Keywords

1 Introduction

A variety of methods are there to determine the load bearing capacity of piles that might be either analytical or empirical. The former requires an evaluation of soil and pile interaction along with several underlying assumptions. On other hand, the latter was based on outcomes from various in-situ test procedures. Generally pile load tests are conducted to confirm the design loads of pile obtained from empirical formulas. Before constructing the bearing piles at site, the design loads calculated theoretically need to be confirmed by conducting initial pile load test on initial load test piles. From the initial pile load test results, it was decided whether to redesign the pile or proceed construction with existing design data. Compression pile load tests normally carried out by two methods such as kentledge method or reaction pile method. This paper present the case study of initial pile load test conducted using kentledge type method where liner piles are used for kentledge support due to low SBC value of soil at existing ground level.

2 Project Scope and Stratigraphy

The project consisted of construction of pile foundation for 2 Nos of 89 m dia LNG tanks. Details of pile foundation for each tank are given in Table 1.

Table 1 Details of pile foundation
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The soil profile of the project location shows that top 20 m is very soft clay. The site location is a filled-up area using dredged soil from sea. Due to the presence of very soft clay & loose sand strata for a depth of approximately 20 m, permanent steel casing is provided for working piles up to 24–30 m and pile is terminated in sand strata at 56 m in some locations and in other location pile is terminated at deeper depth of 61 m as dense sand strata is available after 60 m. Typical soil profile is shown in Fig. 1. Due to the presence of soft soil, it was decided to replace top 3 m soil with compacted engineering fill (GSB) for enhancing the lateral capacity of the pile.

Fig. 1
Three stacked bars of B 03, B 04, and B 05 at base S P T N in two factors C H and S M. The maximum C H value is in B 04 and maximum S M value is in B 03. The minimum C H value is in B 03 and S M value is in B 04.

Typical soil profile of site

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3 Design Load and Initial Pile Load Test

Design load for the piles is given in Table 2.

Table 2 Design load for pile foundation
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To confirm the theoretical design load, it was proposed to carry out an initial compression load test at site near to the tank location. A total of 4nos of initial compression load test were proposed by the client. Due to huge test load (2160t) and availability of sufficient rebars and steel plates, kentledge system was adopted for initial compression load test as per IS 2911 Part 4: 2013. Figure 2 shows the initially proposed test scheme for the load test. The kentledge weight of 2700t is placed over 48Nos of secondary beams which are supported by concrete blocks. The concrete blocks are placed over the GSB fill.

Fig. 2
A cross sectional view of the test arrangement included secondary beams, grade filling, the main beam, the test pile in the center, the wooden sleeper, and the concrete block.

Typical initial load test arrangement

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4 Site Issue in Initial Load Test and Solution

4.1 Site Issue

The base pressure acting on ground surface below concrete blocks on one side was 20 t/m2. From theoretical calculations the settlement for 20 t/m2 was 75–80 mm. So, a gap of 100 mm was kept between the top of the hydraulic jack and the main beam. Since the soil below the GSB fill was soft, as the kentledge loading above secondary beams, the soil below concrete blocks started to settle. Before reaching 20% of kentledge, i.e., 500 t the concrete blocks settled about 95 mm, due to which the main beam also settled, and the main beam almost touched the hydraulic jack. So, from a safety point of view as the excessive settlement of soil will lead to serious accident hence the loading of kentledge was stopped and decided to unload the materials. Figure 3 shows actual site photo during the time of kentledge loading.

Fig. 3
A photograph of loading test sheets placed on blocks with the help of rectangular beams placed between them. A ladder and machine setup are placed near the arrangement.

Load test arrangement in site using concrete blocks

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4.2 Solution

To overcome this excessive settlement, it was decided to support the kentledge above the liner piles instead of using concrete blocks. The 8 mm thick 1200 mm diameter liner piles were driven to a depth of 24 m using ICE-815 vibratory hammer of capacity 72 t. 16Nos of liner piles (8Nos on either side of test pile) were driven. The nearest liner pile is at a clear distance of 2.7 m from test pile. The plan of liner pile location and connecting beams are shown in Fig. 4. The liner piles were connected at top using 4.5 m long built-up beam consisting of 6Nos of NPB 600 beams (Represented as Beam-3 in Fig. 5). Above these built-up Beam-3, another 16 m long built-up beam consisting of 4Nos of NPB 600 beams (Represented as Beam 4 in Fig. 5). Over this Beam 4, 48Nos of secondary beams are placed. The initial load test scheme using liner pile arrangement is shown in Fig. 5.

Fig. 4
A cross sectional top view of sixteen liner piles of 1200 diameter, secondary beams 12 meter long, king beams, grade fill, concrete block four beams with total vertical length 16000 meters.

Plan of load test arrangement using liner piles

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Fig. 5
A cross sectional view of the test arrangement included secondary beams, grade filling, the main beam, the test pile in the center, the wooden sleeper, and the concrete block.

Elevation of load test arrangement using liner piles

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So, with this arrangement system the kentledge load from secondary beams is transferred to a dense stratum which is at about 20 m below EGL. The top levels of four corner liners were monitored using dumpy level at regular interval till the kentledge loading of 2616 t was completed. Figure 6 shows the actual site photo for the load test arrangement as discussed. The settlement of liner piles is monitored regularly.

Fig. 6
A photograph of a front view of the loading arrangement of a series of piles with horizontal sheet supports and concrete blocks placed as the bottom support.

Load test arrangement using liner pile as support to kentledge

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5 Discussion on Liner Pile Settlement During Kentledge Loading

As discussed earlier, the kentledge weight is supported by 16Nos of liner piles. After final kentledge loading load coming on each liner is 167.5 t. With such a huge load on each liner the final settlement observed in liner piles was 16 mm on left side and 20 mm on right side liners. Table 3 shows the left side and right side liner pile settlement at different stages of loading.

Table 3 Liner pile settlement at different stages of loading
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So, comparing with old load test proposal using concrete blocks as kentledge support, the liner pile system as support for kentledge was successful for heavy kentledge loads.

6 Discussion of Initial Pile Load Test Results

As mentioned above the test load for the initial load test is 2600t (250% SWL+100% 2xNSF). Load test was carried out in 3 cycles. First cycle with maximum load of 100% of design load applied in stages of 20% increment and each increment is maintained till rate of movement of pile top is not more than 0.2 mm/h or until 2 h have elapsed, whichever is earlier subject to minimum of 1 h (as per IS 2911 Part 4:2013). Second cycle applying load up to 150% in the same manner and in the third cycle applying load up to 250% and maintaining the final test load for 24 h.

In third cycle after applying a load of 17356kN (200% of SWL+100% of 2XNSF) the pile top settlement reached 123.35 mm (more than 10% of pile diameter) after 30 min of loading. So as per IS 2911 (Part 4): 2013, the loading was stopped and unloaded to zero.

The pile load test result was shared with the client and based on the result they have increased the number of piles in each tank as the actual capacity from the load test is less than the theoretical value (Table 4).

Table 4 Test pile settlement during load test
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7 Conclusion

From the above data obtained during the load test following points can be concluded,

  1. 1.

    For pile load test with heavy kentledge loads, liner pile as support for kentledge weight will be safer compared to kentledge supporting on concrete blocks.

  2. 2.

    For sites where existing ground surface containing soft soil and having low SBC value, liner pile system is recommendable for kentledge type pile load test compared to use of concrete blocks as support to kentledge weight.