Use of CRS test to predict settlement in an Irish silt

R. A. Carroll1 University College, Dublin, Ireland

ABSTRACT

The prediction of settlement is traditionally associated with obtaining parameters from incremental load oedometer testing. Analternative method is the Constant Rate of Strain (CRS) test, which is rapid in comparison to the traditional test. Here anassessment of the predictions for settlement using the CRS test will presented for an Irish estuarine silt from the west of Irelandencountered during road design and construction. A comparison of laboratory based settlement predictions will be made to in-situ measurements from settlement plates in this case study. A detailed characterisation of the silt will be presented. Abackground to the CRS test and the procedures used will be discussed. Janbu's 1D consolidation theory will be explored for use with silts instead of Terzaghi's 1D consolidation theory to analyse settlement. This paper will demonstrate that Janbu's methodof settlement analysis accurately predicts the actual settlement for a typical monitoring location at Foynes. Keywords: silt, CRS, primary settlement

1 School of Architecture, Landscape & Civil Eng., Newstead Bld, Belfield, Dublin 4, Ireland. roselyn.carroll@ucdconnect.ie

1 INTRODUCTION

Selection of parameters to predict settlement in silt and use of 1-D compression theories are not well documented in literature in comparison to clays [2, 3]. Practicing engineers require guidance in design for intermediate soils as conventional 1D settlement theories are not well suited to silts.

The aim of this work is to analyse the results obtained from efficient CRS tests using Janbu's [1] theory for primary settlement and compare it to field results. The behaviour of Foynes silt will be compared to other documented silt deposits using Janbu's framework to profile the range of modulus number (m) for silt.

2 SITE CHARACTERISATION

The site is located at Foynes County Limerick beside the Shannon estuary. The silt deposit is approximately 12 meters (m) thick at its deepest location. It has low plasticity and a low organic content. The deposit overlays a limestone bedrock.

An embankment 2.5 m high was constructed over the silt deposit to provide access to Foynes harbour in 2002.

2.1 Sampling technique

A shell and auger rig was used in conjunction with conventional ELE fixed piston sampler. The thin walled aluminium sampler had a cutting edge of 15 degrees, an area ratio of 6.82% and no inside clearance. The borehole was cleaned to

200-500 mm above the sampling depth for each sample taken between 2.5-8.5 m. Two boreholes were sampled 140m apart; BH-A specimens 02,03 and 04 and BH-B specimen 01.

2.2 Basic properties of Foynes silt

Figure 1 shows the particle size distribution (PSD) for the samples tested. The clay content with depth is given in Figure 3 (a) with average clay, silt and sand content; 15, 80 and 5 (%) respectively.

0.0001 0.001 0.01 0.1 1 10 100Particle size (mm)

0

20

40

60

80

100

PercentPassing(%)

3.99m - 013.99m - 01*6.43m - 027.97m - 034.02m - 04

Fine Med. Coarse

Silt SandFine Med. Coarse Fine Med. Coarse

GravelClay Figure 1. Particle size distribution.

Basic soil parameters are presented in Figure

3 showing organic content, moisture content and bulk density. Bulk density is constant with depth with an average value of 1.9 Mg/m3. Moisture content ranges from 32-38% and shows no change with depth. Organic content is very low and shows a slight decrease with depth, an average value for the site is 1.9%.

The A-line plasticity chart in Figure 2 shows results falling on or just below/above the A-line and having medium to low plasticity.

Based on the PSD and the Atterberg limit results the soil is classified as SILT. The British Standard [4] clause 41.4.4 states that a material classification should be consistent with its engineering behaviour. The material behaviour is

evident in the CRS plot of stress and tangent modulus, which will be discussed later.

0 20 40 60 80 100Liquid limit ( %)

0

10

20

30

40

50

60

Plasticityindex(%)

01020304

Figure 2. 'A-line' plasticity chart.

3 CRS TEST

Standard triaxial equipment is used to carry out loading of the specimen for the CRS test. Measurement of load, compression and pore pressure are taken continuously.

In this series of tests larger specimens, 100 mm in diameter and 50mm high, are used in order to reduce destruction of the specimen and increase accuracy of measurements. Specimens are extruded directly into the confining ring.

One-way drainage of the specimen to the atmosphere occurs through the top porous disc during compression. No initial bedding stress is applied at the beginning of the test. The rate of strain is mechanically controlled at; 3%, 5% and 10% per hour. The procedure used is in line with NGI recommendations [5].

A standard method of analysing the results of a CRS test was developed by Wissa et al. [6]. The parameters obtained are vertical effective stress (σ'v) and coefficient of consolidation (cv) equation (1) and (2) respectively. An assumption that there is a parabolic distribution of pore pressure within the sample is made to resolve for effective stress.

3/2' bvv u−= σσ (1)

tu

Hcb

vv ∆2

'∆2 σ= (2)

Low Int. High

Parameters used are; ub = base pore water

pressure, H is sample height at corresponding stress and t is time between stress levels.

An advantage for the CRS test is an improved plot of stress strain due to continuous measurements and this may result in easier identification of preconsolidation pressure [7]. Determination of coefficient of consolidation and permeability is also more accurate.

4 1D COMPRESSION

4.1 Traditional method

Traditionally design engineers have applied Terzaghi's theory of 1D consolidation analysing primary settlement in all soils, using plots of σ'v

against strain (ε) or void ratio (e) based on oedometer tests. However Long [2] has reported difficulties in using this theory for silts as plots are non-linear and there is separation of primary and secondary consolidation.

4.2 Janbu's theory of consolidation

It is proposed to use Janbu's theory [1] in which there is no separation of primary and secondary consolidation. It is known that creep occurs during primary consolidation but does not dominate results and separation is not representative of actual conditions. His approach is to use a single method to examine all soils

during primary consolidation [1] and is based on stress strain relationship in equation (3).

Primary settlement analysis is based on tangent modulus (M), which is a function of effective stress while secondary consolidation is calculated using the 'time-resistance' concept. This paper deals with primary settlement.

Soil has a resistance to deformation (M), which is characterised by an action reaction process measured in the CRS test. Hence M, known as the constrained tangent modulus for loading, is defined by the stress strain relationship, which accounts for the action reaction process as described in the equation (3):

Mdd

ddM '' σεεσ

== (3)

Janbu [1] characterised silt using M-σ relationship and introduced a general equation (4) that represents the stress and relationship using appropriate dimensionless modulus number (m) and stress exponent (α). σa is a reference stress (100 kPa) and σ' the effective stress (kPa):

α

σσσ

−

=

1'

aamM (4)

M is capable of representing the different stress regime before and after preconsolidation pressure [1] for a silt/sand soil. The constrained modulus plot characterises the material and

10

8

6

4

2

00 1 2 3 4 5

Lo ss o n Igni ti on (% )

1 .6 2 2 .4B u lk Density (Mg /m3 )

1 0

8

6

4

2

020 30 40 50Mo istu re C on ten t (%)

10

8

6

4

2

00 20 4 0 60 80 100

C lay/S ilt/Sandcon ten t (% )

10

8

6

4

2

0

Depth(m)

ClaySiltSand

Figure 3. (a) Particle content; (b) Organic content; (c) Moisture content and (d) Bulk density.

10

8

6

4

2

00 1 2 3 4 5

Lo ss o n Igni ti on (% )

1 0

8

6

4

2

00 1 2 3 4 5

Lo ss o n Igni ti on (% )

1 .6 2 2 .4B u lk Density (Mg /m3 )

1 0

8

6

4

2

01 .6 2 2 .4

B u lk Density (Mg /m3 )

1 0

8

6

4

2

020 30 40 50Mo istu re C on ten t (%)

10

8

6

4

2

020 30 40 50Mo istu re C on ten t (%)

10

8

6

4

2

00 20 4 0 60 80 100

C lay/S ilt/Sandcon ten t (% )

10

8

6

4

2

0

Depth(m)

ClaySiltSand

0 20 4 0 60 80 100

C lay/S ilt/Sandcon ten t (% )

10

8

6

4

2

0

Depth(m)

ClaySiltSand

Figure 3. (a) Particle content; (b) Organic content; (c) Moisture content and (d) Bulk density.Figure 3. (a) Particle content; (b) Organic content; (c) Moisture content and (d) Bulk density.

provides a single modulus number to represent the soil behaviour in 1D primary compression.

In order to calculate settlement due to loading, compression tests are carried out to measure the strain on a specimen when an additional stress is applied. From equation (3) incremental strain is calculated and integration of equation (3) gives:

∫+=

='''

'

'qc

c Mdσσ

σ

σε (5)

Here, q' is a surcharge and σ'v the initial stress in soil before loading. Substituting the general equation for M (4), at any stress level, into equation (5) and integrating gives equation (6). Strain is calculated using equation (6) for the case under consideration here with the associated constant m and positive α from the M-σ plot. This equation is not valid for α = 0.

−

+=

αα

σσ

σσ

αε

a

c

a

c qm

'''1 (6)

Settlement for a soil profile, of depth 0-H, is calculated by summing the strain for each layer times the thickness of each layer:

∫=H

dz0εδ (7)

5 IN-SITU MONITORING

Settlement monuments were monitored at the site for 150 days. A typical settlement profile due to surcharge of 50 kPa applied in a single stage is shown in Figure (4). Settlement transition from primary into creep is established using the settlement potential (S), calculated from rate of settlement, equation (8), and time since initial load applied.

tS δ&= t∆/∆δδ =& (8)

Primary settlement is assumed to be complete and creep dominating when settlement potential is constant [8]. In the case of Foynes, creep effects dominate when the settlement potential is constant, after 27 days with settlement of 137

mm, see Figure (4). Vertical drains were used to speed up the settlement process at Foynes.

0 40 80 120 160Time (days)

2252001751501251007550250

Settlement(mm)

0

1

2

3

4

5

SettlementPotential(mm)

Figure 4. Settlement record & settlement potential.

6 LABORATORY RESULTS

6.1 Traditional representation of results

Figure (5) shows the classic plot of stress against strain with in situ effective stress marked with broken line. It is difficult to select an appropriate value for (Cc/(1+e0)) as the curve is rounded and non linear. Estimation of preconsolidation pressure (p'c) is also difficult to identify due to nature of the curve, similar difficulties are reported by Long et al. [2, 3].

CRS test 01 has a flatter curve compared to other CRS tests indicating poorer structure due to possible densification during sampling in the specimen tested.

6.2 Modulus number

For a normally consolidated silty sandy soil α range is 0.4-0.65, Janbu [9] recommends use of α= 0.5, however in this report α= 0.25 is taken to compare to previous work carried out by Long et al. [2, 3]. The modulus number for Foynes is m = 55. A similar curve would be obtained using m = 45 and α = 0.5. Janbu's method of primary settlement analysis gives a universal system to compare all soils and sedimentary rocks based on

Settlement M15 End primary Primary settlement Settlement potential

m and α which allows comparison of results to be easily made for various soil types.

0.1 1 10 100 1000Applied stress (kPa)

10

8

6

4

2

0

Axialstrain(%)

1 - 3.95 m

2 - 6.47 m

3 - 8.10 m

4 - 4.06 m

0.1 1 10 100 1000Applied stress (kPa)

10

8

6

4

2

0

Axialstrain(%)

1 - 3.95 m

2 - 6.47 m

3 - 8.10 m

4 - 4.06 m

Figure 5. 1D compression behaviour from CRS test.

The curves of M-σ, Figure (6), for the CRS

tests are representative of a silt as the M increases with stress due to compaction of particles together and the trend matches the curve for a silt/sand using appropriate parameters. Similar results with increasing M values have been reported by Long et al. [2, 3] in conjunction with m value in a similar range, for Irish and Norwegian silts, see Table (1).

Table 1. Janbu parameters for silt sites [2, 3]

Site m a Sligo upper 16 0.25 Sligo lower 12 0.25 Dunkettle 25 0.25 Os Silt 70 0.25 Os Clay/Silt 16 0 E39 Silt 20 0

6.3 Rate effects

Foynes CRS tests were carried out over a range of 3-10% per hour. Table (2) shows rate and corresponding cv estimated at σ'v0 plus distribution of 50kPa surcharge with depth using Boussinesq equation for a strip footing.

Change in cv is material and depth dependant as can be seen in Figure (7) with sample 02 and 04 both tested at a rate of 5% but showing a

0 100 200 300 400Applied stress (kPa)

0

4

8

12

16

20

M(MPa)

1-3%

2-5%

3-10%

1-5%

m = 55

α = 0.25

0 100 200 300 400Applied stress (kPa)

0

4

8

12

16

20

M(MPa)

1-3%

2-5%

3-10%

1-5%

m = 55

α = 0.25

Figure 6. Tangent modulus with effective stress.

different cv. The PSD curve shows 04 as a coarser specimen thus having a higher cv.

An average m value of 55 fits rate 5-10 %, with curves falling above and below the M-σ line. While the slowest rate corresponds to a higher m value, suggesting a stiffer soil, which may indicate densification as suggested earlier.

Table 2. CRS rates and cv

Test Depth Rate σ'v0 cv m %/hr kPa m2/yr 01 3.95 3 36 175 02 6.47 5 58 175 03 8.1 10 73 160 04 4.06 5 37 450

6.4 Settlement analysis using Janbu theory

Settlement analysis detail is given in Table (3). Use is made of Boussinesq equation for a strip footing to estimate transfer of stress with depth. The Janbu theory works very well in predicting primary settlement for Foynes matching measured settlement identically, 135 mm, using an average m = 55 and α = 0.25 and embankment height of 2.5m (50 kPa). Each layer is subdivided into 2 m layers to improve accuracy. A surcharge of 60 kPa would increase the settlement prediction to 154 mm. It is difficult to examine the actual amount of surcharge used during construction. However an increase of 19 mm on the initial 135 mm is not very significant.

4-5%

0 100 200 300 400Applied stress (kPa)

0

200

400

600

800

1000

c v(m2 /yr.)

1-3.95 m

2-6.47 m

3-8.10 m

4-4.06 m

0 100 200 300 400Applied stress (kPa)

0

200

400

600

800

1000

c v(m2 /yr.)

1-3.95 m

2-6.47 m

3-8.10 m

4-4.06 m

Figure 7. cv with effective stress

Table 3. Settlement analysis with Janbu general equation (6) for strain estimate using 2.5m embankment - no surcharge.

Depth σvo' q' ε ∆δ

m kPa kPa % mm

-1m 10 49 2.30 46

-3m 27 47.5 1.50 30

-5m 45 42.5 1.08 21

-7m 63 37.5 0.80 16

-9m 80 32 0.60 12

-11m 98 28.5 0.48 10

Total 135

7 CONCLUSION

The CRS test provides a fast and efficient method to analyse settlement at Foynes. Janbu theory has been successfully applied to Foynes silt to predict the settlement and results correlate very well with measured data predicting a settlement of 135 mm for the 12 m deposit.

The estimated m = 55 fits within the statistical plot of m and w correlation [9]. Janbu suggests m = 50 to 100 for a silt and m = 100 to 500 for a sand, variation is due to loose or dense soil. However Janbu notes that for a loose inorganic silt m can be below 50. The m also correlates with previously measured m for Irish and Norwegian silts. It is evident that Foynes is on

the lower end of Janbu's range of m values. Long et al [3] found similar m values in the silt range. However Long [2] reported m values falling into the clay range suggesting the Irish estuarine silt has a loose density and organic content may also have reduce the m value.

More CRS tests are required to verify the effect of rates on measured m values.

ACKNOWLEDGEMENTS

The author would like to thank Mike Long for his advice and support, and George Cosgrove for his assistance.

IRCSET is kindly acknowledged for funding the research and the Geotechnical Trust Fund of the Engineers Ireland for their support.

REFERENCES

1. Janbu, N. Soil compressibility as determined by oedometer and triaxial test. in Proc. 3. ECSMFE. 1963. Wiesbaden 1, 19-25.

2. Long, M., Engineering characterization of estuarine silts. Quarterly Journal of Engineering Geology and Hydrogeology, 2007. 40(2): p. 147-161.

3. Long, M., Gudjonsson, G., Donohue, S., and Hagberg, K., Engineering characterisation of Norwegian glaciomarine silt. Engineering Geology, 2010. 110(n 3-4): p. 51-65.

4. BSI, BS5930. Code of practice for site investigations. 1999: British Standards Institution, London.

5. Sandbaekken, G., Berre, T., and Lacasse, S., Oedometer testing at Norwegian Geotechnical Institute,. ASTM Special Technical Publication, 1986. 892.

6. Wissa, A.E.Z., Christian, J.T., Davis, E.H., and Heiberg, S., Consolidation at Constant Rate of Strain. Journal of Soil Mechanics and Foundations Division, ASCE, 1971. 97(No. SM10): p. 1393-1412.

7. Davison, L.R. and Atkinson, J.H., Continuous loading oedometer testing of soils. Quarterly Journal of Engineering Geology, London, 1990. 23: p. pp. 347-355.

8. Janbu, N. The resistance concept applied to deformations of soils. in Proc. 7. ICSMFE. 1969. Mexico City.

9. Janbu, N., Soil models in offshore engineering (Rankine 25th Lecture). Géotechnique, 1985. 35(3): p. 241-281.