LOAD TESTS ON INSTRUMENTED 600mm DIAMTER BORED PILES IN THE CITY CENTRE OF KUALA LUMPUR
The design of bored piles by geotechnical engineers was based mainly on empirical formula in correlation with Standard Penetration Tests (SPT) established either locally or by others around this region. Leach (1989) in his paper mentioned that SPT correlation with Cu, the undrained shear strength, and estimated adhesion factor α, had been used successfully in bored piles design in overconsolidated soil and also in soft siltstone rocks. In addition to SPT values, static load tests with or without instrumentation were also carried out to confirm their design assumptions. Examples of load tested instrumented bored piles in Kenny Hill formation were by Toh (1989), Tan (1998) and Liew (2004). The other reported cases of load tests on instrumented bored piles were Yong (1982), Chin (1985), Chin (1996) Shi (1996) in Singapore old alluvium, Radhakrishnan (1985) in highly weathered rock and Poh (1993) in fractured granite, both of these tests were carried out in Singapore.
This paper presents load test results on one 600 mm diameter instrumented bored pile which was installed in Kenny Hill formation along the Sungai Ampang/Klang in Kuala Lumpur. Figure 1.0 shows the location of this load tested instrumented bored pile. The objectives of this instrumented test piles are as follows:-
(i) To establish the skin friction distribution along the pile shaft.
(ii) Load settlement behavior
2.0 SUBSOIL PROPERTIES
Residual soils derived from Kenny Hill formation occurs extensively in part of the Kuala Lumpur area such as that area along Sungai Ampang. The distribution of this Kenny Hill formation in Kuala Lumpur and it adjacent area is shown in Figure 2.0. The Kenny hill formation is a sequence of interbedded sandstone, siltstones and shales/mudstones believed to be the Upper Paleozoic age. Detailed of this residual soil properties can be found in James’s paper (1995)
Next to this instrumented test pile number 1 (TP1), a bore hole was carried out by rotary boring. The depth of rotary boring for this bore hole was about 47 meters below the existing ground level. Standard penetration tests (SPT) were carried out at 1.5 meters interval. In addition pressuremeter tests (three cycles) were also carried out. The soils in this bore hole were clayey silt with SPT values vary from 20 to 41 blows per 300 mm at the depth of 8 meters to 16 meters below existing ground level. SPT values increases to more than 50 blows / 300 mm from the depth of 16 meters to 35 meters. The plastic index varies from 19 to 22. The SPT values versus depth was plotted and shown in Figure 3.0. In addition mazier samples were obtained from this bored hole for the stiff clayey silt for laboratory testing.
The pressuremeter test was carried out using Menard pressuremeter. This pressuremeter can achieve a maximum pressure of 80 bars. The initial pressuremeter moduli vary from 3.67 to 57.69 Mpa while the 1st reload pressuremeter moduli vary from 16.58 to 130.4 Mpa. The limit pressure varies from 0.41 to 5.59 Mpa. The pressuremter modulus versus SPT was plotted in Figure 4.0. The initial and reload moduli versus depth for was plotted in Figure 5.0 and the yield and limit pressure versus depth were shown in Figure 6.0 and 7.0 respectively.
3.0 600 mm DIAMETER BORED PILES INSTALLATION
The test pile was constructed by a local bored piling company using mechanical boring machine. The top 12 meters was temporary cased and from 12 meters downwards, the hole was stabilized using pro CDP gel. This test pile was bored to a depth of 35 meters without encountering any rock formation. This test pile was concreted by trimie method. The characteristic cube strength of the concrete for this bored pile was 35 N/mm2 with a slump of more than 150 mm.
4.0 PROPOSED INSTRUMENTATION
In order to obtain the load distribution along the pile, vibrating strain gauges were installed along the whole length of the test piles. Vibrating wire strain gauges were chosen because they can have better long term performance as compared to electrical resistance gauges. There were seven levels of vibrating strain gauges for this test pile as shown in Figures 8.0. The strain gauges were attached to the reinforcement bars of the bored piles at different elevations to measure the load at a particular section in the shaft of the axially loaded pile. There were four strain gauges in each level. The details of these strain gauges installation is also shown in 8.0. The strain gauges in level A were used for reference. It was assumed that the skin friction above level A were negligible. For this purpose the soils above level A were excavated and wrapped with polythene sheet to reduce the skin friction/adhesion on the test pile.
5.0 MAINTAINED LOAD TESTS
For the instrumented test pile, maintained load tests were carried out. This pile was loaded to more than 5,000 kN. In the maintained load tests, ground anchors instead of Kentledge were used as the reactions to the test piles. It is because this test pile is located very closed to the river bank. For this test pile 6 no. of ground anchors were used. The length of the ground anchors was 45 meters below existing ground level. A 3 cycles load/unload maintained load tests was applied to this test pile about one and half months after the pile was installed. At the end of each cycle, the loads were maintained for 24 hours before the loads were released. The settlement of the pile head were measured by four numbers of dial gauges.
6.0 RESULTS AND ANALYSIS
6.1 Load - Settlement Analysis
Table 1.0 summarized the results of the 3 cycles maintained load tests. In the 1st cycle at 2,000 kN, the settlement was 4.88 mm and in the 2nd cycle at 2,000 kN, the settlement was about 3.38 mm and at 4,000 kN after 24 hours of maintained load, the net settlement was about 12.80 mm. In the 3rd cycle the settlement at 2,000 kN and 4,000 kN were 4.79 mm and 9.68 mm respectively. At 5,000 kN the net settlement was about 14.87 mm. The Load – settlement plots were shown in Figure 9.0. The ultimate load as estimated from the inverse slope of the stability plot is about 5,900 kN. As mentioned by Chin (1972) the ultimate load obtained from the inverse slope of the stability plot will always give a higher ultimate load as compared to the actual ultimate load. The stability plot for the 3rd cycle loads is shown in Figure 10.0
6.2 Load – Distribution Analysis
The load distribution along pile length and skin friction versus applied loads plots are shown in Figures 11.0 and 12.0 respectively. The maximum ultimate skin friction was observed at level D-E where the average SPT value is 60 blows per 300 mm. From Figure 11.0, it can be seen that the applied loads were carried mainly by skin friction and very little load was carried by end bearing. The end bearing pressure at the applied load of 5000 kN is 520 kN/m2 and the load carried by end bearing was about 3 % of the total applied loads. From Figure 12.0, it was noticed that there is a low skin friction generated between levels C-D at applied load of 5,000 kN. This skin friction is low as compared to other levels even though the average SPT value in this level is higher other levels except at level D-E. If the skin friction at levels A-B and C-D were ignored, and based on Cu equal to 5*N, the calculated α values vary from 0.22 to 0.85 and the average value is about 0.50. This α value is compared to other α values as obtained by others. This comparison is shown in Table 2.0.
(i) The calculated average α (adhesion factor) is about 0.50 for this instrumented test pile based on Cu = 5*N where N is the SPT values.
(ii) Most of the applied loads (5000 kN) to the test pile were carried by skin friction and only 3 % of the total applied loads were carried by end bearing.
(iii) The 600 mm diameter bored pile with a penetration length of 35 meters can carry a test load of not less than 5,000 kN in this Kenny Hill formation.
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