FOUNDATION ENGINEERING PRACTICE IN TAIWAN – HIGH …hkieged.org/HKIE/download/Foundations in...

The state-of-the-practice of geotechnical engineering in Taiwan and Hong Kong 28

FOUNDATION ENGINEERING PRACTICE IN TAIWAN

– HIGH SPEED RAIL EXPERIENCES

Chung-Tien Chin MAA Group Consulting Engineers

Jie-Ru Chen

MAA Group Consulting Engineers

Abstract: Foundation systems used in Taiwan include spread footings, mat foundation, and deep foundation. Spread footings and mat foundation are mostly used as building

foundations, whereas deep foundation has seen extensively usage both for buildings and

infrastructures. Over the past decade, significant foundation engineering practices and

experiences in Taiwan have been obtained through the 345-km Taiwan High Speed Rail

(THSR) project, which involved installation of more than 30,000 large diameter bored piles.

In this paper, a brief overview of the foundation systems used in Taiwan is first given. Then

discussions will be focused mainly on various topics related to pile foundation design and

construction of the THSR project.

INTRODUCTION

Foundation systems used in Taiwan include shallow and deep foundations. Shallow foundations including footings and mat foundations are mostly used as building foundations for various types of subsoil conditions encountered in Taiwan. Due to the scope of this paper, relevant shallow foundation practices are referred elsewhere (Moh and Ou 1979; Moh and Song 1980; Chin et al. 2005). With increased development of tall buildings and significant infrastructures as a result of booming economy in Taiwan, deep foundations have been extensive used in various types of projects. Figure 1 illustrates some commonly used deep foundation systems in Taiwan. For convenience, the deep foundation can be categorized into three groups: driven piles, drilled shafts, and barrette foundation. A detailed review of these deep foundation practices can be found in a few papers (Moh 1994; Chin 1994; Chin et al. 2005).

Deep Foundations

Barrette Foundations

Bored Piles

Driven Piles

Steel Pipe Piles

Precast Prestressed Concrete Piles

Reverse Circulation Method

Full Length Casing Method

Figure 1 Commonly Used Deep Foundations in Taiwan Commonly used driven piles include precast prestressed concrete piles and steel pipe piles.


Driven piles in cities like Taipei were mostly used for bridge foundation and machine foundation before 1980s. This type of deep foundation, however, has been rarely used in urban area in recent years because of the concerns on noise and vibration. The driven piles are popular on reclaimed sites (Woo et al. 1990) and industrial plants and facilities, where they have been used in the steel plant (Moh 1987), power plant (Yen et al. 1989), chimney (Duann et al. 1994), etc. Drilled shafts are almost always referred to as bored piles in Taiwan. Most common construction method is the reverse circulation method. The reverse circulation method was introduced to Taiwan in early 1960s, but this construction method was soon became the most popular drilling method for bored piles in Taiwan. The full length casing method was introduced to Taiwan in the early 1990s and soon it became widely used in public or industrial construction projects. One of the first documented cases in Taipei region was the Bitan Bridge, a river-crossing bridge of the Expressway No. 3 built in 1990 (Fang and Chen 2003). In general, a number of advantages were taken using full length casing method in bored pile construction. The casing method makes installation in gravelly soils or bedrocks much easier than that by the reverse circulation method. In addition, soils surrounding the pile are protected by the casing from caving in through the entire construction process. Barrette foundations differ from other types of cast-in-place reinforced concrete piles in that barrettes are rectangular in shape and are installed by diaphragm wall machines. The advantages of the barrette foundations are basically the same as the diaphragm wall such as high precision in vertical alignment and workability in gravelly soils. In addition, they can be combined to form sections with different geometry, for example, the cruciform, T-shape and H-shape. Barrette foundation often can provide better lateral resistance and carry huge loads from the superstructure. In recent years, use of barrettes or a combination of barrettes and bored piles as building foundation system was found in a number of projects. In Taiwan, the most widely used deep foundations for infrastructures are bored piles. For the Taipei Rapid Transit Systems, bored piles installed by full length casing method are used extensively for the on going construction project of Neihu Line CB410. For highway, the viaducts mostly were supported by bored piles that may be installed either by the reverse circulation method or the full length casing method. Bored piles also were used widely for the Taiwan High Speed Rail (THSR) Project. Along the 345-km Taiwan high speed rail, more than 30,000 piles were installed. Extensive technical matters regarding bored piles have been learned through this project. For the rest of the paper, the foundation engineering practices through this significant project in Taiwan are discussed in details. An overview of the THSR Project will first be provided, and then important design requirements and considerations are discussed. This is followed by discussions of load test programs and quality control procedures for pile installation implemented in this infrastructure project. Finally, several issues related to design and analysis of foundation systems were discussed from a retrospective perspective.

THE TAIWAN HIGH SPEED RAIL PROJECT

The Taiwan High Speed Rail Project is considered one of the largest BOT (Build-Operate-Transfer) projects in the world with an estimated construction cost of US$ 15 billions. The THSR runs through populated west coast of the Taiwan Island, and its 345-km guideway links several major cities from Taipei to the southern city of Kaohsiung (Figure 2). The maximum design speed of the THSR is 350 km/hr, which is expected shorten the


commute time between Taipei and Kaohsiung to be within 90 minutes. With a capacity of carrying 300,000 passengers per day, the Rail is expected to contribute significantly in alleviating the overcrowded traffic of the west corridor. In addition, the Rail is also expected to improve the efficiency of intercity transportation and enhance resource redistribution along the west coast, where in turn it would boost the economic growth and balance regional development. The design and construction of the system commenced in Year 2000 with the time of operation targeting at the end of 2005. However, due to some complications at the stage of electric/mechanical system implementation, the schedule of operation has been postponed one year. The geological conditions along the route of the THSR vary considerably. The Taiwan Island is located on the western edge of the Pacific-rim earthquake belt and sits over the juncture of the Eurasian and Philippine Sea Plates (Figure 3), which is an extremely active tectonic region with seismic activities among the highest in the world. Complex geological structures of the island were formed by the active tectonic activity, which also created active faults islandwide. Figure 2 also shows the general geology of Taiwan. As can be seen, the THSR route passes through provinces of Terraces and Mountainous Hills on the north and Coastal Plains with thick sedimentary deposits on the south. The continuous collision of plates also has produced numerous earthquakes annually. Figure 4 shows the distribution of earthquake hypocenters recorded in the vicinity of Taiwan over the past century. More than 200 earthquakes with tremors sensible to human (usually with Level 2 intensity and above) each year in Taiwan were recorded. In the past 15 years, more than 30 earthquakes of Ritcher’s scale of 5.0 and above were recorded (Moh and Yao 2005). As a result, seismic forces and earthquake related effects are major concerns for the design of the THSR.

Figure 2 General Geology of Taiwan and Route of the Taiwan High Speed Rail

(Seah et al. 2005)


Figure 3 Tectonic Plates Near Taiwan (Moh and Yao 2005)

Figure 4 Earthquake hypocenter distribution in vicinity of Taiwan (Moh and Yao 2005)

PILE FOUNDATIONS FOR THE TAIWAN HIGH SPEED RAIL PROJECT

Owing to the variation of the geological conditions along the rail route, methods of construction employed for the THSR also varied. Approximately three-quarters of the route were carried by viaducts and bridges with 14% in tunnels and 9% on embankments (Seah et al. 2005). Accounting for one-fifth of the route, the northern section is constructed mostly by cut-and-fill construction in hilly areas with bridges across rivers and tunnels through mountains. The rest of the route goes through the coastal plain, where elevated viaducts with the support of large bored piles were mostly used. Due to significant variations in ground conditions, different construction methods also were adopted for the installation of piles along the THSR route. In the north segment of the viaduct section, the route runs along foothills on mostly terrace deposits with a layer of gravel near the surface underlain by layers of sandstone or mudstone. In these ground conditions, the bored holes would have to be protected by casings, and hammer grab (within gravel layer) or drilling bucket (within rock) would need to be used for excavation. The bored holes were either fully or partially cased dependent on the stability and seepage condition of the hole. For the southern 155 km-segment of the route, over 20,000 bored piles with diameters of 1.5 to 2 m and lengths of 35 to 72m have been installed to support the viaducts. In these regions, the ground

Depth (km)

Eurasian Plate

Philippine Sea Plate


conditions are composed mainly of interbedded sand and silty clay layers, and bored piles were mostly installed by reverse-circulation method due to its popularity, efficiency and availability in Taiwan. Rotary bucket type of drilling also was used in one of the contract sections for pile installation, but this method was primarily used for pile length not exceeding 60 m in sedimentary deposits. Higher skin resistance has been achieved through the use of this method (Seah et al. 2005). In subsequent sections, various issues related to the design and construction of viaduct foundations are discussed.

DESIGN CONSIDERATIONS

Because of the importance of this infrastructure, all the structures were designed with a life of 100 years. In addition, the trains were set to operate at a high speed while maintaining running safety and comfort level for passengers, more stringent criteria than regular railways and highways were adopted in the design. In this section, particular requirements and considerations that were addressed in the design of viaduct foundations are discussed, including: (i) design requirements, (ii) seismic effect, (iii) near/crossing fault effect, (iv) ground subsidence, and (v) foundation scouring.

Design Requirements

Since seismicity is of great concern in the island of Taiwan as mentioned previously, seismic forces are major factors to be considered in designing infrastructures. For the THSR project, the primary purpose of earthquake design is to safeguard against major failures and loss of life, and two levels of earthquakes are specified in the design specification (THSRC, 2001): (a). Type I Earthquake – Design for repairable damage

Type I Earthquake is the ground acceleration corresponding to a return period of 950 years, which has a 10% probability of exceedance in 100 years. In this earthquake level, structures are allowed to yield but damages to structures, if any, shall be repairable. The design peak ground accelerations (PGA) in the horizontal direction for type I earthquake is different for different zones in Taiwan, as shown in Figure 5. The peak ground accelerations in the vertical direction are two-third of those in the horizontal direction.

(b). Type II Earthquake – Design for safe operation at maximum speed and no yielding

Type II Earthquake is intended for the serviceability limit state design for structures carrying THSR tracks, in which no yielding are allowed and the operation of the system subsequent to earthquakes is expected to be unaffected. To make sure that trains still can run at their full speed of 350 km/hr, it is mandatory to inspect the deformations of tracks after earthquakes and make sure they are within allowable ranges. The design PGA in the horizontal direction for Type II earthquakes are one-third of those specified for the Type I earthquakes and the design ground acceleration in the vertical direction are two-thirds of those in the horizontal direction.

For the design of foundation capacity and stability and for the settlement design, five unfactored loading combination cases were specified. Case 1 is the combination for “Normal Load” condition, Cases 2 and 3 are combinations for “Exceptional Load” conditions, while Case 4 is the combination for “Ultimate Load” condition. All these load cases are essentially design conditions corresponding to the ultimate limit state. Case 5 is the loading combination for verification of settlement criteria. For the safety measure of pile foundations and barrettes, some empirical safety factors were specified, as shown in Table 1. For the structural design of foundation components, factored loading combinations along with


the load and resistance design may be used.

Figure 5 Design Ground Acceleration for Type I Earthquake (THSRC 2001)

Table 1 Safety Factors for design of pile foundations and barrettes (THSRC 2001)

Safety Factor

Normal Load Exceptional Load Ultimate Load

End Bearing Capacity 3.0 2.0 1.25

Skin Friction 2.0 1.5 1.25

Pullout Resistance No tension forces are

permitted on piles.

2.5 1.5

One of the significant requirements for the foundation design is to meet the strict deflection control, vertical settlement and horizontal displacement criteria, to ensure running safety of trains at their maximum speed of 350 km/hr. It is prescribed in the design specification of bridge and viaduct foundations that the differential vertical settlements between adjacent piers after completion of construction shall not exceed 1:1000 for simply supported spans and 1:1500 for continuous spans (THSRC, 2001). For the computation of the vertical settlements, all superimposed dead loads including trackwork shall be accounted for, which also are stipulated to ensure that the vertical alignment of rails meets the criteria. As a serviceability limit state criterion under Type II earthquakes, maximum lateral displacement shall not exceed: 50 mm tilt from vertical axis at top of caissons, 50 mm relative displacement between pile head and pile toe, and 50 mm relative displacement between barrette head and barrette toe. The design of the THSR structures has the basic concept of performance-based design, in

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5.D

WG

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ZONE2

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1BZONE

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ZONE

1B

ZONE 1B

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0.40

0.34

0.28

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which different performance requirements were considered for different earthquake levels. However, the performance requirements were not implemented in an explicit form of performance matrix. Table 2 attempts to summarize the loosely specified design requirements in the context of the performance matrix. Further discussions on the performance-based design concept are given in a later section.

Table 2 Performance Matrix Attempted to Summarize Foundation Design Requirements

Limit State Serviceability Ultimate (Repairable)

Long Term Condition Case 5 Cases 1, 2, 3

Type II Earthquake *specified for structural

design

Type I Earthquake Case 4

Seismic Effect

Under earthquake loading, various seismic effects should be considered in the design in addition to the seismic forces. Since thick sedimentary deposits with interbedded sand and clay layers were encountered over the southern portion of the THSR route, liquefaction susceptibility for the soils is a significant effect to be assessed. During strong earthquakes, pore water pressure in saturated soils may increase due to the application of cyclic shear stresses induced by ground motions. This may cause reduction of effective stresses leading to partial or total loss of shear strength of soils. The soil deformation accompanied may be limited (cyclic mobility) or unlimited (liquefaction). As a consequence of liquefaction, loss of foundation support, flow slides, slumping, lateral spreading, ground subsidence, etc., may occur. For the THSR project, contractors are specifically required to evaluate the soil liquefaction potential and address its effects in foundation design. It is stipulated that where potential of liquefaction is identified under either Type I or Type II earthquake, considerations shall be taken to remove the liquefiable soils, implement soil improvement techniques, or utilize piles or cassions to transfer loads from superstructures to a stable stratum. Liquefaction potential is basically expressed in terms of the ratio of soil resistance to the shearing stress induced. The average earthquake-induced shearing stress is normally estimated using the expected peak ground acceleration at the site by empirical correlations with earthquake magnitudes. The cyclic soil resistance normally is determined either by analytical method through laboratory tests (e.g., cyclic triaxial or simple shear tests) or by empirical method utilizing in-situ tests (e.g., standard penetration test and cone penetration test). In Taiwan, various simplified methods proposed by Seed et al. (1985), Tokimatsu and Yoshimi (1983), Japan Road Association (1996), and National Center for Earthquake Engineering Research, USA (Youd et al. 2001) often were considered for liquefaction evaluation. For the THSR project, the method proposed by Japan Road Association (1996) is adopted. This method has updated with experiences from the Hyogoken-Nanbu Earthquake in 1995, and it has the advantage of estimating reduced soil strength parameters as a result of liquefaction. For piles, reduction in soil strength will lead to decrease in bearing capacities. In the mean time, the reduced soil stiffness will result in larger lateral movements and larger bending moments when piles are subjected to lateral seismic forces. Consequently, the pile length, reinforcement, and even the pile diameter have to be increased (Moh 2004).

Negative Skin Friction


The design of piles needs to take into consideration the effects of negative skin friction resulting from dewatering or liquefaction of surrounding soils. Analysis should be carried out to compute ground subsidence caused by dissipation of excess pore pressure within liquefied soil layers after an earthquake and the resultant negative skin friction on piles. If negative skin friction were developed, it is considered as an addition to the working load as stipulated in the THSR design specifications. However, the Chinese Building Codes issued by the Ministry of Interior (1998a, b) stipulate that negative skin friction due to liquefaction is considered as short-term forces and is not necessarily to be combined together with other short term forces, such as wind loads, impacts, and traffic loads, at the same time. Therefore, checks are undertaken separately. In general, it is found that for the THSR Project, negative skin friction does not govern the pile design (Moh 2004).

Liquefaction Lateral Resistance According to the THSR design specifications, it is necessary to study the potential influence of flow slide caused by soil liquefaction. Based on Seismic Design Specification for Railway Bridges issued by the Ministry of Transportation and Communications (1999), and codes by Japan Road Association (1996), flow slide should be considered if one of the following situations is encountered:

(a) The differential height between the bottom of the sea (river in the case for the Rail) and seashore (or riverbank) revetment exceeds 5m.

(b) The thickness of liquefiable soil within 100 m from seashore (riverbank in the case for the Rail) exceeds 5 m and the liquefaction soil is continuous.

Because most rivers along the route have more than 5 m of height difference between the river bottom and adjacent revetment, effects of flow slide due to soil liquefaction have to be considered in design of bridge foundations. The dynamic behavior of pile foundations in liquefiable zone is very complicated because liquefaction advances with time during earthquakes. At the beginning of the earthquake, both reactions and displacements of pile foundations are mainly controlled or forced by the inertia force transferred from the superstructure. After peak of the earthquake excitation, the soil may have already been liquefied and even the phenomenon of flow slide of stratification may start to occur. In this phase, the inertial force transferred from the superstructure is possibly decreased due to shock absorption effect of soil liquefaction. Displacement of the foundation caused by cyclic seismic loading will increase due to weakened soil condition. After the earthquake diminished, loads on the pile foundations are mainly those due to permanent displacements resulted from the flow slide. It is understandable that different time frame exists between the development of flow slide and occurrence of the peak of earthquake. Therefore, the effect of flow slide needs not be included together with the earthquake forces so as to prevent over-conservative design. According to both seismic design specifications mentioned above, the forces acting on piles and the resultant displacements should be checked for the following three conditions:

(a) no liquefaction or flow slide (b) only liquefaction occurs (c) only flow slide occurs

After investigating details for some design cases in the THSR project, it is found that the forces produced by flow slide are relatively small as compared to the seismic forces.


However, when the range and depth of liquefaction are great and the liquefaction is continuous from shallow stratification, the amount of steel reinforcement in the pile head may need to be increased (Moh 2004).

Near/Crossing Fault Effect

The THSR route crosses 3 major faults and ground motions near these faults could be significantly different from those away from the faults. During the THSR design, the National Center for Research on Earthquake Engineering was engaged by the Design Consultant to study ground motions for the THSR section near the Hsinhua Fault (Moh and Associates 2001). Figure 6 shows the near field attenuation of ground motion with distance from the Hsinhua Fault, in which the Boore-Joyner-Funel relationship and the Campbell relationship developed by using Taiwan local strong motion data are both shown. Based on results of this study, it is recommended that the near source effect should be considered when the distance is less than 2.5 km from the fault line. This means that, within the influence range, the viaduct shall be designed based on the near-field seismic forces. However, the THSR specifications also refer to the use of the method proposed by the Japan Road Association (1996). The JRA method takes into account the effect of in-situ stress, so that the near source effect has unique properties of higher seismic intensity and lower number of cycles. Using the JRA method, although the PGA may be increased from 0.34 to 0.50g, the liquefaction potential for a near source site may in fact become even slightly lower. However, it should still be noted that while soil liquefaction potential may not increase at locations near faults, the seismic loads on structures do become larger because of the increased PGA. For convenience, this effect is illustrated for a case near the Hsinhua Fault. For a bridge pier foundation consisting of four piles of 2.0 m diameter, the designed pile length will be increased from 59 m to 61 m when the near fault effect is considered, and the amount of reinforcing steel will be increased by 8 percent (Moh 2004).

Figure 6 Near-Field Ground Motion Attenuation versus Distance from Fault (Moh 2004)

Ground Subsidence

The adverse effect of ground subsidence on foundation piles of the THSR structure is the generation of negative skin friction. According to the design standards for Japanese National Railways, design of foundation piles should consider effect of negative skin friction when the rate of land subsidence exceeds 2 cm/year. When the subsidence rate is more than 4 cm/year, 100 percent negative skin friction should be considered.


Along the THSR route, the most serious ground subsidence region is in Yunlin area, which is located south of the Choshui River. The subsidence is a result of excessive pumping for irrigation and fish farming. Since there are no reservoirs in the area, groundwater had become the sole source of water supply. This area is generally underlain by thick layers of clay, hence consolidation settlements become eminent. The groundwater levels along the THSR route at Kecuo, Yunlin from 1968 to 2000 is shown in Figure 7. It can be seen that the underground water level in the area has dropped by 15 m since 1968. The drawdowns of groundwater level in the Yunlin area along the THSR route from 1968 to 2000 are plotted in Figure 8. Also shown in this figure is the accumulative ground subsidence in the period from October 1990 to September 2001. As can be seen, the total land subsidence in the 11-year period varies from 20 cm to 100 cm.

Figure 7 Monitoring Results of Observation Wells along the THSR Route at Kecuo

(Moh 2004)

Figure 8 Observed ground subsidence and drawdown of groundwater level

along the THSR route in Yunlin area (Moh 2004)


For the section of THSR between TK218+000 and TK+237+000, the rate of land subsidence ranges on average from 0.5 to 16.0 cm/year, hence development of negative skin friction is a potential problem. However, it is important to identify the source of this land subsidence as a result of groundwater withdrawal. In the case that the ground surface subsidence is caused by consolidation of deep soil strata, negative skin friction may not need to be considered. Results of investigation analysis (Moh and Associates 2002) indicated that about 30 percent of the consolidation settlement occurred within the depth above the toe for most piles, and this settlement is less than 2 cm/year. Therefore, it is concluded that the effect of negative skin friction induced by ground subsidence on piles need not be considered (Moh 2004).

Foundation Scouring

The THSR passes across several major rivers, and the design specifications required that the safety of foundation shall be checked against riverbed scoring. According to specifications issued by the Water Conservancy Agency of the Ministry of Economic Affairs (2001) and the design specifications for the THSR project, structures should be founded at depth below the lowest point of riverbed cross-section and below the depth of long-term scouring at the location of the foundation, whichever is deeper. This requirement is deemed reasonable under long-term loading, but it appears to be too conservative when these criteria are combined with requirements for other temporary loading conditions, such as earthquakes. Even it is stipulated in the design specification that only half of the scouring depth be taken for design under earthquake loading. The structural design of substructure systems shall take into consideration both the existing ground elevation and the planned riverbed elevation after scouring. When pier foundations are constructed in water, it is only necessary to bury the foundations with an overburden of 0.6 m if protective measures are taken against scouring. However, such protective measures must not raise the water to a level that would cause concerns on safety of flood control works or the ecological environment of the river (Moh 2004).

PILE LOAD TESTS

Pile load test is the significant part in bridge and viaduct foundation design. It is required in the design specifications that “an appropriate number of advance piles shall be tested to ultimate load to verify design assumptions”. The number of piles to be tested is selected on the basis of one test per 1000 piles. Since the THSR project was divided into twelve contract sections with different designers, the load test programs as well as methods of load tests also varied. The mostly used approach is the conventional static pile load test with instruments. The conventional loading tests for the THSR project normally were carried out to a load over 4,000 tons (Moh 2004; Duann et al. 2004; Seah et al. 2005). Test piles were instrumented with rebar stress transducers mounted at different depths along their shafts for measuring the strains induced together with displacement transducers at top for measuring total settlements. With sufficient instrumentation, it was possible to compute distribution of skin friction and end bearing of the piles from the test results. In addition, Osterberg Cells (O-Cell) were used in two contracts as an alternative to kentledges as loading devices. Dynamic Pile Load Test Method and Statnamic Load Test Method also were acceptable. However, their validity had to be verified by conventional loading tests on similar piles. Additional information on merits and limitations for these various types of load test methods were given in Seah et al. (2005). Some key information learned from these load tests are highlighted in the following sections.


Verification of Design Equations

The primary objective of the load tests is to optimize pile design by establishing appropriate design correlations. A typical load test program for the THSR Contract Lot 291 has been comprehensively covered by Duann et al. (2004). This contract lot included the construction of 28-km viaduct stretching from Chainage TK284 to TK312. In this program, six compression in conjunction with 4 uplift and 2 lateral load tests were conducted. In these test piles, rebar stress transducers were installed at eight and seven different depths for the compression and uplift piles, respectively. With additional instrumentations, the distribution of the skin friction along the pile and the end bearing transferred to the pile toe during loading can be analyzed to develop and verify design correlations. After detail study of the site, four locations were identified for carrying out these load tests. Figure 9 shows the soil profiles for these four test locations. The subsoil conditions along this project route consist mainly of alluvial deposits of interbedded sand and silty clay, except at some locations where mudstone is present as deeper depth.

Figure 9 Soil Profiles at THSR C291 Pile Load Test Locations (Duann et al. 2004)

288K 296K 304K 312K

Chanage (km)

SCALE (km)

0 2 4SAND

CLAY

Legend:

Note: Number next to borehole denotes SPT N value

MUDSTONE

Location 2TP-2

4

3

3

2

5

7

5

7

9

30

28

30

26

29

29

29

18

11

21

38

20

14

14

22

17

18

17

17

26

26

26

31

61

88

63

48403529

32

37

37

46

Location 1TP-1 and TP-1B

4

4

7

6

7

6

6

7

10

10

14

12

21

22

24

23

39

34

37

32

29

69

45

46

29

37

65

65

59

26

57

28

31

44

57

25

39

39

63

63

63

59

32

Location 3TP-3 and TP-3B

10

6

2

18

24

29

22

33

27

28

33

27

24

42

36

39

44

46

48

51

55

28

17

20

19

36

18

26

29

34

41

34

21

24

21

24

36

38

22

20

23

27

30

34

69

8184

Location 4TP-4

7

16

5

15

12

19

18

28

20

26

28

25

21

12

22

24

12

14

11

14

12

12

14

15

13

17

38

29

100

100

100

100

+20

+10

0

-10

-20

-30

-40

-50

-60

Chainage (km)


All piles for this load test program were installed by reversed circulation method. It should be noted that all test piles were constructed without any use of polymer as stabilizing agent, except for test piles TP-1B and TP-3B. Toe grouting was carried out on all compression test piles by injecting grout under pressure through sleeved pipes uniformly spaced over the base of the pile. This was done to re-compact any soil loosened during the boring process and to better mobilize the pile toe resistance. Details of toe grouting procedures are given elsewhere (Duann, et al. 2004). Four compression test piles (TP-1, TP-2, TP-3 and TP-4) were subjected to single stage injection due to inappropriate rubber sleeves, whereas multiple injections were applied on two additional compression test piles (TP-1B and TP-3B).

Figure 10 shows the load settlement curves for these compression load tests. As can be seen, test piles TP-1B and TP-3B show much stiffer response than the other four piles and have no sign of reaching the ultimate capacity at the capacity of the loading frame, whereas tests piles TP-1 and TP-2 have very distinct gradients beyond the yield points giving an indication of low end bearing resistance (Duann, et al. 2004). These differences in loading response are attributed to several construction effects. It is well known that workmanship has profound influences on the capacity of piles. Relaxation of geostress tends to reduce the strength of soil with time, so that it should be desirable to cast a pile as short as possible. The construction records of these six compression test piles are given in Table 3. As can be seen, the low capacity of TP-1 and TP-2 is related somewhat to their longer construction time. Additionally, water tends to soften soils and normally bentonite is used to stabilize the bored hole. The stiffer initial response of TP-1B and TP-3B suggested that better skin friction may be achieved if polymer is instead used to stabilize the hole (Duann et al. 2004). Another factor that would influence the load settlement response of piles is toe grouting, which also could contribute to improve the end bearing capacity where soft toe is a common problem for cast-in-place piles. As a result of slurry mud sediment or wall collapsing, a layer of slime can accumulate at the pile bottom. If the slime was not cleaned up effectively before placing concrete, a “soft toe” will be formed below the pile tip. This problem can be corrected through injecting cement grout at high pressure in stages to strengthen the weak zone surrounding the pile toe. Figure 11 shows the unit bearing capacity versus tip movement for the six test piles. As can be seen, with better grouting technique (multi-stage injection) in TP-1B and TP-3B, stiffer end bearing response has been achieved compared to the other piles. The results clearly indicated the importance of base (or toe) cleaning and the effectiveness of toe grouting as well as the presence of sound bearing stratum. Normally, less emphasis has been placed on the end bearing and higher factor of safety is adopted for the end bearing compared with the skin friction (Duann et al. 2004)

0

50

100

150

200

250

0 1,000 2,000 3,000 4,000 5,000 Load (ton)

Settlement (mm)

TP-1

TP-1B

TP-2

TP-3

TP-3B

TP-4

Figure 10 Load Settlement Curves for Compression Load Tests (Duann et al. 2004)


Table 3 Construction Records of Compression Test Piles (Duann et al. 2004)

Location TP-1 TP-1B TP-2 TP-3 TP-3B TP-4

Diameter (m) 1.8 2.0 2.0 2.0 1.8 1.8

Length (m) 65.0 64.0 62.0 56.5 60.0 54.5

Use of Polymer No Yes No No Yes No

Toe Grouting Single Stage Multi stage Single Stage Single Stage Multi stage Single Stage

Installation Stages Time Taken (Hours)

Drilling 15.5 9.0 9.0 8.5 7.5 9.0

Halting Time 33.0 0.0 38.0 14.0 0.0 14.0

Base Cleaning 2.0 0.5 0.5 0.5 0.5 0.5

Halting Time 1.0 1.0 1.0 0.0 0.5 0.5

Sonic Logging 1.0 0.5 0.5 1.0 0.5 0.5

Halting Time 0.0 0.0 1.0 0.0 0.5 0.0

Caging 11.0 6.0 15.0 6.0 4.0 6.0

Halting Time 0.0 0.0 0.5 0.0 0.0 0.0

Tremie Pipe Installation

0.5 1.0 0.5 1.5 0.5 2.0

Base Cleaning 0.5 1.0 0.5 1.0 0.5 2.0

Halting Time 0.0 0.0 0.5 0.0 0.5 0.5

Concreting 4.0 5.0 5.0 3.5 3.0 3.0

Total Time 68.5 24.0 72.0 36.0 18.0 38.0

Figure 11 Effect of Grouting on End Bearing Capacity of Piles (Moh 2004)


Since all test piles were instrumented, the load distribution curves were available and unit skin frictions can be reduced from the test results for further analyses. From compression test results, various design correlations have been developed. Figure 12 shows the plot of adhesion factor versus the undrained shear strength for cohesive layers, while Figure 13 presents the result of unit skin friction versus the SPT N value for the cohesionless deposits. It also can be seen that improved construction methods have positive effect on both the adhesion factor and the unit skin friction. Similar correlations were developed from other contracts. Figure 14 compiled results obtained from three other contracts for both cohesive and cohesionless soils. The analyses of load test results had led to recommendations of correlations for design of bored piles installed by the reverse circulation method. The recommended correlations adopted for a few contracts are summarized in Table 4.

0.0

0.5

1.0

1.5

2.0

0 10 20 30 40

Undrained Shear Strength, cu (ton/m2)

Adhesion Factor, a

TP-1

TP1B

TP-2

TP-3

TP3B

TP-4

Average for TP-1, TP-3

Average for TP-1B, TP-3B


Figure 12 Adhesion Factor Versus Undrained Shear Strength for Cohesive Soil Layers

(Duann et al. 2004)

0

10

20

30

0 10 20 30 40 50 60SPT N Value

Measured Unit Skin Friction, fs (ton/m

2) TP-1

TP-1B

TP-2

TP-3

TP-3B

TP-4

Slope=0.33

Slope=0.3

Slope=0.15


Average for TP-1B, TP-3B


Figure 13 Unit Skin Friction Versus SPT N Value for Cohesionless Soil Layer

(Duann et al. 2004)

Adhesion Factor, a

Measured Unit Skin Friction, f (ton/m

2)


0

5

10

15

20

25

0 20 40 60 80 100

SPT N-Value

Un

it S

kin

Fri

cti

on

(to

n/m

2)

Design - Contract ADesign - Contract BDesign - Contract CContract AContract BContract C

fs = 0.131 N + 2.6 (ton/m2)

fs = 0.27 N + 2.6 (ton/m2)

fs = 0.14 N + 2.6 (ton/m2)

Cohesive

Soils

0

5

10

15

20

25

0 20 40 60 80 100

SPT N-Value

Un

it S

kin

Fri

cti

on

(to

n/m

2)

Design - Contract BDesign - Contract A,CContract AContract BContract C

fs = 0.33 N (ton/m2)

Cohisionless

Soils

Figure 14 Correlation between Unit Skin Friction and SPT N values (Seah et al. 2005)

Uplift Load Tests

Since the uplift capacity was a governing factor in design of piles under seismic condition in several cases and that it is stipulated in the design specifications that the pullout skin friction cannot be greater than 40% of the downward skin friction. Great attention then had been focused by several contractors to conduct uplift load test so that this limit may be raised up. For uplift load tests also were carried out at the four test locations for the C291 contract. Figure 15 shows the comparisons between the compression and uplift skin frictions, which indicated that the ratio of uplift to compression skin friction in general is greater than 40%. Statistics showed that this ratio ranges from 62 to 105%, with a mean of 81% and a standard deviation of 16 % (Moh 2004; Duann et al. 2004; Seah et al. 2005).


Table 4 Recommended Design Correlations for Shaft Resistance and End Bearing Estimation (Duann et al. 2004; Seah et al. 2005)

Soil Type Resistance Contract C291 Contracts A, B, and C

Shaft Resistance

fs = α cu ≤ 12 ton/m2

α = Adhesion factor

= 0.21+2.6/cu ≤ 1

fs = 0.27N+2.6 ≤ 15 ton/m2

for N ≥ 5

Cohesive

End Bearing qb = 9 su ≤ 160 ton/m2

su = Undrained shear strength (ton/m2)

qb = 5.3 N ≤ 250 ton/m2

Shaft Resistance fs = N/3 ≤ 15 ton/m2

N = SPT-N value

fs = 0.33N (ton/m2) with limit of 16.5 ton/m2

or N ≤ 50

Cohesionless

End Bearing

qb = 7.5N ton/m2

N = Average SPT-N value at depths of 4D above and 1D below pile toe. D = pile diameter

qb = 5.3 N ≤ 250 ton/m2

0

5

10

15

20

0 5 10 15 20Mobilized Compression Skin Friction (ton/m2)

Mobilized Tension Skin Friction (ton/m2)

Clay TP-1 Sand TP-1

Clay TP-2 Sand TP-2

Clay TP-3 Sand TP-3

Clay TP-4 Sand TP-4

Mudstone TP-4

Figure 15 Comparison of Mobilized Skin Friction between Uplift and Compression Piles

(Duann et al. 2004)

Lateral Load Tests

Lateral pile load tests were conducted in ground condition with both mostly sandy profiles and mostly clayey profiles to determine horizontal modulus of subsoil reaction and to verify the design parameters. In these tests, horizontal displacements and lateral loads were measured by displacement transducers and load cells, respectively. An inclinometer also was installed in each test pile to determine the profile of horizontal displacement. Test results revealed that the responses of test piles are quite similar to those predicted by the “p-y” curve method (Figure 16); hence this method was adopted by some of the designers for predicting response of piles in design (Moh 2004).

Mobilized Tension Skin Friction (ton/m

2)


0

5

10

15

20

25

30

-50 0 50 100 150 200

Lateral Displacement (mm)

Dep

th (

m)

60 ton (Prediction)

120 ton (Prediction)





60 Ton (Measurement)

120 ton (Measurement)





CL

Su = 5 t/m2

γ = 2.0 t/m3

SM

φ = 32o

γ = 2.0 t/m3

SM

φ = 32o

γ = 1.9 t/m3

Figure 16 Comparison between Predicted and Measured Lateral Displacement

(Duann et al. 2004)

QUALITY CONTROL ISSUES

To ensure quality of bored pile installation, better construction methods had been employed and quality check were implemented at several stages. Issues related to quality control are discussed briefly below. For stabilization of bored hole, most contractors had used polymer as the stabilizing fluid along with excavated ponds for mud settling. Settling steel tanks only had been adopted in a few occasions. However, the quality of the stabilizing fluid had been checked throughout the drilling process in most contracts. Once the hole was drilled to the required depth, echo sounding or drilling monitoring was performed to check the verticality and dimensions of the hole (Seah et al. 2005). Figure 17 shows a photo of the verticality checking performing at a site. To ensure minimum sediment in the bored hole, airlifting was conducted to remove loose sediment at the bottom of the hole. A photo showing operation of toe cleaning by airlifting is shown in Figure 18. In addition, some contractors adopted special sampling of base sediment to check the cleanliness of the base before concreting (Seah et al. 2005). Honeycombing, segregation and discontinuity of concreting are common problems with bored piles. For this project, over 20 % of the piles were subjected to non-destructive integrity tests. In addition, piles suspected to be defective as indicated by piling records also were subjected to integrity tests. If test results indicated that the pile might be defective, remedial


measures would be taken accordingly. In some cases, continuous coring had been performed to check the quality of the suspected piles (Seah et al. 2005).

Figure 17 Verticality Checking by Echo Sounding

Figure 18 Toe Cleaning by Airlifting

RETROSPECTIVE EVALUATION OF THE THSR PROJECT

Through all the tasks of implementing this significant infrastructure, vast amount of data and experiences have been accumulated. While the civil works for the THSR project have been completed, it is worthwhile to take a look back to this endeavor and evaluate a few aspects of this project from a different perspective. Retrospective discussions are given in the following section on aspects of contractual pattern, design concept and certain earthquake related design of this project.

Project Contractual Pattern

The Taiwan High Speed Rail Project is a mega size infrastructure development in terms of investment, construction size, and complexity, and it was handled by the BOT contract pattern. To meet the operation time target, this project was carried out on a fast track turn-key basis,


and the design and construction had to be proceeded at about the same timeline. This requires extremely close coordination between designers and contractors. Due to the high operation speed, safety becomes the top concern. Design Specifications for the project are therefore much more stringent than conventional highway and railway development (Moh 2004). On the other hand, simplicity in design procedures is an inevitable product for meeting such a tight schedule. An example is the development of design correlations for capacity evaluation of bored piles (e.g., Figures 13 and 14). Many contractors have implemented a very comprehensive advance pile load test program so as to develop design correlations that can optimize design of foundation systems. However, time was limited for thorough and detailed exploration of the accumulated data from the test program. As a result, the developed design correlations as shown by Figures 13 and 14 are very simplified relationships that utilize the uncorrected SPT N values. However, it is well know that the SPT N values are affected by various factors that included the overburden pressure (Skempton 1986). In addition, significant scatter is present for correlations shown in Figures 13 and 14, but the variability associated with these transformation models was not characterized. Additional safety margin is then the only measure to take account of the uncertainty of design. It is interesting to raise this question asking whether the project will be done differently, were it conducted in a different type of contractual pattern, say traditional Engineer’s design (detailed design then build) projects, or governmental turnkey projects. Would more time allow for detailed explorations on data accumulated resulted in much more rational design equations that have uncertain factors better addressed?

Design Code Concept

Since Taiwan is highly susceptible to earthquakes, seismic resistance design is of great importance to the THSR infrastructure. To prevent over-conservatism in design, it is rational considering different performance requirements under different levels of earthquake excitation. This concept was considered in the seismic design of the THSR structures, where two types of earthquakes with different performance requirements were specified in the design specifications. Although the basic performance-based design concept was attempted in the THSR design of civil works, some redundancy may still exist because performance requirements and verification methods were not addressed in a systematic manner. With experiences of the THSR design, it seems importance considering the essence of performance-based design concept and making critical review on current practice. The result of retrospective evaluation on the design code concept would provide useful information for future upgrade of the seismic design code. The performance-based engineering and the development of performance-based design code have been a major pursue in earthquake countries (especially U.S. and Japan) in the past decade. Buckle (2002) stated that there are two fundamental issues that should be addressed for the performance-based design:

1. Selecting the specified ground motions (hazard level) and the corresponding damage states (performance objectives)

2. Developing methods of evaluation for the verification of damage states and performance objectives

An effective way to organize the desired performance requirements is arranging them in a performance (criteria) matrix. This concept has been implemented by Caltran and


considered by AASHTO for the seismic design specification (Buckle 2002). In Japan, the performance matrix also has been used in recently developed geotechnical engineering and civil engineering codes, respectively Geo-code 21 and PLATFORM (Honjo 2004). Both codes are fully performance-based, and the performance matrix used in Geo-code 21 is shown in Figure 19. This matrix consists of three levels of design situations in the vertical axis and three limit states in the horizontal axis, and performance requirements to be specified can be clearly identified based on importance of the structure. In Geo-code 21, serviceability limit state requirements must be specified for both high and low frequency earthquakes, and repairable limit state requirements must be specified for low frequency earthquakes. An attempt to summarize the performance requirements specified in the THSR design specifications into a performance matrix was given in Table 2. With that, it can be compared to the performance matrix of Geo-code 21, and it is suggested that some load cases can be modified and reallocated so as to attain a more complete and effective design consideration.

Figure 19 Performance Matrix for Japanese Geo-code 21 and PLATFORM (Honjo 2004)

In addition, safety measures stipulated in the THSR design specifications are not consistent for structural and geotechnical design. Empirical safety factors were used for the geotechnical design, whereas the load factored design may be used to design the structural components. Essentially, reliability levels associated with these two safety measures are different, which may lead to an over-conservative design either on the geotechnical or the structural components. To attain a harmonized design, it is required that the reliability levels of both the structural and geotechnical designs be known explicitly. It should be known that theoretical developments in structural reliability and applications of probabilistic design are being pursued actively in the structural community (Phoon 2004). Therefore, it warrants that the geotechnical community takes serious efforts in similar endeavor, so that a rational and harmonized design may be achieved in both geotechnical and structural aspects.

Earthquake Related Design

An important aspect of the design is to address relevant seismic effects. Among them, the effect of flow slide as a result of soil liquefaction has been concluded less significant in this project, where it was found that forces produced by flow slide are relatively small compared to the seismic forces. Therefore, the flow slide induced forces were not included in design


check for most cases. However, a special measure was implemented when the range and depth of liquefaction are great and the liquefaction is continuous from shallow stratification, the amount of steel reinforcement in the pile head is increased. One might still have doubt about the reliability of this approach that is without proven verification of safety against flow slide. Another approach related to seismic effect that may be deemed concerned is the design for crossing fault effect. The THSR route crosses two active faults as shown in Figure 2, however the rail was carried by viaducts in both locales. Although the crossing fault effect was considered through elevated seismic impacts, rupture or excessive movement of these faults may still result in adverse event that is beyond the measure of design for viaducts. In this case, embankments may be proven to be a better alternative in terms of repairability and restoration for operation.

CONCLUSIONS

Foundation systems used in Taiwan included shallow and deep foundations. Shallow foundations have been used mainly for buildings, whereas deep foundations have been employed both for buildings and infrastructures. In the past decade, the most significant investment and development is the THSR project, where more than 30,000 piles have been installed. Vast among of data and significant experiences on foundation engineering have been accumulated through the execution of this mega size infrastructure project. Moreover, the state-of-the-practice in foundation engineering in Taiwan also has been updated through the THSR project. Therefore, this paper is focused on the foundation engineering experiences acquired in this project. After providing brief reviews of commonly used foundation systems in Taiwan, the THSR foundation systems were concentrated. An overview of the THSR project was first provided, and then the foundation systems used and their design requirements and considerations were discussed. That is followed by discussions of pile load test programs and quality control issues implemented during the installation of piles. Finally, several issues related to foundation engineering in this project were discussed from a retrospective point of view, now that the civil works have been completed. With the benefit of hindsight, it can be concluded that improved foundation engineering may be achieved with more elaborated studies of the load test data. The project also revealed the need for sound and realistic design concepts, and the performance-based design with due considerations of explicit reliability levels should be the framework for implementation in future geotechnical seismic design. The geotechnical community is way behind the structural community in this subject area. It calls for serious efforts in conducting relevant investigations and developments such that a harmonized design on both structural and geotechnical aspects may be achieved.

REFERENCES

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ACKNOWLEDGEMENTS

The authors are grateful to Dr. Z.C. Moh for his valuable support and advice. Dr. H.W. Yang of Taiwan High Speed Rail Corporation is greatly appreciated for providing useful comments and suggestions. The authors would also like to thank their colleagues at MAA, Messrs. Mark Wang, M.S. Chen, S.W. Duann, T.C. Su, and G.R. Yang, for providing information and suggestions during the preparation of this paper. Appreciation also is due to the Organizing Committee for the invitation to present this paper.

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