investigations on the performance of raft and piled raft model … · 2016-11-05 · raft...

22 D. Prabavathi*, S.P. Jeyapriya, P.Soundrapandiyan

International Journal of Computer & Mathematical Sciences

IJCMS

ISSN 2347 – 8527

Volume 4, Issue 7

July 2015

Investigations on the Performance of Raft and Piled Raft

Model Footings on Loose Sand

D. Prabavathi1*

, S.P. Jeyapriya2, P.Soundrapandiyan

3

1& 2

Department of Geotechnical Engineering, Government College of Technology, Coimbatore, India; 3PG Scholar, Anna University BIT Campus, Trichy, India.

ABSTRACT Raft foundation is specially used in structures like chimneys, silos, cooling towers and buildings with basements were

individual footings may touch or overlap each other. Raft foundation is mainly used to imp rove the bearing capacity of

weak soil and to arrest the settlement. For several foundations in non cohesive soils, although a raft provides adequate

bearing capacity, piles are introduced for the purpose of reducing settlement. In this research, experiments were carried

out to study the load - settlement behaviour of both rafts and piled rafts on loose sand condition.Totally,12 model tests

were performed on rafts o f different sizes and with piled rafts of different con figurations. The performance of above

model footings on the load vs settlement behaviour on loose sand was observed experimentally and also numerically

using PLAXIS 2D.From the results, it was found that as the size of raft increased, the ultimate load increases and the

settlement decreases for same value of load.But at constant pressure, with increase in load the settlement increases. It

was also observed that, introducing piles to rafts results in large settlement decrease.Reduction in settlement was

expressed as settlement reduction ratio and this ratio was observed to decrease with increase in pile to raft area ratio.

Results concluded that, among the different sizes and configurations of footing used in the study,for the piled raft of

110mm × 110mm with 3 × 3 configuration the ultimate load is 270%greater than the unpiled raft used and also the

percentage reduction in settlement was observed to be 89%. Keywords: raft foundation; piled raft; weak sand; PLAXIS 2D

Introduction The most common way of designing a foundation system where loose / soft deposits occurs is a raft foundaton. Alternatively, pile foundations are adopted in situations where the structural loads are heavy. In the recent years, combinations of both piles and rafts are used where the column loads are shared by the raft as well as by the piles. The piles transfer a part of the building loads into deeper and stiffer layers of soil and thereby allow the reduction of settlement and increases the ultimate load in a very economic way. Thus, the piled raft foundation system is an economical foundation system to support structures sensitive to settlement. During the past decade some high-rise and heavy buildings around the world, founded on pile raft system, have been monitored and their settlements were measured and reported. Recently, Poulos and Davis reported a case study, the twin tower in Dubai, in which a disagreement between measured settlements and predicted settlements calculated by boundary element method, were observed.Considering this problem, it seems the best way to certify about the accuracy of an analysis method of pile-raft foundation is to perform a physical model test according to real conditions of soil in project site. Katzenbach et al. (2005) reported that, piles may reduce the differential settlement when raft alone exceed the allowable settlement and the raft may increase the lateral stress between the underlying piles and the soil, and thus can increase the ultimate load capacity of a pile as compared to free-standing piles. MosaJ.Al-Mosawi et al. (2011) investigated that load carrying capacity of the unpiled raft increases with the increase of raft size while the raft thickness slightly affects the load carrying capacity. Baleshwar singh (2011) suggested that, to control the differential settlement, optimum performance can be achieved with a small number of piles placed in the central portion, rather than using a large number of piles evenly distributed over the raft area.S.R.Gandhi (1995) by increasing the rigidity of the raft, the differential settlement of piled raft can be reduced but the load shared by middle pile reduces. In order to study the increase in ultimate load and reduction in settlement of piled raft systems, it is essential to conduct a small scale model tests in laboratory. Hence this study was undertaken with the aim of investigating the performance of different sizes of rafts and with different configurations of piled raft system on loose sand. Experimental results were compared with numerical analysis using PLAXIS 2D.



IJCMS

ISSN 2347 – 8527

Volume 4, Issue 7

July 2015

Materials Soil

Cohesionless soil used as foundation medium was collected from Cauvery river basin. The collected sample was cleaned, air dried and preserved for laboratory investigations and for experimental model studies. The basic properties of soil was evaluated conforming to IS Standards and the values are reported in (Table 1).

Table -1 Properties of Cohesionless soil. S.No Properties Values 1. Specific gravity 2.65 2. Gravel 1.6% 3. Coarse sand 10.4% 4. Medium sand 62% 5. Fine sand 25.6% 6. Combined silt and clay 0.4% 7. Uniformity coefficient 2.96 8. Coefficient of curvature 0.98 9. IS classification Poorly graded sand (SP) 10. Maximum unit weight (γ max) 17.26 kN m

-3

11. Minimum unit weight (γmin) 15.20kN m-3

12. Maximum void ratio 0.80 13. Minimum void ratio 0.53

Raft footing

Model rafts of three different sizes using mild steel was fabricated. All the model rafts are of same thickness of 8mm and the sizes used are 70mm × 70mm, 90mm × 90mm and 110mm × 110mm. Piled Raft models

Model piled rafts used in this study was made of mild steel with same diameter of 8mm and of length 180mm.The pile configuration and pile spacing for each raft sizes are represented in (Table 2) and shown in (Figure 1).

Table - 2 Model Piled Rafts. S.No

Raft dimension (mm) Pile raft configuration identification

Pile spacing

1. 70× 70 PRF 1×1 3.5d 3.5d

2. 70 × 70 PRF 2×2 3. 70 × 70 PRF 3×3 4. 90 × 90 PRF 1×1

4d 4d

5. 90 × 90 PRF 2×2 6. 90 × 90 PRF 3×3 7. 110 × 110 PRF 1×1

4.5d 4.5d

8. 110 × 110 PRF 2×2 9. 110 × 110 PRF 3×3

Fig. 1Raft and piled rafts configurations adopted in the experimental work.



IJCMS

ISSN 2347 – 8527

Volume 4, Issue 7

July 2015

Methodology Preparation of foundation medium

The foundation medium used for the experiment was loose sand. Sand as loose condition was achieved by height of fall method. For each height of fall of sand, the parameters such as unit weight, void ratio, relative density and angle of shearing resistance were found out and shown in (Table 3).

Table - 3Evaluation of soil properties relative to height of fall of sand.

S.No Height of fall(cm)

Unit weight (kN m

-3)

Relative density (%)

Void ratio Angle of shearing resistance

1. 0 15.13 14 0.80 27°

2. 2.5 15.20 16 0.79 28°

3. 5 15.21 19 0.78 28°

4. 7.5 15.22 22 0.77 28°

5. 10 15.23 24 0.76 29°

6. 12.5 15.27 27 0.75 29°

7. 15 15.31 30 0.75 29°

8. 17.5 15.38 33 0.74 31°

9. 20 15.44 36 0.73 32°

As the height of fall of sand increases, the unit weight, relative density and angle of shearing resistance

increases while the void ratio decreases. For the foundation medium to be maintained in loose sand condition, the height of fall varied from 0 to 20cm. Among the varying height, 10cm fall of sand and the corresponding parameters shown in (Table 4) are chosen for further experimental work.

Table - 4 Parameters of loose sand used in experimental study.

S.No

Properties Normal range Chosen value

1. Relative Density % 15-35 24

2. Angle of shearing resistance 28°- 31° 29°

3. Unit weight (kN m-3

) 13-16 15.23

4. Void ratio 0.7 – 0.9 0.76

Application of vertical load

The model raft was placed on the foundation medium and its level was checked by sprit level. Proving ring was positioned over the model raft and a seating pressure of 70g cm

-2 was applied and removed before starting

the load test. The reading in the proving ring was reset to zero before commencing the actual test. The load was applied through a 50kN proving ring in equal increments on the model raft using hydraulic jack. As per IS1888-1982, for soils other than clayey soils, each load increment shall be kept for not less than one hour or up to a time when the rate of settlement gets appreciably reduced to a value of 0.02 mm/min. The settlement of the raft was read by two dial gauges of 0.01 mm sensitivity,which is placed on the loading platen as shown in (Figure 2). The test was continued until the raft undergoescontinuous displacement under constant load. This process was repeated for all other rafts and piled raft models.



IJCMS

ISSN 2347 – 8527

Volume 4, Issue 7

July 2015

Fig.2Loading Assembly.

Results And Discussions This research work was carried out to study the settlement behaviour of piled raft foundation.For this purpose,

12small scale model tests were conducted in the laboratory as per IS 1888 - 1982. Discussions were made on the effect of raft size, introduction of piles to raft of different configurations,Settlement ratio and pile to raft area ratio on the load carrying capacity and settlement behaviour of the model footings on loose sand foundation medium.

Effect of raft size

The model tests were carried out for three raft sizes namely 70mm × 70mm, 90mm × 90mm and 110mm × 110mm. From the results, it was observed that by increasing the size of raft the settlement value increases for constant pressure and decreases for constant load.For example, the pressure under the raft of 70mm × 70mm was 47kPa and the maximum settlement corresponds to this pressure was 7mm.This settlement value was compared with other raft sizes and found that the settlement value increased by 8% and 17% respectively for 90mm × 90mm raft and 110mm × 110mm raft models.Referring to (Table 5), it was also observed that the small size of raft has reached to its ultimate load at smaller settlement than larger size raft.

Table - 5 Ultimate load and settlement of different sizes of rafts.

S.No Piled Raft Dimensions(mm) Ultimate Load(N) Settlement(mm)

1. 70 × 70 231 7

2. 90 × 90 456 9 3. 110 × 110 1020 11 Influence of number of piles Piled raft foundation is an economical foundation option where the performanc of the raft foundation is not satisfactory. The addition of limited number of piles reduces the total settlement and also increases the ultimate load of raft foundation.(Table 6) showed that by introducing piles of 1×1, 2×2 and 3×3 configurations with 70mm × 70mm raft, the ultimate load was increased by 75%, 122% and 146% and percentage of settlement reduction was observed to be 56%, 68% and 70% respectively, when compared to unpiled raft of same size. For 90mm × 90mm raft with the introduction of 1×1, 2×2 and 3×3 group piles, the ultimate load increased by 174%, 242% and 295% while the settlement was decreased by 65%, 74% and 86% respectively. Similarly pile configurations 1×1, 2×2 and 3×3 in 110mm × 110mm raft showed 176%, 235% and 270%increase in ultimate load with reduction in settlement of 72%,78% and 89% respectively.The load – settlement relationship of these combinations are showed in (Figures 3 to 5). It is inferred from the observed values that using limited number of piles to rafts, large reduction in settlement can be obtained.



IJCMS

ISSN 2347 – 8527

Volume 4, Issue 7

July 2015

Table - 6 Ultimate load and settlement for all raft and piled raft configurations. S.No

Piled Raft Dimensions(mm) Ultimate Load(N) Settlement (mm)

% of Settlement reduction

1. 70 × 70 PRF 1×1 403 3.09 56 2. 70 × 70 PRF 2×2 512 2.24 68 3. 70 × 70 PRF 3 ×3 569 1.98 70 4. 90 × 90 PRF 1×1 1250 3.15 65 5. 90 × 90 PRF 2×2 1560 2.37 74 6. 90 × 90 PRF3×3 1800 1.29 86 7. 110 × 110 PRF1×1 1800 3.04 72 8. 110 × 110 PRF2×2 2400 2.41 78 9. 110 × 110 PRF 3×3 2760 1.17 89

Fig. 3Load – Settlement relationship of 70mm × 70mm raft and its piled raft configurations.

Fig .4Load – Settlement relationship of 90mm × 90mm raft and its piled raft configurations.



IJCMS

ISSN 2347 – 8527

Volume 4, Issue 7

July 2015

Fig.5 Load – Settlement relationship of 110mm × 110mm raft and its piled raft configurations.

Settlement Ratio

The behaviour of model rafts and piled rafts are studied using two important terms namely the Settlement ratio and pile to raft area ratio. Sratio=S pr/S r P/R area ratio=A pr/A r

where, S pr - Settlement of piled raft (mm) S r - Settlement of raft (mm) A pr - Area of piled raft (mm

2)

A r - Area of raft (mm2)

(Figure6) illustrates the variation of settlement ratio of model piled rafts with the pile to raft area ratio for various size of raft. From this graph it was understood that, the settlement ratio not only decreases with the increase in the pile to raft area ratio but also increases with raft size. This effect is more pronounced for model raft size 70mm × 70mm in comparison with 110mm× 110mm raft.

Fig.6Variation of settlement ratio with pile to raft area ratio for different rafts..



IJCMS

ISSN 2347 – 8527

Volume 4, Issue 7

July 2015

Simulation Study Using PLAXIS 2D The finite element method is a numerical analysis technique for obtaining approximate solutions for the wide variety of engineering problems. In this study, the PLAXIS FEM program was used to analyze the settlement behaviour of model rafts and piled rafts on loose sand. Mohr – Coulomb model was used preferably for sandy soil which involves five input parameters such as Young’s modulus, poison’s ratio, angle of shearing resistance, cohesion and angle of soil dilatancy. In the plane strain analysis, the raft and piled raft was modeled as a plate element with the geometrical parameters.In addition, the boundary was placed far away from the region of interest in order not to affect the deformations within the region. In this analysis, prescribed load was used to study the settlement behaviour and to get the output from the numerical simulation. For simulation study, all the raft model and its piled raft configurations were chosen. (Figures 7 to 9) represent the geometry, finite element mesh and vertical displacement of 110mm × 110mm raft with 3 × 3 pile configuration.

Fig.7Geometry of piled raft.

Fig. 8 Finite element mesh for piled raft.



IJCMS

ISSN 2347 – 8527

Volume 4, Issue 7

July 2015

Fig. 9Vertical displacement of piled raft.

Table- 7Settlement values of all the raft sizes and its piled raft configurations.

S.No Models Experimental values

settlement (mm)

Numerical results settlement (mm)

1. 70 × 70 7 6.16 2. 70 × 70 PRF 1×1 3.09 2.75 3. 70 × 70 PRF 2×2 2.24 1.93 4. 70 × 70 PRF 3×3 1.98 1.72 5. 90 × 90 9 8.14 6. 90 × 90 PRF1×1 3.15 2.77 7. 90 × 90 PRF 2×2 2.37 2.08 8. 90 × 90 PRF 3×3 1.29 1.10 9. 110 × 110 11 9.90 10. 110 × 110 PRF 1×1 3.04 2.70 11. 110 × 110 PRF 2×2 2.41 2.10 12. 110 × 110 PRF 3×3 1.17 1.06

From thenumerical modeling, settlement values for the prescribed load was taken and compared with the experimental results.Results tabulated in (Table 7) showed that the settlement values observed from experimental model study and numerical simulation compares well and are in close agreement with lesser error. Therefore numerical simulation can be used to study the settlement behaviour of rafts and piled rafts in loose sand.



IJCMS

ISSN 2347 – 8527

Volume 4, Issue 7

July 2015

Conclusions The effect of raft size and introducing piles to raft on the settlement behaviour was studied. The following conclusions were drawn both from experimental model study and numerical modeling. 1. Large size raft showed high ultimate load and the settlement value was also found to be high. The load

was found to increase from 231N for 70mm × 70mm raft to 1020N for 110mm × 110mm raft. 2. Introducing piles to raft effects in reducing the settlement. The percentage reduction in settlement was

found to be greater by increasing the number of piles beneath the raft. For the same size of raft (70mm × 70mm), the settlement reduced from 56% to 72 % when the number of piles increased from 1 (PRF 1×1) to 9 (PRF 3×3).

3. It was also observed that it is advantageous to have increase in number of piles rather than increasing the size of the raft. As the size of the raft is increased, minimum number of piles are sufficient to produce reduction in settlement values, beyond which the percentage reduction in settlement becomes negligible.

4. Settlement ratio decreased with increase in pile to raft area ratio and settlement reduction was high for large size raft.

5. Numerical Analysis using PLAXIS 2D produced settlement values which compared well with experimental data and hence can be used for simulation studies.

References Al-mosawi, J, Fattah, Y, Al-Zayadi, A.O., 2011. Experimental observations on the behavior of a piled raft

foundation.Journal of Engineering,17(4), 807 – 828.

Baleshwar singh., 2011. Influence of piles on load – settlement behaviour of raft foundation. International Journal of Engineering Science and Technology,3(12), 8385 – 8394.

Balakumar, V., 2011. “Experimental studies of model piled rafts on sand and field study of prototype behaviour.” Ph.Ddiss., AnnaUniversity, Chennai.

Bajad, S.P, Sahu, R.B., 2011. Time dependent settlement of piled raft foundation. Indian Geotechnical Conference Kochi, 225–228.

Fattah, Y., et.al., 2013.Bearing Capacity of Pile Group and Piled Raft Foundations on Sandy Soil.Journal of Engineering and Development,17(2),64 – 96.

Gandhi, S.R., 1995. Behaviour of piled raft under uniform loading. Indian Geotechnical Conference, Bangalore,1, 169 –172.

MoghaddasTafreshi, S.N., et.al., 2011. Experimental and numerical investigation on circular footing subjected to incremental cyclic loads. International Journal of Civil Engineering,9(4), 265 –274.

Website Geotechnical Information, 2013. Available from :http://www. geotechnicalinfo. com/ [Accessed18October2013].

investigations on the performance of raft and piled raft model … · 2016-11-05 · raft...

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