The Evaluation of Displacement Ductility of Low Confinement Spun Pile to Pile Cap Connections

Experimental study was carried out on three low confinement spun piles to pile cap connections.  The detail followed the typically fixed connection in Indonesia. Reinforced concrete was filled to the spun pile to strengthen the connection region, except SPPC01. Different concrete types were used, shrinkage and non-shrinkage for SPPC02 and SPPC03, respectively. SPPC02 and SPPC03 could reach the targeted drift of 3.5% whereas SPPC01 was stopped at a drift of 2.75%. There was no shear failure detected during the test. The connection behaved as a fixed connection indicated by the fracture failure of the prestressed bars near the connection region. Analysis of the test results focused on displacement ductility. Two definitions of yield and ultimate displacement were employed to seek the possible ductility values. It varied from 3.05 to 6.04 for SPPC01 and from 3.01 to 4.95 for SPPC02 and SPPC03. The non-shrinkage concrete did not affect the strength of the connection but slightly improved the post-peak behavior. The ductility is 6–12% higher than spun piles with ordinary concrete.  According to the limited ductility referring to ATC 96, JRA 2002, and AASHTO 2011, all specimens could achieve target ductility 3. Hence, it can be concluded that the low confinement spun pile connections performed well in displacement ductility.

Displacement ductility; Experimental study; Low-confinement; Spun pile

     The connection of pile to pile cap in the foundation plays an important role to transfer the force from the upper to the bottom structure and vice versa.  This part is critical since the change of area, stress, and stiffness occurs suddenly (Bang et al., 2016). It needs rigorous detail and, usually, it is designed as a rigid connection that induces maximum curvature.  Designing this part as a linear structure during a severe earthquake is costly. Currently, the design concept of the foundation has been moved forward to performance-based design (PBD).  The pile is allowed to behave beyond its elastic stage to absorb the earthquake energy.  Sufficient strength and ductility are essential to survive during a severe earthquake.  Several countries have implemented PBD although the research is still carried out as indicated by international journal articles until 2021. Ductility is one of the important parameters to describe the seismic performance of a structure. It defines the ability of a structure to experience large amplitude cyclic deformation in the inelastic range without a substantial reduction in strength (Ling et al., 2023).       
      NEHRP (FEMA P-750, 2009) classifies ductility demand according to the seismic zone.  High seismic risk requires a ductility capacity of more than eight and the moderate seismic risk category requires more than four. According to Article C4.7.1 (AASHTO, 2011) for the life safety performance level, inelastic deformation in the piles is permitted but it should be limited to prevent severe damage. Hence, four is the maximum ductility suggested for the foundation.  ATC 96 and JRA 2002 limit the ductility to three and the damage is allowed near the ground surface for accessible repair. Similar regulation is also adopted in the New Zealand code for highway bridges, where design ductility is limited to four for plastic hinges expected at a depth less than two meters below the ground level. For deeper plastic hinges, it should be limited to three (Chai and Hutchinson, 2002).

        Several methods have been proposed to estimate the displacement ductility of the pile. Curvature ductility is one of the main factors affecting it.  To be ductile, the pile section should meet the required curvature ductility demand (Budek-Schmeisser and Benzoni, 2008). A simplification approach to determine the displacement ductility of a pile embedded in single-layer soil was conducted by  (Chiou et al., 2011). Three parameters affect the values, which are curvature ductility (CD), overstrength ratio (OSR), and pile-soil interaction. Curvature ductility contributes the most, followed by the overstrength ratio.  The moment-curvature is assumed as bilinear and the ductility is determined as the ratio of peak to the yield displacement.  If the pile is in non-cohesive soil, estimation of the ductility is purely based on CD and OSR, while soil structure interaction affects the pile in cohesive soil.  Although the equation could determine the ductility accurately only in cohesive soil, in general, the equation can be used to predict the ductility capacity of the pile. 
        The spun pile is a precast prestressed pile that is massively used in bridges and wharves. Experimental and numerical studies of this pile and its connection to the pile cap have been performed by many researchers.  In China, the study was performed on different connection details (Wang et al., 2014; Yang and Wang, 2016).  There were six specimens tested with lateral cyclic loading and axial load.  All of the specimens showed flexural damage. The study found that the ductility of the connection was in the range of 2.5 to 3.00. The study conducted by (Guo et al., 2017) improved the ductility of the connection by adding two different strengthening to the connection area. The ductility increased from 3.07 to 4.31 and 5.48.
      (Bang et al., 2016) conducted testing of spun pile connection with no axial load. The pile was strengthened with different reinforcements, a deformed bar (PHC-B), and more shear reinforcement (PHC-C).  Better energy absorption and ductility were observed on PHC-C where the value was 4.15 meanwhile PHC-B had a ductility of 2.75.  A recent study on improved spun pile connection was conducted by Yang, Li, and Nan (2020). Four specimens with different improvements were tested until failure.  Overall, the ductility of all specimens was in the range of 2.42 to 3.52.
    In Indonesia, the spun pile is produced with a limited amount of transverse reinforcement and below the minimum requirement in accordance with (ACI Committe, 2019). This is because Indonesia still adopts the elastic concept where ductile performance is considered not necessary. Inconsistently, the code requires sufficient transverse reinforcement. It is known that the appropriate confinement is necessary to gain ductile performance.  An experimental study of low confinement of the spun pile in Indonesia has been conducted by (Irawan et al., 2018). The amount of transverse reinforcement was 0.24% which is about 21% of the minimum requirement. The study found that the confinement was insufficient to resist the explosion of the pile’s concrete at the ultimate state due to compression. The ductility of the spun piles was 2.50 and 4.50.  Since the value was below 5, the study concluded that the piles were only suitable for low seismic-risk regions (Irawan et al., 2017) ono. onoA onostudy of insufficient confinement of spun piles was also reported by (Budek and Benzoni, 2009)  slightly below the minimum requirement of ACI 318-05 which was 1.2%. The research reported that the pile performed ductile where the ductility was two.
        Indonesia should move forward to PBD for the bottom structure since based on the recent seismic risk map, where the ground acceleration tends to increase (Pramono et al., 2020). Hence, the structural component should have adequate ductility. The spun pile with limited transverse reinforcement needs an assessment.  The study aimed to obtain the performance of the spun pile to pile cap connection based on the common practice in Indonesia.  The results could provide insight into the implementation of PBD in Indonesia. The evaluation was focused on displacement ductility based on the experimental result.  Several values of ductility from different methods were presented to get a comprehensive result. Thus, the adequacy of the piles under severe seismic can be clarified.

    Three full-scale spun pile connections were tested until failure to evaluate their seismic performance. To represent the real condition, the pile was picked from the stocking yard.  A length of 220mm was cut from the middle part which has less confinement according to the research objective.  The spiral pitch is 120mm where the volumetric ratio is 0,113%.  To clarify the quantity of transverse reinforcement, the amount is compared to three equations and presented in Table 1. As can be seen, the equations result in different required quantities. The revised equation proposed by (Fanous et al., 2010) results in higher minimum reinforcement. The required confinement of the spun pile used in this study cannot be determined since the equation proposed is only for piles with curvature ductility capacity greater than 18. Based on these equations, the spun pile employed in this study had less than 15% of the minimum requirements.

Table 1 The requirement of transverse reinforcement

2.1.  The Specimens 
      Figure 1 shows the DED of the specimens.  The 450mm in diameter spun pile was chosen with a wall thickness of 80mm. The spun pile was made of 57Mpa of concrete strength, reinforced by a 10@7.1mm PC bar, and confined by a spiral of 4mm in diameter. The connection between the spun pile and the pile cap was designed based on the common practice in Indonesia. The spun pile was embedded in the pile cap at a depth of 100mm. The required embedment length of the rebar was 620mm. To reduce the depth of the pile cap, the length was 500mm straight and the 200mm was bent 30 degrees as shown in Figure 1. SPPC01 was an empty spun pile, whereas SPPC02 and SPPC03 were filled with concrete and reinforced by 6D19 as shown in Figure 2.

Figure 1 The connection details (a) SPPC01; (b) SPPC02/3 and (c) the test set-up
     SPPC02 represents the typical connection where for ease in construction the concrete infill was cast together with the pile cap. Additional rebar of 6D19 was added and embedded into the pile cap to improve the connection strength.  Shrinkage of the concrete infill was a concern and therefore in the preceding research, non-shrink concrete was used (Guo et al., 2017; Bang et al., 2016; Yang and Wang, 2016; Wang et al., 2014).  In this research, SPPC03 was filled with non-shrink concrete with fc’ as 54.3MPa to see the effect of the concrete type on the behavior of spun pile connection.

Figure 2 The cross-section of SPPC01 and SPPC02
      The pile cap was cast by 30Mpa of concrete strength and the actual strength of 28 days age was 34MPa. Due to the pandemic situation, the experimental test was delayed and the concrete age of SPPC03 was based on a 56-day test which was 36.5 MPa. The strength of steel employed in the experiment is presented in Table 1.  
2.2.  The Test Set-Up 
      Figure 1 shows the test setup. The specimen was attached to a strong floor and tied with 10 anchoring bolts. It was loaded vertically as 500kN which was equal to 0.1fc’Ag.  A reverse cyclic lateral load was applied after the vertical force was fully applied. The horizontal loading protocol followed the ACI 437-07, where the test was conducted until a targeted drift of 3.5% was achieved or until the strength of the specimen was dropped by more than 25%. Seven and two transducers were employed to measure horizontal and vertical displacement, respectively. The concrete strain gauge was put on the spun pile and pile cap which was located 100mm from the connection. The strain of the reinforcement bar was measured through 6 strain gauges which were placed next to the connection in the loading plane. Meanwhile, four strain gauges were attached to the prestressed wire at a similar location.

Figure 3 The hysteretic curves  a) SPPC01; (b) SPPC02, (c) SPPC03, (d) Comparison of the envelopes curves 

3.1. The Hysteretic Curves 
    SPPC02 and SPPC03 were tested until they reached a drift of 3.5% whereas SPPC01 was stopped at a drift of 2.75% since its strength drop more than 50%. Figure 3 shows the hysteretic curve and the envelope of all specimens.  As can be seen, the presence of the reinforced concrete infill in SPPC02 and SPPC03 changes the performance of the spun pile connection significantly. It improves strength and energy absorption. The envelope of SPPC02 and SPPC03 are very closed which indicates that different concrete type does not affect the strength of the connection.
3.2. The Crack Pattern 
      The crack pattern on the spun pile is shown in Figure 4. There was a slight shear-flexural crack was detected at several places. The crack initiated from the tensile face and propagated to the center of the spun pile. The majority of the crack was a result of flexural failure. The crack propagated until 650mm from the connection region of SPPC02 and almost 800mm of SPPC03. Meanwhile, the last crack of SPPC01 was detected at depth of 400mm above the connection.
      Light damage was found on the pile cap of SPPC01. Concrete crushing of the pile at the connection region was observed when drift reach 2%.  A similar failure mode occurred on SPPC02 and SPPC03. The first crack of the spun pile was at a drift of 0.35% and 0.5%. The pile cap suffered moderate damage where a crack was detected on its surface with a depth of less than 100mm. It started from the connection region and then it propagated to a radius of 150 to 180mm from the pile face.