Influence of compaction procedure on elastic anisotropy

Ahmed Hasan and Simon Wheeler

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University of Glasgow, Glasgow, UK

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Engineering

• Natural deposition process or compacted fills will produce soils that are initially

cross-anisotropic (transversely isotropic).

• Soil properties in all horizontal directions are equal, whereas properties in vertical

direction are different.

• 5 independent elastic parameters (e.g Ghh, Ghv, Eh , Ev and vvh)

• The degree of elastic anisotropy of such soils can be expressed, for example, by

Ghh/Ghv

Introduction

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Isotropic soils Anisotropic soils

Small strain elastic behaviour for saturated and unsaturated

Figure 3. Rotated yield curves for saturated

and unsaturated soils (after D’Onza et al.

2011)

Figure 2. Yield curves for saturated and

unsaturated soils (after Alonso et al 1990)

Inclined of yield curves

Ghh

Ghv =1

Ghh

Ghv >1 or

Ghh

Ghv <1

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yield curves

Large strain plastic behaviour for saturated and unsaturated

Evolution of plastic anisotropy for saturated Otaniemi clay has been studied by, for example, Wheeler et al (2003)

Evolution in

saturated soils

Evolution in

Unsaturated soils

Evolution of plastic anisotropy for Unsaturated kaolin clay has been examined by, for example, Al-Sharrad (2013)

Evolution of elastic anisotropy for unsaturated completely decomposed tuff (CDT) has been explored by, for example, Ng & Yung (2009) using BEs

Elastic Anisotropy

Plastic Anisotropy

Evolution of elastic anisotropy for saturated Chicago clay has been investigated by, for example, Kim & Finno (2012) using bender elements (BEs)

For both saturated and unsaturated soils, the evolutions of large strain plastic anisotropy and small strain elastic anisotropy have been generally studied independently, without any interconnection.

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Can the evolutions of plastic anisotropy and elastic anisotropy both be related to a single fabric tensor or do different fabric tensors control elastic and plastic anisotropy.

To answer this crucial question

• An experimental research programme has been planned to investigate this issue for compacted soils under both saturated and unsaturated conditions. The programme is being undertaken in a suction-controlled triaxial apparatus fitted with bender elements.

• The main testing programme has recently commenced.

• Some preliminary results presented here already throw some interesting light on the key question of whether a single fabric tensor controls anisotropy of both small strain elastic behaviour and large strain plastic behaviour.

A fundamental question:

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Testing programme

Sample preparation method Comment Sample diameter

Modified Proctor dynamic compaction (BS) 1000cm3 mould, 50mm

(cored from larger sample)

Extra heavy dynamic compaction (56 blows/layer instead of 27 as in BS) 1000cm3 mould, 50mm

(cored from larger sample)

Static compaction in a mould (Wheeler & Sivakumar 1995)

9 layers / 400kPa, applied on each layer

50mm (direct compaction into a mould)

Anisotropic static compaction in a triaxial cell (Al-Sharrad 2013)

1st stage (isotropic) 2nd stage (anisotropic)

50mm (cored from larger sample)

Isotropic static compaction in a triaxial cell (Al-Sharrad 2013)

1st stage (isotropic) 2nd stage (isotropic)

50mm (cored from larger sample)

Reconstituted from slurry in a mould Vertical total stress 70kPa 38mm (direct consolidation into a mould)

Figure 8 Isotropic and anisotropic

static compaction stress paths

(after Al-Sharrad 2013)

Material: kaolin clay (LL=68%, PL=36%) , compaction water content =25%

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2shvhv VG

2shhhh VG

where ρ is the bulk density of the soil.

Vs: Shear wave velocity

L: Tip to tip distance between BEs

t: travel time

t

LVs

Bender element testing

Figure 9 shear wave velocity transmission and polarization

L

a b

c

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Sample preparation method

Ghh (MPa)

Ghv (MPa) Ghh /Ghv

Modified Proctor dynamic compaction 206.9 177.8 1.16

Extra heavy dynamic compaction 326.6 241.8 1.35

Static compaction in a mould 52.1 77.0 0.68

Anisotropic static compaction in a triaxial cell 72.6 81.4 0.89

Isotropic static compaction in a triaxial cell 75.9 76.7 0.99

Reconstituted from slurry in a mould 9.7 7.0 1.39

Figure 10. BE test results for sample prepared

by anisotropic static compaction in a triaxial

cell

Table 1. Bender element test results.

But, for plastic anisotropy the yield curve has a positive inclination in all 5 cases.

For 5 anisotropic samples: elastic anisotropy shows

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Ghh

Ghv >1 or

Ghh

Ghv <1

Results

Bender element testing performed under unconfined condition (zero net stress)

at as-compacted water content (i.e as-compacted suction)

• BEs results on unsaturated soil samples indicate that Modified Proctor dynamic compaction produced samples with Ghh > Ghv (similar to saturated reconstituted samples), whereas anisotropic static compaction in a mould or in a triaxial cell produced samples with Ghh < Ghv.

• All four types of anisotropic compaction (also reconstituted samples) produce a single form of anisotropy of large strain plastic behaviour (a positive inclination of yield curves). Therefore, it seems likely that anisotropy of small strain elastic behaviour and anisotropy of large strain plastic behaviour are controlled by different aspects of soil fabric.

Conclusions

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Thank you for your attention