Recent Advances in Non-linear Soil

Structure Interaction Analysis

using LS-DYNA

Michael Willford

Richard Sturt

Yuli Huang

Ibrahim Almufti

Xiaonian Duan

Arup

Global Firm of Design, Planning and Management Consultants

10,000 staff worldwide

LS-DYNA

• Multi-physics simulation software developed by LSTC

• Typical Design Applications

– Impact

– Blast

– Seismic

– Numerical prototyping

• Arup collaborating with LSTC since 1982

• Arup enhancements made in our development version of the

code, later ported to LSTC commercial version

Impact

• Nuclear Transport Containers

• Automotive Crashworthiness

• Impact and Penetration

Seismic performance of structures

Incorporating Non-linear SSI Direct Method

• Soil is non-linear, can be

layered and site specific

• Mesh density designed to

transmit frequencies desired

• Motion input via Lysmer

dampers at ‘bedrock’

• Vertical cut faces of soil are

distant (requiring large model)

and subjected to free-field site

response motions

• wave passage and incoherency

can be included via spatial

variation of input motions

Horizontal InputForce Time History

Ch

Distant SoilDomain Edge

Moving asFree-Field

Soil FE Mesh

Basementand Piles

Nonlinear Structure

Numerical Simulation of Traditional Site

Response using LS-DYNA

• Non-linear hysteretic soil model of layered site over bedrock

• 1-D Vertically propagating shear wave

• Transmitting bedrock boundary

• Amplitude dependence of stiffness and material damping simulated

Typical soil hysteresis

Validation of Site Response Simulation

• Comparison with measurements

in the Chiba borehole array

• Similar results to SHAKE for

moderate levels of ground motion

• Excellent comparison with

DeepSoil for strongly non-linear

response

Porewater pressure generation - validation

Example - Dobry et al (1995) centrifuge test on sloping site

Excellent simulation of

generation and dissipation

of pore pressures

Validation: Reinforced Concrete Simulation

• UCSD full scale 7 story rc shear wall shake table test

Comparison of test and simulation

• Progressive stiffness and strength

degradation under successive cycles

• Crack intensity also well predicted

Validation: Squat Shear Wall

NUPEC shake table test (c.1994)

• Cyclic degradation

• Shear failure

Project Applications

• LNG tanks

– Soft soil acting like lateral seismic isolation

– Uplift of flexible foundation

• Heavy building subjected to adjacent deep excavation and earthquake

– Effect of initial stress state in soils

– Strain rate effects

– Interaction of adjacent structures

– Permanent deformation

• Offshore Gravity Petrochemical Platform

– Foundation sliding and seismic isolation

Point Fortin LNG Tanks, Trinidad (1995)

• Two 72m dia. Tanks

• Hazardous product

• Total mass 100,000t

each

• Shallow soft-soil

layer

Point Fortin LNG Tanks, Trinidad

1-D soil column site response analysis

Point Fortin LNG Tanks, Trinidad

Site Response Results

• Natural period of

primary inertial

mode c. 0.4 secs

• Soft site provides

natural isolation –

elastic spectral

demands are halved

• But foundation must

support gravity load

and will stiffen site

Point Fortin LNG Tanks, Trinidad

• Non-linear modeling of soft soil and driven steel pipe piles

• Non-linear local p-y soil-pile springs

• Upper, lower and best estimate soil properties

• Linear elastic halfspace for class B bedrock

Effect of pile group on site response

Point Fortin LNG Tanks, Trinidad

Effect of pile group on site response

Point Fortin LNG Tanks, Trinidad

• Soil is highly non-linear and inertia forces are very high

• Add Housner mass-spring analogy for tank slosh and impulsive

• Assume rigid basemat

Effect of tank and contents

Point Fortin LNG Tanks, Trinidad

Include tank inertia forces with complete SSI Model

Conclusions

• Non-linear modeling of soft soil enables benefit to be

taken of natural ‘Isolation’

• Steel pipe piles support gravity and overturning and do

not yield in SSE

• Ground improvement would have stiffened site and

increased demand on tanks

Soil mesh

Outer tank

wall

Bedrock level transmitting boundary

earthquake motion input system

Distant side boundaries

Symmetry plane

Piles and nonlinear pile-

soil interaction springs

LNG Tank: 3D SSI simulation with explicit

fluid and tank wall uplift (2004)

Analysis now performed in 3D with explicit

modeling of tank wall, base and LNG

Cross-section overview of response

• Sensitivity to edge boundary

distance checked

• Cf. Wolf’s cone analogy for

practical purposes

Detail of uplift of flexible base plate

Effect of construction of large

excavation adjacent to existing tall

building (2009) • Tall reinforced

concrete building

with basement

• Adjacent

excavation 55’ deep

185’ wide

• Secant pile buttress

to be installed to

bedrock to control

movements due to

construction

• Earthquake to be

considered

Effects of concern

• Movement of existing building due to excavation

• Forces in props across excavation

• Design of ‘buttress’ to prevent slip-circle failure

• Effect of M7.5 earthquake

Modeling issues

• 3-D problem with non-horizontal surface (after excavation)

• Previous experience shows non-linear soil behavior essential for

accurate ground movement predictions

• Soil properties vary across site due to different effective stress

states (weight of building, unloading beneath excavation)

• Buttress is segmented concrete secant pile wall – potential sliding

interfaces - and

• Soil properties at slow strain rate (excavation) and dynamic strain

rate (seismic) are different

Sequence of Simulation

• Initialize free field soil pressures

• Simulate construction of existing building

• Simulate construction of secant buttress and shoring walls

• Simulate excavation and insertion of props

• Apply specified earthquake

Visualization of Simulation

• Vertical deflection

contoured

• Green/Blue

=settlement

• Orange/Red/Purple

• =heave

Results

• Predicted settlement profile due to construction of existing

building match ongoing measurements very well

• Additional permanent settlement and rotation are induced by

excavation and by earthquake

Use in design

• Permanent increase in

prop forces due to

earthquake

• Buttress design is

optimized to control

movements

• Effect of buttress and time

varying soil properties are

incorporated in seismic

response

•

0 5 10 15 20 25 30 35 40

Time, s

-0.2

-0.1

0.0

0.1

0.2

Accele

rati

on

, g

0 10 20 30 40 50 60

Time, s

-8,000

-4,000

0

4,000

8,000

Ba

se S

hear,

kip

s

0 10 20 30 40 50 60

Time, s

-800

-600

-400

-200

0

Str

ut

Fo

rce, kip

s/s

tru

t

LegendStrut 1

Strut 2

Strut 3

Strut 4

Malampaya Gas Platform – Philippines

(1994)

• Massive reinforced concrete

structure to support 13,000t

topsides

• Sea-towed to offshore site

• Seabed leveled with engineered

gravel fill

• Ballasted to seabed

Malampaya – Seismic design issues

• Conventional design would place ballast offshore to prevent

sliding in design earthquakes (i.e. fixed base)

• This design would require seismic isolation of deck to reduce

equipment responses

• Alternative is to reduce quantity of ballast and permit limited

sliding in SSE

Malampaya CGS, Philippines

FEED Study - 3D Model

Malampaya CGS, Philippines

without seismic isolators with seismic isolators

Malampaya CGS, Philippines

SSE response analysis

Seismic isolator hysteresis sliding soil layer hysteresis

Similar beneficial effect on topsides acceleration

Outcome

• Isolation associated with sliding on engineered soil layer is

sufficient to control topsides equipment responses

• Sliding deflections easily accommodated in flexible seabed

pipeline design

• Cost is saved by reduction of requirement for offshore placed

ballast, the cost of seismic isolators and multiple flexible

topside connections

Summary

• We have conducted extensive development and (importantly)

validation of LS-DYNA to improve the design of major

construction projects

• Non-linear soil structure interaction analysis is feasible, and is

being used in design practice to find realistic and economic

solutions to complex design issues

• Non-linear analysis is the only means of predicting important

effects such as permanent deformation, sliding, uplift etc.

• In some cases very significant performance and/or cost benefits

can be realized by taking account of non-linear effects explicitly

LRS Mass

Bridge Bearing LRS Column

(Modelled using

Seismic Beam

Elements)

Pile Cap

Soil Layers

(Hysteretic

Soil Model)

Piles Embedded

in Soil (Modelled

using Seismic

Beam Elements)

Bedrock Motion

Applied at Base

Free Field Motion

Applied to End

Boundaries

27.5m 10.0m

29.5m

Description of SSI Model

JFK Airport - Bridge Structures

LS-DYNA - Validation of Soil Model

Pile - soil interaction

• Required by client to

demonstrate capacity of

pile group.

• Test examining static

push-over condition.

• We used same DYNA

model as for dynamic

study to examine pile

group test.

Loading

Direction

Pile group lateral load test simulation at JFK Airport Quadrant 4

LS-DYNA - Validation of Soil Model

Push-over Analyses

Pile Test

Leading Piles

Trailing Piles

Pile group test simulation -comparison of Results