A novel cushioned piled raft foundation to protect buildings subjected to normal fault rupture

Computers and Geotechnics 106 (2019) 228–248

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Computers and Geotechnics journal homepage: www.elsevier.com/locate/compgeo

Research Paper

A novel cushioned piled raft foundation to protect buildings subjected to normal fault rupture Habib Rasoulia, Behzad Fatahib, a b

T

⁎

School of Civil and Environmental Engineering, University of Technology Sydney (UTS), Sydney, Australia Centre for Built Infrastructure Research (CBIR), School of Civil and Environmental Engineering, University of Technology Sydney (UTS), Sydney, Australia

A R T I C LE I N FO

A B S T R A C T

Keywords: Fault rupture Piled raft foundation Cushioned piled raft foundation ABAQUS

Recent earthquake events have shown that besides the earthquake forces, interaction between the fault rupture and structure could cause a lot of damage to the surface and underground structures. Field observations have revealed a need to design structures for fault induced loading in regions with active faults. In this present study, three-dimensional numerical modelling using ABAQUS finite element software is used to study the interactive mechanism of normal fault rupture with a 20-story moment-resisting building frame sitting on a raft, connected piled raft, and cushioned piled raft foundations. The performance of a foundation-structure system is examined by considering geotechnical and structural performance objectives such as structural inter-story drift, raft displacement, and the bending moment and shear forces within the raft and piles. In order to improve the geotechnical and structural performance of foundations and buildings, a new foundation system with cushioned piles below the raft is proposed because of its superior performance with regards to raft rocking and permanent structural inter-story drifts under normal fault rupture. This proposed foundation system also curtailed the bending moments induced in the piles.

1. Introduction There are reports on the destructive impact of fault ruptures on above surface and underground structures during large earthquakes; for example, the 1999 Chi-Chi earthquake, and Düzce-Bolu, Kocaeli, and the 2008 Wenchuan earthquakes [1–5]. While a lot of research effort has been devoted to studying the performance of structures subjected to earthquake-induced dynamic forces and seismic structural protection, the research into minimising the detrimental impact that fault rupturing has on structures is limited. The interaction between shallow foundations such as rafts and isolated footings and normal and reverse fault ruptures has been studied extensively using numerical simulation [6–8], experimental modelling [9–12], and field evidence from recent earthquakes [1–4]. Bransby et al. [9,12] used centrifuge modelling to study the interaction between normal and reverse fault ruptures and strip foundations. They found that the foundation bearing pressure, the thickness of the soil, the relative location of the raft to the free-field outcrop, and the width and rigidity of rafts has a huge impact on the geotechnical performance of foundations subject to fault rupture. Ahmend et al. [11] carried out centrifuge tests to assess the interaction between the reverse fault

rupture and shallow foundations and concluded that their response to reverse faults is similar to normal faults, and is a function of the relative distance between a fault outcrop and the foundations. Faccioli et al. [1], compared their field evidence from recent earthquakes with numerical simulation and showed that structures that rest on flexible foundations such as isolated foundations could experience structural damage ranging from partial to full collapse, depending on the severity of differential settlement at the ground surface. However, a structure sitting on a rigid foundation or raft and caisson foundations that is subjected to the substantial settlement at the surface could resist applied large amount of structural distress [2,3]. Through a comprehensive comparison between numerical predictions and field measurements, Anastasopoulos et al. [2,3] showed that the rigidity and continuity of foundations had the most significant effect on foundations response under a specific fault rupture incident because the rigid raft foundation tended to deviate fault rupture dislocation further away from foundations, rather than an isolated footing. Moreover, the structural load transfer to the ground via the raft foundations strengthening the ground due to the higher confining pressures, intensifying the diversion of the fault rupturing path from the raft. Increasing the surcharge load (e.g. foundation embedment) can lead to decreasing differential settlement

⁎ Corresponding author at: School of Civil and Environmental Engineering, Faculty of Engineering and Information Technology, University of Technology, Sydney (UTS), City Campus PO Box 123, Broadway, NSW 20077, Australia. E-mail address: [email protected] (B. Fatahi).

https://doi.org/10.1016/j.compgeo.2018.11.002 Received 28 June 2018; Received in revised form 5 November 2018; Accepted 5 November 2018 0266-352X/ © 2018 Elsevier Ltd. All rights reserved.

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H. Rasouli, B. Fatahi

[1–3,7]. Unlike the interaction between shallow foundations and fault ruptures, only a few studies have addressed the performance of deep foundations subjected to fault rupture [13–15]. Anastasopoulos et al. [13] numerically studied the effect of normal fault rupture on a strip piled raft foundation with 2 × 4 pile configurations, and showed that the location of the piled raft relative to the fault outcrop and the magnitude of fault slip, could have a significant effect on the performance of piled raft foundations. On this basis, they predicted that a piled raft could experience significant damage (e.g. raft rocking and the pile bending moment could exceed the allowable limits) if a fault outcrop falls between two rows of piles because in this situation the piles embedded in the hanging wall would be dragged down due to the moving block, while the piles in the footwall would remain in their original location. Gazetas et al. [15] compared a series of centrifuge test results with numerical modelling predictions and showed the capability of caisson foundations in diversion, bifurcation, and diffusion of normal and thrust fault rupture paths. In recent fault rupture incidents, despite structures being destroyed by fault ruptures, some structures have successfully resisted fault ruptures [1,3]. Over the years, these observations have encouraged researchers to design new structural systems that will withstand fault ruptures [6,16–18]. For example, Bray [16,17] proposed a framework to mitigate the effect of fault ruptures on buildings such as a setback of surface-fault rupture, reinforcing soil, or using compressive material beneath the building. Baziar et al. [7] carried out numerical research studies and showed that the differential settlement of foundations could be reduced when the deeper layers of soil near the bedrock are reinforced with high strength geogrid. They also showed that increasing the depth of the geogrid and the structural load could enhance the performance of a foundation. Fadaee et al. [18] proposed building a soil-bentonite wall in front of shallow foundations to reduce the effect that an inverse fault rupture has on a shallow foundation; this proposed mitigation technique can divert fault ruptures away from the foundation. Loli et al. [14] carried out centrifuge tests to investigate how a caisson foundation interacts with a normal fault rupture and found that the caisson foundation could divert the fault rupture, but with regards to rocking, this foundation was very sensitive to its position with regards to the location of the free field fault outcrop. This study proposes a new cushioned piled raft foundation that will reduce the detrimental effects of normal fault rupture on the buildings sitting on deep foundations. The ability of this proposed solution to reduce raft rocking and overall permanent structural deformation, as well as the bending moments in the piles induced by fault rupture, was evaluated numerically. Initially, the interaction between the fault rupture and connected and cushioned piled raft options were studied by assessing the structural and geotechnical performance objectives of the building and foundations by capturing raft displacement and the forces and moments in the piles and the raft. This was followed by a comprehensive parametric study to capture the effects of fault position relative to the foundation to further assess the performance of a cushioned piled raft foundation. 2. Proposed cushioned piled raft foundation to resist fault rupture Interaction between a fault rupture and a connected piled raft can be categorised by three different mechanisms [13] depending on the location of fault rupture outcrop relative to the foundation, as shown in Fig. 1. In Fig. 1a the first mechanism is observed when a fault rupture emerges at the side of the raft closer to the fault (i.e. left side of foundation in the configuration shown in Fig. 1a), where the fault rupture path is being deviated by the piles at one side of the foundation (i.e. PI in Fig. 1a). Although the soil under the raft could potentially settle, the raft does not experience any significant displacement or rocking due to the presence of piles adjacent to the hanging wall. The second mechanism acts when a fault rupture emerges at the other side of the raft (i.e. right side of the foundation in the configuration shown

Fig. 1. Schematic representation of a normal fault rupture interaction with a connected piled raft (a) fault outcrops in front of foundation (b) fault outcrops beyond the foundation (c) fault outcrops in the middle of the foundation.

in Fig. 1b), so all the piles are embedded in the hanging wall. In this instance, the structure would be entirely embedded in the hanging wall and therefore, by referring to Fig. 1b, the foundation would experience 229

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footwall. In this instance, those piles embedded in the hanging wall would be dragged down or bent due to the moving block, while those piles in the footwall will remain in their original locations. Therefore, even with a small bedrock slip, the displacement of piles embedded in the hanging wall could cause large bending moments in the piles and raft rocking that exceeds the design limits, as shown in Fig. 1c. In order to improve its performance, a new foundation system shown in Fig. 2 is proposed in which a cushion layer is placed between the raft and the piles. In this composite foundation system, the piles adjacent to the hanging wall are connected non-structurally (e.g. using the elastomeric bearing pads shown in Fig. 2c) to the raft while the other piles are disconnected from the raft by placing a layer of soil between the piles and the raft. In this design, when a fault rupture emerges at the left corner of the raft, as shown schematically in Fig. 2a, the piles connected non-structurally help to reduce raft rocking. However, when the fault outcrop is beyond the middle of the raft (see Fig. 2b), the connected non-structurally piles could easily separate from the raft or slide, without dragging the raft down and causing any excessive rocking and bending moments in the raft. The key contribution of the soil cushion is separating the building from the pile foundation, so the raft would experience less differential settlement while the system is subjected to the fault rupture. To construct a cushioned piled raft foundation, in the first step, the ground should be excavated equal to the thickness of the cushion below the proposed raft level. In the next stage, the piles could be installed. After installing the piles in the required configuration and to the desired depth, the cushion soil could be placed and compacted. The mechanical properties of the cushion such as thickness, particle grading and stiffness have a significant effect on the performance of cushioned piled raft foundation [19,20]. The cushion should have enough strength to carry the load of the structure, and piles should not punch through the cushion. By conducting these checks, the thickness of the cushion, material to be used and the compaction level can be finalised. Lastly, before the construction of the raft, the elastomeric bearing pads could be placed on the pile heads adjacent to the hanging wall as proposed in Fig. 2. Note that although adding settlement reducing piles below rafts has been reported in the literature, this proposed composite cushioned piled raft foundation with specific pile arrangements is a novel solution for construction on fault lines. In the foundation design space, when the ground bearing capacity is sufficient while raft settlement would be excessive and would not satisfy the serviceability limits, a few piles are placed under the raft to control settlement [21–23]. In reality, a small number of piles below the raft could cause a high-stress concentration where the piles connect to the raft [24–26], and this would induce significant plasticity and deformation in this connection. Therefore, to moderate the induced stresses and position them below the acceptable limit, Wong et al. [27] proposed to adopt soil reinforcement elements and piles that are not connected directly to the raft as a geotechnical solution. Since then many researchers have studied the behaviour of cushioned piled raft foundations under static and seismic loading experimentally [24,25,28] and numerically [19,20,29,30,32], while no attempt has been made to determine whether this composite foundation system will actually protect structures built on fault lines.

Fig. 2. Proposed cushioned piled raft foundation with piles connected nonstructurally to the raft using elastomeric bearing pad; (a) fault outcrops in front of foundation; (b) fault outcrops in the middle of the raft; (c) non-structural connection of piles to the raft with elastomeric bearing pad.

3. Overview of adopted soil-pile–structure system In this study, a 13.5 m wide by 60 m high moment resisting concrete building (corresponding to 20 stories) sitting on a piled raft foundation that was divided into three equal 4.5 m-spans in each direction, was considered (See Fig. 3). The structural sections of this building were designed based on AS3600 [31], AS1170.1 [33] and AS1170.4 [34] by conducting analysis and design using SAP2000 v19 [35]. The specific compressive strength (f 'c ) and mass density of the concrete members were considered to be 32 MPa and 2400 kg/m3 respectively, and the modulus of elasticity of concrete was estimated to be equal to 30.1 GPa

settlement and horizontal displacement that is almost equal to the vertical component of fault slip (i.e. fault throw) and the horizontal component of fault slip (i.e. fault heap) without any significant raft rocking. In Fig. 1c, the third mechanism occurs when the fault rupture protrudes from the middle of the raft or where a portion of piles are embedded in the hanging wall while the remainders are fixed inside the 230

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Fig. 3. Schematic representation of a 20-story moment-resisting building adopted in the numerical model. Table 1 Designed sections for structural columns and slabs. Section type

C. I

C. II

C. III

C. IV

C. V

C. VI

Slab

Dimensions (m) Distribution level Cross section area, A (m2)

0.75 × 0.75 1–3 0.5625

0.7 × 0.7 4–7 0.49

0.65 × 0.65 8–11 0.4225

0.6 × 0.6 12–15 0.36

0.55 × 0.55 16–18 0.3025

0.5 × 0.5 19–20 0.25

13.5 × 13.5 × 0.25 All levels 0.25 (1-m width)

Flexural rigidity, EI (GPa·m4)

0.794

0.602

0.448

0.325

0.230

0.157

0.039

heads allows relative displacement between the pile heads and the raft, obviously increasing the settlement of the building foundation in comparison to the conventional connected piled raft. In this study, numerical analyses were conducted to assess the impact of introducing the cushion layer on settlement of the building under dead and live loads according to Australian Standards AS1170.1 [33] and AS1170.4 [34]. The maximum settlement of the raft alone excluding piles subjected to these loads was 71 mm, which exceeded the allowable foundation settlement (i.e. 50 mm) referring to Skempton and Macdonald [37] and O'Brien [38]. This clearly indicated that piles were required to reduce the building settlement. The numerical predictions showed that the adopted connected piled raft and cushioned piled raft successfully reduced the building settlement to 31 mm and 47 mm (corresponding to 56% and 34% reduction), which were less than the allowable settlement. Therefore, under the static loads, both options of pile foundations indicated satisfactory performance. The schematic features of adopted piled raft foundations are shown in Fig. 3. The piled raft foundations (i.e. connected and cushioned) consist of a square raft, 15 m wide by 1.5 m thick supported by a group of piles in a 4 × 4 configuration where the piles are 1 m in diameter by 20 m long. The soil cushion that separates the raft from the piles for the

Table 2 Soil properties adopted in the present study. Soil Property

Value

Reference

Young’s modulus (MPa) Poisson’s ratio (µ) Unit weight (kN/m3) Friction angle (ϕ ) (degree) Cohesion (kPa)

40 0.3 16.3 30 10

[35]

by referring to Eq. (1) as reported in Australian Standard AS3600 [31]:

Ec = (ρ1.5) × (0.043 f cmi )

when

f cmi ≤ 40 MPa

(1)

where ρ and f cmi are the mass density and mean value of the compressive strength of concrete. The dimensions and distribution of columns, as well as the floors, are listed in Table 1 and shown in Fig. 3. This structure sits on a 30 m thick deposit of sandy soil, and its mechanical properties are summarised in Table 2. The properties of this soil were obtained from a real project based on laboratory and site investigations, and therefore have merits over the assumed parameters [36]. The insertion of a deformable layer between the raft and the pile 231

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Fig. 4. Schematic representation of a piled raft foundation with normal fault rupture interaction with a 0.6 m fault slip and a dip-slip angle of 60° and boundary condition during fault rupturing.

(ii) The piled raft foundation and structure were placed on top of the soil deposit and then subjected to a gravity load, (iii) A load of 5 kPa was applied onto the floor slabs as live load, according to Australian Standard 1170.1 [34], and (iv) Displacements were applied to the base and side boundaries of the hanging wall to simulate a fault rupture in the rock layer beneath, with the dip of 60° as shown in Fig. 4.

corresponding case of a cushioned piled raft was considered to be 1 m thick. 4. Numerical simulation Over the previous decades, many researchers have used different numerical methods [1,2,6–8,13,14,39–44] to simulate fault ruptures. Anastasopoulos et al. [39] compared the results of centrifuge tests and finite element predictions to validate the numerical model used to simulate fault ruptures passing through a uniform layer of sand. They could predict where the faults were outcropped at the surface, as well as the settlement and the minimum fault slip needed at the bedrock that would cause the fault rupture to reach the ground surface. Another study by Anastasopoulos et al. [7] compared the performance of finite element software packages such as PLAXIS 2D, ABAQUS, and DYNAFLOW when simulating the interaction between shallow foundations and fault ruptures through centrifuge tests. Their results indicated that the accuracy of numerical modelling of fault-foundation interaction depends highly on the type and size of the elements used in the mesh, the type of interface between soil and structure that would allow both sliding and separation, and also considering the strain softening behaviour of soil after failure. In this study, finite element modelling using ABAQUS - version 2016 [45] software was used to simulate the three-dimensional interaction of normal fault rupture and piled raft foundations in sandy soil. The numerical simulation in this study was carried out in the following steps:

4.1. Characteristics of the Numerical Model The columns of the building were modelled using 2-node linear beam elements (B31), and four-node reduced-integration shell elements (S4R) were used to model the floors. The mechanical behaviour of structural elements of the building (i.e., the columns and floors) were modelled using an elastic-perfectly plastic constitutive model to capture any possible inelastic behaviour of the structural elements; these structural elements could behave elastically until they reached yielding stress that was equal to the compressive strength of concrete (i. e. f 'c = 32 MPa) referring to Shing and Tanabe [47]. In order to capture the bending moment and shear forces, the raft structure was modelled using four-node reduced-integration shell elements (S4R). Solid elements with eight-node reduced-integration ‘‘brick’’ elements (C3D8R) were used to simulate the subsoil and cushion layers, and an elastic-perfectly plastic Mohr-Coloumb constitutive model was used to simulate the soil. In recent years, many researchers used numerical modelling to study the interaction mechanism of different structures such as foundations, pipelines, bridges, tunnels and embankments with fault ruptures [8,15,18,48,49]. Among different constitutive models for modelling soil behaviour, the Mohr-Coloumb constitutive model was used widely by other researchers as in this study [6,14,15,18,39]. They

(i) The initial stresses of the soil were established by applying a gravity load and the coefficient of lateral earth pressure using the Jaky [46] formula (i.e. K 0 = 1 − sinφ ), 232

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H. Rasouli, B. Fatahi

Soil and piled raft detachment

Raft

P1

P3

=0.75) ft (s/B a r d e il cted p Conne

Fault path P1 P3 P2 P4

(a)

P5 P7 P6 P8

(a) Soil and piled raft detachment Raft Cushion P1 P3 P5 Plastic deformation of soil at pile head of P3 to P8 5) B=0.7 aft (s/ r d e il ned p Cushio

P7

a t io n form e 2 d t ic 1, P Plas of P e o t le P1 P3 P5 P7 at pi

h t pat Faul

(b)

P2 P4 P6 P8

(b) Fig. 6. Interaction of connected and cushioned piled raft foundations with a normal fault slip of 0.6 m at s/B = 0.75; (a) FE computed plastic strain contours of the connected piled raft and (b) cushioned piled raft; (c) settlement contours of the connected piled raft and (d) cushioned piled raft.

It should be noted that the key purpose of this study is proposing a novel foundation system for buildings to reduce determinantal effects of fault rupture and therefore enhance the performance of structural components. In this study, for simplicity and considering the available real project site investigation and laboratory test results [36], an elastic-perfectly plastic Mohr-Coulomb model with dilation angle of zero was adopted. It should be noted that previous research studies [39,43,50,51] have indicated that using more rigorous constitutive models capturing strain softening behaviour of granular materials can lead to a more accurate prediction of fault propagation through the soil and the location of the fault outcrop on the ground surface. In this study, the comprehensive parametric study was conducted investigating cases with different distances between the fault outcrop and the foundation edge, covering a wide range of possible cases. Obtaining the bending moment and shear forces within piles

(c) Fig. 5. Finite element model and meshing. (a) Soil model and structure, (b) model of piled raft, (c) model of hybrid piles.

confirmed the suitability of the Mohr-Coulomb model to simulation fault rupture problems, by validating the numerical predictions against experimental results and case studies during recent fault rupture incidents [6–8,13,15,43]. 233

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+6.000e-01 +5.500e-01 +5.000e-01 +4.500e-01 +4.000e-01 +3.500e-01 +3.000e-01 +2.500e-01 +2.000e-01 +1.500e-01 +1.000e-01 +5.000e-02 +0.000

After fault rupture

Fault path

s/B=0.75

(c)

Before fault rupture

0.6

H

Bedrock

0.2

0.05

0.0 0.6

0.00

P7 P8

0.1

0.2

0.3

0.4

0.5

Fault slip (m)

(a)

Cushioned piled raft

Horizontal displacement of center of raft, x (m)

+6.000e-01 +5.500e-01 +5.000e-01 +4.500e-01 +4.000e-01 +3.500e-01 +3.000e-01 +2.500e-01 +2.000e-01 +1.500e-01 +1.000e-01 +5.000e-02 +0.000

After fault rupture

Fault path

s/B=0.75

P5 P6

H

0.4

0.35

P3 P4

z

0.10

0.0

Settlement (m)

P1 P2

B

P7 P8

Connected-PR, x Cushioned-PR, x Connected-PR, z Cushioned-PR, z

0.30 0.25

s/B=0.75

A

x

θ

0.35 B

z

H

H

0.25

Bedrock

0.20

0.20

0.15

0.15

0.10

0.10

0.05

0.05

0.00 0.0

(d)

0.30

0.1

0.2

0.3

0.4

0.5

Settlement of center of raft , z (m)

P5 P6

0.15

θ

x

Foot wall

P3 P4

s/B=0.75

A

moving block

P1 P2

Connected-PR Cushioned-PR

Raft rocking, θ (degree)

Settlement (m)

0.20

moving block

Connected piled raft

Differential settlement between Points A, B (m)

Before fault rupture

Foot wall

H. Rasouli, B. Fatahi

0.00 0.6

Fault slip (m)

Fig. 6. (continued)

(b)

simulated by solid elements can be complex, so many researchers have used a hybrid method to model piles by embedding a flexible beam element into the centre of the simulated pile using a solid element [13,52,53]. The piles in this study were modelled using solid eight-node brick elements, and by embedding a beam element with a very small flexural rigidity (i.e. EI 1,000,000 times less than the solid elements of the pile so it does not affect the pile’s response) into a solid element so that the bending moments and shear forces in the piles could be readily obtained (see Fig. 5c). The movement of a hanging wall during fault rupture enables the piles, and the raft, to become detached and slide from the surrounding soil, so considering appropriate soil-structure interfaces was essential. In the connected piled raft, the interfaces were defined between the piles and raft and the surrounding soil, including the interfaces between soil and the base of the raft, as well as the shaft and toe of the piles with the surrounding soil. In addition, in the cushioned piled raft foundation, interfaces between the head of the piles and the base of the cushion layer and the raft were defined. The soil-structure interfaces in this study were modelled using a hard contact algorithm that would allow sliding and separation to occur during analysis. The tangential behaviour of the contacting surfaces between the piles and surrounding soils was simulated using a linear elastic-

Fig. 7. Interaction of connected and cushioned piled raft foundations with a normal fault slip of 0.6 m at s/B = 0.75; (a) evolution of raft rocking and differential settlement and (b) settlement and horizontal displacement of the center of raft with fault slip.

perfectly plastic Mohr-Coulomb frictional model utilising the coefficient of friction of μ = 0.4 , that corresponds to an interface friction angle of 22°. 4.2. Boundary conditions and fault simulation The soil profile was a single 30 m thick layer that covered the hard bedrock which was modelled using a boundary condition because the focus was only on the propagation of fault ruptures through the soil, and since the numerical model was symmetrical in shape, only a half model was modelled to reduce the computation time, as Fig. 5 shows. Referring to the recommendation of Bary [44], to minimise the impact of any possible boundary effects on the predictions, the end to end size of the numerical model was four times the thickness of the soil and eight times the width of the raft (Fig. 5a). Since the primary goal of this study is the interactive mechanism 234

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H. Rasouli, B. Fatahi

3

P2

P4

P6

P8

P1

P3

P5

P7

A

s/B=0.75

15

x/B 1

-0.5 -0.25

0

0.25

0.5

0

-1

-2

P1 -3 -0.50

P5

P3 -0.25

Cushioned-PR Connected-PR s/B=0.75 Life safety level of 1%, FEMA 273 [55]

A

Story Number

Bending moment (MN m)

2

20

Connected-PR Cushioned-PR

60° fault outcrop

5

P7

0.00

10

0.25

0.50

Normalised distance from raft center (x/B)

0 -1

(a)

0

1

2

3

Inter-story Drift (%) 3

Connected-PR Cushioned-PR

60° fault outcrop A

Shear force (MN m)

2

P2

P4

P6

P8

P1

P3

P5

P7

Fig. 9. Permanent inter-story drift of building sitting on connected and cushioned piled raft foundations for s/B = 0.75.

A s/B=0.75

the hanging wall (i.e. the moving block) moved downwards parallel to the dip angle (i.e. α = 60°). Note that to satisfy equilibrium, the bottom and left boundaries of the hanging block were subjected to displacement to simulate fault rupturing. To optimise the computation time, the mesh close to the potential fault rupture path was refined to 0.5 m, whereas a larger mesh size was used in those regions farther away from the potential fault rupture path. Fig. 5 shows a typical discretised model that includes the soil layer, the piled raft foundation, and the building used in this study; they consist of 165,744 elements and 203,131 nodes. Since the numerical model was very large, the fast parallel computation facility at the University of Technology Sydney with 1330 cores and 7 TB of memory was used to carry out this series of finite element simulations.

x/B -0.5 -0.25

1

0

0.25

0.5

0

-1

-2

P1 -3 -0.50

P5

P3 -0.25

0.00

P7 0.25

0.50

5. Results and discussion

Normalised distance from raft center (x/B) In this study, the distance between the left corner of the foundation and free-field fault outcrop was quantified by introducing the breadth of the raft (i.e. s/B in Fig. 3, where s is the distance between free-field fault outcrop and the edge of the raft adjacent to hanging wall and B is the breadth). Initially, a model consisting of only a 30 m thick soil layer without building and foundation was analysed to determine the freefield fault outcrop by measuring the horizontal distance between fault rupture at the bedrock and the fault outcrop at ground surface determined through the fault propagation through the soil observed in the numerical model. After the free-field fault rupture outcrop being determined, the connected and cushioned piled raft foundations and the building were placed and analysed at three different relative distances (i.e. s/B = 0.75, 1.1, and 1.5). Referring to Figs. 6, 11, and 16, the values of s/B were selected so that the fault rupture outcrop could be simulated in three different conditions, i.e., (i) the fault deviated away from the pile group (Fig. 6), (ii) the fault emerged in the middle of the foundation (Fig. 11), and (iii) the fault emerged at the right corner of the raft after passing through the piles (Fig. 16). In the following sections, the predictions for raft rocking, horizontal displacement and settlement at the centre of the raft, permanent inter-story drift in the building, and the bending moment and shear forces in the piles and rafts for the connected and cushioned piled raft foundations are presented and discussed with a fault slip of 0.6 m, as shown in Fig. 4.

(b) Fig. 8. Structural response of raft for connected and cushioned piled raft foundations with a normal fault slip of 0.6 m at s/B = 0.75; (a) raft bending moment and (b) shear force per unit length in cross-section A-A. Table 3 Details of designed sections for adopted piled raft. Structural member

Pile

raft

Dimensions (mm) Longitudinal reinforcement Transverse reinforcement Maximum bending moment capacity (MN m) [49] Maximum shear capacity (MN) [49]

1000 (dia.) 24N32 N14 @150 3 MN m 2.10 MN

1500 (Thk.) N32 @125 N32 @125 4.9 MN m 5.5 MN m

between the fault rupture and the foundation, the bedrock was simulated as a rigid boundary. To model fault rupturing in the bedrock, the bottom boundary which corresponded to the surface of the bedrock was split into two parts, as shown in Fig. 4, so that the left part represented the hanging wall (i.e., the moving block), and the other part simulated the footwall. As Fig. 4 shows, during fault rupturing the boundaries of 235

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H. Rasouli, B. Fatahi

Connected PR s/B=0.75

0.6

60°

fault outcrop

P1 P2 P3 P4 P5 P6 P7 P8

0.2

0.4

Mmax, pile=3 MN m, AS 3600 [31]

0.4

Mmax, pile=3 MN m, AS 3600 [31]

0.2

Cushioned PR s/B=0.75

0.6

60°

fault outcrop

0.8

0.8 P2

P4

P6

P8

P1

P3

P5

P7

P2

P4

P6

P8

P1

P3

P5

P7

1.0

1.0 -8

-6

-4

-2

0

2

-8

4

-6

-4

(a)

Normalised pile length

0.2

0.4

Vmax, pile=2.1 MN, AS 3600 [31]

P1 P2 P3 P4 P5 P6 P7 P8 Connected PR s/B=0.75

0.6

60°

P1 P2 P3 P4 P5 P6 P7 P8 Cushioned PR s/B=0.75

0.2

fault outcrop

0.8

4

0.4

0.6

60°

fault outcrop

P2

P4

P6

P8

P1

P3

P5

P7

P2

P4

P6

P8

P1

P3

P5

P7

1.0 -6

-4

-2

0

2

4

-6

-4

Shear force (MN)

-2

P2 P4 P6 P8 P2 P4 P6 P8

Cushioned PR

6

Connected PR 60°

2

4

(d) 7

Bending moment (MN m)

P1 P3 5 P5 P7 P1 4 P3 P5 3 P7 s/B=0.75

0

Shear force (MN)

(c) 6

Shear force (MN)

2

0.8

1.0

2

0

(b) 0.0

Normalised pile length

0.0

-2

Bending moment (MN m)

Bending moment (MN m)

Vmax, pile=2.1 MN, AS 3600 [31]

Normalised pile length

0.0

P1 P2 P3 P4 P5 P6 P7 P8

Normalised pile length

0.0

fault outcrop P2

P4

P6

P8

P1

P3

P5

P7

Vmax, pile=2.1 MN, AS 3600 [31]

1

5 4 3

P1 P3 P5 P7 P1 P3 P5 P7 s/B=0.75

P2 P4 P6 P8 P2 P4 P6 P8

Cushioned PR Connected PR 60°

fault outcrop P2

P4

P6

P8

P1

P3

P5

P7

Mmax, pile=3 MN m, AS 3600 [31]

2 1

0

0 0.0

-1 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.1

0.2

0.3

0.4

0.5

0.6

Fault slip (m)

Fault slip (m)

(e)

(f)

Fig. 10. Structural response of piles for connected and cushioned piled raft foundations with a normal fault slip of 0.6 m at s/B = 0.75; bending moment in piles of (a) connected piled raft and (b) cushioned piled raft; (c) shear force in piles of connected piled raft and (c) cushioned piled raft; (e) evolution of shear force and (f) bending moment in piles with fault slip.

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Soil and piled raft detachment

Before fault rupture Connected piled raft

After fault rupture

Settlement (m) P1

P3

P5

+6.000e-01 +5.500e-01 +5.000e-01 +4.500e-01 +4.000e-01 +3.500e-01 +3.000e-01 +2.500e-01 +2.000e-01 +1.500e-01 +1.000e-01 +5.000e-02 +0.000

P7

Raft Plastic deformation of soil at pile toe of P3 to P6 =1.1) aft (s/B r d e il cted p Conne

s/B=1.1 P1 P3 P5 P7

h P2 P4 P6 P8 t pat Faul

Secondary failure path due to building weight

Fa ul tp at h

(a) Raft

P1 P2

P3 P4

P5 P6

P7 P8

(c) Before fault rupture

P1 P3 P5 P7 Plastic deformation of soil at pile head of P3 to P8 /B=1.1) iled raft (s p d e n io Cush

Cushioned piled raft

After fault rupture

Secondary failure path due to building weight Settlement (m)

path Fault

+6.000e-01 +5.500e-01 +5.000e-01 +4.500e-01 +4.000e-01 +3.500e-01 +3.000e-01 +2.500e-01 +2.000e-01 +1.500e-01 +1.000e-01 +5.000e-02 +0.000

P7 P1 P3 P5 P8 P6 P4 P2

(b) Fa ult pa th

Fig. 11. Interaction of connected and cushioned piled raft foundations with a normal fault slip of 0.6 m at s/B = 1.10; (a) FE computed plastic strain contours of the connected piled raft and (b) cushioned piled raft; (c) settlement of connected piled raft and (d) cushioned piled raft.

5.1. Fault deviating away from the pile group (s / B = 0. 75)

P1 P2

P3 P4

P5 P6

(d)

P7 P8

s/B=1.1

Fig. 11. (continued)

Fig. 6 shows that in this model the structure was located so that the fault rupture would emerge in front of the raft, and it only struck piles P1 and P2. Fig. 6a and b show the plastic strain contours of the connected and cushioned piled raft foundations. As shown, the fault rupture path was modified by the foundation options, and being forced to deviate towards the P1 and P2 piles. The piles and raft detached from the surrounding soil in both foundations, as shown in the enlarged sections in Fig. 6a and b. Since the fault rupture deviated away from the foundation, the piles in the connected piled raft only experienced small displacements, as shown in Fig. 6c. In the connected piled raft, the rigid connection between the piles and the raft means that the maximum differential settlement of the raft (i.e. the differential settlement between Points A, B, as shown in Fig. 1) would be a function of the relative movement of piles. The sensitivity analysis reported in Fig. 7a shows that beyond the fault slip of 0.23 m, the differential settlement of the raft barely

increased. Numerical predictions showed that when the fault slip was 0.23 m, the fault rupture emerged at the surface alongside the edge piles without passing through the foundation system (see Fig. 6a and c) and thus any further increase in the fault slip would not make any difference in the foundation performance. In contrast to the connected piled raft option, the results reported in Fig. 7a shows that the differential settlement of the cushioned piled raft only increased slightly when the fault slip was less than 0.23 m. This occurred because the fault rupture gradually propagated from the bedrock level towards the ground surface, so the piles were dragged down in the early stages (i.e. fault rupture less than 0.23 m) before the fault rupture reached the surface. Therefore, the piles in the connected piled raft could drag the raft down as soon as the fault rupture reached the piles, whereas, with the cushioned piled raft, the raft did not

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3

1.50

B

Foot wall

z

H

Connected-PR Cushioned-PR 2 s/B=1.10 A

1.25 H

1.00

Bedrock

0.2

0.75 0.50

0.1

0.25 0.0

Bending moment (MN m)

A

0.3

θ

x

Raft rocking (degree)

0.4

moving block

Differential settlement between Points A, B (m)

H. Rasouli, B. Fatahi

0.00

s/B=1.10

0.0

0.1

0.2

0.3

0.4

0.5

θ

0.6

Bedrock

0.2

s/B=1.10 Connected-PR, x Cushioned-PR, x Connected-PR, z Cushioned-PR, z 0.1

0.2

0.3

0.4

0.5

0.1

2

Shear force (MN m)

0.3

0.2

0.0

P7

0

0.25

0.5

P5

P3

P7

-0.25

0.00

0.25

0.50

(a)

H

0.0

P5

Normalised distance from raft center (x/B)

3

0.1

P3

-1

-0.50

Settlement of center of raft, z (m)

H

P1

A

0

0.4 z

P8

-3

B

Foot wall

moving block

Horizontal displacement of center of raft, x (m)

0.3

x

P6

-0.5 -0.25

(a)

A

P4

x/B

1

P1

Fault slip (m)

0.4

P2

-2

Connected-PR -0.25 Cushioned-PR

-0.1

60° fault outcrop

Connected-PR Cushioned-PR s/B=1.10 A

60° fault outcrop P2

P4

P6

P8

P1

P3

P5

P7

A

1

x/B -0.5 -0.25

0

0.25

0.5

0

-1

-2

P1

0.0 0.6

-3 -0.50

Fault slip (m)

(b)

P5

P3 -0.25

0.00

P7 0.25

0.50

Normalised distance from raft center (x/B)

(b)

Fig. 12. Interaction of connected and cushioned piled raft foundations with a normal fault slip of 0.6 m at s/B = 1.1; (a) evolution of raft rocking and differential settlement and (b) settlement and horizontal displacement of the center of raft with fault slip.

Fig. 13. Structural response of raft for connected and cushioned piled raft foundations with a normal fault slip of 0.6 m at s/B = 1.1; (a) raft bending moment and (b) shear force per unit length in cross-section A-A.

experience any notable deformation because the piles were not connected to the raft. Moreover, as Fig. 7a shows, for any given fault slip, the differential settlement of the raft with a cushioned pile was always less than the corresponding differential settlement for the raft with connected piles. This observation was because the differential settlement of the raft was reduced by plastic deformation of the soil at the piles head, as shown in the enlarged section in Fig. 6b. In summary, Fig. 7a shows that due to the gap between the piles and the raft in the cushioned piled raft, there was a delay in the raft experiencing differential settlement due to fault slip, however, the maximum differential settlement of the cushioned piled raft (i.e. 0.071 m) was 12.5% less than the corresponding value for the connected piled raft (i.e. 0.082 m). The horizontal displacement and settlement at the centre of the cushioned and connected piled rafts are shown in Fig. 7b. Note here that while the settlement and horizontal displacement of the cushioned piled raft were 0.172 and 0.181 m respectively, they were higher than the corresponding values for the connected piled raft,

which were 0.053 and 0.082 m, as shown in Fig. 7b. Regarding raft settlement it should be noted that, many researchers using experimental [9,11,18] and numerical [6] modelling as well as field evidence [1,3,54] showed that a building sitting on a rather rigid raft (as used in this study) could withstand a fault rupture incident even with a large raft settlement without a significant structural distress. Although the rigid foundation could guard the structure from considerable structural distress, it could not solve the raft rotation and permanent inter-story drift of the building as stated by Fadaee et al. [18]. In addition, the excessive permanent inter-story drifts of the building could cause a significant structural distress, repair costs and life-threatening injuries due to possible building collapses [55,56]. Fig. 8 shows the structural response of the raft, including the bending moments and shear forces along Section A-A, which correspond to a fault slip of 0.6 m at s/B = 0.75. The bending moment and shear forces generated within the rafts for both foundation options were 238

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H. Rasouli, B. Fatahi

20

s/B=1.10

Life safety level of 1%, FEMA 273 [55]

15

Story Number

Fig. 11a shows, the fault rupture path was bifurcated when it struck the piles under the connected piled raft in which one fault branch had deviated towards the surface alongside the piles on the left (i.e. P1, P2) and slowly diffused, while the second branch had propagated and become diffused. Like the connected piled raft, the fault rupture bifurcated as soon as it reached to the piles on the left side of the cushioned piled raft (i.e. P1, P2), while the second fault rupture branch propagated and then emerged alongside piles P5 and P6 (see Fig. 11b). In addition, a secondary failure path also propagated through the soil from the right-hand edge of the foundation due to the weight of the building. This secondary failure path reduced the differential settlement of raft, as shown in the enlarged section in Fig. 11b. Referring to Fig. 11a and c, the downward movement of the hanging wall dragged the embedded piles down, including P1 to P4, whereas piles P5 to P8 were embedded in the footwall. Therefore, as shown in Fig. 11, the rigid connection between the piles and the raft forced the raft to follow the movement of the piles, so the raft experienced a significant differential settlement. However, Fig. 11d shows that the cushioned piled raft had a different mechanism whereby the piles on the left (i.e. P1 and P2), became totally embedded in the footwall and thus experienced significant displacement, while the remaining piles only experienced a minor displacement. However, as Fig. 11b shows, the differential displacement of the raft did not increase considerably due to the gap between the piles and the raft. Fig. 11b also shows that the plastic deformation of the soil at the heads of the piles increased the settlement of the raft and reduced the differential settlement. In contrast to the previous scenario reported in the last section (i.e. fault deviated out of a group of piles when s/B = 0.75, Fig. 7a), the predictions shown in Fig. 12a indicated that the differential settlement between Points A and B of the connected piled raft (inducing anticlockwise raft rocking) increased continuously to 0.363 m when the fault slip reached 0.60 m. Moreover, the settlement and horizontal displacement at the centre of the connected piled raft reported in Fig. 12b, steadily expanded to their maximum values (i.e. to 0.196 and 0.371 m, respectively), as the fault rupture increased to 0.60 m. Figs. 12a and 11d show that the piles on the left-hand side (which were embedded in the hanging wall (i.e. P1 to P4) of the cushioned pile raft) experienced a significant deformation and displacement due to fault rupture. However, the differential settlements of the cushioned piled raft between Points A and B were much less than the corresponding values for the connected piled raft. Unlike the connected piled raft, Fig. 12a indicates that the differential settlement of the cushioned piled raft barely increased when the fault slip was less than 0.3 m, and then gradually increased to 0.062 m at a fault slip of 0.60 m, which was 83% lower than the corresponding value for the connected piled raft. Fig. 12b shows that horizontal displacement at the centre of the cushioned pile raft due to fault slip was less than the corresponding value for the connected pile raft, whereas settlement at the centre of the cushioned piled raft was higher than the connected piled raft. This could be due to disconnecting of piles from the raft when the cushion layer was introduced which resulted in a reduction in raft differential settlement. Referring to Fig. 12b, when the fault slip increased to 0.6 m, the horizontal displacements and settlement of the cushioned piled raft gradually increased to 0.346 m and 0.378 m respectively, unlike the corresponding values of 0.373 m and 0.196 m for the connected piled raft. As reported in Fig. 13, when the building was positioned at s/B = 1.10 , the bending moments and shear forces within the raft in both foundation options were well below the bending moment capacity and shear capacity of the raft Mmax,raft = 4.9 MN m , Vmax,raft = 5.5 MN m , according to AS3600 [31], and as tabulated in Table 3. Fig. 14 shows the permanent inter-story drift of the building sitting on connected and cushioned piled raft foundations subjected to a fault slip of 0.6 m at s/B = 1.10. In contrast to the case previously reported for s/B = 0.75 (see Fig. 9), the maximum permanent inter-story drift of the building sitting on the connected piled raft was 2.72%, which

Cushioned-PR Connected-PR

10

5

0 -1

0

1

2

3

Inter-story Drift (%) Fig. 14. Permanent inter-story drift of building sitting on connected and cushioned piled raft foundations at s/B = 1.10.

smaller than the bending moment (i. e. Mmax,raft = 4.9 MN m) and shear capacity (i. e. Vmax,raft = 5.5 MN m) of the raft, as presented in Table 3, according to AS3600 [31]. Fig. 9 shows the permanent inter-story drifts of the structure resting on connected and cushioned piled raft foundations when it was positioned at s/B = 0.75. The structure’s permanent inter-story drifts can be defined as the normalised deferential deflection of two adjacent stories [55] as follows:

Drift = (di + 1 − di)/h

(2)

where di+1, di, and h, are deflection at level i + 1, and deflection at level i and the height of the story, respectively. Since excessive permanent inter-story drifts could incur significant structural distress and repair costs, many structural design codes put stringent limitations on residual drift to control the performance of different structures. According to FEMA273 [55], the maximum allowable permanent interstory drift (also known as residual drift) for concrete frame structures is limited to 1% to satisfy the life safety level. In Fig. 9 the maximum permanent inter-story drifts of the building sitting on a cushioned piled raft was 34.3% less than the corresponding drift for the foundations of the connected piled raft when the foundation was positioned at s/ B = 0.75. Fig. 10 shows the diagrams of the bending moment and shear force along the piles that correspond to a fault slip of 0.6 m at s/B = 0.75. Fig. 10 shows that the bending moment and shear forces mobilised in the piles were less than the capacity of the bending moment (i. e. Mmax,pile=3 MN m) and shear force (i. e. Vmax,pile=2.1 MN) of the piles adopted in this study and listed in Table 3. The bending moments and shear forces mobilised in the piles connected rigidly to the raft barely increased when the fault slip exceeded 0.2 m, which is similar to the raft displacement reported in Fig. 7, while the corresponding bending moments and shear forces of piles for cushioned pile rafts gradually and continuously increased as soon as fault rupture began (see Fig. 10e and f). 5.2. Fault rupture emerging in the middle of pile group (s / B = 1. 10 ) In this scenario, the structure was placed closer to the hanging wall such that a fault rupture emerged below the centre of the raft, as shown in Fig. 11, this means that almost half of the piles were embedded in the hanging wall, while the remaining piles were in the footwall. As 239

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0.0

0.0

P1 P2 P3 P4 P5 P6 P7 P8 Connected PR s/B=1.10

0.6

0.8

1.0 -4

60°

fault outcrop

P2

P4

P6

P8

P1

P3

P5

P7

Mmax, pile=3 MN m, AS 3600 [31]

0.4

0.2

Normalised pile length

Mmax, pile=3 MN m, AS 3600 [31]

Normalised pile length

0.2

0.4

0.6

0.8

1.0

-2

0

2

4

6

P1 P2 P3 P4 P5 P6 P7 P8

8

-4

Cushioned PR s/B=1.10

60°

-2

0

Bending moment (MN m)

(a)

P6

P8

P1

P3

P5

P7

2

4

6

8

(b)

fault outcrop

0.8 P4

P6

P8

P1

P3

P5

P7

P1 P2 P3 0.2 P4 P5 P6 P7 0.4 P8 Cushioned PR s/B=1.10

Vmax, pile=2.1 MN, AS 3600 [31]

0.6

P2

Normalised pile length

Vmax, pile=2.1 MN, AS 3600 [31]

Normalised pile length

P4

0.0

P1 P2 P3 0.2 P4 P5 P6 P7 0.4 P8 Connected PR s/B=1.10

0.6

60°

fault outcrop

0.8

1.0

P2

P4

P6

P8

P1

P3

P5

P7

1.0 -6

-4

-2

0

2

-6

-4

Shear force (MN)

-2

2

2

(d) 7

P2 P4 Cushioned PR P6 P8 Connected PR P2 60° fault outcrop P4 P6 P2 P4 P6 P8 P8 P1

P3

P5

6

Bending moment (MN m)

P1 P3 5 P5 P7 P1 4 P3 P5 3 P7 s/B=1.10

0

Shear force (MN)

(c) 6

Shear force (MN)

P2

Bending moment (MN m)

0.0

60°

fault outcrop

P7

Vmax, pile=2.1 MN, AS 3600 [31]

1 0

5 4

P1 P3 P5 P7 P1 P3 P5 P7 s/B=1.10

3

P2 P4 Cushioned PR P6 P8 Connected PR 60° fault outcrop P2 P4 P2 P4 P6 P8 P6 P8 P5 P7 P1

P3

Mmax, pile=3 MN m, AS 3600 [31]

2 1

-1 0.0

0.1

0.2

0.3

0.4

0.5

0

0.6

0.0

Fault slip (m)

0.1

0.2

0.3

0.4

0.5

0.6

Fault slip (m)

(e)

(f)

Fig. 15. Structural response of piles for connected and cushioned piled raft foundations with a normal fault slip of 0.6 m at s/B = 1.10; bending moment in piles of (a) connected piled raft and (b) cushioned piled raft; (c) shear force in piles of connected piled raft and (c) cushioned piled raft; (e) evolution of shear force and (f) bending moment in piles with fault slip.

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H. Rasouli, B. Fatahi

Soil and piled raft detachment

Before fault rupture

Connected piled raft

Raft

Settlement (m)

P3

P5

P7

After fault rupture

Plastic deformation of soil at pile toe of P5 to P8

Conne

iled r cted p

aft (s/B

+6.000e-01 +5.500e-01 +5.000e-01 +4.500e-01 +4.000e-01 +3.500e-01 +3.000e-01 +2.500e-01 +2.000e-01 +1.500e-01 +1.000e-01 +5.000e-02 +0.000

=1.5)

s/B=1.5

path Fault

Fault path

P1 P3 P5 P7 P2 P4 P6 P8

P1 P2

(a) Plastic deformation of soil at Raft pile head of P3 to P8

P3 P4

P5 P6 (c)

Before fault rupture

P7 P8 Cushioned piled raft

After fault rupture

P1 P3 P5 Secondary failure path

Cushio

ed r ned pil

Secondary failure path due to building weight

P7

aft (s/B

Settlement (m) +6.000e-01 +5.500e-01 +5.000e-01 +4.500e-01 +4.000e-01 +3.500e-01 +3.000e-01 +2.500e-01 +2.000e-01 +1.500e-01 +1.000e-01 +5.000e-02 +0.000

=1.5)

path Fault

P5 P7 P1 P3 P6 P8 P2 P4

s/B=1.5

Fault path

P1 P2

(b)

P3 P4

P5 P6

P7 P8

(d)

Fig. 16. Interaction of connected and cushioned piled raft foundations with a normal fault slip of 0.6 m at s/B = 1.50; (a) FE computed plastic strain contours of the connected piled raft and (b) cushioned piled raft; (c) settlement of connected piled raft and (d) cushioned piled raft.

Fig. 16. (continued)

bending moment of piles in the connected piled raft that became embedded in the moving block (i.e. P1, P2, P3, P4), occurred at their upper portions, whereas the maximum bending moment of piles embedded in the footwall (i.e. P5, P6, P7, P8) was at the mid-depth of the piles. In contrast, referring to Fig. 15b, the bending moment of piles in the cushioned piled raft foundation were well below their capacity. Moreover, the maximum bending moment induced in the piles due to fault rupture in the cushioned piled raft occurred in their mid-depth (see Fig. 15b). For a fault slip exceeding 0.5 m, the shear forces predicted within piles connected to the raft (Fig. 15e) revealed that the two piles fully embedded into the hanging wall (i.e. P1 and P2) experienced shear failure where they connected to the raft, whereas the shear force of piles in the cushioned piled raft, shown in Fig. 15d and e were well below the shear capacity of piles reported in Table 3.

exceeded the 1% life safety level margin in FEMA 273 [55], and therefore it falls into the unsafe/unacceptable region. However, the building sitting on the proposed cushioned piled raft only experienced 0.354% of permanent inter-story drift (87% lower than connected piled raft), which was well below the life safety level, and hence has an acceptable/safe performance. Therefore, the superiority of the proposed cushioned piled raft in controlling the deformation and stability of the building, as well as guarding the structure against fault rupture is quite evident. Fig. 15 compares the structural responses of piles for connected and cushioned piled raft foundations that correspond to a fault slip of 0.60 m at s/B = 1.1. Referring to Fig. 15a, the maximum bending moments experienced by piles P1, P2, and P8 of the connected piled raft were more than the structural bending moment capacity of the piles reported in Table 3 (i. e. Mmax,pile=3 MN m) that were determined based on AS3600 [31]. Moreover, Fig. 15a shows that the maximum 241

Computers and Geotechnics 106 (2019) 228–248

z

0.3

Foot wall

0.4

H

1.5

3

A

H

1.0 Bedrock

0.2 0.5 0.1 0.0

0.0

s/B=1.50

P2

P4

P6

P8

P1

P3

P5

P7

A

x/B 2

-0.5 -0.25

0

0.25

0.5

1

0

-0.1

Connected-PR -0.5 Cushioned-PR

-0.2 0.0

0.1

0.2

0.3

0.4

0.5

-1

P1

P5

P3

-2 -0.50

(a)

-0.25

0.00

4

H

0.5 H

0.4

Bedrock

0.3

0.3

0.2

0.2

0.1

s/B=1.50 Connected-PR, x 0.1 Cushioned-PR, x Connected-PR, z Cushioned-PR, z

0.0 0.0

0.1

0.2

0.3

0.4

0.5

Connected-PR Cushioned-PR s/B=1.50

fault outcrop 60°

3

Shear force (MN m)

z

Settlement of center of raft, z (m)

0.4

B

Foot wall

moving block

0.5

θ

0.50

(a)

0.6 x

0.25

Normalised distance from raft center (x/B)

0.6 A

P7

0.6

Fault slip (m)

Horizontal displacement of center of raft, x (m)

Connected-PR Cushioned-PR s/B=1.50

fault outcrop 60°

B

Bending moment (MN m)

x

A

4

θ

Raft rocking, θ (degree)

0.5

moving block

Differential settlement between Points A, B (m)

H. Rasouli, B. Fatahi

A

P2

P4

P6

P8

P1

P3

P5

P7

A

x/B 2

-0.5 -0.25

0

0.25

0.5

1

0

-1

0.0 0.6

P1

P5

P3

P7

Fault slip (m) -2 -0.50

(b) Fig. 17. Interaction of connected and cushioned piled raft foundations with a normal fault slip of 0.6 m at s/B = 1.5; (a) evolution of raft rocking and differential settlement and (b) settlement and horizontal displacement of the center of raft with fault slip.

-0.25

0.00

0.25

0.50

Normalised distance from raft center (x/B)

(b) Fig. 18. Structural response of raft for connected and cushioned piled raft foundations with a normal fault slip of 0.6 m at s/B = 1.5; (a) raft bending moment and (b) shear force per unit length in cross-section A-A.

5.3. Fault rupture emerging in the right corner of the group of piles (s / B = 1. 50 )

differential settlement with fault slip reported in Fig. 17a shows that it increased gradually and continuously up to the fault slip of 0.20 m, and then the differential settlement increased hastily to 0.433 m, corresponding to a fault slip of 0.60 m. As illustrated in Fig. 16b when a fault emerged at the right-hand corner of the raft(i. e. s/B = 1.50) , the fault path in the presence of the cushioned piled raft split into two branches. The first fault rupture path diffused intensely while the second branch propagated to the surface along the third row of piles (i.e. P5 and P6), while a secondary failure path propagated through the cushion and subsoil due to the weight of the building, as shown in the enlarged section of Fig. 16b. The differential settlement of the raft between Points A and B barely increased during the fault rupture, and the differential settlement was only 0.025 m when a fault slip of 0.60 m occurred, as shown in Fig. 17a. Similar to the previous case (i.e. s/B = 1.10 in Fig. 12b), the horizontal

Fig. 16c shows that in this scenario the building was much closer to the fault rupture such that two rows and most of the third row of piles were entirely embedded in the hanging wall, and only one row of piles (i.e., P7, P8) was embedded into the footwall. Similar to the predictions reported in the previous section for s/B = 1.1 and shown in Fig. 11, the fault rupture path had split into two branches when the connected piled raft was adopted, as shown in Fig. 16a. The first branch of rupture propagated and then diffused quickly, whereas the second branch gradually diffused along piles P5 and P6 (Fig. 16a). According to Fig. 16a and c, the downward movement of the hanging wall dragged three rows of piles down (i.e. P1 to P6), while the row of piles on the right-hand side (i.e. P7 and P8) were fixed into the footwall. This caused the raft to experience intense anti-clockwise rocking due to the differential settlement of piles, as shown in Fig. 16c. The evolution of 242

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the extent to which a fault rupture can influence the performance of nearby structures is the relative distance between the foundation and the fault rupture outcrop. In this section, a parametric study was carried out to determine how the relative distance between a fault rupture outcrop and a building would influence the performance of the building and the foundation systems. As a supplementary study, an additional case of a structure on a shallow raft foundation was analysed, and the results were compared with the predictions for connected and cushioned piled rafts to investigate the influence that piles have on the interaction of a building with a normal fault rupture. In particular, the distance between the building and the free-field fault outcrop (i.e. s as shown in Fig. 3) changed from 0 to 33 m in increments of 1 m, and the corresponding results for displacement of the raft, permanent interstory drift of the building, and the maximum bending moment and shear forces within piles were determined. Fig. 21 shows the effect of a normalised distance between the building and free-field fault outcrop (i.e. s/B) on the differential settlement between points A and B, horizontal displacement and settlement at the centre of both piled raft foundations, and the shallow raft foundation. Fig. 21 shows that the performance of these three foundations subjected to a fault rupture was completely different, as will be discussed below. According to Fig. 21a, raft rocking of the connected piled raft foundation only had a half sinusoidal response when s/B varied, but when s/B increased from zero to 0.75, the foundation experienced minor differential settlement. However, as s/B increased further, differential settlement between Points A and B and horizontal displacement at the centre of the raft increased significantly and then reached their maximum values (i.e. 0.5 m and 0.4 m, respectively) at s/B = 1.3; this was similar to the observations reported by Anastasopoulos et al. [13]. Beyond s/B = 1.3, the differential settlement decreased significantly and eventually dropped to zero at about s/B = 2. Referring to Fig. 21c, by increasing s/B from zero to 0.75, the centre of the raft only experienced minor settlement (i.e. 0.0531 m), but beyond s/B = 0.75 raft settlement increased to the vertical component of fault slip (i.e. 0.519 m) at s/B = 2. Fig. 21 shows that interaction between the proposed cushioned piled raft and fault rupture resulted in a sinusoidal variation (1.5 cycles) of the differential settlement with s/B. As evident, the proposed cushioned piled raft successfully reduced the maximum predicted differential settlement to less than 0.1 m, unlike the alternative foundations (i.e. 0.5 and 0.292 m for the connected piled raft and raft respectively). As Fig. 21b shows, the horizontal displacement of the cushioned piled raft was higher than the connected piled raft when s/B < 1.05, but beyond that, the horizontal displacement of the cushioned piled raft was less than the connected piled raft. Moreover, the maximum settlement at the centre of the raft for the proposed cushioned piled raft was more than the corresponding values for the connected piled raft, when 0.5 < s / B < 2 (Fig. 21c). Fig. 22 shows the effect that s/B had on the structural response of piles in the cushioned and connected piled raft foundations options. Referring to Fig. 22a the maximum bending moments of piles in the connected pile raft occurred at s/B = 1.3 where the piles closer to the hanging wall were more susceptible to failure; in this case, most of the piles near the hanging wall could not escape from bending moment failure (i.e. P1–P6). Moreover, the maximum shear force in these piles at the s/B = 1.3, exceeded their shear capacity, as shown in Fig. 22c, whereas the bending moments and shear forces demands of piles in the proposed cushioned piled raft were within the safety limits and certainly well below the corresponding predictions for the connected piled raft case irrespective of where the foundation was located relative to the free-field fault outcrop on the ground surface, as can be seen in Fig. 22b and d. Loli et al. [14] using centrifuge testing and numerical modelling studied the interaction mechanism of a caisson foundation with dimensions of 5 m × 5 m × 10 m embedded in a 15 m thick layer of dense sand with a normal fault rupture with a dip angle of 60°. The

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displacement at the centre of the connected piled raft foundation was higher than the corresponding values for the cushioned piled raft, whereas settlement of the connected piled raft was less than the cushioned piled raft; in fact, by gradually increasing the fault slip to 0.60 m, the settlement and horizontal displacement of the connected piled raft foundation progressively increased to 0.351 and 0.378 m, respectively (see Fig. 17b), while the corresponding values for cushioned piled raft were 0.468 m and 0.316 m, as shown in Fig. 17b. Fig. 18 presents the maximum bending moment and the shear forces mobilised in the connected and cushioned piled rafts when s/B = 1.5. It is worth noting that the maximum bending moment and shear forces in the raft along Section A-A occurred where the piles are located, but as Fig. 18 shows, similar to previous cases (i.e. s/B = 0.75 and 1.1, reported in Figs. 8 and 13), the bending moment and shear forces within the raft were well below the capacity of the raft reported in Table 3. Fig. 19 shows the permanent inter-story drifts of the structure sitting on both foundation systems where s/B = 1.50. Here, the maximum inter-story drift of the building sitting on the connected piled raft was 3.15% when the fault slip was 0.60 m, excessively exceeding the life safety limit of 1% recommended by FEMA 273 [55]. However, as evident in Fig. 19, the proposed cushioned piled raft foundation could successfully reduce the maximum inter-story drift from an unacceptable/unsafe level to a safe/acceptable value of 0.1%. Thus, the proposed cushioned piled raft foundation outperformed the conventional connected piled raft and could hold a building subjected to the dip-slip fault rupture within the safety margins referred to the relevant design codes. Fig. 20 shows the structural response of piles for both foundation options subjected to a fault slip of 0.60 m at s/B = 1.50. Referring to Fig. 20a, the bending moment demands of most piles in the connected piled raft exceeded their capacity. In addition, piles P1 to P6 could not escape from failure when the fault slip reached 0.45 m, as reported in Fig. 20f, whereas Fig. 20b shows that the bending moment demands of piles in the cushioned piled raft were less than their capacity and were therefore safe. Moreover, the shear forces mobilised along the piles for the connected and cushioned piled rafts were well below their shear capacity, as shown in Fig. 19c–e. 5.4. Further Investigation into the Relative Position of Fault Rupture In the existing literature [6,7,9,11–14], the key factor that affects 243

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foundation dimensions were selected in such a way as to simulate a bridge pier foundations. They carried out a parametric study using numerical modelling to study the effect of relative location of the foundation to the free-field fault outcrops at the ground surface. The obtained results showed the capacity of the rigid caisson foundations to deviate or bifurcate the fault rupture paths irrespective of the relative distance of the foundation to the free-field fault rupture. However, they showed that the response of the foundation in terms of rotation and vertical and horizontal displacement was very sensitive to its position relative to the free-field fault outcrops. The foundation rotation for different relative distances between the foundation to the fault rupture varied from 0 to 2° for a normal fault slip of 0.580 m. Referring to Fig. 21, in this study, the foundation rotation for different relative distances between the foundation and the fault rupture for a normal fault slip of 0.600 m, ranged from 0 to 0.42° which was significantly less than the values for the caisson foundation presented by Loli et al. [14]. Fig. 23 shows the effect of s/B on the permanent inter-story drift of a building sitting on different types of foundations, including only a raft, a connected piled raft, and a cushioned piled raft. In Fig. 23a, when the structure was sitting on a raft foundation and positioned at 0.15 < s/ B < 0.65, the permanent inter-story drift of a structure exceeded the life safety limit of 1%. For example, the building on the raft foundation reached a maximum permanent inter-story drift of 2.5% at s/B = 0.55, while the structure sitting on a connected piled raft foundation experienced even more significant permanent inter-story drifts when 0.95 < s/B < 1.7, in which a maximum inter-story drift of 3.7% was observed at s/B = 1.3. However, according to Fig. 23c, the proposed cushioned piled raft foundation could curtail structural inter-story drifts handsomely and maintained them below the acceptable limit of 1% for the entire range of s/B used in this study. Indeed, as Fig. 23c shows, the maximum inter-story drift of the building sitting on the proposed cushioned piled raft was almost 0.6%, which is well below the life safety limit. Loukidis et al. [43] using numerical modelling of propagation fault rupture through a uniform soil layer for different soil types including loose and dense sands as well as normally and overconsolidated clays, studied the effect of fault dip angle, soil layer thickness, and soil type on the fault rupture path propagation. They showed that for a normal fault rupture incident, the fault rupture paths might be different in sandy soils and clayey soils. In addition, they showed that the peak inclination of the ground surface caused by the fault rupture in the sand layer is much larger than clayey soils. The effect of fault dip angle on the fault rupture path propagation through a uniform soil layer without considering any structure studied by Anastasopoulos et al. [39], Loukidis et al. [43], and Hazeghian & Soroush [41]. They showed that for a normal fault with a dip angle of 45 or less in addition to the primary fault rupture path, a secondary path would be induced and propagated in the opposite direction of the primary fault rupture. They showed that depending on the fault slip, the distance between the primary fault rupture outcrop and the secondary one on the ground surface would be about 0.65H (where H is the thickness of the soil deposit). Loukidis et al. [43] showed that the maximum ground surface inclination usually takes place in the vicinity of the primary fault rupture path somewhere between the primary fault path outcrop and a straight line projected from the fault location in the bedrock; this means that the maximum raft rotation occurs due to the interaction between the primary fault rupture path and the foundation. It should be noted that when the foundation and building fall between primary and secondary fault rupture paths, the foundation usually experiences significantly smaller raft rotation since the inclination of the ground surface is much smaller than for the case where the structure is in the vicinity of the primary fault rupture path as stated by Loukidis et al. [43]. The failure mechanism of a connected piled raft under a normal fault rupture is considerably impacted by the differential displacement of the connected piles. For a specific fault rupture path outcropping in the middle of the connected piled raft, as shown in Fig. 1c, the raft

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The predictions in this study showed that a higher differential displacement would cause higher structural distress such as inter-story drifts, bending moments and shear forces in the piles and the raft. For instance, by increasing the pile length from 20 m to 25 m, the maximum bending moment in the piles increased from 6.1 MPa to 7.3 MPa. In contrast, referring to Fig. 24, the differential displacement of the cushioned piled raft was insignificant until the fault slip of 0.4 m, while beyond which the foundation differential settlement slightly increased from 0.031 to 0.087 m by increasing the pile length from 20 m to 25 m. These values are considerably less than the corresponding values for the connected piled raft. Moreover, by increasing the pile length from 20 m to 25 m, the maximum bending moment in the piles for the cushioned piled raft foundation increased from 1.96 MPa to 2.61 MPa, while still remaining below the pile capacity reported in Table 3. In summary, the piles in the connected foundation dragged the raft down in the early stages of the fault propagation from the bedrock level towards the ground surface, and this mechanism could be more significant when the pile tips were closer to the bedrock. However, in the case a cushioned foundation, by disconnecting the piles from the raft, the drag down of the piles would not directly transfer to the raft, hence the differential settlement would be significantly reduced as shown in

rotation is a function of the relative displacement between piles embedded in the moving block and the piles fixed in the footwall. In this condition, the piles embedded in the hanging wall and therefore the connected raft would be dragged down due to the negative skin friction caused by the moving block. It seems that longer piles (which means closer to bedrock) would experience more significant negative skin friction or drag down forces, resulting in more structural damage and raft rotation. In stark contrast, the failure mechanism of a cushioned piled raft foundation under a normal fault rupture would not be as sensitive to pile length due to the disconnection of the piles and the raft. In order to study the impact of pile length (or the distance between the pile tip and bedrock) on the foundation differential settlement, two extra models of connected and cushioned piled rafts at s/B = 1.50 with longer piles of 25 m (which means the distance between the pile tip and the bedrock would be 5 m) were analysed. Fig. 24 compares the differential displacements of both connected and cushioned piled raft for 20 m and 25 m long piles. As Fig. 24 shows, by increasing the pile length from 20 m to 25 m (i.e. reducing the distance between the bedrock and pile tip from 10 m to 5 m), the differential settlement of connected piled raft considerably increased from 0.42 m to 0.544 m (i.e. 29.5% increase) at the fault slip of 0.6 m. 246

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In this study, three-dimensional numerical simulations using ABAQUS software were carried out to evaluate the performance of a proposed cushioned piled raft foundation to protect a 20-story momentresisting frame building subjected to a normal fault rupture. The interaction between a normal fault rupture and a building sitting on a cushioned piled raft was investigated, and the results were compared with conventional foundations such as raft and connected piled raft. The raft sat on top of a sandy soil, and then piles were added to control its settlement below the acceptable level. The responses of the building and foundation in terms of permanent inter-story drifts, raft displacement and rocking, and forces and bending moments in the raft and pile elements were assessed separately for a normal fault rupture slipping up to 0.60 m along a dip-slip angle of 60°. In the conventional connected piled raft foundation, as expected some of the piles near the hanging wall could not escape from bending failure, and the raft experienced significant differential settlement up to 0.5 m. Furthermore, the building experienced excessive permanent inter-story drifts in the order of 2.5% to 3.5%, which well exceeded the life safety level of 1% according to FEMA273 [55]. Indeed, the most critical condition happened when s/B = 1.3 (where s is the distance between a free-field fault outcrop and the edge of the raft adjacent to the hanging wall, and B is the breadth of the raft), here the raft experienced a differential settlement of 0.5 m, and the piles experienced a maximum bending moment of 7.3 MN m, which exceeded their maximum bending moment capacity of Mmax,pile = 3 MN m . Similarly, maximum raft rocking and inter-story drift of a structure sitting on a raft foundation exceeded the life safety limit, with the maximum occurring when the structure was at s/B = 0.55. In this case, the maximum differential settlement experienced by the raft and the maximum permanent structural inter-story drift were 0.292 m and 2.5% respectively. Although the building sitting on a raft alone achieved an enhanced structural and geotechnical performance compared to the connected piled raft foundation, neither option was satisfactory and could not guard the building against normal fault rupture. In order to find a solution to address the unacceptable/unsafe performance of foundation-structure systems in term of raft differential settlement, inter-story drift and failing piles, a new piled raft foundation with a 1 m thick cushion of soil was proposed as to protect buildings against fault rupture. In this cushioned piled raft foundation,

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(c) Fig. 23. Effect of selected s/B on permanent inter-story drifts of the building sitting on (a) raft alone (b) connected piled raft (c) cushioned piled raft for a normal fault slip of 0.6 m.

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H. Rasouli, B. Fatahi

those piles near the hanging wall were connected non-structurally to the raft with elastomeric bearing pads, and there was a soil cushion between the raft and the piles in other locations. The numerical predictions proved that this proposed foundation system resulted in an acceptable/safe performance while considering structural inter-story drifts, raft rocking, and bending moments and shear forces in the foundation elements. Indeed, in the most critical conditions, the maximum permanent inter-story drifts and raft differential settlement of 0.6% and 0.1 m were predicted for this proposed foundation, respectively. Furthermore, the wide range of s/B adopted in this study proved that the maximum bending moments and shear forces mobilised in the piles of this foundation system remained well below their structural capacity. This paper, therefore, provides an alternative solution for practising engineers who design structures on deep foundations in seismic regions that are subjected to large fault ruptures.

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