Preface

    At 05:40 Beijing time on June 27, 2009, a 13-floors small high-rise building that had been basically completed but not yet delivered was built on the south side of the Jingyuan Riverside Garden, and then collapsed rapidly and the pile was broken [1 ]. This is the case of a reverse building in Shanghai. The building is based on pre-stressed piles (China called pre-stressed concrete piles, hereinafter referred to as foundation piles, piles) as the basic support structure. The purpose of this paper is to specifically discuss the structural mechanics of the pre-stressed pile failures of the foundation piles as the foundation support in this case, in order to be able to use the piles as building structures, bridge structures or other for the future engineering community. When supporting large structures, it is possible to improve and obtain a more complete and stable supporting force.

1. The collapse of the building

1.1 Historical records of the building collapse

    In this case, the investigated results show that the main reason for the overturning is that the north side of the building is up to 10 meters in the short term, and the underground garage pit of 4.6 meters deep is being excavated on the south side. The pressure difference on both sides causes the horizontal displacement of the soil, which is too large. The horizontal force exceeds the anti-slide ability of the pile foundation, causing the house to fall [1]. (See Fig. 1-1, 1-2)

Fig. 1-1 shows the building collapse event [2]

    In addition to the damage caused by strong earthquakes in the Japan early years, in the case of modern architecture, it is very difficult to see the damage of pre-stressed piles. This is an extremely rare case. In particular, the case where the entire core of the pre-stressed pile head is pulled out is even more rare, even in the laboratory (the experiment is generally stopped when the strain gauge reaches the displacement) . If the core-filling segment is better handled, it may be possible to make the friction of the core-filling segment more thoroughly, and it may be possible to stay longer. In this case, there is no low excavation/high filling method to support the side block, and the occurrence of the disaster should be inevitable.

    In terms of structural design, the design engineer hopes that the initial damage point will occur on the "members" rather than the "joints." Therefore, in this case, the pre-stressed pile break is regular, and only “the core-filling segment is pulled out" is the biggest anomaly. Once the core-filling segment of pile head is pulled out (frictional failure), the allowable tensile force is down to zero, and the tensile force of the adjacent pre-stressed pile is greatly increased, the pre-stressed pile is gradually broken, then the building collapse is inevitable. Pre-stressed piles can increase the tensile strength of the pile by increasing the number of pre-stressed steel bars. However, the treatment of core-filling segment of the current construction method (known in Taiwan as pile head treatment, and the mainland is called pile head strengthening) is not feasible. The cement slurry in the inner hole of the pile head and the inferior concrete after the soil mixture is dry and hard are not easily removed, and the tensile strength of the core portion of the implanted pile head is worse than that of the striking pile. In the case of 30 years of implanted pile construction experiences in Taiwan, only a few tensile test reports of the core-filling segment [5][6] were used to affirm and consolidate the construction results of the pile heads, and the rest of the construction cases were mostly unknown.

    After the secondary drilling of pile core and the high-pressure water clear, the interface friction of the pile core and the pile wall is conservatively taken as 0.20N/mm2 (about 2.0kg/cm2) [5], and the estimated friction of fill core of 2M depth of 600mm pile about 491 KN (about 50 tons). In order to increase the friction, only the depth of the core-filling segment is increased; but the deeper the core-filling segment is, the more difficult the cleaning of the inner hole surface is, and the rigor of the construction of the worker is more tested.

2. Improvement of the head joint of pre-stressed piles

    Disasters are always sudden, abnormal, and exceed the allowable range. No one can predict the occurrence of disasters. The pre-stressed pile is an excellent building material with high cost performance (C/P value), but there are still three major defects in the upper, middle and lower parts. The pile head treatment part is the most prone to the upper end of the pre-stressed pile. In the improved DH-PHC pile construction method is to solve the problem, try to consolidate the design of the pile head joints, so that the structural components can get better performance.

    Unlike Taiwan's general implanted piles (the mainland is called the buried type), the pre-stressed piles of buildings in the mainland are mostly piles of driven type (hammer type) or pressed-into type. This is similar to the construction method prevailed in Taiwan in the 1970s. Compared with the implantable method, the construction cost of the striking method is relatively lower (about 40% reduction) [4], but the pile is easily damaged due to hitting, affecting the pile durability.

    In the case of this paper, we focus on the sudden damage of the pile head of pre-stressed pile. In the tensile failure of pre-stressed pile, the core-filling concrete (hereinafter referred to as the core-filling segment) of the pile head core-filling segment is pulled and pulled out of the pile. This is the most unusual case. Since the traditional core-filling segment and the pile wall are mainly transmitted by the frictional force of the interface, in addition to the material, the construction quality of the site is considerably affected, including the construction rigor of the workers/staff. When the pile head tension occurs, since the pile wall is made of concrete, and there is no proper tension connection between the pile and the upper foundation, it can only be relied on by the core-filling segment. The possibility of occurrence of tensile brittle failure increases when the tensile force of the pile exceeds the friction force of the core-filling segment of pile (including the loss or reduction of friction). In the structural design, the engineers will strive to allow the bearing capacity of the component to be greater than the external force, in order to achieve structural safety, at least to achieve the situation of early warning damage, in order to reduce casualties in the disaster.

    In the treatment of core-filling segment of pre-stressed piles, there has been no better way than the design of the friction of the core-filling segments for decades. Most of the engineering designs can only use “Designed to withstand vertical pressure rather than tension” as a design criterion of foundation piles to circumvent the design of the tension. This also caused the stereotype of the pile to be pressed only without being pulled. However, the disaster does not occur as expected only in the pressure bearing. When the tensile load occurs and exceeds the designed tensile capacity, or the friction of the inner hole of the pile is lost or degraded due to the construction of the core-filling segment, the disaster will occur. It will be sudden and significant.

    In research of Zhang Jiaqi (2013) [3], the tensile strength of pile head can be increased by pre-embedded the horizontal reinforcement in the pile head section when prefabricated in the pile factory (Fig. 2-1, 2-3). The combination of vertical steel bars and concrete at the construction site and the core-filling segment becomes a more reliable structural joint, and the tensile load capacity of the pile-head core-filling segment is greatly improved. Since the pile core-filling segment is completed, it is almost perpendicular to the solid concrete section (referring difference of the hollow section of pile). The anchor bars are firmly anchored into the pile wall by core-filling concrete and horizontal steel bars. With sufficient horizontal reinforcement and proper anchoring, the tensile strength of the core-filling segment can be as high as 400 tons or more. However, the pile generally is not designed as a tensile pile, and the required allowable tensile force is not high, and only the tensile capacity of the pre-pressed steel bars can be satisfied.

    Fig. 2-2 shows the compacted type of the horizontal reinforcements of the DH-PHC pile head. The horizontal bars no longer use the hooks, only the straight bars, which are simplified and lower the cost. The pile core-filling segment is subjected to tensile force. When the horizontal bar has 1-2 hooks, the horizontal bar can exert the full shear capacity. The shear capacity of horizontal bars will be controlled by the compressive stress of the projected area of the horizontal bar into the pile wall concrete (diameter*penetration depth) if the horizontal bars don’t have any hook (straight type),. Although the tensile strength of the core-filling segment is slightly reduced, the tensile capacity of pre-stressed bars can still be satisfied. Of course, in the case of special needs (for example as a tensile pile design), the horizontal bars can still be set to provide greater tensile strength.

Fig. 2-1 Horizontal bars of DH-PHC pile head Fig. 2-2 Horizontal bars of compact DH-PHC pile head

Fig. 2-3 DH-PHC pile horizontal bars are assembled and embedded into the pile wall concrete

    In design, the pile core-filling segment will adopt the pre-embedded horizontal bars. After combined with core-filling concrete, the horizontal bars will be embedded in the pile wall concrete, and the steel section will exert sufficient strength to bear the pressure. The bearing type transfers the tension of the core-filling segment to the pile wall. The transmission mode of its pulling force is as follows:

The pile head is subjected to tensile force --> vertical anchoring bar is pulled --> transferred by the reinforcing force of the bar to the core concrete --> transferred to the horizontal bar --> transferred to the pile wall concrete --> pulled by the high-strength pre-stressed bar in the pile wall --> transferred to the full length of pile body --> transmitted to the soil by the friction of the pile body.

    In the design of the horizontal reinforcement of pile head, the depth of the horizontal bars into the pile wall and the width of the steel bars are taken as the bearing area, and the allowable bearing capacity is controlled by the number of layers, the numbers and the diameter of the horizontal bars. The size can be adjusted by the design engineer.

    The bearing pressure of a single end point of a single horizontal bar can be controlled by the following two values:

Shear strength of steel bars, Fhbs1 = As1*fva*Φss ...... (1)
Pile wall concrete bearing pressure, Fhbc1 = Asc*fc' *Φcb ...... (2)
Fhbv1 = min( formula (1) , formula (2) ) ...... (3)
The total supporting force is
Fhbv = Fhbv1*Nhb ...... (4)

    In the formula, As1 = single bar area (mm2). fva = allowable shear stress (N/mm2), taken as bar's yielding strength fy. Φss = steel shear reduction factor, taken as 0.75. Asc = Pile wall concrete bearing area (mm2), taken as db*Lhb, db=rebar nominal diameter (mm), Lhb=depth of horizontal bar embedded into pile wall (mm). fc' = concrete compressive strength of pile wall (N/mm2), taken as 78.5 N/mm2 (or 800 kg/cm2). Φcb = concrete bearing pressure reduction factor, taken as 0.65. Nhb=the total numbers of support points of horizontal bars, taken as (the number of horizontal bar layers * the number of steel bars per layer * 2 end points). [8]

    Since the horizontal bars of the DH-PHC pile is limited by the thickness of pile wall, it will affect the protective cover thickness of the bar [7], and it is not enough to penetrate the pile wall. Therefore, the pile at the smaller outer diameter (i.e. the pile wall is thin), the shear capacity of bars will be limited by the concrete bearing pressure of pile wall. Fortunately, the concrete used in pre-cast piles is high-strength concrete (such as 78.5N/mm2), which can exert a considerable bearing pressure. The total capacity of horizontal bars is still large and stable compared with the traditional core-filling concrete friction. Moreover, the design engineer can change the total number of horizontal bars to increase the allowable tension.

Table 2-1: Tensile capacity of various pile diameter DH-PHC pile core-filling segments under the recommended horizontal bars

    From Table 2-2, we can find effect is better that the horizontal bars of the core-filling segment use 2 smaller diameters (such as 2-D16mm) than the one with a larger diameter (such as 1-D19mm). This is based on the fact that the total vertical projection area of horizontal bars embedded into the pile wall is large. However, if the horizontal bars are provided with hooks, the effect is different.

2.2 Advantages of the implantable construction method using DH-PHC piles

    The implanted type of construction method (buried type) has been implemented in Taiwan and Japan for more than 30 years. Although the construction cost is high, it is recognized as a method to maintain the optimum component performance of the pile. In the out-drilling implanted method, after the pile is implanted into the soil, it is necessary to do a secondary work use a large drilling machine to drill a hole in the pile head, and then cleaning the hole, so that the inner wall surface of the hole may have enough good contact surface.

Fig. 1-2 Photo of the building collapse event [2]

Fig. 1-3 Photo of the building collapse event [3]

1.2 Discussion on the failure reasons of the building supported by pre-stressed piles

    According to the investigated report of the year, this case was caused by the “pressure difference” on both sides of the building due to the excavation of the underground garage (4.6M) on one side and the overfilling side (about 10M) on another side. The "pressure difference" on both sides of the building is the cause, and it has been apparent for some time. The construction of the 13-storys building is definitely more than 28 days, and the strength of the concrete members has been fully utilized. The "pressure difference" on both sides caused the horizontal shear force to be too large, and the building had an "abnormally large overturning moment", which eventually led to the failure of foundation piles as a geological improvement.

    There may be three reasons for failure of the pre-stressed pile: failure of compression, failure of tension, failure of shearing. According to the scene photos published in the media in the same year (see Fig. 1-3), only the exposed parts can be seen. This part is subject to the pull side. We have no way of knowing the failure of the pre-stressed pile on the pressure side of the building (because being pressed under the building, perhaps the photo is also blind to some shooting angles)

    When the pre-stressed pile fails to be pulled, the point of maximum tensile force occur at the higher portion of pile body, generally occurs near the interface of pre-stressed pile and foundation cap. There are several situations:

(1) Occurs in the pre-stressed pile and the foundation cap interface: it may be that the vertical anchoring steel bar fails to break.

(2) Occurs in the core-filling segment of the pre-stressed pile: it may be that the concrete friction of the core-filling segment fails, and the worst part is that the entire core-filling segment is “pulled out”.

(3) Occurs below the core-filling segment of the pre-stressed pile: it may be that the pre-stressed pile itself has failed to break. Generally, the pre-stressed pile body is designed to withstand the "pressure", and the allowable tensile force may be only about 1/5 to 1/10 of the allowable bearing pressure. Since the concrete material of the pile wall is not design to the tensile force, the tensile force is absorbed by the pre-stressed steel bars. The reason why most of the pre-stressed bars of piles hasn’t be seen in Fig. 1-3 is that the pre-stressed steel bars are “shrink back” into the pile walls after the break, and it don’t mean there is not steel bars.

    When the "pressure difference" on both sides of building has been present, the pre-stressed pile on the tension side is subjected to "abnormal tension". The driven type of pre-stressed piles are mainly made of high-strength concrete (fc'=78.5N/mm2 or 800kg/cm2, similar to C80 specifications for hammer piles in mainland China) and high-strength steel bars (fy=1,227N/mm2 or 12,500kg/cm2) and spiral bars are combined and pre-stressed. The pile body material is quite rigid and basically has no ductility. Once the damage(tension failure) occurs, it is directly "brittle destruction". The building is mainly composed of beam-column-slab-wall members, and the overall structure has a certain degree of flexibility and elasticity, which is softer than the pre-stressed pile. If the building fails as a single pre-stressed pile in the support, the allowable tensile force of this pile is “returned to zero” and must be shared by the adjacent piles. In terms of structural design, usually the piles are not designed to "withstand the tension force", and the allowable tension force is lower, so it is prone to "chain breaking and collapse without warning". Of course, it will also fail to the "abnormal pressure" of the pre-stressed pile on the pressure side of the building; when the pile is damaged by pressure, the force will be transmitted to the soil layer, because the pile body strength is much greater than the soil. Intensity, unless the bottom of pile enters the rock (Note: the site of this case is the alluvial soil layer of the river bank), otherwise it will eventually lead to soil collapse and subsidence. However, this case has been judged to be a case of tensile damage of pre-stressed pile.

    According to the photo (see Fig. 1-3, the estimated pile diameter is estimated from the proportion of the human in the photo). The pre-stressed pile diameter is about Φ600mm, the pipe wall thickness is 100mm, and the inner hole (core-filling segment) diameter is Φ400mm. There are two failures of piles on the pull side:

(1) The pre-stressed pile itself is "broken". If the exposed tubular member is, the breakpoint should be near or below the junction of the core-filling segment;
(2) The core-filling segment of the pre-stressed pile is “pulled out”. The member exposed in the upper right corner of the photograph in Fig. 1-3 is a flat, light-colored cylinder of smaller diameter;

    When the pre-stressed pile is damaged by tension, it is normal for the pile body to be pulled off. But the core-filling segment is broken, which is abnormal. Especially, the whole section of the core-filling segment is pulled out, the friction did not play or fail at all on the pile internal hole interface. The construction method of pre-stressed pile is mainly driven type or pressed-into type in the mainland China. The pre-stressed pile internal hole after piling is still clean, and the interfacial friction of core-filling segment is relatively high. In Taiwan, implanted pile always used, the secondary core-drilling process is still required after complete piling. It can increase the roughness of the inner hole, but the inner hole surface of the pre-stressed pile is still inferior cement slurry, and the friction of the core-filling segment after casting concrete is relatively poor (refer to the same case of core-filling concrete) (see Fig. 1-4). [5][6]

    Fig. 1-4 The situation of traditional implanted pre-stressed pile head secondary core clearing residual inferior concrete

    The pre-stressed pile wall distributed with high-strength shaped steel bars. When the pile body is subjected to tensile force, although the allowable tensile strength of the pile-wall concrete is lower, the pre-stressed steel bars can still withstand the tensile force. Table 1-1 shows the pile tensile capacity of the TYPE C pre-stressed pile (been pre-stressed at 7.85 N/mm2, or about 80 kg/cm2). It can be seen that the tensile capacity of the 600 mm pre-stressed pile is 638 KN or about 65 tons. It’s abnormal if the pile body is damaged when the tensile force does not exceed this allowable value. For example, the core-filling segment of pile has been pulled out. The foundation pile is damaged by tension and is brittle failure without warning. It is extremely easy to cause chain damage. In the upper right corner of the picture in Fig. 1-3, it can be seen that the core-filling segment of pile is pulled out and is broken, but may be the piles near pressured side of building have not been damaged. Otherwise, it is presumed that the core-filling segment will be fail due to the inclination of the building, rather than being pulled out.

Table 1 -1 Tensile capacity of existing pre-stressed piles

     Except the damage caused by strong earthquakes in the early earthquakes in Japan, in the case of modern architecture, it is very difficult to see the damage of pre-stressed piles. This case is an extremely rare case. In particular, the case where the entire core-filling segment of the pile head is pulled out is even more rare, even in the laboratory (the experiment is generally stopped when the strain gauge reaches the displacement) . If the core-filling segment is better handled, it may be possible to make the friction of the core-filling segment more thoroughly, and the building may be possible to stay longer. In this case, there is no low-excavation/high-filling method to support the side block, and the occurrence of the disaster should be inevitable.

    In terms of structural design, the design engineer hopes that the initial damage point will occur on the "member" rather than the "joint". Therefore, in this case, the pre-stressed pile break is regular, but the "filling core-filling segment is pulled out" is the biggest anomaly. Once the core-filling segment is pulled out (frictional failure), the allowable tensile force is down to zero suddenly, and the tensile force of the adjacent piles is greatly increased. These pre-stressed pile is gradually broken, and the building damage is inevitable. Pre-stressed piles can increase the allowable tensile force of the pile body by increasing the number of pre-stressed steel bars. However, the core-filling segment of the current construction method (known in Taiwan as pile head treatment, and the mainland is called pile head strengthening) is not feasible. The cement slurry in the inner hole of head head and the inferior concrete after the soil mixture is dry and hard are not easily removed, and the tensile capacity of the core-filling segment of the implanted pile is worse than that of the driven pile. In the case of pass 30 years of implanted piling construction in Taiwan, only a few of the core-filling tensile test reports [5][6] were used to affirm and consolidate the construction results of the pile heads, and the rest of the construction cases were mostly unknown.

    After the secondary drilling and high-pressure water clearing of pile head core, the friction on the interface of the core-filling segment and the pile wall is conservatively taken as 0.20N/mm2 or about 2.0kg/cm2 [5], and the estimated tension capacity of a 600mm pile with core-filling segment of 2M depth is about 491 KN or 50 tons. Increasing the depth of core-filling segment is the only way to increase the friction force. But the deeper the core-filling segment is, the more difficult the cleaning of the inner hole surface is. The rigor of the construction of the worker would have been more tested.

2. Improvement of the joint of pre-stressed piles

    Disasters are generally sudden, abnormal, and exceed the allowable range. No one can predict the occurrence of disasters. Pre-stressed piles are an excellent building material with high cost performance (C/P value), but there are still three major defects in the upper, middle and lower parts. The pile head treatment part is the most prone to the upper end of the pre-stressed pile. In the improved method of DH-PHC pile is to solve the problem, try to consolidate the design of the pile head joints, so that the structural components can perform better.

    Unlike Taiwan's general implanted piles (it is called the buried type in mainland China), the pre-stressed piles of the buildings in mainland china are mostly piles of driven type (or pressed-into type). This is similar to the construction method prevailed in Taiwan in the 1970s. Compared with the implanting construction method of piling, the construction cost of the driven method is relatively lower (about 40% reduction) [4]. But the pile is easily damaged due to hitting, and it would affect the durability of the pile.

    In the case of this paper, we focus on the sudden damage of the pile head of pre-stressed pile. In the pre-stressed pile tensile failure, the core-filling segment of reinforced concrete is pulled and pulled out of the pile. This is the most unusual case. Since the traditional core-filling segment and the pile wall are mainly transmitted by the frictional force of the interface. Except the material, the construction quality of the site is considerably affected, including the construction rigor of the workers/staffs. When the pile head tension occurs, since the pile wall is made of concrete, and there is no proper tension connection between the pile and the upper foundation, it can only be relied on by the core-filling segment of pile. When the tensile force of the pile exceeds the friction capacity of the core-filling segment of pile (including the loss or reduction of friction), the possibility of occurrence of tensile brittle failure increases. In the structural design, the engineer will strive to allow the bearing capacity of the component to be greater than the external force, in order to achieve structural safety, at least to achieve the situation of early warning damage, in order to reduce casualties in the disaster.

    In the treatment of the core-filling segment of piles, there has been no better way than the design of the friction of the core-filling segments for decades. Most of the engineering designs the foundation piles to “withstand vertical pressure rather than tension” as a design criterion to circumvent the design of the tension. This also caused the stereotype of the pile to be pressed only without being pulled. However, the disaster does not occur as expected only in the pressure bearing. When the tensile load occurs and exceeds the designed tensile capacity, or the friction of the inner hole of pile is lost or degraded due to the poor construction of the core-filling segment, the disaster will occur. It will be sudden and significant.

    In research of Zhang Jiaqi (2013) [3], the tensile capacity of pile head can be pre-embedded by the horizontal reinforcement in the pile head section when pre-casted by the pile factory (Fig. 2-1, 2-3), with the combination of vertical steel bars and concrete at the construction site and the core-filling segment becomes a more reliable structural joint, and the tensile capacity of the core-filling segment is greatly improved. Since the core-filling segment of pile head is completed, it is almost perpendicular to the solid concrete section (referring to the hollow tubular section of pile). The vertical anchor bars are firmly anchored into the pile head by combination with core-filling concrete and horizontal steel bars. With sufficient horizontal bars and proper anchoring, the tensile capacity of the core-filling segment can be as high as 400 tons or more. However, the general pile is not designed as a tensile pile, and the required tensile force is not so high, and it’s enough to simply meet the tensile capacity of the pre-stressed steel bars.

    Fig. 2-2 shows the compacted type of the horizontal bars of DH-PHC pile head. The horizontal bars no longer use the hook or hooks, only the straight bars, which are simplified and lower cost. The pile core-filling segment is subjected to tensile force. When the horizontal bar has a hook or hooks, the horizontal bar can exert the full shear capacity. However, when the horizontal bar does not have a hook (straight type), the shear capacity of the horizontal bar will be controlled by the compressive stress of the projected area of the horizontal bar into the concrete of the pile wall (diameter * penetration depth). Although the tensile strength of the core-filling segment is slightly reduced, the tensile capacity of the pre-stressed steel bar can still be satisfied. Of course, in the case of special needs (for example as a tensile pile design), the horizontal bars can still be set to provide greater tensile capacity.

Fig. 1-4 The situation of the traditional implanted pre-stressed pile head with secondary core clearing residual inferior concrete

 (omitted below)