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Proceedings of the Institution of Civil Engineers

Geotechnical Engineering 166 August 2013 Issue GE4

Pages 343356 http://dx.doi.org/10.1680/geng.10.00018

Paper 1000018

Received 09/02/2010 Accepted 19/09/2011

Published online 05/07/2012

Keywords: field testing & monitoring/geotechnical engineering/land

reclamation

ICE Publishing: All rights reserved

Geotechnical Engineering

Volume 166 Issue GE4

Design, prediction and monitoring of deep

fill settlement

Waddell

Design, prediction andmonitoring of deep fillsettlementPeter Waddell MEngScPrincipal Geotechnical Engineer, Coffey Geotechnics Pty Ltd, Lane Cove West, New South Wales, Australia

Creep settlement is an important aspect to consider where development is planned over deep compacted fill. This

paper presents a case study of an earthworks design for infilling a quarry pit with sandstone and shale derivedmaterial and a comparison of predicted and monitored creep settlements. Laboratory testing was carried out during

filling to assess the creep characteristics of available infilling materials. Based on the laboratory testing the

influence of saturation on creep rates is discussed. Settlement monitoring points across the infilled quarry were

monitored for up to 455 days. Predicted creep rates based on the results of the testing are compared with

predictions based on monitoring. Monitored settlements within a few years of fill placement were variable and

substantially greater than predictions based on the laboratory testing. Longer term monitoring data indicates that

settlements are likely to be less than predictions based on laboratory testing. The results suggest that predictions

over 30 years based on laboratory testing are conservative and lie within the normal range of accuracy of

geotechnical predictions.

Notationk constant based on either pre- or post-saturation strain

t time (days)

creep strain rate per log cycle time (%)

creep strain (%)

v applied total vertical stress (MPa)

9v effective vertical stress in units consistent with

atmospheric pressure

ratm atmospheric pressure assumed to be 100 kPa

1. Case study background

The settlement design, laboratory testing and monitoring resultspresented in this paper are from a project involving the infilling

of a former clay and shale quarry pit used to win materials for

brick making. The pit shown in the aerial photograph, Figure 1,

covers an area of about 15 ha and has been filled to depths

ranging from about 13 m to 26 m.

Filling was undertaken over a period of about 6 years with fill

sourced from tunnelling and basement excavations in the Sydney

region. The pit area is to be developed with the construction of

services, kerb and gutter and residential dwellings, typically

comprising two-storey brick buildings.

2. Earthworks designIn the design of the earthworks it was recognised that the

volumetric stability of the fill would be an important considera-

tion. The following elements of volumetric stability were consid-

ered.

j Shrink swell characteristics: limitations were placed on the

plasticity of the fill and the moisture content at which it was

placed, to limit the potential for changes in volume due to

variation in moisture content.

j Hydroconsolidation settlements: compacted fills can

experience large settlements that occur rapidly under constant

stress if the fill is compacted relatively dry of standard

optimum moisture content (SOMC) and/or at relatively low

density. To limit the potential for hydroconsolidation, a

relatively high degree of compaction and moisture content

relatively close to SOMC was specified in the design.

j Long-term creep settlements: owing to the thickness of the

fill, long-term settlements due to the self-weight of the fillcould impact on services and structures.

Predominantly fresh sandstone and shale rock sources, providing

low-plasticity materials, were preferred to high-plasticity clays in

the fill design from the perspective of shrink swell and creep

characteristics. Two types of fill were defined.

j Type 1 has a maximum plasticity index of 20% and

maximum percentage passing the 0.075 mm sieve of 30%.

j Type 2 has a maximum plasticity index of 30%.

A limit was placed on the amount and distribution of type 2 fill

in the profile. Type 2 fill was limited to a maximum thickness of

1 m before it had to be overlain by a similar or greater thickness

of type 1 fill. This layering was designed to reduce the risk of

relatively thick, lower quality fill pockets resulting in significant

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differential settlements across areas of dissimilar fill types. In

reality, the bulk of the fill used in the pit was type 1, with theearthworks contractors finding it convenient to manage a single

fill type. Some fill sourced from other parts of the site was type

2, but was often disposed of in areas where no structures were

proposed.

The fill compaction specified was a minimum of 98% standard

maximum dry density (SMDD) at moisture content within 1%

to +2% of SOMC. Field wet density was determined using a

nuclear density gauge in accordance with AS1289.5.8.1 (Stan-

dards Australia, 1995f) and SMDD and SOMC were determined

in accordance with AS1289.5.1.1 (Standards Australia, 1993).

Infilling of the pit was achieved by spreading and compacting fill

into maximum 300 mm thickness layers, generally using CAT815

and CAT825 pad foot compactors. Field density tests were carried

out on each compacted layer under a Level 1 monitoring and

testing regime as defined in AS3798 (Standards Australia,

1996a).

The objective of the earthworks design was to produce compacted

fill capable of supporting raft foundations, where a relatively

uniform thickness of fill existed beneath a residential lot.

Predictions of long-term creep made during the design of the

earthworks and before laboratory testing ranged from 30 mm to

95 mm for a 20 m fill thickness.

It was considered that settlement gradients should be flatter than

1:500 where the fill thickness was relatively uniform. AS2870

(Standards Australia, 1996b) Residential slabs and footings

indicates that articulated brick veneer construction on stiffened

rafts (the common building design for residential structures inSydney) should be able to tolerate settlement gradients of up to

1:400 and differential settlements of up to 30 mm.

3. Prediction of creep ratesA programme of laboratory settlement testing was carried out in

a purpose-built rig to assess the long-term settlement character-

istics of the typical fill being used on the site. This testing was

described by Waddell and Wong (2005). A purpose-built rig was

constructed to apply a vertical pressure of up to 400 kPa through

a lever arm system to 75 mm diameter samples. The test rig

shown in Figure 2 is similar to a conventional oedometer test

cell.

Samples of Hawkesbury sandstone, weathered Ashfield shale and

fresh Ashfield shale were pre-treated by repeated compaction

using the method in RTA T102 (NSW Roads and Traffic

Authority, 1999) to simulate the breakdown of rock fill that

occurs under compaction plant. The following tests were then

carried out

j Atterberg limits determined in accordance with

AS1289.3.1.1 (Standards Australia, 1995a), AS1289.3.2.1

(Standards Australia, 1995b) and AS1289.3.3.1 (Standards

Australia, 1995c)

j linear shrinkage determined in accordance with

AS1289.3.4.1 (Standards Australia, 1995d)

j particle size distribution determined in accordance with

AS1289.3.6.1 (Standards Australia, 1995e)

Pit area during filling works

Aerial image source:Aerial image :

Google Earth Pro 2010Sinclair Knight Merz 2010

Scale: m

20 0 20 40 60 80 100

Figure 1.Aerial view of former quarry pit during filling

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j standard compaction tests carried out in accordance with

AS1289.5.1.1 (Standards Australia, 1993).

The test results are summarised inTable 1.

The test results presented in Table 1 indicate that the sandstone

was a non-plastic silty, gravelly sand, the weathered shale was agravelly, sandy clay and the fresh shale was a silty, sandy gravel.

After pre-treatment the samples were sub-sampled to obtain

material passing a 20 mm sieve for compaction into polyvinylchloride (PVC) test cells with 75 mm internal diameter and

150 mm high. The sides of the cell were perforated with 1 mm

diameter holes at about 25 mm spacing to allow saturation of the

samples by adding water to an outer PVC containment cell.

The samples were compacted to a target density of 98% SMDD

at moisture contents 1% dry of SOMC determined in accordance

with AS1289.5.1.1 (Standards Australia, 1993). The samples were

initially loaded dry (at compaction moisture content) and were

inundated to produce a saturated state after settlement reached a

linear trend.

The impact of side friction between the test samples and the cells

has not been considered in the analysis of results presented in this

paper. Penman and Charles (1976) indicate that in large-scale

floating ring oedometer tests, where the ratio of sample height to

diameter was 0.5, the impact of side friction is likely to be a

310% reduction in the applied vertical load. In the testing

programme, where a fixed ring was used and the ratio of sample

height to diameter was 2, the impact of side friction is likely to

be greater. However, a reduction in the ratio of sample height to

diameter is not necessarily desirable when testing relatively

coarse material. Parkin (1990) indicates that there are conflicting

requirements with respect to sample dimensions. Height to

diameter ratios should be kept small to reduce friction but needto be larger to produce better seating conditions. Parkin (1990)

adds that in spite of the impacts of friction on oedometer tests

and measures required to alleviate frictional resistance, rockfill in

situ generally proves to be significantly stiffer than predicted

based on oedometer tests. It could be the case that differences

between tests and field particle sizes and grading have more

impact than friction on settlement characteristics.

In Waddell and Wong (2005) the creep strain rate per log cycle

time relationship relative to total vertical stress was proposed for

three fill types, derived from sandstone, fresh shale and weathered

shale, described usingEquation 1

kv1:

Figure 2.Settlement test rig

Sample Liquid limit:

%

Plasticity

index: %

Linear

shrinkage:

%

Passing

2.36 mm

sieve: %

Passing

0.075 mm

sieve: %

SMDD: t/m3 SOMC: %

Sandstone 22 NP NP 58 19 2.03 8.9

Weathered shale 40 23 10 52 35 1.94 13

Fresh shale 25 10 4.5 30 13 2.13 7.5

Note: NP denotes non-plastic.

Table 1.Summary of laboratory test results on sandstone and

shale samples

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where k was based on post-saturation strain and was calculatedbased on the applied stress in MPa.

In this paper, settlement data have been obtained from deep fill in

which a water table has established. The laboratory test data have

been re-examined to consider the impact of saturation on creep

behaviour. Average effective stress analysis, as is commonly

applied to saturated soils, has been used to examine both the

saturated and unsaturated zones within the fill. The expression for

creep strain rate per log cycle time has been revised and is

expressed asEquation 2

k9v=r

atm2:

where k is selected from either pre- or post-saturation values

depending on the position of the groundwater table.

The applied stress has been divided by atmospheric pressure to

avoid the potential for errors in the calculation of creep strain

rate that could occur if a unit other than MPa was used for the

applied stress, as was adopted in Waddell and Wong (2005). The

creep strain over a particular time period can be calculated using

Equation 3

log(t2=t1)3:

Figures 35 show plots of strain against time, pre-saturation and

post-saturation for the sandstone, fresh shale and weathered shale

derived fill types.

The weathered shale sample subject to a vertical stress of400 kPa pre-saturation was unloaded to 250 kPa to simulate and

assess the impact of removal of a surcharge prior to saturation.

Hence, no post-saturation data are presented for a vertical stress

of 400 kPa.

Based on the pre- and post-saturation strain trend line in Figures

3, 4 and 5, the creep strain rate per log cycle time, , was

calculated for each stress level. Creep rates are reported in the

literature by various authors as either stress dependent or

independent. Charles (1990) indicates that creep of heavily

compacted rock fills is stress dependent. Assuming a linear

relationship between creep and stress,Figure 6shows creep strain

4

3

2

1

010 100 1000

Strain,:%

00058log ( ) 0020710 t 00018log ( ) 0028910 t

00041log ( ) 001710 t 00014log ( ) 0022210 t

00023log ( ) 0013210 t 00014log ( ) 0015110 t

Time, : dayst

200 kPa post-saturation 300 kPa post-saturation 400 kPa post-saturation200 kPa pre-saturation 300 kPa pre-saturation 400 kPa pre-saturation

Figure 3.Creep strain sandstone

00028log ( ) 00113 10 t

00016log ( ) 0009 10 t

10 100

Time, : dayst

200 kPa unsaturated 200 kPa saturated

0

1

2

3

4

Strain,:%

1000

Figure 4.Creep strain fresh shale

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rate per log cycle time plotted against the effective vertical stress

divided by atmospheric pressure.

Figure 6shows that the measured creep strain rates are dependent

on the vertical stress level. The slope of the trend line is k in

Equation 2 and ranges from 0.0005 to 0.0027 for the saturated

sandstone and shale samples tested. Table 2shows creep rates for

compacted fills from Charles and Watts (2001) expressed in the

format ofEquation 2.

Comparing the k values obtained from the laboratory testing

with those in Table 2 it can be seen that laboratory-derived

values range from an order of magnitude greater to about four

times the values reported by Charles and Watts (2001) based on

4

3

2

1

010 100 1000

Strain,:%

00115log ( ) 0034810 t

00039log ( ) 0050810 t

00108log ( ) 0028610 t

00081log ( ) 0037410 t

00097log ( ) 0017710 t 00055log ( ) 0026310 t

Time, : dayst

200 kPa post-saturation 300 kPa post-saturation 250 kPa saturated

200 kPa pre-saturation 300 kPa pre-saturation 400 kPa unsaturated

5

6

7

8

Figure 5.Creep strain weathered shale

Fresh shale post-saturationWeathered shale post-saturation

00034 / v atm

00027 / v atm

00014 / v atm 00008 / v atm

00005 / v atm

04

03

02

01

0

05

06

07

08

09

10

1112

13

14

Creepstrainrateperlogcycletime,:

%

0 1 2 3 4 5Effective vertical stress/atmospheric pressure

Sandstone pre-saturation Sandstone post-saturationFresh shale pre-saturationWeathered shale pre-saturation

Figure 6.Creep strain rate per log cycle time plotted against

effective vertical stress/atmospheric pressure

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field monitoring. The value for stiff clay in Table 2 i s o f a

similar order of magnitude to the laboratory test results for the

shales.

The creep strain rates based on the laboratory testing within the

range of stress levels tested are of a similar magnitude to the

value of 0.2% reported bySowers et al. (1965) for rock fill dams

constructed from well sluiced (compacted) fill. The values are

less than the value of 1.05% reported bySowers et al. (1965) for

poorly sluiced (compacted) fill and less than the range of

0.51% for uncompacted fill reported by Charles and Watts(2001).

Based on these comparisons with creep values for both com-pacted and uncompacted fills in the literature, the laboratory test

results suggest that the sandstone and shale behaviour in the

laboratory tests lies somewhere between that of compacted stiff

clay and compacted rock fill.

Figure 6 illustrates the difference in the pre- and post-saturation

creep, with the strain rate trend lines flattening for the three

materials tested. The mechanism by which the creep rates have

reduced in the samples tested is beyond the scope of this paper.

However, the reduction may be attributed to slight swelling of the

particles resulting in impacts such as reduced void size, redis-

tribution of particles and changes in permeability. No special

precautions were taken to prevent drying of the shale and

sandstone samples by way of the perforations in the test cells.

This is a potential shortcoming in the test methodology and may

have impacted on pre-saturation creep rates.

No significant hydroconsolidation was observed as a result of the

samples being compacted to a high relative density and at

moisture content close to SOMC determined in accordance with

AS1289.5.1.1 (Standards Australia, 1993).

A relationship between pre- and post-saturation creep strain rates

and plasticity is proposed for the sedimentary rocks tested in the

laboratory, as illustrated in Figure 7, where creep strain rateconstant, k, is plotted against plasticity index (PI).

Material type Nature of compaction Creep rate per logcycle time,

Sandy gravel Heavy vibrating roller 0.00004 9v=ratmMudstone Heavy vibrating roller 0.00012 9v=ratmSandstone/

mudstone

Heavy vibrating roller 0.00013 9v=ratm

Stiff clay Heavy dynamic

compaction

0.005

Table 2.Creep rates derived from values presented inCharles and

Watts (2001)

35 10 3

25 10 3

15 10 3

50 10 4

30 10 3

20 10 3

10 10 3

0

Strainratecon

stant,k

0 5 10 15 20 25

Plasticity index: %

Pre-saturation Post-saturation Sandstone range Shale range

Figure 7.Creep strain rate constant, k, plotted against plasticity

index derived from sandstone, fresh shale and weathered shale

fill

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The trends inFigure 7may be consistent for shale and sandstonebut not for other materials. The present author has conducted

tests on argillite, which is a metamorphosed sedimentary rock

unlike the shale and sandstone from the Sydney region. In these

tests on argillite, the pre- and post-saturation creep rate trends

were

j similar for a sample with a PI of 6% and containing 13%

material passing the 75 m sieve

j different for a sample with a PI of 4% and containing 19%

material passing the 75 m sieve.

Thus, in fill derived from the same argillite rock source, a sample

with coarser grading and lower silt and clay (fines) content

exhibited no significant variation in pre- and post-saturation creep

rate. In contrast, a sample with higher fines content resulted in a

significantly lower post-saturation creep rate.

The source rock for the argillite sample was a higher strength

rock than the shale or sandstone, which may impact on the

behaviour of the argillite once it is broken down under compac-

tion. This may explain the difference in the pre- and post-

saturation creep behaviour of the argillite compared to the shale

and sandstone.

4. Settlement monitoring

4.1 Monitoring installations

Settlement monitoring points were installed across the infilled pit

after the completion of filling at the locations shown in Figure 8.

Contours of fill thickness and the locations of two piezometers

are also shown inFigure 8.

The settlement monitors consisted of 12 mm diameter steel

reinforcement rods, concreted into a pad at the base of a 2 m

deep hole augered in the fill and protruding about 1 m above the

ground surface. A PVC pipe was placed over the steel rod to

isolate the rod from the upper 1.5 m of the soil profile, thus

reducing the impact of near-surface shrinkswell movements on

the settlement monitoring results. The elevation of the monitoring

points was surveyed for periods of up to 455 days by a registered

surveyor using conventional survey techniques relative to site

bench marks that were located on natural ground outside the

infilled pit area. The accuracy of the survey levelling method was

of the order1 mm.

4.2 Monitoring results

Settlements of up to 34 mm were measured over a period of 423

days in the north-west area of the pit. In other areas of the pit

Settlement monitoring point

Note: Fill thickness in metres

6260300

6260200

6260100

6260000

6260250

6260150

6260050

125 113

22

124 20

112

114

111110

107

106

18

108

109

16

115116

120 119

24

22

20

117

118

136135

137 138

134

22

22

20

20

1816

129132

131

23

222221 20

Piezometer no. 1 2018

1021210314

104

101121

161412

10

123

122 Piezometer no. 2

1312

18

105A

130

133

321300 321400 321500 321600 321700321350 321450 321550 321650

N

126 127

128

Figure 8.Monitoring point locations and fill thickness contours

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negligible settlement occurred over monitoring periods of up to455 days. Figure 9 shows settlement contours for a 316 day

period when all monitoring points were being surveyed concur-

rently. During this period the maximum monitored settlement was

about 18 mm in the north-west area of the infilled pit, with

negligible settlement in other areas.

The settlement of 18 mm over 316 days produced a maximum

settlement gradient of about 1:1800 in the infilled pit. The

maximum settlement of 34 mm over 455 days would probably

produce a maximum gradient of about 1:1000, if data for all

points were available. If settlement is assumed to be linear with

log time, similar gradients should occur over a further log cycle

of time (10 years). Such gradients due to creep settlement

compare favourably with the limit of 1:400 in AS2870 (Standards

Australia, 1996b) for articulated brick veneer construction. The

further analysis of the settlement monitoring results presented

below suggests that total settlements will be less than predicted

assuming a linear trend with log time.

Figure 10shows strain plotted against elapsed time for a selection

of monitoring points across the infilled pit area. Creep strain was

assumed to commence from the time the fill reached half the final

bulk fill height at a particular monitoring point.

The nature of vertical load developing within the fill over aperiod of many months as it is placed will result in early phase

settlement that occurs in a relatively elastic fashion. Where load

was applied instantaneously in the test samples, the early phase

settlement was generally complete within 2040 days. In the field

90 days elapsed between the end of filling and the commence-

ment of monitoring. Linear settlement trends were monitored over

a period of up to 455 days. Therefore, the early phase settlement

was assumed complete in the field before the commencement of

monitoring. The fill was assumed to be within the creep phase

and linear trend lines have been drawn through the calculated

strains for the data sets over the monitoring period.

Based on the compaction test records, the fill was placed at

different rates and monitoring commenced at different times,

post-completion of filling. Figure 11 illustrates some typical

filling and monitoring histories and the results of monitoring

groundwater levels in the two piezometers.

Figures 9and10show that monitored creep strains are relatively

variable across the infilled pit. Strains in the more recently filled

areas (such as monitoring point 115) were considerably higher

than the areas where filling was carried out early in the filling

period (such as monitoring point 20). A possible explanation for

Settlement monitoring point

Note: Fill thickness contours in millimetres

6260300

6260200

6260100

6260000

6260250

6260150

6260050

125 113

124 112

114

111110

107106

108

109

115116

120 119

12

14

6

6

2

117

118

136135

137 138

134

2

22

10

129

132

131

23

2221

Piezometer no. 120

102

103104

101121

123

122 Piezometer no. 2

1312

2

2

4

4

4

8

0

0

0

0

105A

130

133

321300 321400 321500 321600 321700321350 321450 321550 321650

N

126 127

128

Figure 9.Settlement contours over a 316 day period

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the variability in the monitored strain rates is provided where the

monitored and predicted strains are compared below.

5. Predicted as opposed to monitoredsettlements

5.1 Fill propertiesTo assess the similarity of the laboratory test samples to the fill

used in the pit, index and grading test results have been com-

pared. Index and grading tests were carried out on samples from

each source site that supplied fill during the filling works to check

source sites for compliance with the fill engineering specification.

The samples were subject to pre-treatment by repeat compaction

using RTA Method T102 (NSW Roads and Traffic Authority,

1999) before testing. A summary of the median test results is

presented inTable 3and the index and grading test results for the

laboratory samples are repeated inTable 4.

Comparing the results in Tables 3 and4 it can be concluded that

the laboratory test samples were similar to the imported fill.

However, the laboratory test samples were sub-sampled to mater-

ial passing the 20 mm sieve and the fill contained cobbles and

boulders, hence some scale and grading impacts are to be

expected.

The median values of PI from Table 3are shown as dashed lines

onFigure 7and have been used to select pre- and post-saturation

creep strain rates for the shale and sandstone presented in

Table 5.

The mix of shale and sandstone used to fill the pit was not

recorded during the filling works. Based on the current authorsknowledge of the site filling process, sandstone was the predomi-

nant fill type in the early phases of filling (eastern third of the

pit) and shale predominated in the latter stages of filling. This

assessment is supported by the relative proportion of shale and

sandstone source test results presented inTable 3.

However, this does not allow an accurate assessment of the

relative proportions of shale and sandstone in particular areas of

the infilled pit, as information on the relative volumes of material

sourced from each site was not readily available. For the

prediction of settlement, ratios of shale to sandstone have been

assumed to be

j 20:80 shale to sandstone in the period before 2005

j 80:20 shale to sandstone in the period including and after

2005.

020

018

016

014

012

010

008Stra

in,:%

006

004

002

0

002400 4000

Time, : dayst

133

20

128

135

115

113

112

101 00026log ( ) 00071 10 t

00048log ( ) 00013 10 t

00014log ( ) 00034 10 t

00012log ( ) 00031 10 t 00007log ( ) 00021 10 t

00021log ( ) 00066 10 t

00001log ( ) 00003 10 t

00018log ( ) 00006 10 t

00028log ( ) 00076 10 t

Figure 10.Strain plotted against time for selected monitoring

points

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Piezometer no. 1

Piezometer no. 2

Area near SMP12

Area near SMP115

Area near SMP135

Area near SMP23

SMP12

SMP115

SMP135

SMP235/06/02

5/12/02

5/06/03

5/12/03

4/12/04

4/12/05

4/12/06

4/12/07

3/12/08

3/12/09

4/06/04

4/06/05

4/06/06

4/06/07

3/06/08

3/06/09

3/06/10

Date

70

75

80

85

90

95

100

Approximateelevation:mAHD

Filling periodSettlement

monitoring period

Piezometer monitoring

Figure 11.Examples of filling and settlement monitoring histories

Material type Date Liquid limit:

%

Plasticity

index: %

Linear

shrinkage:

%

Passing

2.36 mm

sieve: %

Passing

0.075 mm

sieve: %

Proportion of

source sites: %

Shale Pre-2005 34 15 8.5 45 35 36

2005 and after 32 13 7 43 21 58

Sandstone Pre-2005 21 3 2.5 57 13 64

2005 and after NP NP NP 57 15 42


Table 3.Summary of median material properties from fill source

site samples

Material type Liquid limit:

%

Plasticity index:

%

Linear shrinkage:

%

Passing 2.36 mm

sieve: %

Passing 0.075 mm

sieve: %

Weathered shale 40 23 10 52 35

Fresh shale 25 10 4.5 30 13

Sandstone 22 NP NP 58 19


Table 4.Summary of material properties of laboratory test

samples

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5.2 Creep predictions compared with monitored

settlements

InFigure 10a dashed line represents a predicted trend line based

on the selected strain rates, the fill thickness and the groundwater

conditions at monitoring point 115 (assumed 80:20 ratio of shale

to sandstone).

Examining the range of strain at the various monitoring points

illustrated in Figure 10 and the distribution of settlement across

the infilled pit illustrated in Figure 9, it is evident that measured

creep strain rates for the fill are

j generally greater than or equal to the values predicted

assuming a linear trend line within about 1200 days of

reaching half fill height

j generally less than or equal to the values predicted assuming

a linear trend line in the period from 1200 to 1800 days of

reaching half fill height

j significantly less than predicted assuming a linear trend line

at greater than 1800 days from half fill height.

This suggests that there is a flattening trend in the creep phase

rather than a linear trend. Charles and Watts (2001) comment that

some researchers propose non-linear creep trends that would lead

to creep settlement eventually ending.

In the case study, there are a number of factors that could be

influencing the apparent non-linear trend including

j the proximity of some of the monitoring points to the pit

walls, which may reduce the creep rate

j changes in groundwater and moisture content within the fill

above the groundwater table

j variability in the composition of the fill

j filling history

j the time that has elapsed between the end of filling and the

commencement of monitoring.

The filling history provides data spanning three log cycles of time

and the flattening trend with elapsed time is considered signifi-

cant. The deeper fills generally exhibit the steeper strain trends

compared to the predicted trend line, suggesting that fill thickness

independent of elapsed time may also be significant in the creepbehaviour of deep fill.

To illustrate more clearly the strain trends that are inferred from

the data,Figure 12shows a linear trend line over a 30 year period

based on the laboratory test results and the following trend lines

derived from the settlement monitoring

j the maximum strain rate measured extending to 1200 days

(based on monitoring point 115)

j a strain rate half the rate of the linear trend line extending to

1800 days (based on monitoring point 112)

j a strain rate equal to the linear trend line from 1200 to 1800

days (based on monitoring point 135)

j a trend line based on one of the relatively flat trend lines

from 1800 days (based on monitoring point 20).

As the creep settlement post-completion of filling is of interest,

strains have been set to zero at the date of the completion of

filling (an average of 500 days after the fill reached half height in

the present case study).

If the trend lines based on the settlement monitoring inFigure 12

are considered upper and lower bounds, then predictions of creep

strain can be made for periods of interest, for example at

intervals of 2, 7 and 30 years post-completion of filling. Table 5presents the predicted strains for shale to sandstone mix of 80:20,

with a groundwater table at half the fill depth. The settlements

corresponding to a 20 m fill thickness are also presented in Table

6to give an appreciation of the order of magnitude of settlement

in a deep fill.

Settlements predicted based on monitoring trends within about 2

years of fill completion vary by 45208% from settlement based

on a linear trend with log time. In the intermediate term (7 years)

the range is 34127% and in the longer term (30 years) the total

post-end of filling settlement ranges from 23% to 77% of the

value predicted based on a linear trend with log time.

This analysis assumes that the groundwater table remains con-

stant. The groundwater monitoring data indicate that groundwater

is rising and this could cause the overall fill creep rate to reduce

as more fill becomes saturated over time.

6. GroundwaterThe piezometer readings shown in Figure 11 indicate that

seepage from the surrounding shale bedrock is re-establishing a

groundwater table within the pit. During the monitoring period

the average groundwater elevation was about RL 85 m, compared

to the elevation in the base of the pit before filling, which was

about RL 72 m.

No assessment of hydroconsolidation was possible in this case

study as settlement monitoring was not carried out during filling.

However, as a groundwater table has established during the

Fill type Creep strain rateconstant,k,

pre-saturation

Creep strain rateconstant,k,

post-saturation

Pre-2005 shale 0.0022 0.00125

All sandstone 0.0014 0.0005

Post-2005 shale 0.0018 0.00125

Table 5.Assumed creep strain rate constants for settlement

predictions

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filling, hydroconsolidation is no longer a significant risk in the

saturated lower fill. Furthermore, the relatively high level of

compaction of the fill at moisture content close to standard

optimum should reduce the risk of hydroconsolidation resulting

from ongoing rises in groundwater or localised saturation due to

broken services.

7. Conclusion

Based on a laboratory testing programme, creep strain rates have

been assessed for fill sourced from sedimentary rock. The addition

of a database of field data that can be correlated with the

laboratory testing offers further value for the assessment of fill

performance.

Trend line derived fromsettlement monitoring

2 years

7 years

000262log ( ) 00071 10 t

000262log ( ) 00059 10 t

000576log ( ) 00155 10 t

000115log ( ) 00031 10 t

000023log ( ) 00001 10 t

000023log ( ) 00019 10 t

30 years

Trend line derived fromlaboratory testing

040

036

032

028

024

020

016

012

008

004

0500 5000

Time, : dayst

Strain,:%

Trend line derived fromsettlement monitoring

Figure 12.Creep rate trend lines derived from laboratory testing

and settlement monitoring

Years post-filling Assumed strain rate Creep strain, : % Settlement for 20 m fill

thickness: mm

Ratio monitoring trend to

linear trend

2 Linear with log time 0.11 21

Upper bound based on

monitoring trends

0.21 43 208%

Lower bound based on

monitoring trends

0.046 9 45%



monitoring trends

0.29 52 127%

Lower bound based onmonitoring trends

0.07 14 34%



monitoring trends

0.30 55 77%

Lower bound based on

monitoring trends

0.08 17 23%

Table 6.Predicted strains and settlements compared to

monitoring trends

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The controlled fill monitored in this case study exhibited variablecreep strains. Settlements within about 2 years of the completion

of filling could exceed predictions based on a single linear trend

line. The estimated settlement within 2 years post-filling for this

case study ranged from 45% to 208% of that predicted assuming

a single linear trend with log time. In the longer term (30 years),

the data indicate a flattening settlement trend. The total post-

filling settlement should be less than is predicted, with an

estimated range for this case study of 2377% of the settlement

calculated using a single linear trend with log time.

The data from this case study indicate that settlements of

2050 mm could occur for 20 m fill thickness over 30 years,

which is of a similar magnitude to shrinkswell movements of

expansive clays. If construction is not commenced until at least 2

years post-filling, the total settlements up to 30 years should be

less than 20 mm for a 20 m fill thickness. The suitability of

particular footing systems will depend on factors that impact on

differential settlement such as variability in material properties,

fill thickness, shrinkswell movement and building loads. The

data indicate creep settlement gradients should be significantly

less than the limit of 1:400 for articulated masonry veneer

structures supported on stiffened rafts.

This case study illustrates that knowledge of the filling history is

essential for settlement prediction in the early period post-fillcompletion, particularly where creep behaviour deviates from a

linear trend with log time. If planning to construct over deep

engineered fill, the risk of settlements exceeding predictions in

the longer term does not appear to be significant, given the

settlement trends presented in this case study. The settlement data

show the benefits of allowing deep fill time (preferably up to 2

years post-filling) before commencement of construction.

The laboratory test data may be of use for assessments where fill

is sourced from sedimentary rock, similar in nature to the

sandstone and shale of the Sydney region and compacted to at

least 98% SMDD and moisture content near SOMC, determinedin accordance with AS1289.5.1.1 (Standards Australia, 1993).

Further testing would be of value to provide additional data to

test the relationship between pre- and post-saturation and plasti-

city index. Aspects of the test methodology should be developed

to consider the potential for drying of samples before saturation

and the impact of friction in the test cells.

Further long-term monitoring of filled sites would be of benefit to

assess whether the deviation from a linear creep rate with log

time is common and, if there is a deviation, at what point in time

does the trend line flatten.

AcknowledgementsThe author would like to thank Austral Brick Pty Ltd for allowing

the data on laboratory testing and site settlement monitoring to

be used in this paper.

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