had a good performance under a load of 780 kN. When the anchors were initially locked, the anchor prestress had an exponential relationship with time. The prestress loss increased in three stages: the fast loss stage, the slow loss stage, and the stable stage. The holding-load tension method and overload tension method are effective in reducing the prestress loss. Engineers need to pay attention to the fast loss stage of tieback anchors and retension the tieback anchors when the prestress loss is excessive. According to the observed deformations of the pile heads, the deformations of the structures and ground in sandy soils showed only a slight variation with time. The consolidation behavior of sandy soil was shown to mainly involve primary consolidation. Therefore, the frequency of deformation observation is suggested to be increased after the soil excavated and the retaining structure needs to be installed in time.
No potential conflicts of interest were reported by the authors.
This work was supported by the National Natural Science Foundation of China (Grant no. 51578116).