Second Seminar on

Analysis and Design of Piled Raft Foundation

Submitted by

Contents

1. Overview

2. Literature Review

3. Numerical Studies

4. Work to be done

1. Overview

1. Literature Review

Noh et al (2008) carried out analysis of un-piled and piled raft

foundations. They observed

• The raft thickness affects diff. settlement and bending moments,

but has little effect on load sharing or maximum settlement.

• Piles spacing plays an important role on the performance of

piled raft foundation

• Differential settlement, the maximum bending moment and

the load sharing are not affected much by increasing the pile

lengths.

“ Finite element modeling for piled raft foundation in sand.” Proc., 11th East Asia-Pacific Conference on Structural Engineering and Construction (11EASEC-2008), Taipei,Taiwan,19-21.

Gowri (2011) analyzed raft foundation using FEM.

• load carrying capacity increased with the increase

in thickness up to 1.25m only

• Also the load carrying capacity increased up to 3m

x 3m size raft and later load carrying capacity

decreased with increase in raft size.

“Analysis of Mat foundation using finite element method”, J. Earth Science and Eng., 04, 113-115.

Shukla et al (2011)

• Parametric study of piled raft foundations

• Increase in raft thickness increases settlement

• Increase in pile diameter reduces the settlement

“Behavioral study of pled raft foundation in layered soil deposits.” J. Advanced Eng. Tech., 2(4), 191-195.

3. Numerical Studies

• The foundation is designed for a storage 5 story building. The bearing stress assumed around 100 kN/m2

.

• The raft is modeled in SAFE software. All analysis and design are based on the ACI code.

• “Conventional Rigid Method” is used for design.

Parameter Notation Value

Yield strength of steel Fy 400 MPa

Strength of concrete fc 30 MPa

Young modules of elasticity E E 2000000 kN/m2

Dear load factor D.L.F D.L.F 1.2

Live load factor L.L .F L.L .F 1.6

Soil Unit weight γ soil γ soil 15 kN/m3

Allowable Bearing stress qa qa 100 kN/m2

Concrete Unit weight γ concrete γ concrete 25 kN/m3

Design Parameters

Raft Modeling in SAFE

Columns loads on Raft

Load type Load case Load value (kN/m2)

Services Dead 2.5 kN/m2

Slab own weight assumed

Dead (25kN/m3)(0.2m) =5 kN/m2

Flooring Dead 1 kN/m2

Live loads Live 7 kN/m2

Punching Shear Factors

All Punching shear factors are less than 1

Soil Pressure Check

< 100 kN/m2

q1 = 95 /𝑘𝑁 𝑚2 < = 100 /𝑞𝑛𝑒𝑡 𝑘𝑁 𝑚2

q2 = 75 /𝑘𝑁 𝑚2 < = 100 /𝑞𝑛𝑒𝑡 𝑘𝑁 𝑚2

q3 = 71 /𝑘𝑁 𝑚2 < = 100 /𝑞𝑛𝑒𝑡 𝑘𝑁 𝑚2

q4 = 92 /𝑘𝑁 𝑚2 < = 100 /𝑞𝑛𝑒𝑡 𝑘𝑁 𝑚2

In SAFE analysis stresses are less than 100kN/m2 at

all the points in raft .

SAFE Settlement Analysis

Maximum settlement for 800mm Raft width ∆ manual = 30 mm. ∆ SAFE = 12 mm

Raft thickness effect

Thickness(mm) Settlement (mm)200 30300 19400 16500 14600 13800 12

1000 111200 10.81400 10.6

020

040

060

080

010

0012

0014

0016

000

5

10

15

20

25

30

Thickness v/s Settlement

Settlement

Raft Thickness V/s Settlement

Conclusion

• Increase in raft thickness beyond a particular limit has little or no effect on settlement of foundation.• In such a case, alternative method such as introducing piles in raft could prove effective.

. Work to be done

• To study the effect of introduction of piles in a raft foundation settlement

• Parametric studies of piled raft foundations.

References

Gowri, S. (2011) “Analysis of Mat foundation using finite element method”, J. Earth Science and Eng., 04, 113-115.

IS 2950. (Part I) (1981). Code of practice for design and construction of raft foundations (second revision), BIS, New Delhi, India.

Noh, E. Y., Huang, M., Surarak, C., Adamec, R. and Balasurbamaniam, A. S. (2008). “ Finite element modeling for piled raft foundation in sand.” Proc., 11th East Asia-Pacific Conference on Structural Engineering and Construction (11EASEC-2008), Taipei,Taiwan,19-21.

Shukla, S. J., Desai, A. K. and Solanki, C. H. (2011). “Behavioral study of pled raft foundation in layered soil deposits.” J. Advanced Eng. Tech., 2(4), 191-195.

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