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Equivalent pile load–head settlement curve using a bi-directional pile load test
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Computers and Geotechnics 35 (2008) 124–133 www.elsevier.com/locate/compgeo
Equivalent pile load–head settlement curve using a bi-directional pile load test Jong-Sub Lee b
a,*
, Yung-Ho Park
b
a Civil and Environmental Engineering, Korea University, Seoul 136-701, Republic of Korea Hyundai Engineering and Construction Co., Ltd., Yongin-Si, Gyounggi-Do 449-716, Republic of Korea
Received 20 February 2007; received in revised form 1 May 2007; accepted 5 June 2007 Available online 23 July 2007
Abstract An equivalent pile load–head settlement curve to predict pile capacity has been obtained by using test results from the Osterberg Cell (O-cell). The method assumes that the pile is a rigid body and predicts very stiff pile behavior. This study describes a new method for predicting an equivalent pile load–head settlement curve considering the elastic shortening of piles. The static pile load test is compared to the O-cell test using two piles constructed 10 m apart (center-to-center distance). A measured O-cell test settlement consists of an elastic shortening under the load applied inside the pile and a net settlement. The measured net settlement is added to the elastic shortening, which has a different unit skin friction distribution under the top-down load on the pile head, to predict the equivalent pile load–head settlement curve. The new equivalent pile load–head settlement curve, considering the elastic shortening under top-down load, is similar to the pile load–head settlement curve obtained by the static pile load test before the pile yields. The new method can be used to effectively estimate the pile head settlement as well as pile capacity by using the bi-directional load test results. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Bi-directional pile load test; Elastic shortening; Failure; Rigid body; Settlement; Skin friction distribution
1. Introduction The static pile load test using dead weight or reaction piles is a well accepted and common method to determine the load–settlement curve and pile capacity [1,2]. The maximum test load generally applied to piles is 35–40 MN due to the safety and cost. However, along with an increase in typical pile size (diameter and length), the design load of piles has also increased. For a large diameter pile, the capacity can be investigated with the O-cell test developed by Dr. Jori Osterberg [19]. The O-cell test has been used since 1984 for drilled shafts and driven piles [10]. The loading device used for the O-cell test (also called the bi-directional load test) is a hydraulic jack-like device. It is a sacrificial tool placed at the bottom and/or distance *
Corresponding author. Tel.: +82 2 3290 3325; fax: +82 2 928 7656. E-mail addresses: [email protected] (J.S. Lee), [email protected] (Y.H. Park). 0266-352X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compgeo.2007.06.008
up from the bottom of the pile [7,14,16]. As the hydraulic pressure is applied to the device, an equal upward and downward force is applied to the pile. The end bearding is resisted by the skin friction and vice versa. Therefore, dead weight or reaction piles are not required in the O-cell test. The O-cell test results are used to construct an equivalent top-down load–settlement curve. The original method suggested by Dr. Osterberg [19] assumes that the pile is a rigid body. Therefore, top-down settlement may be underestimated by this method. However, pile settlement is a critical issue in large diameter piles. To overcome the deficiency of the original method, finite element simulations can be used to predict the equivalent pile load–head settlement curve [8,18,21]. However, many steps are required for the numerical simulations: assumption of geo-material properties, matching the measured and computed O-cell displacements, and adding the computed elastic shortening components. Thus, numerical simulations have been only
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ment. After the pile head settlement reaches 5 mm, the load-transfer curves are parallel.
used as a research tool. A new simplified method for constructing a realistic pile load–head settlement curve is required. This paper describes a relatively simple method for calculating the equivalent pile load–head settlement curve from O-cell test results. The paper also includes a review of load-transfer characteristics, and compares results from a static load test and an O-cell test conducted under similar conditions.
2.2. Perfectly rigid pile with linearly increasing unit skin friction If the pile is assumed to be perfectly rigid, the settlement is the same along the whole pile shaft. And unit skin friction linearly increases with depth as shown in Fig. 2b. However, ultimate values of unit skin friction reach the same displacement as shown in Fig. 2c. End bearing is assumed to increase linearly with displacement as shown in Fig. 2d. The load-transfer curve for this condition is shown in Fig. 2e. The curve characteristics are similar to the constant skin friction condition except for the shape (parabola instead of trapezoid).
2. Load-transfer characteristics Load-transfer characteristics in piles depend on pile rigidity and unit skin friction distribution along the pile shaft. Figs. 1 and 2 summarize the load-transfer characteristics for a rigid pile with different unit skin friction distributions.
2.3. Elastic pile with constant unit skin friction or linearly increasing unit skin friction
2.1. Perfectly rigid pile with constant unit skin friction
As the pile undergoes elastic shortening, the displacement of the pile decreases with depth under a top-down load. Therefore, the skin friction produced changes with depth before ultimate skin friction is generated (see details in [6,20]). Since the elastic shortening depends on the pile stiffness, the load applied, and the resistance generated, the load-transfer curves can be obtained through numerical simulation or by the trial and error method.
If the pile is assumed to be a perfectly rigid body, the settlement of the pile head is exactly the same as the settlement at the pile toe. Unit skin friction is held constant along the pile shaft in Fig. 1b. Unit skin friction versus pile movement along the pile shaft is modeled as an elasto-plastic element as shown in Fig. 1c. Therefore, unit skin friction increases linearly with displacement until ultimate skin friction is reached. The displacement for ultimate skin friction is about 5–15 mm for clays and 20 mm for sand [16]. After this displacement, ultimate skin friction is constant (see details in [17]). For this analysis, the displacement for ultimate skin friction is set at 5 mm. It is assumed that end bearing increases linearly with displacement as shown in Fig. 1d. As a result, the slope of the load-transfer curve is linear as shown Fig. 1e. Since the settlement of the pile is constant during loading, the developed skin friction is the same along the whole pile shaft. The shape of the load-transfer curves is that of a trapezoid. The slopes of the load-transfer curves increase with pile head displace-
3. Comparison tests Two pile load tests, the static load test and O-cell test, were conducted at the early stages of Mass Rapid Transit (MRT) project in Singapore. The main contractor of the MRT project is Hyundai Engineering and Construction Co., Ltd. Center-to-center distance between the two piles is 10 m. The static load test and the O-cell test were carried out by CEP Instruments PTE Ltd and LOADTEST Inc., respectively. Fig. 3 shows a schematic drawing of the sub-
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Fig. 1. Load-transfer characteristics for a perfectly rigid pile with constant unit skin friction; (a) pile, (b) ultimate unit skin friction versus depth, (c) skin friction versus displacement, (d) end bearing versus displacement, (e) load-transfer curves with different pile head settlements. Note the load-transfer curves are parallel after skin friction reaches the ultimate value.
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Fig. 2. Load-transfer characteristics for a perfectly rigid pile with linearly increasing unit skin friction; (a) pile, (b) ultimate unit skin friction versus depth, (c) skin friction versus displacement, (d) end bearing versus displacement, (e) load-transfer curves with different pile head settlements. Note the loadtransfer curves are parallel after skin friction reaches the ultimate value.
Pile for static load test
Pile for O-cell test
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The SPT tests were terminated after a blow count of 100 at any test location was obtained. Where the effective blow count exceeded 100, the corresponding penetration depth in mm at 100 blows are shown in Fig. 3.
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Fig. 3. Schematic profile of soil conditions and test piles. When SPT Nvalues are greater than 100, the corresponding penetration depth is noted in mm unit (with /).
surface conditions and pile construction records for the static and O-cell tests. 3.1. Subsurface conditions One borehole with standard penetration testing (SPT) was drilled to investigate the subsoil conditions between the two test piles (Fig. 3). The soils encounted during borehole drilling and during pile excavation consisted of 18.5 m of silty clay overlying 18.5 m of stiff clayey silt. The stiff clayey silt is underlain by a layer of very stiff silty clay at a depth 37.0 m. The SPT N-values of this site gradually increase with depth until 31.8 m. Below 31.8 m depth, SPT N-values are greater than 100 blows per 300 mm.
The test pile drilling was carried out with a rotary table and bucket. As the rotary table drilled into the soil, the cuttings gradually filled the bucket. The bucket was lifted and emptied when full. A temporary steel casing of 2.2 m length was inserted into the ground after pre-boring to prevent collapse of the drilled hole near the ground surface. Polymer based drilling fluid was used to prevent collapse along the length of the borehole. Preassembled steel cages were lowered into the borehole, and concrete was placed by tremie pipe. The pile dimensions for the static load test were a diameter of 1.2 m and a length of 37.0 m (Fig. 3). The static load test was performed 20 days after concrete placement. The loading procedure for the static pile test was similar to the maintained load (ML) method described in ASTM D 1143 [1] and BS 8004 [2]. The designed working load (WL) was 11.3 MN and the maximum test load was two and half times the working load, 28.25 MN. A first attempt was made at three loading cycles at 1.0WL, 1.5WL, and 2.5WL. However, the short stroke of the hydraulic jack required that four loading cycles were necessary. The test results are shown in Fig. 4. Settlement is often a governing factor for determining pile length or the number of the piles. The specifications for this project (M&W Spec) set settlement criteria as follows: 12 mm at working load, 24 mm at 1.5 times the working load, and 0.1 D at 2.5 times the working load [12]. The results of the static pile load test satisfy the specification. 3.3. O-cell test The pile construction procedures for the O-cell test are similar to those adopted for the static pile load test except
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Load (MN)
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1.0×WL 6 hours (1st cycle) 6 hours (2nd cycle)
1.5×WL 6 hours (2nd cycle) 1 hour (3rd cycle)
2.5×WL 24 hours
Fig. 4. Load–settlement curve from the static load test. WL = working load.
O-cells were installed at two levels as shown in Fig. 3. The bottom O-cell was installed at 36.9 m on top of a thin concrete layer (0.5 m thickness) at the end of the drill shaft. The middle O-cell was installed at 26.9 m depth. Note that the length of the O-cell test pile is 0.4 m longer than that of the static load test pile. Concrete was poured into the drill shaft and allowed to cure for 30 days before performing the O-cell load test. The O-cell test method loads the pile internally in two directions by hydraulically pressurizing the installed Ocells. Hence, the toe of the pile below the O-cell resists the O-cell load through end bearing and skin friction which resist downward movement. The shaft of the pile above the O-cell resists the O-cell load through shaft friction which resists upward movement [15,16]. The trend of pile design toward greater lengths and greater capacities have fostered the popularity of multilevel testing [3–5]. Two-level configurations of O-cells are employed in this study as shown in Fig. 3. For the two levels of O-cells, three stages of loading were proposed as follows [13]: 3.3.1. Stage 1: Confirming end bearing (EB) The bottom O-cell is pressurized to measure end bearing by providing reaction with the lower side shear and upper side shear (LSS + USS) as shown in Fig. 5. At this stage the middle O-cell is closed, so all the skin friction is mobilized as a reaction force. The downward displacement corresponding to end bearing is measured by telltale and denoted by D1 in Fig. 5. The displacements of the shaft are denoted by D2 (upward total displacement) and D3 (upward net displacement), respectively. Details of displacement measurements for the single O-cell are represented in [9].
D3
D7
D7
D6
D6
Close D4
D2 Open D1
D5
Fig. 5. Multi-level O-cell test: EB = end bearing of the pile; LSS = lower side shear (skin friction between O-cells); USS = upper side shear (skin friction above middle O-cell). D1 = downward displacement corresponding to end bearing during stage 1; D2 and D3 = upward total and net displacements of the shaft during stage 1; D4 and D5 = downward total and net displacements for LSS during stage 2; D6 and D7 = upward total and net displacements for USS during stage 2 and 3.
3.3.2. Stage 2: Confirming lower side shear (LSS) between the O-cells The middle O-cell is pressurized to measure the LSS using the upper side shear (USS) above the middle O-cell under the open condition of the bottom O-cell as shown in Fig. 5. Therefore, no load is applied to the pile toe. The corresponding displacements for LSS are D4 and D5. D4 is the downward total displacement which is the summation of the elastic shortening induced by skin friction
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and downward net displacement D5. The corresponding displacements for USS are D6 and D7. D6 is the upward total displacement which is a summation of the elastic shortening induced by skin friction and upward net displacement D7 as shown in Fig. 5.
lower than the lower side shear, the ultimate value of the upper side shear was measured during stage 2 as shown in Fig. 6b. Since lower side shear is greater than the end bearing and upper side shear, the ultimate value of lower side shear is not confirmed. In summary, the test pile is successfully loaded to ultimate capacity in one sections: upward upper side shear. To determine the upward direction skin friction, the upper side shear (USS), the buoyant weight of the pile above the middle O-cell is subtracted from the measured value. For the estimation of lower side shear (LSS), the buoyant weight of the pile between O-cells is added to the measured value. However, the end bearing value can be directly taken from the measured value [10].
3.3.3. Stage 3: Confirming upper side shear (USS) above the middle O-cell The middle O-cell is again pressurized to measure the USS using the lower side shear (LSS) between the O-cell and the end bearing below the bottom O-cell (closed condition of the bottom O-cell) as shown in Fig. 5. In this stage, D6 and D7 are continuously monitored. Loading procedures for the O-cell test are similar to the quick test method described in ASTM D 1143 [1]. Loads are held for 4 min and then increased to next load increment. When the movement reaches 6–7 mm and 12– 13 mm, the load is sustained for 1 h. Test results are presented in Fig. 6. During stage 1, end bearing is successfully measured until the movement of the pile toe reaches at about 60 mm as shown in Fig. 6a. Since the upper side shear is
4. Equivalent pile load–head settlement curve An equivalent pile load–head settlement curve can be constructed from the combination of the upper side shear, lower side shear, and end bearing obtained from the O-cell test. The original and elastic methods are discussed and compared below.
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Fig. 6. Multi-level O-cell test results: (a) stage 1; (b) stage 2 and 3. EB = measured end bearing value; USS=measured upper side shear value – buoyant weight of the pile above the middle O-cell (0.66 MN); LSS = measured lower side shear value + buoyant weight of the pile between O-cells (0.15 MN).
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4.1. Original method
4.2. Elastic method (new method)
This method is commonly used to obtain an equivalent pile load–head settlement curve from the O-cell test. This is done by assuming that the pile is incompressible (or elastic shorting is less than 3 mm) and the upward ultimate skin friction is equal to the downward ultimate skin friction [10,16,19]; see also [11]. Procedures for constructing the equivalent pile load–head settlement curve are as follows:
The elastic shortening of the pile may be less than 3 mm if the pile diameter is huge and the pile length is short. However, the portion of the elastic shortening in the total head settlement is not negligible (see [21]). The equivalent settlement obtained by the original method is smaller than that by static load test and so should not be used to check whether the settlement criteria is satisfied (12 mm at 1.0WL and 25 mm at 1.5WL). The lower settlement estimated by the original method is due to the reduced elastic shortening. The reduced elastic shortening in the O-cell test results from the different load-transfer characteristics as shown in Fig. 8. Fig. 8 presents the distribution of axial load in the pile during the Ocell test and the static pile load test under an applied maximum load. Note that unit skin friction is assumed constant at each segment (upper side shear and lower side shear). While ultimate values of the lower side shear and end bearing appear to be greater than 9.5 MN and 10 MN (see Fig. 6), they are assumed as 9.5 MN and 10 MN, respectively. Ultimate upper side shear is 9 MN. Therefore, the ultimate capacities from the O-cell test and the static load test produce the same value (28.5 MN). However, the applied load at each pile segment is very different for each test. For example, upper side shear during the O-cell test and the static pile load test are 4.5 MN (average of 0 and 9 MN) and 24 MN (average of 28.5 MN and 19.5 MN), respectively, as shown in Fig. 8. Thus, the amount of elastic shortening differs according to the type of test employed: the elastic shortening from the static load test is 5.3 times higher than that from the O-cell test. Therefore, to predict the precise equivalent pile load–head settlement curve using the O-cell test, the elastic shortening should be properly considered. Note this confirms that the original method is simply the rigid body method with limited elastic shortening.
Step 1: Select an arbitrary movement from D1, D4, and D 6. Step 2: Add all the resistance components at this movement regardless of the direction of movement. This load corresponds to the selected movement at the equivalent pile load–head settlement curve. Step 3: Repeat steps 1 and 2 up to the maximum deflection (or maximum extrapolated deflection) of the component. If the selected displacement is beyond the maximum displacement of any other resistance component that does not reach an ultimate, the resistance is conservatively assumed constant after the maximum load applied. The equivalent pile load–head settlement curve obtained from this method is shown in Fig. 7. This method underpredicts the settlement measured during the static load test at the same load. Note that the equivalent curve obtained by this method includes elastic shortening even though the pile is assumed to be a rigid body. For example, the arbitrary deflection of the upper side shear consists of elastic shortening due to compression load from the middle O-cell and head movements. On the other hand, the numerical simulation indicates that the equivalent pile load–settlement curve by the original method is satisfactory when the slenderness ratio (length divided by diameter) is less than about 20 [18].
Load (MN) 0
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Fig. 7. Equivalent pile load–head settlement curve and static load test results.
30
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J.S. Lee, Y.H. Park / Computers and Geotechnics 35 (2008) 124–133 O-cell test
Static load test
Laod (MN)
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Fig. 8. Conceptual load-transfer characteristics. (a) O-cell test; (b) static load test. Assumption: unit skin friction is uniform above the middle O-cell level (0–27 m deep) and between the two O-cells (27–37 m deep); skin friction in the upward direction is the same as the skin friction in the downward direction.
Fig. 9 presents the concept of pile settlement due to top load (static load test). The pile head settlement, that is, the settlement of the upper shaft (SUSS) is the summation of the elastic shortening of the upper shaft (dUSS) and the net settlement of the upper shaft (nUSS). The net settlement of the upper shaft (nUSS) is the movement of the bottom of the upper shaft. Therefore, the movement of the toe of the upper shaft should be equal to the total movement of the top of the lower shaft (SLSS = nUSS). Again, the total settlement of the lower shaft (SLSS) is the summation of the elastic shortening of the lower shaft (dLSS) and the net settlement of the lower shaft (nLSS). The net settlement of the lower shaft (nLSS) should be the settlement of the pile toe (SEB = nLSS). Therefore, the settlement at the pile head is the summation of the settlement of the pile toe (SEB), the elastic shortening of the lower shaft (dLSS), and the elastic shortening of the upper shaft (dUSS). The load corresponding to this settlement is the summation of end bearing (EB), lower side shear (LSS), and upper side shear (USS). The similar method was also suggested by [6]. The procedure to construct the equivalent pile load– head settlement curve considering the elastic shortening by using the O-cell test is as follows: Step 1: Select the arbitrary pile toe settlement (SEB) and corresponding end bearing (EB) as shown in Fig. 10a. SEB is denoted by D1 in Fig. 5. Step 2: Select or interpolate the lower side shear (LSS) corresponding to this settlement from the lower side shear curve. Note that the corresponding settlement should be the net settlement of the lower shaft (nLSS) instead of the measured total settlement of
Step 3:
Step 4:
Step 5:
Step 6:
the lower shaft. The measured total settlement of the lower shaft is used in the original method. nLSS is denoted by D5 and the total settlement of the lower shaft is represented by D4 in Fig. 5. Calculate the elastic shortening of the lower shaft (dLSS) under end bearing (EB) at the bottom, and the force of end bearing and lower side shear (EB + LSS) at the top of the lower shaft as shown in Fig. 10b. The total settlement of the lower shaft (SLSS) is the summation of the calculated elastic shortening of the lower shaft (dLSS) and the settlement of pile toe (SEB). Select or interpolate the upper side shear (USS) corresponding to SLSS. Note that the corresponding settlement should be the net settlement of the upper shaft (nUSS) instead of the total measured settlement of the upper shaft. nUSS is denoted by D7 and the total measured settlement of the upper shaft is represented by D6 in Fig. 5. Calculate the elastic shortening of the upper shaft (dUSS) under a force of EB + LSS at the bottom and EB + LSS + USS at the top of the upper shaft as shown in Fig. 10c. The total settlement at the pile head (SUSS) is the summation of the calculated elastic shortening of the upper and lower shaft, and the settlement of pile toe (dUSS + dLSS + SEB). The corresponding load in the pile head is the summation of end bearing, lower side shear, and upper side shear (EB + LSS + USS) Repeat steps 1–5 until enough points are obtained to construct the equivalent pile load–head settlement curve.
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Actual top-down load
Upper shaft
Middle O-Cell
Lower shaft Bottom O-Cell
Pile toe
Fig. 9. Principle of the pile settlement evaluation based on the elastic method.
Fig. 10. Procedures for pile load–head settlement evaluation using the elastic method.
Note that the general form of elastic shortening (d) in the pile can be expressed in terms of the area of the pile cross section A, pile length L, the elastic modulus of the pile material E, the force acting on the bottom of the pile or segment QB, and skin friction along the pile shaft QS,
ðQB þ nQS ÞL ð1Þ AE where n is a shape factor depending on skin friction distribution. n = 0.5 and n = 0.67 are applied to uniform skin friction distribution and the triangular distribution (line-
d¼
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Head Movement (mm)
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O-Cell: Elastic Method (ξ = 0.67) Static Load Test
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120 1.5WL 6 hours (2nd cycle) 1 hour (3rd cycle)
1.0WL 6 hours (1st cycle) 6 hours (2nd cycle)
2.5WL 24 hours
Fig. 11. Comparison of equivalent pile head-settlement curves. Elastic method is derived based on uniform unit skin friction (n = 0.5) and linearly increasing unit skin friction (n = 0.67).
arly increasing), respectively. Skin friction distribution can be obtained using strain gauges. The equivalent pile load–head settlement curves obtained by the elastic method are shown in Fig. 11 for different shape factors (n = 0.5 and n = 0.67). For comparison, the curve obtained by the original method is plotted. The results by the elastic method are similar to the static load test results. The settlement is slightly larger for the triangular distribution than the uniform distribution. 4.3. Perfectly rigid pile If the pile is a perfectly rigid body and upward ultimate skin friction is equal to downward ultimate skin friction, another equivalent pile load–head settlement curve may be obtained. The procedures are identical to the original method with the exception of using the net settlements (nUSS, nLSS, and SEB) instead of measured values (see Fig. 9). The equivalent pile load–head settlement curve using this method is shown in Fig. 11. The equivalent settlement obtained by the perfect rigid body assumption is similar to that by the original method.
Maintained time: The maintained time for each loading stage is relatively short in the O-cell test (typically 4 min, 1 h at two points) compared with the static load test (1– 24 h). Table 1 summaries the maintained time effect on the settlement during the static load test. Pile settlement increases with holding time and these effects increase with loading level. Note that the equivalent pile load–head settlement curve is developed for 4 min holding time. Therefore, the settlement should be smaller for the O-cell test than for the static load test. Overrun: The concrete volume during construction was measured to obtain the overrun. Overrun is the ratio of the difference between theoretical and used concrete volume to the theoretical volume. Overrun of the pile for the static pile load test was 3.35% and overrun for the Ocell test was 4.56%. When the volume of the sacrificial hydraulic jacks installed is considered, the overrun of the two piles are almost the same. Therefore, differences due to overrun are considered to be negligible. Construction effects: The pile length was 0.4 m longer in the O-cell test pile than in the static load test pile. Therefore,
5. Discussion
Table 1 Settlement increments due to maintained time during the static load test
The elastic method predicts a similar pile load–head settlement curve compared to the static pile load test. However, the predicted curve does not exactly match the static load test results. As explained below this may be attributed to several factors which include the maintained time at each loading stage, overrun during concrete placement, construction effects and the size of influence zone.
Maintained time (h)
Loading step (mm) 11.3 MN (1.0WL)
17.0 MN (1.5WL)
20.0 MN (1.75WL)
1 6 7 12
0.54 1.04 1.15 1.94
1.56 2.61 3.47 –
6.11 – – –
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in the O-cell test, slightly higher skin friction and less settlement would be expected. This effect may become more significant at higher test loads as the pile approaches maximum capacity. Influence zone: The pile head diameter is slightly larger than the pile toe diameter due to the bucket movement during the drilling process. The size of the influence zone of the O-cell test is smaller than that of the static load test due to load-transfer characteristics. Therefore, the settlement of the static load test may be smaller than that of the O-cell test. This effect is probably insignificant if the loading level is minor (less than 1.0WL). Therefore, a larger settlement is expected during the O-cell test if the maintained time and pile length are the same. These factors in combination affect the pile load–settlement curve: the settlement during the static load test may be smaller in the lower loading step than that in the O-cell test due to the size of the influence zone if the holding time is short. But the settlement in the static load test may be larger even in the lower loading step as holding time increases (see Fig. 11): At the 11.3 MN, 17 MN and 20 MN, the settlement of the static test is smaller than that of the O-cell test at the beginning of the static test. But the settlement becomes greater during the same loading step after 12 h (11.3 MN), 6 h (17 MN) or 1 h (20 MN). Since the maintained time effects are more severe with each loading step, the gap should increase with the loading step. 6. Conclusions Comparison tests between a static load test and an Ocell test were carried out during early stages of the MRT project in Singapore. As a result of the analyses of the test results, a new method of constructing an equivalent pile load–head settlement curve was developed. The main observations from the tests were: The original method suggested by Dr. Osterberg and LOADTEST is useful to evaluate the ultimate capacity of the pile including skin friction and end bearing. Since the settlement predicted by the original method is relatively small in the long pile, the original method is not appropriate to evaluate the settlement of the pile, especially for a long pile. The new O-cell test analysis method described in this paper considers the elastic shortening of the pile and produces an equivalent pile load–head settlement curve which is similar to that obtained from a static load test. This analysis method can be easily modified to accommodate changes in the shape of unit skin friction response curves. This method provides a promising tool to calculate an equivalent static pile load–head settlement curve by using the O-cell test or similar other bidirectional loading test results.
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References [1] ASTM D 1143-81, Standard test method for piles under static axial compressive load. American Society for Testing and Materials, Annual book of standards; 2000. p. 96–106. [2] BS 8004. British standard code of practice for foundations. BS 8004 British Standard Institution; 1986. [3] Camp WM. Drilled and driven foundation behavior in a calcareous clay. In: Tunner JP, Mayne PW, editors. GEOSUPPORT 2004. Florida: Geotechnical Special Publication No. 124; 2004. [4] Castelli RJ, Wilkins E. Osterberg load cell test results on base grouted piles in Bangladesh. In: Tunner JP, Mayne PW, editors. GEOSUPPORT 2004, Florida: Geotechnical Special Publication No. 124; 2004. [5] Castelli RJ, Fan K. O-cell test results for drilled shafts in marl and limestone. In: O’Neill MW, Townsend FC, editors. Deep Foundations 2002: An international perspective on theory, design, construction, and performance, Florida: Geotechnical Special Publication No. 116; 2002. p. 807–23. [6] Coyle HM, Reese LC. Load-transfer for axially loaded piles in clay. J Soil Mech Found Div ASCE 1966;92(2):4702–11. [7] England M. Bi-directional static load testing – State of the art. Van Impe WF, editor. Deep foundation on bored and auger piles, Belgium; 2003. p. 309–13. [8] Fellenius BH, Altaee A, Kulesza R, Hayes J. O-Cell testing and FE analysis of 28-m-deep barrette in Manila, Philippines. J Geotech Geoenviron 1999;125(7):566–75. [9] Gunnink B, Kiehne C. Capacity of drilled shafts in Burlington limestone. J Geotech Geoenvironm 2002;128(7):539–45. [10] Hannigan PJ, Goble GG, Thendean G, Likins GE, Rausche, F. Design,and construction of driven pile foundations – Volume III. Publication No. FHWA-HI-96-033, US Department if Transportation, Federal Highway Administration; 1996. [11] Kishida H, Tsubakihara Y, Ogura T. Pile loading tests at Osaka Amenity Park Project. Mitsubishi Co; 1992. [12] M&W Spec. North-East Line Contract 701, Materials & Workmanship Specification for Architectural, Civil & Structural Works, Contract 701, Tebder Addendum No. 01. [13] Molnit T, Lee JS. Comparison report: Osterberg cell test method (PTP14) versus Kentledge test method (PTP15), Singapore; 1998. [14] Osterberg JO, Hayes UA. The Osterberg load cell as a research tool. In: Proceedings of the fifth international conference on soil mechanics and geotechnical engineering, vol. 2, 2001. p. 977–9. [15] Osterberg JO. The Osterberg load test method for drilled shafts and driven piles. The first ten years. In: Proceedings of the 7th international conference and exhibition on piling and deep foundations, Austria; 1998. p. 1–11. [16] Osterberg JO. Load testing high capacity piles: What have we learned? In: Proceedings of the fifth international conference on deep foundation practice, Singapore; 2001. [17] Poulos HG, Davis EH. Pile foundation analysis and design. New York: John Wiley and Sons; 1980. [18] Russo G, Recinto B, Viggiani C, de-Sanctis L. A contribution to the analysis of Osterberg’s cell load test. In: Van Impe WF, editor. Deep foundation on bored and auger piles. Belgium: 2003. p. 331–8. [19] Schmertmann JH, Hayes, JA. The Osterberg cell and bored pile testing – A symbiosis. In: The third international geotechnical engineering conference, Cairo, Egypt; 1997. p. 139–66. [20] Tomlinson MJ. Pie design and construction practice. 4th ed. London: E & FN Spon; 1994. [21] Zuo G, Drumm EC, Islam MZ, Yang MZ. Numerical analysis of drilled shaft O-cell testing in mica schist. In: Tunner JP, Mayne PW, editors. GEOSUPPORT 2004, Florida: Geotechnical Special Publication No. 124; 2004.
Computers and Geotechnics 35 (2008) 124–133 www.elsevier.com/locate/compgeo
Equivalent pile load–head settlement curve using a bi-directional pile load test Jong-Sub Lee b
a,*
, Yung-Ho Park
b
a Civil and Environmental Engineering, Korea University, Seoul 136-701, Republic of Korea Hyundai Engineering and Construction Co., Ltd., Yongin-Si, Gyounggi-Do 449-716, Republic of Korea
Received 20 February 2007; received in revised form 1 May 2007; accepted 5 June 2007 Available online 23 July 2007
Abstract An equivalent pile load–head settlement curve to predict pile capacity has been obtained by using test results from the Osterberg Cell (O-cell). The method assumes that the pile is a rigid body and predicts very stiff pile behavior. This study describes a new method for predicting an equivalent pile load–head settlement curve considering the elastic shortening of piles. The static pile load test is compared to the O-cell test using two piles constructed 10 m apart (center-to-center distance). A measured O-cell test settlement consists of an elastic shortening under the load applied inside the pile and a net settlement. The measured net settlement is added to the elastic shortening, which has a different unit skin friction distribution under the top-down load on the pile head, to predict the equivalent pile load–head settlement curve. The new equivalent pile load–head settlement curve, considering the elastic shortening under top-down load, is similar to the pile load–head settlement curve obtained by the static pile load test before the pile yields. The new method can be used to effectively estimate the pile head settlement as well as pile capacity by using the bi-directional load test results. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Bi-directional pile load test; Elastic shortening; Failure; Rigid body; Settlement; Skin friction distribution
1. Introduction The static pile load test using dead weight or reaction piles is a well accepted and common method to determine the load–settlement curve and pile capacity [1,2]. The maximum test load generally applied to piles is 35–40 MN due to the safety and cost. However, along with an increase in typical pile size (diameter and length), the design load of piles has also increased. For a large diameter pile, the capacity can be investigated with the O-cell test developed by Dr. Jori Osterberg [19]. The O-cell test has been used since 1984 for drilled shafts and driven piles [10]. The loading device used for the O-cell test (also called the bi-directional load test) is a hydraulic jack-like device. It is a sacrificial tool placed at the bottom and/or distance *
Corresponding author. Tel.: +82 2 3290 3325; fax: +82 2 928 7656. E-mail addresses: [email protected] (J.S. Lee), [email protected] (Y.H. Park). 0266-352X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compgeo.2007.06.008
up from the bottom of the pile [7,14,16]. As the hydraulic pressure is applied to the device, an equal upward and downward force is applied to the pile. The end bearding is resisted by the skin friction and vice versa. Therefore, dead weight or reaction piles are not required in the O-cell test. The O-cell test results are used to construct an equivalent top-down load–settlement curve. The original method suggested by Dr. Osterberg [19] assumes that the pile is a rigid body. Therefore, top-down settlement may be underestimated by this method. However, pile settlement is a critical issue in large diameter piles. To overcome the deficiency of the original method, finite element simulations can be used to predict the equivalent pile load–head settlement curve [8,18,21]. However, many steps are required for the numerical simulations: assumption of geo-material properties, matching the measured and computed O-cell displacements, and adding the computed elastic shortening components. Thus, numerical simulations have been only
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ment. After the pile head settlement reaches 5 mm, the load-transfer curves are parallel.
used as a research tool. A new simplified method for constructing a realistic pile load–head settlement curve is required. This paper describes a relatively simple method for calculating the equivalent pile load–head settlement curve from O-cell test results. The paper also includes a review of load-transfer characteristics, and compares results from a static load test and an O-cell test conducted under similar conditions.
2.2. Perfectly rigid pile with linearly increasing unit skin friction If the pile is assumed to be perfectly rigid, the settlement is the same along the whole pile shaft. And unit skin friction linearly increases with depth as shown in Fig. 2b. However, ultimate values of unit skin friction reach the same displacement as shown in Fig. 2c. End bearing is assumed to increase linearly with displacement as shown in Fig. 2d. The load-transfer curve for this condition is shown in Fig. 2e. The curve characteristics are similar to the constant skin friction condition except for the shape (parabola instead of trapezoid).
2. Load-transfer characteristics Load-transfer characteristics in piles depend on pile rigidity and unit skin friction distribution along the pile shaft. Figs. 1 and 2 summarize the load-transfer characteristics for a rigid pile with different unit skin friction distributions.
2.3. Elastic pile with constant unit skin friction or linearly increasing unit skin friction
2.1. Perfectly rigid pile with constant unit skin friction
As the pile undergoes elastic shortening, the displacement of the pile decreases with depth under a top-down load. Therefore, the skin friction produced changes with depth before ultimate skin friction is generated (see details in [6,20]). Since the elastic shortening depends on the pile stiffness, the load applied, and the resistance generated, the load-transfer curves can be obtained through numerical simulation or by the trial and error method.
If the pile is assumed to be a perfectly rigid body, the settlement of the pile head is exactly the same as the settlement at the pile toe. Unit skin friction is held constant along the pile shaft in Fig. 1b. Unit skin friction versus pile movement along the pile shaft is modeled as an elasto-plastic element as shown in Fig. 1c. Therefore, unit skin friction increases linearly with displacement until ultimate skin friction is reached. The displacement for ultimate skin friction is about 5–15 mm for clays and 20 mm for sand [16]. After this displacement, ultimate skin friction is constant (see details in [17]). For this analysis, the displacement for ultimate skin friction is set at 5 mm. It is assumed that end bearing increases linearly with displacement as shown in Fig. 1d. As a result, the slope of the load-transfer curve is linear as shown Fig. 1e. Since the settlement of the pile is constant during loading, the developed skin friction is the same along the whole pile shaft. The shape of the load-transfer curves is that of a trapezoid. The slopes of the load-transfer curves increase with pile head displace-
3. Comparison tests Two pile load tests, the static load test and O-cell test, were conducted at the early stages of Mass Rapid Transit (MRT) project in Singapore. The main contractor of the MRT project is Hyundai Engineering and Construction Co., Ltd. Center-to-center distance between the two piles is 10 m. The static load test and the O-cell test were carried out by CEP Instruments PTE Ltd and LOADTEST Inc., respectively. Fig. 3 shows a schematic drawing of the sub-
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Fig. 1. Load-transfer characteristics for a perfectly rigid pile with constant unit skin friction; (a) pile, (b) ultimate unit skin friction versus depth, (c) skin friction versus displacement, (d) end bearing versus displacement, (e) load-transfer curves with different pile head settlements. Note the load-transfer curves are parallel after skin friction reaches the ultimate value.
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Fig. 2. Load-transfer characteristics for a perfectly rigid pile with linearly increasing unit skin friction; (a) pile, (b) ultimate unit skin friction versus depth, (c) skin friction versus displacement, (d) end bearing versus displacement, (e) load-transfer curves with different pile head settlements. Note the loadtransfer curves are parallel after skin friction reaches the ultimate value.
Pile for static load test
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The SPT tests were terminated after a blow count of 100 at any test location was obtained. Where the effective blow count exceeded 100, the corresponding penetration depth in mm at 100 blows are shown in Fig. 3.
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Fig. 3. Schematic profile of soil conditions and test piles. When SPT Nvalues are greater than 100, the corresponding penetration depth is noted in mm unit (with /).
surface conditions and pile construction records for the static and O-cell tests. 3.1. Subsurface conditions One borehole with standard penetration testing (SPT) was drilled to investigate the subsoil conditions between the two test piles (Fig. 3). The soils encounted during borehole drilling and during pile excavation consisted of 18.5 m of silty clay overlying 18.5 m of stiff clayey silt. The stiff clayey silt is underlain by a layer of very stiff silty clay at a depth 37.0 m. The SPT N-values of this site gradually increase with depth until 31.8 m. Below 31.8 m depth, SPT N-values are greater than 100 blows per 300 mm.
The test pile drilling was carried out with a rotary table and bucket. As the rotary table drilled into the soil, the cuttings gradually filled the bucket. The bucket was lifted and emptied when full. A temporary steel casing of 2.2 m length was inserted into the ground after pre-boring to prevent collapse of the drilled hole near the ground surface. Polymer based drilling fluid was used to prevent collapse along the length of the borehole. Preassembled steel cages were lowered into the borehole, and concrete was placed by tremie pipe. The pile dimensions for the static load test were a diameter of 1.2 m and a length of 37.0 m (Fig. 3). The static load test was performed 20 days after concrete placement. The loading procedure for the static pile test was similar to the maintained load (ML) method described in ASTM D 1143 [1] and BS 8004 [2]. The designed working load (WL) was 11.3 MN and the maximum test load was two and half times the working load, 28.25 MN. A first attempt was made at three loading cycles at 1.0WL, 1.5WL, and 2.5WL. However, the short stroke of the hydraulic jack required that four loading cycles were necessary. The test results are shown in Fig. 4. Settlement is often a governing factor for determining pile length or the number of the piles. The specifications for this project (M&W Spec) set settlement criteria as follows: 12 mm at working load, 24 mm at 1.5 times the working load, and 0.1 D at 2.5 times the working load [12]. The results of the static pile load test satisfy the specification. 3.3. O-cell test The pile construction procedures for the O-cell test are similar to those adopted for the static pile load test except
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Load (MN)
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0
Ist Cycle 2nd Cycle 3rd Cycle 4rd Cycle
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1.0×WL 6 hours (1st cycle) 6 hours (2nd cycle)
1.5×WL 6 hours (2nd cycle) 1 hour (3rd cycle)
2.5×WL 24 hours
Fig. 4. Load–settlement curve from the static load test. WL = working load.
O-cells were installed at two levels as shown in Fig. 3. The bottom O-cell was installed at 36.9 m on top of a thin concrete layer (0.5 m thickness) at the end of the drill shaft. The middle O-cell was installed at 26.9 m depth. Note that the length of the O-cell test pile is 0.4 m longer than that of the static load test pile. Concrete was poured into the drill shaft and allowed to cure for 30 days before performing the O-cell load test. The O-cell test method loads the pile internally in two directions by hydraulically pressurizing the installed Ocells. Hence, the toe of the pile below the O-cell resists the O-cell load through end bearing and skin friction which resist downward movement. The shaft of the pile above the O-cell resists the O-cell load through shaft friction which resists upward movement [15,16]. The trend of pile design toward greater lengths and greater capacities have fostered the popularity of multilevel testing [3–5]. Two-level configurations of O-cells are employed in this study as shown in Fig. 3. For the two levels of O-cells, three stages of loading were proposed as follows [13]: 3.3.1. Stage 1: Confirming end bearing (EB) The bottom O-cell is pressurized to measure end bearing by providing reaction with the lower side shear and upper side shear (LSS + USS) as shown in Fig. 5. At this stage the middle O-cell is closed, so all the skin friction is mobilized as a reaction force. The downward displacement corresponding to end bearing is measured by telltale and denoted by D1 in Fig. 5. The displacements of the shaft are denoted by D2 (upward total displacement) and D3 (upward net displacement), respectively. Details of displacement measurements for the single O-cell are represented in [9].
D3
D7
D7
D6
D6
Close D4
D2 Open D1
D5
Fig. 5. Multi-level O-cell test: EB = end bearing of the pile; LSS = lower side shear (skin friction between O-cells); USS = upper side shear (skin friction above middle O-cell). D1 = downward displacement corresponding to end bearing during stage 1; D2 and D3 = upward total and net displacements of the shaft during stage 1; D4 and D5 = downward total and net displacements for LSS during stage 2; D6 and D7 = upward total and net displacements for USS during stage 2 and 3.
3.3.2. Stage 2: Confirming lower side shear (LSS) between the O-cells The middle O-cell is pressurized to measure the LSS using the upper side shear (USS) above the middle O-cell under the open condition of the bottom O-cell as shown in Fig. 5. Therefore, no load is applied to the pile toe. The corresponding displacements for LSS are D4 and D5. D4 is the downward total displacement which is the summation of the elastic shortening induced by skin friction
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and downward net displacement D5. The corresponding displacements for USS are D6 and D7. D6 is the upward total displacement which is a summation of the elastic shortening induced by skin friction and upward net displacement D7 as shown in Fig. 5.
lower than the lower side shear, the ultimate value of the upper side shear was measured during stage 2 as shown in Fig. 6b. Since lower side shear is greater than the end bearing and upper side shear, the ultimate value of lower side shear is not confirmed. In summary, the test pile is successfully loaded to ultimate capacity in one sections: upward upper side shear. To determine the upward direction skin friction, the upper side shear (USS), the buoyant weight of the pile above the middle O-cell is subtracted from the measured value. For the estimation of lower side shear (LSS), the buoyant weight of the pile between O-cells is added to the measured value. However, the end bearing value can be directly taken from the measured value [10].
3.3.3. Stage 3: Confirming upper side shear (USS) above the middle O-cell The middle O-cell is again pressurized to measure the USS using the lower side shear (LSS) between the O-cell and the end bearing below the bottom O-cell (closed condition of the bottom O-cell) as shown in Fig. 5. In this stage, D6 and D7 are continuously monitored. Loading procedures for the O-cell test are similar to the quick test method described in ASTM D 1143 [1]. Loads are held for 4 min and then increased to next load increment. When the movement reaches 6–7 mm and 12– 13 mm, the load is sustained for 1 h. Test results are presented in Fig. 6. During stage 1, end bearing is successfully measured until the movement of the pile toe reaches at about 60 mm as shown in Fig. 6a. Since the upper side shear is
4. Equivalent pile load–head settlement curve An equivalent pile load–head settlement curve can be constructed from the combination of the upper side shear, lower side shear, and end bearing obtained from the O-cell test. The original and elastic methods are discussed and compared below.
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Fig. 6. Multi-level O-cell test results: (a) stage 1; (b) stage 2 and 3. EB = measured end bearing value; USS=measured upper side shear value – buoyant weight of the pile above the middle O-cell (0.66 MN); LSS = measured lower side shear value + buoyant weight of the pile between O-cells (0.15 MN).
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4.1. Original method
4.2. Elastic method (new method)
This method is commonly used to obtain an equivalent pile load–head settlement curve from the O-cell test. This is done by assuming that the pile is incompressible (or elastic shorting is less than 3 mm) and the upward ultimate skin friction is equal to the downward ultimate skin friction [10,16,19]; see also [11]. Procedures for constructing the equivalent pile load–head settlement curve are as follows:
The elastic shortening of the pile may be less than 3 mm if the pile diameter is huge and the pile length is short. However, the portion of the elastic shortening in the total head settlement is not negligible (see [21]). The equivalent settlement obtained by the original method is smaller than that by static load test and so should not be used to check whether the settlement criteria is satisfied (12 mm at 1.0WL and 25 mm at 1.5WL). The lower settlement estimated by the original method is due to the reduced elastic shortening. The reduced elastic shortening in the O-cell test results from the different load-transfer characteristics as shown in Fig. 8. Fig. 8 presents the distribution of axial load in the pile during the Ocell test and the static pile load test under an applied maximum load. Note that unit skin friction is assumed constant at each segment (upper side shear and lower side shear). While ultimate values of the lower side shear and end bearing appear to be greater than 9.5 MN and 10 MN (see Fig. 6), they are assumed as 9.5 MN and 10 MN, respectively. Ultimate upper side shear is 9 MN. Therefore, the ultimate capacities from the O-cell test and the static load test produce the same value (28.5 MN). However, the applied load at each pile segment is very different for each test. For example, upper side shear during the O-cell test and the static pile load test are 4.5 MN (average of 0 and 9 MN) and 24 MN (average of 28.5 MN and 19.5 MN), respectively, as shown in Fig. 8. Thus, the amount of elastic shortening differs according to the type of test employed: the elastic shortening from the static load test is 5.3 times higher than that from the O-cell test. Therefore, to predict the precise equivalent pile load–head settlement curve using the O-cell test, the elastic shortening should be properly considered. Note this confirms that the original method is simply the rigid body method with limited elastic shortening.
Step 1: Select an arbitrary movement from D1, D4, and D 6. Step 2: Add all the resistance components at this movement regardless of the direction of movement. This load corresponds to the selected movement at the equivalent pile load–head settlement curve. Step 3: Repeat steps 1 and 2 up to the maximum deflection (or maximum extrapolated deflection) of the component. If the selected displacement is beyond the maximum displacement of any other resistance component that does not reach an ultimate, the resistance is conservatively assumed constant after the maximum load applied. The equivalent pile load–head settlement curve obtained from this method is shown in Fig. 7. This method underpredicts the settlement measured during the static load test at the same load. Note that the equivalent curve obtained by this method includes elastic shortening even though the pile is assumed to be a rigid body. For example, the arbitrary deflection of the upper side shear consists of elastic shortening due to compression load from the middle O-cell and head movements. On the other hand, the numerical simulation indicates that the equivalent pile load–settlement curve by the original method is satisfactory when the slenderness ratio (length divided by diameter) is less than about 20 [18].
Load (MN) 0
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Fig. 7. Equivalent pile load–head settlement curve and static load test results.
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J.S. Lee, Y.H. Park / Computers and Geotechnics 35 (2008) 124–133 O-cell test
Static load test
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Fig. 8. Conceptual load-transfer characteristics. (a) O-cell test; (b) static load test. Assumption: unit skin friction is uniform above the middle O-cell level (0–27 m deep) and between the two O-cells (27–37 m deep); skin friction in the upward direction is the same as the skin friction in the downward direction.
Fig. 9 presents the concept of pile settlement due to top load (static load test). The pile head settlement, that is, the settlement of the upper shaft (SUSS) is the summation of the elastic shortening of the upper shaft (dUSS) and the net settlement of the upper shaft (nUSS). The net settlement of the upper shaft (nUSS) is the movement of the bottom of the upper shaft. Therefore, the movement of the toe of the upper shaft should be equal to the total movement of the top of the lower shaft (SLSS = nUSS). Again, the total settlement of the lower shaft (SLSS) is the summation of the elastic shortening of the lower shaft (dLSS) and the net settlement of the lower shaft (nLSS). The net settlement of the lower shaft (nLSS) should be the settlement of the pile toe (SEB = nLSS). Therefore, the settlement at the pile head is the summation of the settlement of the pile toe (SEB), the elastic shortening of the lower shaft (dLSS), and the elastic shortening of the upper shaft (dUSS). The load corresponding to this settlement is the summation of end bearing (EB), lower side shear (LSS), and upper side shear (USS). The similar method was also suggested by [6]. The procedure to construct the equivalent pile load– head settlement curve considering the elastic shortening by using the O-cell test is as follows: Step 1: Select the arbitrary pile toe settlement (SEB) and corresponding end bearing (EB) as shown in Fig. 10a. SEB is denoted by D1 in Fig. 5. Step 2: Select or interpolate the lower side shear (LSS) corresponding to this settlement from the lower side shear curve. Note that the corresponding settlement should be the net settlement of the lower shaft (nLSS) instead of the measured total settlement of
Step 3:
Step 4:
Step 5:
Step 6:
the lower shaft. The measured total settlement of the lower shaft is used in the original method. nLSS is denoted by D5 and the total settlement of the lower shaft is represented by D4 in Fig. 5. Calculate the elastic shortening of the lower shaft (dLSS) under end bearing (EB) at the bottom, and the force of end bearing and lower side shear (EB + LSS) at the top of the lower shaft as shown in Fig. 10b. The total settlement of the lower shaft (SLSS) is the summation of the calculated elastic shortening of the lower shaft (dLSS) and the settlement of pile toe (SEB). Select or interpolate the upper side shear (USS) corresponding to SLSS. Note that the corresponding settlement should be the net settlement of the upper shaft (nUSS) instead of the total measured settlement of the upper shaft. nUSS is denoted by D7 and the total measured settlement of the upper shaft is represented by D6 in Fig. 5. Calculate the elastic shortening of the upper shaft (dUSS) under a force of EB + LSS at the bottom and EB + LSS + USS at the top of the upper shaft as shown in Fig. 10c. The total settlement at the pile head (SUSS) is the summation of the calculated elastic shortening of the upper and lower shaft, and the settlement of pile toe (dUSS + dLSS + SEB). The corresponding load in the pile head is the summation of end bearing, lower side shear, and upper side shear (EB + LSS + USS) Repeat steps 1–5 until enough points are obtained to construct the equivalent pile load–head settlement curve.
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Actual top-down load
Upper shaft
Middle O-Cell
Lower shaft Bottom O-Cell
Pile toe
Fig. 9. Principle of the pile settlement evaluation based on the elastic method.
Fig. 10. Procedures for pile load–head settlement evaluation using the elastic method.
Note that the general form of elastic shortening (d) in the pile can be expressed in terms of the area of the pile cross section A, pile length L, the elastic modulus of the pile material E, the force acting on the bottom of the pile or segment QB, and skin friction along the pile shaft QS,
ðQB þ nQS ÞL ð1Þ AE where n is a shape factor depending on skin friction distribution. n = 0.5 and n = 0.67 are applied to uniform skin friction distribution and the triangular distribution (line-
d¼
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5
10
15
20
25
30
0
Head Movement (mm)
20
40 O-Cell: Perfect Rigid Pile O-Cell: Original Method
60 O-Cell: Elastic Method (ξ = 0.5)
80
O-Cell: Elastic Method (ξ = 0.67) Static Load Test
100
120 1.5WL 6 hours (2nd cycle) 1 hour (3rd cycle)
1.0WL 6 hours (1st cycle) 6 hours (2nd cycle)
2.5WL 24 hours
Fig. 11. Comparison of equivalent pile head-settlement curves. Elastic method is derived based on uniform unit skin friction (n = 0.5) and linearly increasing unit skin friction (n = 0.67).
arly increasing), respectively. Skin friction distribution can be obtained using strain gauges. The equivalent pile load–head settlement curves obtained by the elastic method are shown in Fig. 11 for different shape factors (n = 0.5 and n = 0.67). For comparison, the curve obtained by the original method is plotted. The results by the elastic method are similar to the static load test results. The settlement is slightly larger for the triangular distribution than the uniform distribution. 4.3. Perfectly rigid pile If the pile is a perfectly rigid body and upward ultimate skin friction is equal to downward ultimate skin friction, another equivalent pile load–head settlement curve may be obtained. The procedures are identical to the original method with the exception of using the net settlements (nUSS, nLSS, and SEB) instead of measured values (see Fig. 9). The equivalent pile load–head settlement curve using this method is shown in Fig. 11. The equivalent settlement obtained by the perfect rigid body assumption is similar to that by the original method.
Maintained time: The maintained time for each loading stage is relatively short in the O-cell test (typically 4 min, 1 h at two points) compared with the static load test (1– 24 h). Table 1 summaries the maintained time effect on the settlement during the static load test. Pile settlement increases with holding time and these effects increase with loading level. Note that the equivalent pile load–head settlement curve is developed for 4 min holding time. Therefore, the settlement should be smaller for the O-cell test than for the static load test. Overrun: The concrete volume during construction was measured to obtain the overrun. Overrun is the ratio of the difference between theoretical and used concrete volume to the theoretical volume. Overrun of the pile for the static pile load test was 3.35% and overrun for the Ocell test was 4.56%. When the volume of the sacrificial hydraulic jacks installed is considered, the overrun of the two piles are almost the same. Therefore, differences due to overrun are considered to be negligible. Construction effects: The pile length was 0.4 m longer in the O-cell test pile than in the static load test pile. Therefore,
5. Discussion
Table 1 Settlement increments due to maintained time during the static load test
The elastic method predicts a similar pile load–head settlement curve compared to the static pile load test. However, the predicted curve does not exactly match the static load test results. As explained below this may be attributed to several factors which include the maintained time at each loading stage, overrun during concrete placement, construction effects and the size of influence zone.
Maintained time (h)
Loading step (mm) 11.3 MN (1.0WL)
17.0 MN (1.5WL)
20.0 MN (1.75WL)
1 6 7 12
0.54 1.04 1.15 1.94
1.56 2.61 3.47 –
6.11 – – –
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in the O-cell test, slightly higher skin friction and less settlement would be expected. This effect may become more significant at higher test loads as the pile approaches maximum capacity. Influence zone: The pile head diameter is slightly larger than the pile toe diameter due to the bucket movement during the drilling process. The size of the influence zone of the O-cell test is smaller than that of the static load test due to load-transfer characteristics. Therefore, the settlement of the static load test may be smaller than that of the O-cell test. This effect is probably insignificant if the loading level is minor (less than 1.0WL). Therefore, a larger settlement is expected during the O-cell test if the maintained time and pile length are the same. These factors in combination affect the pile load–settlement curve: the settlement during the static load test may be smaller in the lower loading step than that in the O-cell test due to the size of the influence zone if the holding time is short. But the settlement in the static load test may be larger even in the lower loading step as holding time increases (see Fig. 11): At the 11.3 MN, 17 MN and 20 MN, the settlement of the static test is smaller than that of the O-cell test at the beginning of the static test. But the settlement becomes greater during the same loading step after 12 h (11.3 MN), 6 h (17 MN) or 1 h (20 MN). Since the maintained time effects are more severe with each loading step, the gap should increase with the loading step. 6. Conclusions Comparison tests between a static load test and an Ocell test were carried out during early stages of the MRT project in Singapore. As a result of the analyses of the test results, a new method of constructing an equivalent pile load–head settlement curve was developed. The main observations from the tests were: The original method suggested by Dr. Osterberg and LOADTEST is useful to evaluate the ultimate capacity of the pile including skin friction and end bearing. Since the settlement predicted by the original method is relatively small in the long pile, the original method is not appropriate to evaluate the settlement of the pile, especially for a long pile. The new O-cell test analysis method described in this paper considers the elastic shortening of the pile and produces an equivalent pile load–head settlement curve which is similar to that obtained from a static load test. This analysis method can be easily modified to accommodate changes in the shape of unit skin friction response curves. This method provides a promising tool to calculate an equivalent static pile load–head settlement curve by using the O-cell test or similar other bidirectional loading test results.
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References [1] ASTM D 1143-81, Standard test method for piles under static axial compressive load. American Society for Testing and Materials, Annual book of standards; 2000. p. 96–106. [2] BS 8004. British standard code of practice for foundations. BS 8004 British Standard Institution; 1986. [3] Camp WM. Drilled and driven foundation behavior in a calcareous clay. In: Tunner JP, Mayne PW, editors. GEOSUPPORT 2004. Florida: Geotechnical Special Publication No. 124; 2004. [4] Castelli RJ, Wilkins E. Osterberg load cell test results on base grouted piles in Bangladesh. In: Tunner JP, Mayne PW, editors. GEOSUPPORT 2004, Florida: Geotechnical Special Publication No. 124; 2004. [5] Castelli RJ, Fan K. O-cell test results for drilled shafts in marl and limestone. In: O’Neill MW, Townsend FC, editors. Deep Foundations 2002: An international perspective on theory, design, construction, and performance, Florida: Geotechnical Special Publication No. 116; 2002. p. 807–23. [6] Coyle HM, Reese LC. Load-transfer for axially loaded piles in clay. J Soil Mech Found Div ASCE 1966;92(2):4702–11. [7] England M. Bi-directional static load testing – State of the art. Van Impe WF, editor. Deep foundation on bored and auger piles, Belgium; 2003. p. 309–13. [8] Fellenius BH, Altaee A, Kulesza R, Hayes J. O-Cell testing and FE analysis of 28-m-deep barrette in Manila, Philippines. J Geotech Geoenviron 1999;125(7):566–75. [9] Gunnink B, Kiehne C. Capacity of drilled shafts in Burlington limestone. J Geotech Geoenvironm 2002;128(7):539–45. [10] Hannigan PJ, Goble GG, Thendean G, Likins GE, Rausche, F. Design,and construction of driven pile foundations – Volume III. Publication No. FHWA-HI-96-033, US Department if Transportation, Federal Highway Administration; 1996. [11] Kishida H, Tsubakihara Y, Ogura T. Pile loading tests at Osaka Amenity Park Project. Mitsubishi Co; 1992. [12] M&W Spec. North-East Line Contract 701, Materials & Workmanship Specification for Architectural, Civil & Structural Works, Contract 701, Tebder Addendum No. 01. [13] Molnit T, Lee JS. Comparison report: Osterberg cell test method (PTP14) versus Kentledge test method (PTP15), Singapore; 1998. [14] Osterberg JO, Hayes UA. The Osterberg load cell as a research tool. In: Proceedings of the fifth international conference on soil mechanics and geotechnical engineering, vol. 2, 2001. p. 977–9. [15] Osterberg JO. The Osterberg load test method for drilled shafts and driven piles. The first ten years. In: Proceedings of the 7th international conference and exhibition on piling and deep foundations, Austria; 1998. p. 1–11. [16] Osterberg JO. Load testing high capacity piles: What have we learned? In: Proceedings of the fifth international conference on deep foundation practice, Singapore; 2001. [17] Poulos HG, Davis EH. Pile foundation analysis and design. New York: John Wiley and Sons; 1980. [18] Russo G, Recinto B, Viggiani C, de-Sanctis L. A contribution to the analysis of Osterberg’s cell load test. In: Van Impe WF, editor. Deep foundation on bored and auger piles. Belgium: 2003. p. 331–8. [19] Schmertmann JH, Hayes, JA. The Osterberg cell and bored pile testing – A symbiosis. In: The third international geotechnical engineering conference, Cairo, Egypt; 1997. p. 139–66. [20] Tomlinson MJ. Pie design and construction practice. 4th ed. London: E & FN Spon; 1994. [21] Zuo G, Drumm EC, Islam MZ, Yang MZ. Numerical analysis of drilled shaft O-cell testing in mica schist. In: Tunner JP, Mayne PW, editors. GEOSUPPORT 2004, Florida: Geotechnical Special Publication No. 124; 2004.