Characterisation of Response of Circular Piled Raft Tested in Sand
Characterisation of Response of
Circular Piled Raft Tested in Sand
K.Ilamparuthi
Professor, Division of Soil Mechanics and Foundation Engineering Anna University Chennai, Chennai–600025, India.
E-mail:
V.Balakumar
Former Ph.D Scholar, Division of Soil Mechanics and Foundation Engineering Anna University Chennai, Chennai–600025, India. E-mail:
ABSTRACT: In order to understand the load sharing and settlement reduction behaviour of circular piled raft resting on sand, 1gmodel tests were conducted on small-scale models. The parameter analysed were length, diameter and number of piles. The load-settlement response curves obtained were analysed and characterized. The characteristic response exhibited three phase behaviour irrespective of pile parameters and density of sand bed. The stiffness of piled raft system in the third phase is almost equal to raft-soil stiffness, which indicates that, piles perform essentially settlement reducer rather than load sharing members. Finite element analysis using MISO idealisation for soil compares well with the experimental findings. At lower settlements piles share more load whereas raft shares higher load with increase in settlement.
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Characterisation of Response of Circular Piled Raft Tested in Sand
1.INTRODUCTION
Traditionally tanks are founded either on raft or piles, depending on the magnitude of settlement. When the settlement exceeds the permissible value, piles provided are designed to take the full load ignoring the contribution of raft resting on reasonably good supporting strata. This conventional approach provides a safe but uneconomical design. Instead of suppressing the entire settlement, the settlement can be restricted to the permissible level by providing pile elements to the raft which is known as piled raft. By this foundation considerable economy can be achieved on foundation design. Even though Burland et al. (1977) proposed the concept of settlement reducing piles, limited understanding of the behaviour and the absence of simplified design has restrained the designers from using this system by default .
Attempts were made by earlier researchers to understand the behaviour of piled raft. They are boundary element approach by Butterfield & Banerjee (1971) and Kuwabara (1989), plate on spring approach by Poulos (1994), and Clancy Randolph (1993), and three dimensional finite element method by Katzenbach et al. (1998) involve complicated interaction analysis. Laboratory tests on 1g model piled raft (Weisner Brown (1980)) and centrifuge model test by Horikoshi Randolph (1996)) were also reported in literature. The work on model tests indicated that the piles in the piled raft has served effectively as a settlement reducer and also reduced differential settlement in the raft. Centrifuge model study of Horikoshi Randolph (1996) indicated that the small centered pile groups are very effective in reducing the settlement of flexible raft under uniformly distributed load. Turek Katzenbach (2003) and Balakumar Ilamparuthi (2004) have conducted tests on piled raft embedded in sandy soil to understand the settlement reduction as well as proportionate sharing of load between the piles and the raft. The literature reviewed here indicated that research work on piled raft in sand are very limited and most of the analytical works are for linear elastic condition of soil. From the literature, it appears that 1g model tests related to piled raftfoundation on sand deposit is very limited and characteristic response of piled raft system is not established. With this background test on piled raft models placed on sand were tested and results were analysed. FE analysis was also carried out using MISO soil model and validated the response with experimental findings.
2. MODEL TESTS
Tests were performed on models of circular piled rafts founded in sand of different densities. Few tests were also conducted on piled raft without the contact of raft on the sand bed that is termed as free standing pile group. All the tests were conducted in a steel tank of 1000mm × 600mm × 600mm using clean uniformly graded sand, which is free from fines. Perspex sheets of 6mm, 8mm and 10mm thickness have been chosen for the raft models and solid rods of diameter (d) 6mm, 8mm and 10mm have been used as piles. The lengths (L) of piles tested are 100mm, 120mm, 160mm and 200mm. The diameter of model raft(D) is 200mm, which represents the prototype tank of diameter 20m.
One of the important parameter, which has strong restrictions on the settlement behaviour of model foundation, is the density of packing of sand grains. In this study tests were conducted for three different densities. Their properties are presented in Table 1. Their densities were achieved by the combination of sand raining and compaction. Pre-weighed sand was rained in layers and controlled compaction was adopted.
Table 1: Properties of Sand Bed
Sl No. / State of compaction / Unit weight, (kN/m3) /
1. / Dense / 16.2 / 41
2. / Medium / 15.5 / 37
3. / Loose / 14.8 / 34
The piles were installed in the sand bed prepared by adopting the procedure as explained above. The installation of pile was so planned that represents a real time pile installation. Piles were connected to the raft with suitable arrangement to ensure monolithic action. The foundation was vertically loaded using a hydraulic jack fitted to a loading frame and the load applied was monitored using a proving ring of
20kN capacity. Settlement of piled raft was measured usingdial gauges having travel of 50mm and least count of 0.01mm.
3. RESULTS AND DISCUSSION
3.1 Pile Arrangement on Load-Settlement Response
In order to select the arrangement of piles to be used in the study, the load-settlement response of piled raft with two independent arrangements of piles of radial and square grid configurations as shown in Figure 1 were analysed by keeping the number of piles 21 and spacing 4d.
Fig. 1: Layout of Piles in Circular Piled Raft
The model tested is a circular piled raft of 200mm diameter and 8mm thick. The diameter and length of the pile were 8mm and 160mm respectively and the area ratio (Ar) of piled raft was kept as 5.2%. Figure 2 presents the load-settlement curves of plain and piled rafts. A study of the curves indicates that the settlement of piled raft is lesser than the plain raft for a given load irrespective of the pile arrangement.
It was found that for the given area ratio both the radial and square grid arrangements exhibited almost identical load- settlement behaviour. Tests were also conducted on piled raft with square and radial arrangement for piles in other densities and compared. The results of tests on other two densities showed virtually no difference in load-settlement response between the two pile arrangements of piled raft.Since the radial arrangement is more commonly used in practice for tank pads, further studies were on piled raft with the piles arranged in the radial directions.
Fig. 2: Comparison of Load Settlement Response of Plain and Piled Raft
3.2Behavior of Circular Piled Raft with Radial
Pile Layout
The load–settlement response of plain raft (unpiled raft) and piled raft obtained from 1g model tests are presented and analysed.Figure 3 compares the load-settlement behaviour of plain raft and the piled raft tested in medium dense sand for the raft thickness of 8mm.Similar, tests were carried out in the case of loose and dense sand also. Table 2 presents the comparison of the load taken by the piled raft and the plain raft for three different settlements and the densities tested.In all the three densities of sand, the load taken by the piled raft is higher than the plain raft irrespective of the magnitude of settlement.
Fig.3:Load-Settlement Response of Plain Raft, Piled Raft
The settlement of piled raft in the initial stages of loading is much lesser than plain raft. For the settlement of 2mm in loose sand the load on piled raft is 0.72kN which is 110% higher than the plain raft load, whereas for the settlement of 20mm, the piled raft load is 35% higher. At the settlement of 20mm the excess load taken by the piled raft varies between 30% and 35% for the three densities of sand.
Table 2: Load Shared by Plain and Piled Raft
Bed Density / Load in kN at Different Settlements2 mm / 6 mm / 20 mm
Plain / Piled / Plain / Piled / Plain / Piled
Loose / 0.34 / 0.72 / 0.84 / 1.30 / 2.00 / 2.70
Medium / 1.30 / 2.30 / 2.80 / 4.05 / 5.10 / 6.80
Dense / 1.60 / 2.90 / 3.70 / 4.90 / 6.30 / 8.20
The trend seen above in load sharing by the piled raft indicates that, in the initial stages of loading, inclusion of piles makes the system stiffer.The piles function as settlement reducer, and the combined interaction between pile–raft-soil makes the raft to take higher load under reduced settlement. However as the load increases, the settlement of piled raft increases; this is due to the reduction in soil- pile stiffness. This indicates that the provision of piles to the raft is very effective when the settlements are less, and in particular settlement less than 2% of the raft size tested.
Fig. 4: Characteristic Response of Plain Raft and Piled Raft
The load-settlement response explained above is seen in sand of all the three densities tested. The characterisation curves for the piled raft and plain raft for medium dense condition are presented in Figure 4.
The response is three phase behaviour. In the three phase behaviour, up to a load level of 1.7kN in the case of medium dense sand, (the first phase) the combined system stiffness is high. Both the piles and the raft are in the elastic phase, and the increase in the settlement is very small. In the second phase reduction on the pile-soil stiffness becomes high; hence the rate of change in the settlement with the load is higher. In the third phase, the stiffness of the piled raft-soil reduced drastically and even for a small increase in the load the piled raft settles rapidly. Beyond 20mm settlement the piled raft settlement was very high even for a small increase in the load. Similar trend was observed in the case of loose and dense sand although the load levels vary with the densities in all the three phases.
Comparing the behaviour of the plain raft and the piled raft through the load-settlement characterization curves, it can be seen that, an addition of a small area (5.2%) of the piles to the raft, enhances the performance of the foundation system. The addition of piles to the raft enhances the stiffness of the combined system and it becomes far higher than the plain raft at any particular level of settlement. It is seen that at the maximum settlement of 20mm (all the tests were conducted upto the settlement of 20mm), the stiffness of the combined system is very close to that of plain raft indicating that at higher settlement the piles tend to behave as settlement reducer and not primarily a load bearing member.
The comparison was also done for the piled raft with area ratio of 2.75%. Nearly 50% of the piles were reduced in this case. It was found that with an addition of 11 piles to the raft, the load carrying capacity of the combined system is higher than the plain raft and the behaviour pattern remains same as that of the previous case (i.e. Ar=5.2%) with three well defined phases. Even though the number of piles added is less in number, the behaviour pattern of the piled raft remains almost the same. This indicates that, even when the area ratio is small the pile group enhances the load carrying capacity of the raft as a combined system.
The first phase of the curve up to a settlement level of around 2mm represents the elastic behaviour of the entire system. The second phase shows (upto 6mm settlement) gradual loss of system stiffness (the pile group loses its elastic behaviour) and beyond this stage the loss of stiffness is rapid and at 20mm settlement (the maximum settlement at which all the tests were terminated) the stiffness is close to that of plain raft. In other words, beyond a settlement level of 3% of the least lateral dimension of the raft, the piled raft system behaves more like plain raft.
3.2.1 Effect of Pile Length
Figure 5 presents the load settlement response for piled raft tested with piles of five different lengths and diameter equal 10mm. The lengths of piles are 200mm, 160mm, 120mm, 100mm and 75mm. It can be seen that the loadsettlement response of the piled raft with piles of various lengths is similar, although the load taken at any settlement varied with the length. In all the cases it is observed that for a given load, the settlement of piled raft is smaller than that of the plain raft. It is seen from the Figure 5, that introduction of piles whose length is as small as 75mm (0.375 times the dia of the raft) makes the piled raft to take a higher load at any given settlement. Similar response is seen for piles of different diameters irrespective of the density.
Fig. 5: Load-Settlement Response of Circular
Piled Raft with Various Pile Lengths
The typical characterisation curve of the piled raft shown with various pile lengths are given in the Figure 6 for a pile diameter of 10mm, which shows that irrespective of the pile length, the behaviour has three phases. Although the settlement up to which the linear elastic stage (portion OA of the curve) remains same as 1mm, the load corresponding to this varies. As can be seen at higher length the linear behaviour extends nearly upto 30% of the load taken by the piled raft corresponding to settlement equal to 10% of the pile length. The second stage of the curve AB is the stage where the behaviour tends to become elasto- plastic, which extends up to a settlement level 9mm for 200mm long pile, 7.5mm for 120mm long pile and 4.5mm for 75mm long pile. The variations in stiffness for different length are presented Table3.
Fig. 6:Characteristic Response of Piled Raft
for VariousPile Lengths
The reduction in the stiffness in the region AB is very high as seen from the Table 4.6 and beyond the level of B the foundation stiffness is close to that of plain raft. This indicates that at a settlement level of 4.5% to 6% of the length of the pile, the piled raft losses its stiffness indicating that the piles do not involve effectively in load sharing and functions mainly as settlement reducer.
Table 3: Variation of Stiffness for Piled Raft
with Piles ofDifferent Lengths (medium dense sand)
Phase OA / Phase AB / Phase BC
200 / 2800 / 471 / 239
120 / 2100 / 343 / 217
75 / 833 / 213 / 96
Figure 7 presents the non dimensional plot where in the load ratio PRand settlement ratio R has been plotted for various lengths of piles of piled raft. The plot indicates that irrespective of the length, the settlement ratio remains same for any given load ratio. The relation between load ratio and settlement ratio appears close to rectangular hyperbolic relation. Thus the relationship between load ratio and settlement ratio is given below.
(1)
where, m = 0.689 and C = 0.35
Figure 8 presents the non-dimensional plot of load ratio vs settlement ratio. A unique relation is seen between them irrespective of the diameter and is as follows:
(2)
Fig. 7: Non-Dimensional Plots for
Various Lengths
This indicates that the load-settlement response of piled rafts tested in this study is almost identical and is independent of pile length and pile diameter and thickness of raft.
Fig. 8: Non-Dimensional Plots for Various Diameter of Piles
4.NONLINEAR ANALYSIS (MISO MODEL)
To have better understanding on load sharing between the raft and pile group of piled raft, three dimensional nonlinear analysis was carried out using ANSYS code. Only quarter model of piled raft was analysed taking advantage of the symmetry (Figure 9). The bed density was kept as medium dense with φ=37.5° and unit weight=15.5kN/m3. MISO material model was used for the soil. The continuum was modelled using solid 45 elements with three degrees of freedom at each node.In the analysis the bed dimensions were kept same as that of the model tested in the laboratory. The raft and piles were also modelled as solid 45 elements in order to maintain the elements compatibility. The load was applied as pressure in small increments till the load on the raft equal to the final test load. Figure 9 shows the quarter model including finite element meshing adopted in the analysis.
Fig. 9: Finite Element Mesh of a Circular Piled Raft
4.1 Load-Settlement Behaviour
Figure 10 presents the load settlement curves of circular piled raft obtained from 1g model test and the numerical model. Figure 11presents comparison of characteristic load-settlement response of circular piled raft between experiment and numerical analysis. The results obtained from the 1g model test and numerical model agree very closely, till the settlement level of 4mm.