Ground Reinforcement with Shallow Timber Piles for Soils Susceptible to Liquefaction

25/02/2015

25/02/15Ground Reinforcement with Shallow Timber Piles for Soils Susceptible to Liquefaction

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Outline of presentation

� Liquefaction & Ground Treatment Methods

� Driven Timber Piles & Christchurch Ground Improvement Trials

� Previous Studies – Shake table models

� Numerical model

� Results of numerical analysis

� Conclusions

25/02/15Ground Reinforcement with Shallow Timber Piles for Soils Susceptible to Liquefaction

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Liquefaction

� Liquefaction is a phenomenon in which the strength and stiffness of a

soil is reduced by dynamic loading (earthquake shaking) or other rapid –

cyclic loading

� It is associated with significant loss of stiffness and strength in the

liquefiable soil and consequent large ground deformation

Liquefaction is primarily a deformation problem

25/02/15Ground Reinforcement with Shallow Timber Piles for Soils Susceptible to Liquefaction

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Ground Treatment Methods

Three broad categories (JGS 1998)

� Treatment of the liquefiable soil to strengthen it (mitigation and / or reinforcement)

� Treatment of the liquefiable soil to accelerate dissipation of excess pore water pressures

� Measures to reduce liquefaction-induced damage to the structures or facility

25/02/15Ground Reinforcement with Shallow Timber Piles for Soils Susceptible to Liquefaction

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Driven Timber Piles & Christchurch Ground Improvement Trials (July & Dec 2013)

8 different ground improvement methods

were trialled in ChCh (S.J. Van Ballegooy,

Dec 2014)

by T-Rex testing and blast induced

liquefaction testing• Rapid Impact Compaction (RIC)

• Low Mobility Grout (LMG)

• Rammed Aggregate Piers (RAP) or Stone

Columns (SC)

• Horizontal Soil Mixing (HSM)

• Driven Timber Piles (DTP)

� EQC Trial (Nov – Dec 2013)

� FY 2013 research program (ODOT)

� Specification for Ground Improvement using Densified Rafts, Stabilised Crusts, Stone Columns and

Driven Timber Poles for residential properties in Canterbury

September 2014, Draft version for comment

by anecdotal information

• 250mm diameter driven piles were

trialled

• 3.6m long – 1.2m c/c grid – capped

with 300mm gravel raft

25/02/15Ground Reinforcement with Shallow Timber Piles for Soils Susceptible to Liquefaction

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Previous Studies / Shake Table Models

Yoshida et al (2010, 2012) – Soil models 800x400x300mm

Top View

Case 1 – untreated ground

Case 2 – piles around the house

▼44%

Case 3 – pile top fixed

▼50%

Case 4 – inclined 15o

▼65%

Case 2 & 3 Case 4

Top View

Case Examined – piles beneath the

house

▼70%

25/02/15Ground Reinforcement with Shallow Timber Piles for Soils Susceptible to Liquefaction

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Numerical Model (PLAXIS 2D AE finite element software)

Reinforced gravel raft

Driven timber piles

Loose SAND

Dense SAND

25/02/15Ground Reinforcement with Shallow Timber Piles for Soils Susceptible to Liquefaction

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Material Properties and Constitutive Model (HSsmall)

Loose SAND Loose SAND

(liq)

Dense SAND Gravel Raft

���� ��� 17.0 17.0 20.0 22.0

�� ���MPa 11.0 1.1 12.5 20.0

� ���MPa 22.0 2.2 25.0 40.0

�� ���MPa 55.0 5.5 100.0 80.0

�′ �ο 29.0 15.0 37.0 38.0

Timber piles 300SED @ 900mm c/c

�����GPa 8.7

������MPa 580.0

Geogrid Tensar RE520

• σ-ε relation approximated with a

non-linear curve (hyperbolic

Duncan-Chang function)

• stress-dependent stiffness

(degradation of Go with

increasing shear strain)

25/02/15Ground Reinforcement with Shallow Timber Piles for Soils Susceptible to Liquefaction

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Dynamic Analysis – Strong Ground Motion

Feb 2011 strong motion records from the Christchurch Hospital (CHHC) station with a firm peak ground

acceleration of 0.36g in the EW direction has been considered• CHHC seismic station was situated in a 2-storey concrete building (235 Antigua St) close to Christchurch Hospital

5sec 10sec

(Smyrnou et al 2013, Structural and

geotechnical aspects of the Christchurch (2011)

and Darfield (2010) earthquakes in New

Zealanand)

Cyclic loading; build-up

of excess pwp

25/02/15Ground Reinforcement with Shallow Timber Piles for Soils Susceptible to Liquefaction

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Numerical model – Stages of analysis

The following analytical stages undertaken to investigate

the system performance and to compare it with other

layouts without the timber piles and / or the gravel raft

� Analysis #01: 20kPa foundation load over untreated

soil

� Analysis #02: 20kPa foundation load over reinforced

gravel raft

� Analysis #03: as per the previous stage but with the

inclusion of the timber piles

� Analysis #04: similar to #03 with the inclusion of

drainage elements around the piles

20 kPa

25/02/15Ground Reinforcement with Shallow Timber Piles for Soils Susceptible to Liquefaction

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Results of numerical analysis - Estimated model displacements

Analysis #01: 20kPa foundation load over untreated soil

25/02/15Ground Reinforcement with Shallow Timber Piles for Soils Susceptible to Liquefaction

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Results of numerical analysis - Estimated model displacements

Analysis #02: 20kPa foundation load over reinforced gravel raft

25/02/15Ground Reinforcement with Shallow Timber Piles for Soils Susceptible to Liquefaction

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Results of numerical analysis - Estimated model displacements

Analysis #03: as per the previous stage but with the inclusion of the timber piles

25/02/15Ground Reinforcement with Shallow Timber Piles for Soils Susceptible to Liquefaction

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Results of numerical analysis - Estimated model displacements

-1.20

-1.00

-0.80

-0.60

-0.40

-0.20

0.00

0 5 10 15 20 25

Su

rfa

ce S

ett

lle

me

nt

(m)

Dynamic Time (s)

20 kPa load on Ground - Liquefaction after 0s

20 kPa load on Ground - Liquefaction after 5s

20 kPa load on Ground - Liquefaction after 10s

20 kPa load on Timber - Liquefaction after 0s

20 kPa load on Timber - Liquefaction after 5s

20 kPa load on Timber - Liquefaction after 10s

Su

rfa

ce

Dis

pla

ce

men

t (m

)

Time (s)

Timber + raft – 10s lq0.26m

Timber + raft – 0s lq0.38m

Untreated soil – 5s lq0.52m

Untreated soil – 10s lq0.60m

Untreated soil – 0s lq0.96m

0.22mTimber + raft – 5s lq

25/02/15Ground Reinforcement with Shallow Timber Piles for Soils Susceptible to Liquefaction

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Conclusions

treated ground; tend to “act” like a stiffer crust on the top of a liquefiable deposit

drainage – dissipation; minor beneficial effects (numerically) from the timber acting as a source of

drainage

important component of the system; the gravel mat above the piles (with some geogrid reinforcement)

proposed simplified procedure of dynamic analyses capture the overall system performance with or

without the inclusion of the timber piles and can be comparatively evaluated in terms of anticipated

displacements

use of the timber piles; plays an important role in reducing the liquefaction-induced foundation

settlements to a considerable degree - the predicted settlement decreased by 50 to 60% with a more

uniform response limiting the potential of differential foundation displacements

piles should be closely spaced and relative large in diameter (>200mm)

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Acknowledgment

� Coffey & my colleagues

� Special thank to

Prof. Harry Poulos

David Sullivan Principal Geotechnical Engineer

Ala’a El-Nahas Lead Geotechnical Engineer at Bechtel CCEE

� Co-authors:

Andreas Giannakogiorgos Associate Geotechnical Engineer

Thayalan Nallarulanantham Principal Geotechnical Engineer

Satha Iyathurai Associate Geotechnical Engineer

� my family

25/02/15Ground Reinforcement with Shallow Timber Piles for Soils Susceptible to Liquefaction

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