- Email: [email protected]
Sustainable Design for Unpiled-Raft Foundation Structure
Available online at www.sciencedirect.com
Procedia Engineering 54 (2013) 353 – 364
The 2nd International Conference on Rehabilitation and Maintenance in Civil Engineering
Sustainable Design for Unpiled-Raft Foundation Structure Tan Kim Leonga* and Chan Swee Huatb a
Building & Construction Authority, Singapore b Nottingham University, Malaysia
Abstract This abstract forms Part 1 of 4 of full report on research work behaviour of piledThe convention approach on piled-raft design tends to ignore load bearing and settlement contribution from the raft slab. Thus, selecting an effective raft size taking into effect of the soil-structure interaction environment is often neglected and this resulted with conservative design, expensive piled-raft foundation structure, depleting of resources and ineffective construction in all, expensive and non-sustainable work This report evaluates on the conventional design approach, problems and limitations faced and propose possible alternative design approach to derive an optimum raft size which are both effective and practical based on the load bearing and settlement criteria. The model would then be used on piled-raft foundation in the next phase of research work to study any significance contribution from the raft and it level of contribution through similar parametrics. Analyses work and charts done would be used to support the selection of the most efficiency raft size model through the use of FEM geotechnical software in both short and long terms design consideration with structure founded on homogeneous normal consolidated soft clay overlaying a thickness of firm clay soil strata. As such, all presentations in this paper would only be encompassing solely on unpiled-raft foundation design under undrained condition since clay soil is expected to consolidate with times and gets firmer and stronger in long term. A sustainable design chart together with self-explained flowchart to serve as quick-reference design guide is developed for completeness.
© 2012ThePublished by Elsevier Ltd.Ltd. Selection and/or peer-review under responsibility of © 2013 Authors. Published by Elsevier Selection and peer-review responsibility of Department of University Civil Engineering, Sebelas Maret University Department of Civil under Engineering, Sebelas Maret Keywords: Unpiled-raft foundation; piled-raft foundation; raft contribution factors; design chart; total permissible load; soil-structure interaction; load-settlement curve.
* Corresponding author. E-mail address: [email protected]
1877-7058 © 2013 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of Department of Civil Engineering, Sebelas Maret University doi:10.1016/j.proeng.2013.03.032
354
Tan Kim Leong and Chan Swee Huat / Procedia Engineering 54 (2013) 353 – 364
1. Introduction 1.1. Background A raft foundation support numbers of columns or load bearing walls so as to transmit approximately uniform loading to the supporting soil. Usually, foundation structures are designed for bearing capacity and piles are then introduced as settlement reducer plus bearing capacity enhancer whenever is requires. The piled-raft is a foundation system consisting of three elements, i.e. piles, raft and soil. The full detailed analysis of a piled-raft is not trivial due to its three-dimensional nature and the complicated interactions among piles, soil and raft. In the conventional design approach, piled-raft foundation designs usually ignore any contribution from the raft, and assume that piles carry all the superimposed loads. As a result, the conventional piled-raft designs are often conservative. The overall settlement of piled-raft in such conventional designs is often very small, owing to the installation of longer or more piles than are necessary. Obviously, more economical solutions can be obtained by accounting for the contribution of the raft. In brief, this initial part of the completed work helps to set pace for the full piled-raft research work. As such, the current work would be focusing solely on raft foundation and the findings would be used in the next Phase of the work to investigate the efficiency of the raft in a floating piled-raft foundation design resting on soft clay conditions which could be used to support large structures such as storage tank, blast furnace and low-rise to medium-rise building. Thus, objective of the research work is to study the effectiveness of raft element and it significance level of contribution on a piled-raft foundation in hope to bring saving and reduce depiction of materials and resources which would contribute to the building of cleaner and greener environment. 1.2. Problem Definition The ultimatum of the research work is to produce some lean foundation design charts for both unpiled-raft and piled-raft structures based on specific geotechnical aspect. In normal circumstances, the ultimate bearing capacity for cohesive soils is most critical immediately after construction; this is when the soil is still undrained. In the long term, as consolidation takes place, the soil gets stronger. Since the clay will checked and thus will not be consider in this research work. To determine an optimum size of a square foundation, various sizes and thicknesses, with and without piles, different pile length and spacing together with ranges of soil and load bearing capacity relationships. The model was loaded with uniformly distributed load and rested on soft clay ground. Short term total vertical settlements
Tan Kim Leong and Chan Swee Huat / Procedia Engineering 54 (2013) 353 – 364
against permissible total loads charts for soil shear strengths of 10 kPa to 40kPa were established. The result of these studies will allow users to choose an optimum model. Total settlement is rarely damaging but not differential settlement. However, this can be reduced by prudent design. As highlighted by Terzaghi & Peck, most buildings can tolerate up to 20mm differential settlement. These differential settlements are unlikely to exceed 75% of average total settlements, thus a maximum settlement of around 25mm will be a safe guide for buildings on isolated foundation. 1.3. Objectives The objectives for this current research work : - To study the influence of thicknesses and sizes of raft structure foundation under loadsettlement relationship; - To study those parametric influence on short term behaviour of the raft foundation with respect to different types of undrained soil shear strength under normal consolidate clay; - To develop series of quick-reference Design Chart (only on Raft foundation at this stage) on short term total vertical settlement correspond with stages of total permissible total load (dead & imposed) against permissible soil shear strength with ranges of raft size to provide user with quick reference especially for designers and practitioners; and - To develop methodology workflow on lean raft foundations design pedagogy. 2. Geology of Singapore 2.1. General Singapore Island covers an area of about 700 km2 that includes the offshore island. The climate is hot and humid with an annual rainfall ranging from 1600mm in the southwest to 2500mm in the central regions. Base on their conditions, the rocks are deeply weathered. Hence, various types of sub-soils can be found, and they range from very soft peat and marine clay in the low lying areas to hard rock such as sandstone and granite. 3. Development of Design and Workflow Charts 3.1. Diagram Considered The basic problem addressed is illustrated in Fig. 3.1.1. The diagram shows crosssection of raft foundation grounded on normal consolidated clay. Following ranges of matrix parameters were identified for desktop analyses and investigation work: - square raft size, L = 5x5, 10x10, 20x20 m2 - square raft thickness, t = 5%L, 10%L, 15%L
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Tan Kim Leong and Chan Swee Huat / Procedia Engineering 54 (2013) 353 – 364
- undrained soil shear strength at top layer, Cu = 10, 20, 30, 40 kN/m2 - undrained soil young modules at top layer, E = 300Cu kPa ~ (3,6,9,12 MPa) - short term maximum vertical settlement, focus would be on 25mm)
(however,
3.2. Finite Element Model A quarter of the model is used (see Fig. 3.2.1) due to symmetry about both axes. It is assumed to be fully embedded and weight of raft is the same as that of the soil to allow study of the raft thickness influence. Vertical uniformly distributed load is applied as total load onto the model. Boundaries were placed sufficiently remote so as not to restrict or constrain movements in the area of interest. 3.3. Numerical Analysis To produce a lean concrete reinforced raft foundation structure, the numerical study was carried out with a structural model based on finite-element method. The soil and the foundation are modelled with finite element, which allows very rigorous treatment of soil-structure interaction. Theoretical calculations would be done on short term settlement to validate against the developed design chart. 4. Facts Finding 4.1. Raft Thicknesses A typical raft size was setup and used to evaluate the sensitivity of the thickness of raft. The 4 graphs (Fig. 4.1.1 to Fig. 4.1.4) showing load-settlement curves were established. It covered raft thicknesses ranging from 5% L to 15% L. In summary, these 4 graphs showed consistency results and proved that the thickness of the raft has little influence on the load-settlement aspects. However, as would be expected, increasing the raft thickness reduces the differential settlement and imposed loads. Conversely, this does not necessarily mean that a thin raft would be an optimum selection because as thickness increases, the raft bending stresses generally decrease. A decrease in stresses with an increase in raft thickness might mean a decrease in the required reinforcing steel with an increase in the required concrete volume, suggesting that the required raft thickness might have to be determined based on the optimum volume of both materials. In addition, increase thickness would increase the dead load and caused decreased in the imposed load altogether.
4.2. Raft Sizes
Tan Kim Leong and Chan Swee Huat / Procedia Engineering 54 (2013) 353 – 364
To investigate the effectiveness of the raft size, vertical loads were uniformly applied and the results were presented on these 3 charts (Fig. 4.2.1 to Fig. 4.2.3). It has to be mindful that increase in weight (thickness and size) of the raft will decreased the allowable imposed load. Thus, it could be concluded that smaller sizes were more effective probably due to smaller contacted clay areas which attributed to faster dissipation of pore water pressure and consolidation which allows drained condition to be achieved earlier. The efficiency is even more observable for stiffer soil. These results were summarised and shown in Fig. 4.2.4. 4.3. Raft Critical Settlement Point With the raft thickness and size investigated, the evolution of a momentous typical raft model of 5mx5mx0.5m was selected to study the critical settlement of a raft using 3D Plaxis Foundation as shown in Fig. 4.3.1. Three critical total vertical settlement points identified for study: i)
Centre-point;
ii)
End edge-point; &
iii)
End diagonal-point.
A uniformly distributed load spread on the raft and the results were than plotted to produce 2 graphs as shown in Fig. 4.3.2 & Fig. 4.3.3. Fig. 4.3.2 - the plotted graph with results generated from the analysed showed the most critical total vertical settlement point occurred at centre of the raft, follows by enddiagonal and end-edge respectively. Fig. 4.3.3 - the graph further highlighted that the differential total vertical settlement is worst at centre-end-diagonal direction. These findings concurred with literature Contribution To Piled Raft Foundation Design (By Widjojo A. Prakoso, Student member, ASCE, and Fred H. Kulhawy, Fellow, ASCE) 4.4. Behaviour of Raft Slab The reference displacement represents a nominal average type of displacement along the raft by considering the displacement at the raft centre, ¼ edge and end edge. From graph shown in Fig. 4.4.1, the load-raft length ratio @ 25mm vertical settlement downward dish-shape curve. Thus, this suggested that the raft model acted and behaved as a flexible structure foundation having non-uniformly total vertical settlement pattern. Moreover, the different in the differential settlement from centre-to-edge is about 60% which concur s suggestion that the differential settlement is unlikely to exceed 75% (refer to Para 1.2).
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5. Findings on Undained Soil Conditions 5.1. Design Stage Consideration for Bearing Capacity Two extreme raft sizes (5m x 5m and 20m x 20m) were selected for the study and the results analysed were plotted and presented in the 4 graphs (Fig. 5.2.1 to Fig. 5.2.4). The settlements of raft increased with the size of the raft increased. It was also observed that the rate of settlement is faster for smaller raft. This could possibly due to faster dissipation of pore water pressure and rate of consolidation since the contacted soffit is smaller. 6. Outcome of the Current Research Work
Lean Raft Slab Design
6.1. Developed of Design Charts and Validation The developed design chart (see Fig. 6.1.1) offers palliative option for conceptual design, tendering and cost estimation uses, or predicting of the total permissible allowable load to prevent any unintended level of differential settlement caused by vironment. The design chart offers a spectrum of upper bound and lower bound load-settlement Comparison with theoretical calculations done on short-term settlement to validate the newly developed design chart is shown in Fig. 6.1.2 & Fig. 6.1.3. Theoretical calculations done had proved that the developed Design Chart is valid. 6.2. Design Pedagogy Design pedagogy is presented in a self-explanation flow chart reflected in Fig. 6.2. 7. Conclusions At this juncture, the research work on the sole raft foundation on normal consolidated soft clay condition for short term has been completed. Simple Design Chart (Fig. 6.1.1) for predicting the permissible loadings against the maximum vertical settlements had been developed based on various parametric studies conducted. A methodology flowchart on the design of lean raft foundation has also been established in Fig. 6.2. 8. Future Research Work The findings on this lean raft foundation system would be use to study the significance of the raft contribution in the piled-raft foundation. A mindful design to include the raft contribution in the piled-raft foundation is expected to generate savings; especially for floating piled-raft design.
Tan Kim Leong and Chan Swee Huat / Procedia Engineering 54 (2013) 353 – 364
Thus, the same raft model dimension would be used for the next Phase of the research work on piled-raft foundation system to develop more similar simplified design charts taking into account of any significant contribution from the raft, and to complete with a methodology workflow. Acknowledgement The author would like to thank his research project supervisor, Ir. Dr. Chan Swee Huat, for his relentless efforts, times and guidance given along the research project journey. Lastly, the author would also like to thanks the Conference for this publication. References Contribution To Piled Raft Foundation Design (By Widjojo A. Prakoso, Student member, ASCE, and Fred H. Kulhawy, Fellow, ASCE) Design Charts for Piles Supporting Embankments on Soft Clay (By H.G. Poulos, F.ASCE) Piled raft foundation: design and applications (By H.G. Poulos, F.ASCE) Bearing Capacity of Piled Rafts on Soft Clay Soils (By Luca de Sanctis and Alessandro Mandolini)) Settlement of structures on clay soils (By C.J. Padfield, M.J. Sharrock) Foundations And Soil Technology (By The Construction Press) Effect of Footing Width on Bearing Capacity Factor N for Smooth Strip Footing (By Jyant Kumar and V.N. Khatri) Numerical Study of the Bearing Behaviour of Piled Rafts (By Oliver Reul) Analysis of Piled Rafts (By M.Smith and A. Wang) Analysis and Performance of Piled Rafts Designed Using Innovative Criteria (By Luca de Sanctis and Gianpiero Russo) Method of Analysis of Piled Raft Foundation (By H.G. Poulos, F.ASCE) Settlement of structures on clay soils (By C.J. Padfield, M.J. Sharrock)
Appendixes 1/5 Related charts, Diagrams, Flowchart, Graphs
Fig 3.1.1: Cross-section diagram of raft foundation modelled
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x-axis a-qtr of raft structure constructed on the ground
y-axis Boundary lines
Fig. 3.2.1 : Plan View
Fig. 4.1.2
Load - Settlement Graph
Total Allowable Loads (MPa)
(for soil shear strength @ C u = 10kN/m 2 ) (5mx5m raft model)
0.12
Raft Thickkness:-
0.10
0.06 0.05
Raft Thickkness:-
0.04
lc 1u ~ 5%L
0.03
lc 5u ~ 10%L lc 9u ~ 15%L
0.02
lc 2u ~ 5%L
0.08
lc 6u ~ 10%L
0.06
lc 10u ~ 15%L
0.04 0.02
0.01 0.00
0.00
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.00
0.05
Maximum Vertical Settlement (m)
Fig. 4.1.3
Fig. 4.1.4
Load - Settlement Graph
lc 3u ~ 5%L
0.15
lc 7u ~ 10%L
0.10
lc 11u ~ 15%L
0.05 0.00 0.05
0.10
0.15
Maximum Vertical Settlement (m)
Total Allowable Load (MPa)
Raft Thickkness:-
0.00
0.15
Load - Settlement Graph (for soil shear strength @ C u = 40k N/m 2 ) (5mx5m raft model)
(for soil shear strength @ C u = 30kN/m ) (5mx5m raft model) 0.20
0.10
Maximum Vertical Settlement (m)
2
Total Allowable Load (MPa)
Load - Settlement Graph
(for soil shear strength @ C u = 20kN/m 2 ) (5mx5m raft model)
Total Allowable Load (MPa)
Fig. 4.1.1
qtr of raft model
Raft Thickkness:-
0.25
lc 4u ~ 5%L
0.20
lc 8u ~ 10%L
0.15
lc 12u ~ 15%L
0.10 0.05 0.00 0.00
0.05
0.10
0.15
Maximum Vertical Settlement (m)
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Tan Kim Leong and Chan Swee Huat / Procedia Engineering 54 (2013) 353 – 364
Appendixes 2/5 Load-Settlem ent Chart (raft model : 5x5x0.5)
Permissible Load (MPa)
Fig 4.2.1 4 0.25 0.20
lc 5u ~ Cu=10kN/m2
0.15
lc 6u ~ Cu=20kN/m2
0.10
lc 7u ~ Cu=30kN/m2 lc 8u ~ Cu=40kN/m2
0.05 0.00 0.000
0.010
0.025
0.050
0.100
Vertical Settlem ent (m )
Fig 4.2.3 4
(raft model : 10x10x1)
0.20 lc 17u ~ Cu=10kN/m2
0.15
lc 18u ~ Cu=20kN/m2 0.10
lc 19u ~ Cu=30kN/m2 lc 20u ~ Cu=40kN/m2
0.05 0.00 0.000
0.010
0.025
0.050
Permissible Load (MPa)
Permissible Load (MPa)
Fig 422
0.100
0.10 lc 29u ~ Cu=10kN/m2
0.08
lc 30u ~ Cu=20kN/m2
0.06
lc 31u ~ Cu=30kN/m2
0.04
lc 32u ~ Cu=40kN/m2
0.02 0.00 0.010
0.025
0.050
Vertical Settlem ent (m )
Fig. 424
centre ree o off raft raftt ra
0.12
0.000
Vertical Settlem ent (m )
Load-Settlem ent Chart (raft model : 20x20x2)
e edge end g of rraft
e diagona end all of raft a
Fig. g 4.3.1- Plan View
0.100
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Tan Kim Leong and Chan Swee Huat / Procedia Engineering 54 (2013) 353 – 364
Appendixes 3/5 Critical Settlement Point (raft model : 5x5x0.25)
Fig. 4.3.3
Settlement Ratio vs Distance Ratio
Fig. 4.3.2 1.20
1.00
0.02
Settlement Ratio
Allowable Load (MPa)
0.03
Centre Point
0.02
Edge Point 0.01 Corner Point
0.0, 1.000
0.80
0.5, 0.754 centre-to-edge
0.60
centre-to-diagonal
0.5, 0.530
1.0, 0.506
0.40
0.01 0.20
0.00 0.00
0.01
0.01
0.02
0.02
0.03
1.0, 0.061
0.00
Vertical Settlement (m)
0.00
0.20
0.40
0.60
0.80
1.00
1.20
Distance Ratio
Fig. 4.4.1
Behaviour of the loaded Raft Raft Length Ratio
0.000
Loading (MPa)
-0.5
-0.25
0
0.25
0.5
-0.050 -0.100 -0.150 -0.200 -0.250 Displacement @ 66kPa
Fig. 5.2.1Design Chart for Unpiled-Raft Foundation
Fig. 5.2.2Design Chart for Unpiled-Raft Foundation (@ 25mm settlement)
(@ 10mm settlement) Total Permissible Load (MPa)
Total Permissible Load (MPa)
0.1
0.04 0.035 0.03 0.025 0.02 0.015 0.01 0.005 0
0.08
5mx5m undrained 5mx5m drained 20mx20m undrained 20mx20m drained
0
10
20
30
40
5mx5m undrained
0.06
5mx5m drained
0.04
20mx20m undrained 20mx20m drained
0.02 0 0
50
10
20
30
40
50
Soil Shear Strength, kPa
Soil Shear Strength, kPa
Fig. 5.2.3Design Chart for Unpiled-Raft Foundation
Fig. 5.2.4Design Chart for Unpiled-Raft Foundation (@ 100mm settlement)
(@ 50mm settlement) Total Permissible Load (MPa)
Total Permissible Load (MPa)
0.25
0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0
0.2
5mx5m undrained 5mx5m drained 20mx20m undrained 20mx20m drained
0
10
20
30
Soil Shear Strength, kPa
40
50
5mx5m undrained
0.15
5mx5m drained
0.1
20mx20m undrained 20mx20m drained
0.05 0 0
10
20
30
Soil Shear Strength, kPa
40
50
Tan Kim Leong and Chan Swee Huat / Procedia Engineering 54 (2013) 353 – 364
Appendixes 4/5 Fig.6.1.1
Fig.6.1.2
Fig.6.1.3
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Tan Kim Leong and Chan Swee Huat / Procedia Engineering 54 (2013) 353 – 364
Appendixes 5/5 Design Pedagogy: - Proposed Design Method (with methodology flowchart) Assumption: The raft is axially loaded, the reactions to design ultimate loads may be assumed to be uniformly distributed. The numerical analysis work would be run by FEM geotechnical software.
Input design conditions
Determine: fcu, fy, safe or allowable soil bearing pressure, raft base cast on ground, design criteria, design code, design standard, total permissible loads(dead + imposed), soil and concrete parameters.
Select raft size from the Pocket Design Chart available (Fig 6.1.1)
Assume initial thickness of the square raft at 10%L.
adjust thickness
adjust thickness
Adjust size
check for raft size
Total permissible loads divided by safe bearing pressure to get the final size.
structural integrity check
Design at Ultimate Limit State
Design at Serviceability Limit Sate
Design Start
No
No
Check the new bearing pressure using total permissible loads divided by raft area < allowable bearing pressure.
Determine the critical bending moment. Determine bending reinforcement. Check whether raft thickness is sufficient?
Determine the critical shear force and shear stress. Determine shear reinforcement. Check whether raft thickness can resist bending moments and shear force? Shear stress < 0.8(fcu)0.5?
Yes
Yes
Design Completed
Fig. 6.2 : Flowchart
No
Procedia Engineering 54 (2013) 353 – 364
The 2nd International Conference on Rehabilitation and Maintenance in Civil Engineering
Sustainable Design for Unpiled-Raft Foundation Structure Tan Kim Leonga* and Chan Swee Huatb a
Building & Construction Authority, Singapore b Nottingham University, Malaysia
Abstract This abstract forms Part 1 of 4 of full report on research work behaviour of piledThe convention approach on piled-raft design tends to ignore load bearing and settlement contribution from the raft slab. Thus, selecting an effective raft size taking into effect of the soil-structure interaction environment is often neglected and this resulted with conservative design, expensive piled-raft foundation structure, depleting of resources and ineffective construction in all, expensive and non-sustainable work This report evaluates on the conventional design approach, problems and limitations faced and propose possible alternative design approach to derive an optimum raft size which are both effective and practical based on the load bearing and settlement criteria. The model would then be used on piled-raft foundation in the next phase of research work to study any significance contribution from the raft and it level of contribution through similar parametrics. Analyses work and charts done would be used to support the selection of the most efficiency raft size model through the use of FEM geotechnical software in both short and long terms design consideration with structure founded on homogeneous normal consolidated soft clay overlaying a thickness of firm clay soil strata. As such, all presentations in this paper would only be encompassing solely on unpiled-raft foundation design under undrained condition since clay soil is expected to consolidate with times and gets firmer and stronger in long term. A sustainable design chart together with self-explained flowchart to serve as quick-reference design guide is developed for completeness.
© 2012ThePublished by Elsevier Ltd.Ltd. Selection and/or peer-review under responsibility of © 2013 Authors. Published by Elsevier Selection and peer-review responsibility of Department of University Civil Engineering, Sebelas Maret University Department of Civil under Engineering, Sebelas Maret Keywords: Unpiled-raft foundation; piled-raft foundation; raft contribution factors; design chart; total permissible load; soil-structure interaction; load-settlement curve.
* Corresponding author. E-mail address: [email protected]
1877-7058 © 2013 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of Department of Civil Engineering, Sebelas Maret University doi:10.1016/j.proeng.2013.03.032
354
Tan Kim Leong and Chan Swee Huat / Procedia Engineering 54 (2013) 353 – 364
1. Introduction 1.1. Background A raft foundation support numbers of columns or load bearing walls so as to transmit approximately uniform loading to the supporting soil. Usually, foundation structures are designed for bearing capacity and piles are then introduced as settlement reducer plus bearing capacity enhancer whenever is requires. The piled-raft is a foundation system consisting of three elements, i.e. piles, raft and soil. The full detailed analysis of a piled-raft is not trivial due to its three-dimensional nature and the complicated interactions among piles, soil and raft. In the conventional design approach, piled-raft foundation designs usually ignore any contribution from the raft, and assume that piles carry all the superimposed loads. As a result, the conventional piled-raft designs are often conservative. The overall settlement of piled-raft in such conventional designs is often very small, owing to the installation of longer or more piles than are necessary. Obviously, more economical solutions can be obtained by accounting for the contribution of the raft. In brief, this initial part of the completed work helps to set pace for the full piled-raft research work. As such, the current work would be focusing solely on raft foundation and the findings would be used in the next Phase of the work to investigate the efficiency of the raft in a floating piled-raft foundation design resting on soft clay conditions which could be used to support large structures such as storage tank, blast furnace and low-rise to medium-rise building. Thus, objective of the research work is to study the effectiveness of raft element and it significance level of contribution on a piled-raft foundation in hope to bring saving and reduce depiction of materials and resources which would contribute to the building of cleaner and greener environment. 1.2. Problem Definition The ultimatum of the research work is to produce some lean foundation design charts for both unpiled-raft and piled-raft structures based on specific geotechnical aspect. In normal circumstances, the ultimate bearing capacity for cohesive soils is most critical immediately after construction; this is when the soil is still undrained. In the long term, as consolidation takes place, the soil gets stronger. Since the clay will checked and thus will not be consider in this research work. To determine an optimum size of a square foundation, various sizes and thicknesses, with and without piles, different pile length and spacing together with ranges of soil and load bearing capacity relationships. The model was loaded with uniformly distributed load and rested on soft clay ground. Short term total vertical settlements
Tan Kim Leong and Chan Swee Huat / Procedia Engineering 54 (2013) 353 – 364
against permissible total loads charts for soil shear strengths of 10 kPa to 40kPa were established. The result of these studies will allow users to choose an optimum model. Total settlement is rarely damaging but not differential settlement. However, this can be reduced by prudent design. As highlighted by Terzaghi & Peck, most buildings can tolerate up to 20mm differential settlement. These differential settlements are unlikely to exceed 75% of average total settlements, thus a maximum settlement of around 25mm will be a safe guide for buildings on isolated foundation. 1.3. Objectives The objectives for this current research work : - To study the influence of thicknesses and sizes of raft structure foundation under loadsettlement relationship; - To study those parametric influence on short term behaviour of the raft foundation with respect to different types of undrained soil shear strength under normal consolidate clay; - To develop series of quick-reference Design Chart (only on Raft foundation at this stage) on short term total vertical settlement correspond with stages of total permissible total load (dead & imposed) against permissible soil shear strength with ranges of raft size to provide user with quick reference especially for designers and practitioners; and - To develop methodology workflow on lean raft foundations design pedagogy. 2. Geology of Singapore 2.1. General Singapore Island covers an area of about 700 km2 that includes the offshore island. The climate is hot and humid with an annual rainfall ranging from 1600mm in the southwest to 2500mm in the central regions. Base on their conditions, the rocks are deeply weathered. Hence, various types of sub-soils can be found, and they range from very soft peat and marine clay in the low lying areas to hard rock such as sandstone and granite. 3. Development of Design and Workflow Charts 3.1. Diagram Considered The basic problem addressed is illustrated in Fig. 3.1.1. The diagram shows crosssection of raft foundation grounded on normal consolidated clay. Following ranges of matrix parameters were identified for desktop analyses and investigation work: - square raft size, L = 5x5, 10x10, 20x20 m2 - square raft thickness, t = 5%L, 10%L, 15%L
355
356
Tan Kim Leong and Chan Swee Huat / Procedia Engineering 54 (2013) 353 – 364
- undrained soil shear strength at top layer, Cu = 10, 20, 30, 40 kN/m2 - undrained soil young modules at top layer, E = 300Cu kPa ~ (3,6,9,12 MPa) - short term maximum vertical settlement, focus would be on 25mm)
(however,
3.2. Finite Element Model A quarter of the model is used (see Fig. 3.2.1) due to symmetry about both axes. It is assumed to be fully embedded and weight of raft is the same as that of the soil to allow study of the raft thickness influence. Vertical uniformly distributed load is applied as total load onto the model. Boundaries were placed sufficiently remote so as not to restrict or constrain movements in the area of interest. 3.3. Numerical Analysis To produce a lean concrete reinforced raft foundation structure, the numerical study was carried out with a structural model based on finite-element method. The soil and the foundation are modelled with finite element, which allows very rigorous treatment of soil-structure interaction. Theoretical calculations would be done on short term settlement to validate against the developed design chart. 4. Facts Finding 4.1. Raft Thicknesses A typical raft size was setup and used to evaluate the sensitivity of the thickness of raft. The 4 graphs (Fig. 4.1.1 to Fig. 4.1.4) showing load-settlement curves were established. It covered raft thicknesses ranging from 5% L to 15% L. In summary, these 4 graphs showed consistency results and proved that the thickness of the raft has little influence on the load-settlement aspects. However, as would be expected, increasing the raft thickness reduces the differential settlement and imposed loads. Conversely, this does not necessarily mean that a thin raft would be an optimum selection because as thickness increases, the raft bending stresses generally decrease. A decrease in stresses with an increase in raft thickness might mean a decrease in the required reinforcing steel with an increase in the required concrete volume, suggesting that the required raft thickness might have to be determined based on the optimum volume of both materials. In addition, increase thickness would increase the dead load and caused decreased in the imposed load altogether.
4.2. Raft Sizes
Tan Kim Leong and Chan Swee Huat / Procedia Engineering 54 (2013) 353 – 364
To investigate the effectiveness of the raft size, vertical loads were uniformly applied and the results were presented on these 3 charts (Fig. 4.2.1 to Fig. 4.2.3). It has to be mindful that increase in weight (thickness and size) of the raft will decreased the allowable imposed load. Thus, it could be concluded that smaller sizes were more effective probably due to smaller contacted clay areas which attributed to faster dissipation of pore water pressure and consolidation which allows drained condition to be achieved earlier. The efficiency is even more observable for stiffer soil. These results were summarised and shown in Fig. 4.2.4. 4.3. Raft Critical Settlement Point With the raft thickness and size investigated, the evolution of a momentous typical raft model of 5mx5mx0.5m was selected to study the critical settlement of a raft using 3D Plaxis Foundation as shown in Fig. 4.3.1. Three critical total vertical settlement points identified for study: i)
Centre-point;
ii)
End edge-point; &
iii)
End diagonal-point.
A uniformly distributed load spread on the raft and the results were than plotted to produce 2 graphs as shown in Fig. 4.3.2 & Fig. 4.3.3. Fig. 4.3.2 - the plotted graph with results generated from the analysed showed the most critical total vertical settlement point occurred at centre of the raft, follows by enddiagonal and end-edge respectively. Fig. 4.3.3 - the graph further highlighted that the differential total vertical settlement is worst at centre-end-diagonal direction. These findings concurred with literature Contribution To Piled Raft Foundation Design (By Widjojo A. Prakoso, Student member, ASCE, and Fred H. Kulhawy, Fellow, ASCE) 4.4. Behaviour of Raft Slab The reference displacement represents a nominal average type of displacement along the raft by considering the displacement at the raft centre, ¼ edge and end edge. From graph shown in Fig. 4.4.1, the load-raft length ratio @ 25mm vertical settlement downward dish-shape curve. Thus, this suggested that the raft model acted and behaved as a flexible structure foundation having non-uniformly total vertical settlement pattern. Moreover, the different in the differential settlement from centre-to-edge is about 60% which concur s suggestion that the differential settlement is unlikely to exceed 75% (refer to Para 1.2).
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5. Findings on Undained Soil Conditions 5.1. Design Stage Consideration for Bearing Capacity Two extreme raft sizes (5m x 5m and 20m x 20m) were selected for the study and the results analysed were plotted and presented in the 4 graphs (Fig. 5.2.1 to Fig. 5.2.4). The settlements of raft increased with the size of the raft increased. It was also observed that the rate of settlement is faster for smaller raft. This could possibly due to faster dissipation of pore water pressure and rate of consolidation since the contacted soffit is smaller. 6. Outcome of the Current Research Work
Lean Raft Slab Design
6.1. Developed of Design Charts and Validation The developed design chart (see Fig. 6.1.1) offers palliative option for conceptual design, tendering and cost estimation uses, or predicting of the total permissible allowable load to prevent any unintended level of differential settlement caused by vironment. The design chart offers a spectrum of upper bound and lower bound load-settlement Comparison with theoretical calculations done on short-term settlement to validate the newly developed design chart is shown in Fig. 6.1.2 & Fig. 6.1.3. Theoretical calculations done had proved that the developed Design Chart is valid. 6.2. Design Pedagogy Design pedagogy is presented in a self-explanation flow chart reflected in Fig. 6.2. 7. Conclusions At this juncture, the research work on the sole raft foundation on normal consolidated soft clay condition for short term has been completed. Simple Design Chart (Fig. 6.1.1) for predicting the permissible loadings against the maximum vertical settlements had been developed based on various parametric studies conducted. A methodology flowchart on the design of lean raft foundation has also been established in Fig. 6.2. 8. Future Research Work The findings on this lean raft foundation system would be use to study the significance of the raft contribution in the piled-raft foundation. A mindful design to include the raft contribution in the piled-raft foundation is expected to generate savings; especially for floating piled-raft design.
Tan Kim Leong and Chan Swee Huat / Procedia Engineering 54 (2013) 353 – 364
Thus, the same raft model dimension would be used for the next Phase of the research work on piled-raft foundation system to develop more similar simplified design charts taking into account of any significant contribution from the raft, and to complete with a methodology workflow. Acknowledgement The author would like to thank his research project supervisor, Ir. Dr. Chan Swee Huat, for his relentless efforts, times and guidance given along the research project journey. Lastly, the author would also like to thanks the Conference for this publication. References Contribution To Piled Raft Foundation Design (By Widjojo A. Prakoso, Student member, ASCE, and Fred H. Kulhawy, Fellow, ASCE) Design Charts for Piles Supporting Embankments on Soft Clay (By H.G. Poulos, F.ASCE) Piled raft foundation: design and applications (By H.G. Poulos, F.ASCE) Bearing Capacity of Piled Rafts on Soft Clay Soils (By Luca de Sanctis and Alessandro Mandolini)) Settlement of structures on clay soils (By C.J. Padfield, M.J. Sharrock) Foundations And Soil Technology (By The Construction Press) Effect of Footing Width on Bearing Capacity Factor N for Smooth Strip Footing (By Jyant Kumar and V.N. Khatri) Numerical Study of the Bearing Behaviour of Piled Rafts (By Oliver Reul) Analysis of Piled Rafts (By M.Smith and A. Wang) Analysis and Performance of Piled Rafts Designed Using Innovative Criteria (By Luca de Sanctis and Gianpiero Russo) Method of Analysis of Piled Raft Foundation (By H.G. Poulos, F.ASCE) Settlement of structures on clay soils (By C.J. Padfield, M.J. Sharrock)
Appendixes 1/5 Related charts, Diagrams, Flowchart, Graphs
Fig 3.1.1: Cross-section diagram of raft foundation modelled
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x-axis a-qtr of raft structure constructed on the ground
y-axis Boundary lines
Fig. 3.2.1 : Plan View
Fig. 4.1.2
Load - Settlement Graph
Total Allowable Loads (MPa)
(for soil shear strength @ C u = 10kN/m 2 ) (5mx5m raft model)
0.12
Raft Thickkness:-
0.10
0.06 0.05
Raft Thickkness:-
0.04
lc 1u ~ 5%L
0.03
lc 5u ~ 10%L lc 9u ~ 15%L
0.02
lc 2u ~ 5%L
0.08
lc 6u ~ 10%L
0.06
lc 10u ~ 15%L
0.04 0.02
0.01 0.00
0.00
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.00
0.05
Maximum Vertical Settlement (m)
Fig. 4.1.3
Fig. 4.1.4
Load - Settlement Graph
lc 3u ~ 5%L
0.15
lc 7u ~ 10%L
0.10
lc 11u ~ 15%L
0.05 0.00 0.05
0.10
0.15
Maximum Vertical Settlement (m)
Total Allowable Load (MPa)
Raft Thickkness:-
0.00
0.15
Load - Settlement Graph (for soil shear strength @ C u = 40k N/m 2 ) (5mx5m raft model)
(for soil shear strength @ C u = 30kN/m ) (5mx5m raft model) 0.20
0.10
Maximum Vertical Settlement (m)
2
Total Allowable Load (MPa)
Load - Settlement Graph
(for soil shear strength @ C u = 20kN/m 2 ) (5mx5m raft model)
Total Allowable Load (MPa)
Fig. 4.1.1
qtr of raft model
Raft Thickkness:-
0.25
lc 4u ~ 5%L
0.20
lc 8u ~ 10%L
0.15
lc 12u ~ 15%L
0.10 0.05 0.00 0.00
0.05
0.10
0.15
Maximum Vertical Settlement (m)
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Appendixes 2/5 Load-Settlem ent Chart (raft model : 5x5x0.5)
Permissible Load (MPa)
Fig 4.2.1 4 0.25 0.20
lc 5u ~ Cu=10kN/m2
0.15
lc 6u ~ Cu=20kN/m2
0.10
lc 7u ~ Cu=30kN/m2 lc 8u ~ Cu=40kN/m2
0.05 0.00 0.000
0.010
0.025
0.050
0.100
Vertical Settlem ent (m )
Fig 4.2.3 4
(raft model : 10x10x1)
0.20 lc 17u ~ Cu=10kN/m2
0.15
lc 18u ~ Cu=20kN/m2 0.10
lc 19u ~ Cu=30kN/m2 lc 20u ~ Cu=40kN/m2
0.05 0.00 0.000
0.010
0.025
0.050
Permissible Load (MPa)
Permissible Load (MPa)
Fig 422
0.100
0.10 lc 29u ~ Cu=10kN/m2
0.08
lc 30u ~ Cu=20kN/m2
0.06
lc 31u ~ Cu=30kN/m2
0.04
lc 32u ~ Cu=40kN/m2
0.02 0.00 0.010
0.025
0.050
Vertical Settlem ent (m )
Fig. 424
centre ree o off raft raftt ra
0.12
0.000
Vertical Settlem ent (m )
Load-Settlem ent Chart (raft model : 20x20x2)
e edge end g of rraft
e diagona end all of raft a
Fig. g 4.3.1- Plan View
0.100
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Appendixes 3/5 Critical Settlement Point (raft model : 5x5x0.25)
Fig. 4.3.3
Settlement Ratio vs Distance Ratio
Fig. 4.3.2 1.20
1.00
0.02
Settlement Ratio
Allowable Load (MPa)
0.03
Centre Point
0.02
Edge Point 0.01 Corner Point
0.0, 1.000
0.80
0.5, 0.754 centre-to-edge
0.60
centre-to-diagonal
0.5, 0.530
1.0, 0.506
0.40
0.01 0.20
0.00 0.00
0.01
0.01
0.02
0.02
0.03
1.0, 0.061
0.00
Vertical Settlement (m)
0.00
0.20
0.40
0.60
0.80
1.00
1.20
Distance Ratio
Fig. 4.4.1
Behaviour of the loaded Raft Raft Length Ratio
0.000
Loading (MPa)
-0.5
-0.25
0
0.25
0.5
-0.050 -0.100 -0.150 -0.200 -0.250 Displacement @ 66kPa
Fig. 5.2.1Design Chart for Unpiled-Raft Foundation
Fig. 5.2.2Design Chart for Unpiled-Raft Foundation (@ 25mm settlement)
(@ 10mm settlement) Total Permissible Load (MPa)
Total Permissible Load (MPa)
0.1
0.04 0.035 0.03 0.025 0.02 0.015 0.01 0.005 0
0.08
5mx5m undrained 5mx5m drained 20mx20m undrained 20mx20m drained
0
10
20
30
40
5mx5m undrained
0.06
5mx5m drained
0.04
20mx20m undrained 20mx20m drained
0.02 0 0
50
10
20
30
40
50
Soil Shear Strength, kPa
Soil Shear Strength, kPa
Fig. 5.2.3Design Chart for Unpiled-Raft Foundation
Fig. 5.2.4Design Chart for Unpiled-Raft Foundation (@ 100mm settlement)
(@ 50mm settlement) Total Permissible Load (MPa)
Total Permissible Load (MPa)
0.25
0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0
0.2
5mx5m undrained 5mx5m drained 20mx20m undrained 20mx20m drained
0
10
20
30
Soil Shear Strength, kPa
40
50
5mx5m undrained
0.15
5mx5m drained
0.1
20mx20m undrained 20mx20m drained
0.05 0 0
10
20
30
Soil Shear Strength, kPa
40
50
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Appendixes 4/5 Fig.6.1.1
Fig.6.1.2
Fig.6.1.3
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Appendixes 5/5 Design Pedagogy: - Proposed Design Method (with methodology flowchart) Assumption: The raft is axially loaded, the reactions to design ultimate loads may be assumed to be uniformly distributed. The numerical analysis work would be run by FEM geotechnical software.
Input design conditions
Determine: fcu, fy, safe or allowable soil bearing pressure, raft base cast on ground, design criteria, design code, design standard, total permissible loads(dead + imposed), soil and concrete parameters.
Select raft size from the Pocket Design Chart available (Fig 6.1.1)
Assume initial thickness of the square raft at 10%L.
adjust thickness
adjust thickness
Adjust size
check for raft size
Total permissible loads divided by safe bearing pressure to get the final size.
structural integrity check
Design at Ultimate Limit State
Design at Serviceability Limit Sate
Design Start
No
No
Check the new bearing pressure using total permissible loads divided by raft area < allowable bearing pressure.
Determine the critical bending moment. Determine bending reinforcement. Check whether raft thickness is sufficient?
Determine the critical shear force and shear stress. Determine shear reinforcement. Check whether raft thickness can resist bending moments and shear force? Shear stress < 0.8(fcu)0.5?
Yes
Yes
Design Completed
Fig. 6.2 : Flowchart
No