On the Evaluation of Pre-consolidation Pressure

HBRC Journal (2015) xxx, xxx–xxx

Housing and Building National Research Center

HBRC Journal

http://ees.elsevier.com/hbrcj

On the evaluation of pre-consolidation pressure

of undisturbed saturated clays

* Corresponding author.

E-mail address: [email protected] (A.H. Hammam).1 Associate Professor.2 Researcher.

Peer review under responsibility of Housing and Building National

Research Center.

Production and hosting by Elsevier

http://dx.doi.org/10.1016/j.hbrcj.2015.02.0031687-4048 ª 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of Housing and Building National Research Center.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article in press as: A.H. Hammam et al., On the evaluation of pre-consolidation pressure of undisturbed saturated clays, HBRC Journal (2015)dx.doi.org/10.1016/j.hbrcj.2015.02.003

Adel H. Hammam *,1, Ashraf I. Abel-Salam 1, Mostafa A. Yousf 2

Housing & Building National Research Center, Cairo, Egypt

Received 27 October 2014; revised 6 February 2015; accepted 12 February 2015

KEYWORDS

Pre-consolidation pressure;

Sample disturbance;

Pocket penetrometer;

Sample quality

Abstract The objective of this research was to show the effect of sample disturbance on the values

of pre-consolidation pressure ,c by using Schmertmann method. A prediction of ,c form pocket

penetrometer is also achieved. This was carried out by comparing the values of ,c that were

estimated from the results of consolidation tests, with the readings of pocket penetrometer for same

samples. Pocket penetrometer is a simple tool that can be easily used in field and laboratory to

initially predict unconfined compressive strength for clayey soils. Before carrying out the consol-

idation tests on undisturbed samples, pocket penetrometer readings were recorded. The correlation

obtained between pocket reading and ,c values that resulted from consolidation tests was found to

be valid for a wide range of clay stiffness, ranging between medium stiff to very stiff clay. As for soft

clay, this correlation was found not to be applicable where its behavior is believed to be greatly

affected by the degree of disturbance occurring to samples during drilling.ª 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of Housing and Building

National Research Center. This is an open access article under the CC BY-NC-ND license (http://

creativecommons.org/licenses/by-nc-nd/4.0/).

Introduction

Pre-consolidation pressure ,c is an important parameter, whichexpresses the stress history of soil especially for the behavior ofcohesive soil. It is well known that accurate estimation of

settlement of cohesive soil depends to large extent on the

accurate value of ,c. Many researchers did their best for how

one can estimate ,c from the results of consolidation tests.

Moreover several researchers tried to predict ,c from empirical

correlations depending on soil properties such as moisture con-tent, Atterberg limits, void ratio, shear strength and overbur-den pressure. Cone penetration test was widely used to

estimate ,c to overcome the disadvantages of disturbances,which occurred during drilling and when carrying out lab-

oratory tests. The concept of pre-consolidation pressure andits importance is well defined in geotechnical engineering for

calculating settlement. Pre-consolidation pressure, ,c is com-monly defined as the highest pressure to which the soil had beenexposed in the past. Consolidation test results are considered

the main laboratory test that can be used for determining the

value of ,c. Several researchers established different methods

for obtaining ,c from the consolidation tests. Casagrande [1]

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Fig. 1 Casagrande method for estimating pre-consolidation

pressure.

2 A.H. Hammam et al.

is the oldest and most commonly used method based on the

relation e � log ,0, as shown in Fig. 1. This relation shows thatthe soil is under elastic behavior up to a certain pressure then itstarts plastic behavior. According to the break point between

the two behaviors, one can estimate ,c graphically.Good results can be obtained if the break point is well defined

on the curve e � log ,0. In general, this curve is affected by

the degree of sample disturbance, as shown in Fig. 2.Many researchers developed other methods depending on

Casagrande method or new ones to define ,c [2,3], as shownin Table 1.

Schmertmann method, e � log ,0 [4] was an attempt tocompensate the effect of sample disturbance by adjusting theresults of consolidation test, as shown in Fig. 2.

Butterfield method [5] is based on plotting the variationbetween effective stress and volume change of specimen,

log (1 + e) � log ,0 or ln (1 + e) � ln,0. The pre-consolidation

Fig. 2 Schmertmann method for predicting sample disturbance.

Please cite this article in press as: A.H. Hammam et al., On the evaluation of pre-consdx.doi.org/10.1016/j.hbrcj.2015.02.003

pressure is defined as the intersection point of the two straightlines.

Pre-consolidation pressure is affected not only by sample

disturbance, but also by load durations and load incrementratios during testing [10,11]. Al-Zairjawi [2] carried out experi-

mental study and he concluded that the values of ,c decreasedas load durations and load increment ratios increased.

Correlations of pre-consolidation pressure

‘‘Settlement analysis – the backbone of FoundationResearch’’, said Terzaghi in 1929 [12]. It is well known that

the accuracy of determining the consolidation settlement isconsidered the corner stone of estimating the total settlementof cohesive soil. Therefore, over past decades researchers

developed several approaches to define the factors includedin the consolidation equations. The amount of settlementdepends mainly on three unknown factors that are included

in the settlement equation, among them are compression indexCc or re-compression index Cr, void ratio e and pre-

consolidation pressure ,c. For saturated soil, void ratio canbe estimated from moisture content Wc and specific gravityGs. Moreover, Bowles [13] reported several correlations for

estimating Cc (accordingly Cr) from simple soil properties suchas moisture content and Atterberg limits. Hammam andAbdul-fadiel [14] have established a new correlation to

estimate Cc from moisture content (Cc = 1.38 Wc � 15). Onthe other hand, to overcome the complicated defects related

to estimating ,c from consolidation test researchers tried to

develop empirical correlations between ,c and some of soil

properties. There are few correlations in the literatures amongthem are as follows:

– Nagaraj and Srinivasa [15] established the followingcorrelation:

log ,c ¼ 5:97� 5:32 ðWc=WLÞ � 0:25 log ,0vo ½kPa� ð1Þ

– Solanki and Desai [16] developed new correlation asfollows:

,c ¼ 137:924� 0:179 ,0vo � 30:48 ðe=eLÞ ½kPa� ð2Þ

where e = void ratio, eL = void ratio at liquid limit (WL),

,0vo = effective over burden pressure.

By examining the above correlations, it can be noticed that

the first one depends mainly on the ratio (Wc/WL) which canbe considered as an indication of soil stiffness and hence itcan be valid for certain soil conditions. While the second cor-

relation assumed that value of ,c should be less than certainvalue (137.924 kPa). Moreover, the authors themselves

reported that this correlation depended on data of alluvialdeposits of south Gujarat region.

Several researchers considered that the best estimation can

be developed from the field tests especially cone penetration

tests, CPT. Therefore, pre-consolidation pressure ,c could be

expressed by the following simple formula (after Mayneet al.) [17]:

,c ¼ 0:33ðqt � ,voÞ ½kPa� ð3Þ

where qt = total cone tip resistance, ,vo = total overburdenpressure.

olidation pressure of undisturbed saturated clays, HBRC Journal (2015), http://



Table 1 Most important methods developed to define pre-consolidation pressure.

Year Method to define Pc Space Comments

1936 Casagrande method [1] e � log,0 Graphical, subjective, most commonly used

1955 Schmertmann method [4] e � log,0 Suitable for soft and stiff soil

1969 Janbu method [6] DH/H � ,0 Empirical, graphical construction

1970 Pacheco Silva method e � log,0 Graphical, easy to use, good results for soft soil, widely in Brazil

1979 Tavenas method [7] ,0 * DH/H � ,0 Regression analysis

1979 Butterfield method [5] log(1 + e) � log,0 Graphical, depends on critical state theory

1989 Jose method [8] loge � log,0 Fitted lines, regression analysis

1997 Van Zelst method DH/H � log,0 Regression analysis, depends on Tavenas method

2000 Senol and Saglamer

method [9]

,0 * DH/H � log,0 The abscissa of point of intersection between the

extensions of first and third lines represents ,c

0.65

0.75

0.85

0.95

1.05

1.15

1.25

1.35

1.45

1.55

1.65

10 100 1000 10000

Applied pressure kPa

Void

Rat

io

OCR=1, Wc=63%, D=15mOCR=1, Wc=57%, D=23mOCR=1, Wc=53%, D=30mOCR=1.2, Wc=60%, D=38m

Fig. 3 e � log ,0 curve for four samples at different depths form

a same borehole.

0.70

0.80

0.90

1.00

1.10

1.20

1.30

1.40

1.50

1.60

10 100 1000 10000

Applied pressure, kPa

Void

s ra

tio

e =0.42eo

Fig. 4 e � log ,0 curve for sample at depth 15 m.

Evaluation of pressure of undisturbed saturated clays 3

Mayne [18] reported at low over-consolidation ratio,OCRs < 2, the back analyses of failure case records involving

corrected shear vane strengths for embankments, footings, andexcavations, indicated that the relation between the mobilized

undrained shear strength and pre-consolidation pressure ,cmay be taken simply as:

,c � 4:5 su ð4Þ

where su = undrained shear strength.

The fourth formula related directly ,c with undrained shearstrength that was derived from CPT data. The constant valueof [4.5] included in the above formula is still debatable.

Experimental study

During the routine daily works of Housing and Building

National Research Center, undisturbed clayey samples havebeen collected from different locations around Egypt. Thepocket penetrometer is considered a main tool for testing the

cohesive samples in the field, besides it is always used duringsample classification in the laboratory. Hundreds of consol-

idation tests were performed and values of ,c were estimatedand compared with the pocket penetrometer readings.

Consolidation tests and sample disturbance

The consolidation test is generally considered the main test forestimating the soil properties related to settlement prediction.The settlement equation included three unknowns, void ratio,

compression index or recompression index and pre-

consolidation pressure ,c. As mentioned above and away fromconsolidation test, one can easily get the void ratio as soon asmeasuring the sample moisture content. The compressionindex can be estimated from simple soil properties and hence

the values of recompression index ranged between 10% and20% of that for compression index. Therefore, the bestestimation of settlement value depends mainly on the sound

choice between compression and recompression index thatdepends mainly on the value of pre-consolidation pressure.

Pocket penetrometer testing is believed to overcome the

difficulties that have been found during the analysis of theresults of consolidation tests. Although well-controlledundisturbed samples were achieved during drilling works atPort-Said city in Egypt, the disturbed behavior dominated

on the shape of e � log ,0 curves. Fig. 3 shows e � log ,0 curvesfor four samples from a borehole at different depths ranging


between 15, 23, 30 and 38 m. Although the samples havesimilar over consolidation ratios OCR and natural moisture

contents, they had different e � log ,0 curves. These differencesare attributed to sample disturbance, which cannot be noticed

or estimated by naked eye. Schmertmann method was used toestimate the disturbance and to correct the compressioncurves in order to match the in-situ stress history, as shown

in Figs. 4–7. The gap between the solid and dashed curves isconsidered to be as criterion for the disturbance, the morethe gap is the more the disturbance.

Although the samples shown in Figs. 4–6 have approxi-

mately same properties with stiffness soft to medium stiff, theyhave different disturbances. It can also be seen that the




0.65

0.75

0.85

0.95

1.05

1.15

1.25

1.35

1.45

1.55

1.65

10000100010010


Void

s ra

tio

e =0.42eo


0.70

0.80

0.90

1.00

1.10

1.20

1.30

1.40

1.50

10000100010010


Void

s ra

tio

e =0.42eo


0.80

0.90

1.00

1.10

1.20

1.30

1.40

1.50

1.60

1.70

10 100 1000 10000

Applied Pressure kPa

Void

Rat

io

e =0.42eo


0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

10 100 1000 10000

Applied pressure kPa

Void

Rat

io

e =0.42eo

OCR=3.1, Wc=33%, D=8m

Fig. 8 e � log,0 curve for stiff sample at depth 8 m extracted by

single core.

0.70

0.80

0.90

1.00

1.10

1.20

1.30

1.40

1.50

10 100 1000 10000Applied Pressure kPa

Void

Rat

io

e =0.42eo

OCR=1.3, Wc=52%, D=10m

Fig. 9 e � log,0 curve for soft to medium sample at depth 10 m

extracted by Shelby tube.


stiffness of the samples minimized to some extent the sampledisturbance as shown in Fig. 7 in which the sample is stiff clay.

Table 2 Quantification of sample disturbance, after Kontopoulos [

Specimen Quality Designation (S.Q.D.) (Terzaghi et al.) [12]

Vertical strain at ,0vo% S.Q.D. level

<1 A

1–2 B

2–4 C

4–8 D

>8 E


According to Sample Quality Designation, S.Q.D. devel-oped by some researchers [12], the rating of the four sampleswas poor to very poor, see Table 2. Sample Quality

Designation can be estimated by measuring the strain of sam-

ple at effective overburden pressure ,0vo and compares themwith the values in Table 2. According to S.Q.D., it was sug-gested that reliable estimate of engineering parameters of soilshould require samples with rating at least good to fair.

Samples with disturbance poor or more should not be trustedfor deriving soil-engineering properties.

The following Figs. 8 and 9 show a comparison between

two different samples from different locations indicating theeffect of soil stiffness on the sample disturbance. Fig. 8 shows

e � log ,0 curve for stiff cohesive soil at relatively shallow

3].

Criteria of De/eo at ,0vo (Lunne et al.) [19]

OCR= 1–2 OCR= 2–4 Rating

De/eo De/eo

<0.04 <0.03 Very good to excellent

0.04–0.07 0.03–0.05 Good to fair

0.07–0.14 0.05–0.10 Poor

>0.14 >0.10 Very poor

Worst




0

50

100

150

200

250

300

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16

∆e/eo

Pre-

cons

olid

atio

n pr

essu

re, k

Pa

Fig. 10 Effect of disturbance of sample De/eo on ,c (after

Kontopoulos [3]).

0

0.2

0.4

0.6

0.8

1

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16

∆e/eo

Vss /

Vs

CPT

U

Fig. 11 Relationship between Vss/Vs CPTU and disturbance of

sample De/eo (after Landon et al. [20]).

Plate 1

Plate 2


depth. Although this sample was extracted by single core of adrilling machine, which is not matching with the minimumrequirement of undisturbed sample, its S.Q.D. was good to fair.

Fig. 9 shows e � log ,0 curve for soft to medium cohesivesoil at relatively shallow depth. Although this sample was

extracted by Shelby tube of auger machine, its S.Q.D. waspoor to very poor.

The above discussion indicated that the disturbance of sam-

ples whether soft or stiff could not be easily avoided. This dis-turbance affects the engineering properties of soil especiallyvoid ratio, compression index and pre-consolidation pressure.

Kontopoulos [3] reported that the value of ,c obviouslydecreased with increasing the disturbance of sample, as shown

in Fig. 10.The disturbance of samples could be estimated through an

interesting nondestructive method achieved by Landon et al.[20]. They used bender elements to measure shear wave veloci-

ties, Vss for samples with different qualities and shear wavevelocities VsCPTU in the field by seismic piezocone at the samedepth. By comparing shear wave velocities for sample and that

for field, it could be seen that the ratio Vss/Vs CPTU decreasedwith increasing sample disturbance De/eo, as shown in Fig. 11.

Correlation between pocket penetrometer readings and pre-consolidation pressure

Pocket penetrometer is a very simple tool by which approxi-

mate unconfined compressive strength of cohesive soil could


be estimated at the field or at laboratory, as shown in Plate1. There are different shapes and models for the pocket, asshown in Plate 2.

The approximate unconfined compressive strength for eachsample was the average of at least three readings taken bypocket penetrometer. The readings were taken during perform-

ing the consolidation test. Table 3 shows samples of testresults, while Fig. 12 shows comparison between the readingsof pocket and the measured pre-consolidation pressures.

It can be seen that there is good agreement between thereadings of pocket and the values of pre-consolidation pres-sure. There are some notes in the Fig. 12 as follows:

Soft clay

Refer to the above discussion of the outcome results, the sam-ples of soft clay having pocket readings less that 50 kPa, have

been exposed to high degree of disturbance so that the pocketcan give false values to its bearing capacity. Therefore, pocketreadings cannot represent the pre-consolidation pressure forsoft clay.

Medium stiff clay

The unconfined compression strength of medium stiff clay

ranged between 50 and 100 kPa. It can be noticed fromFig. 12 that pocket readings indicated conservative values for




0

50

100

150

200

250

300

350

400

450

500

0 50 100 150 200 250 300 350 400 450 500

Pocket Readings, kN/m2

Prec

onso

lidat

ion

Pres

sure

, kN

/m2

Medium clayStiff clayVery stiff clay

Fig. 12 Comparison between pocket readings and measured pre-

consolidated pressure.

Table 3 Samples of values of pocket and pre-consolidation pressure at different locations in Egypt.

Location Depth (m) Pocket (kPa) ,c (kPa) Location Depth (m) Pocket (kPa) ,c (kPa)

Port-said 15 100 100 Middle of delta 8.0 180 200

23 70 Nona 6.0 150 200

29 100 180 10.0 150 150

18 80 100 12.0 60 70

38.5 400 350 6.0 40 80

21.5 80 90 8.0 40 60

37.5 200 300 10.0 200 200

Assiut 7.0 250 200 6.0 220 220

10.0 200 200 5.0 180 250

4.0 300 250 7.0 100 120

5.0 200 200 10.0 120 120

11.0 40 Nona 4.0 250 200

5.0 230 250 3.0 200 240

9.0 50 Nona 6.0 150 150

3.0 180 150 9.0 150 200

Giza 6.0 180 200 6.0 180 160

9.0 180 180 8.0 200 200

7.0 180 180 6.0 170 170

5.0 180 180 8.0 160 150

8.0 200 200 12.0 250 250

3.0 170 180 5.0 150 190

10.0 250 280 8.0 100 100

6.0 200 200 10.0 270 280

7.0 280 270 7.0 200 200

9.0 240 300 4.0 150 170

6.0 50 Nona 5.0 80 90

5.0 150 150 8.0 200 250

4.0 240 250 5.0 250 250

8.0 120 120 4.0 170 200

North of delta 12.0 40 100 11.0 320 320

26.0 40 Nona South of delta 4.0 200 250

10.0 80 80 6.0 230 240

12.0 60 60 9.0 250 250

a ,c cannot be easily estimated from e � log ,c curve.


,c, that could be attributed to the samples disturbance. Thiseffect can be neglected if the pocket readings multiply by a

constant equal to 1.2.


Stiff to very stiff clay

As the stiffness of clay increased, the disturbance of samplesdecreased so that the reading of the pocket can directly give

good estimation for the ,c.

Conclusions

Accurate values of pre-consolidation pressure could be esti-mated from the results of consolidation tests providing thata break point should be found between the initial and virgin

lines of e � log ,0 curve. The disturbance of samples whethersoft or stiff could not be easily avoided and hence affect the

engineering properties of soil especially void ratio, compres-sion index and pre-consolidation pressure so that the esti-

mated values of ,c usually include uncertainties. This paper

showed very simple prediction for ,c depending on pocketreadings. The pocket measures directly the unconfined com-pression strength of cohesive soil. For soft clay, it cannot

be relied on the readings of pocket for estimating ,c. For

medium stiff clay, ,c can be estimated after multiplying the

readings of pocket by a value of 1.2. The values of ,c for stiffto very stiff clay could be directly expressed by the pocketreadings.





These correlations between pocket readings and ,c could beconsidered the first step, and more studies are needed for dif-ferent types of clays.

Conflict of interest

The author declares that there are no conflict of interests.

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On the Evaluation of Pre-consolidation Pressure