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Abu-Farsakh, Pant, Gautreau, Yu, and Zhang

A Case Study on Estimating the Embankment Settlement from1Piezocone Penetration Test Data2

Murad Y. Abu-Farsakh, Ph.D., P.E. (Corresponding Author)3Research Associate Professor4

Louisiana Transportation Research Center5Louisiana State University6

4101 Gourrier Avenue7Baton Rouge, LA 708088

9Rohit Pant10

Former MS Graduate Student11Geotechnical Engineering, HNTB12

Baton Rouge, LA 708091314

Gavin Gautreau, P.E.15Geotechnical Engineering, LTRC16

Louisiana Transportation Research Center17Louisiana State University18


21Xinbao Yu, Ph.D.22

Research Associate23Louisiana Transportation Research Center24

Louisiana State University254101 Gourrier Avenue26

Baton Rouge, LA 708082728

And2930

Zhongjie Zhang, Ph.D., P.E. 31Geotechnical and Pavement Administrator32Louisiana Transportation Research Center33


36Submitted to:37

38

90th

Transportation Research Board Annual Meeting39 January 23-27, 201140Washington, D.C.41

4243

TRB 2011 Annual Meeting Paper revised from original submittal.

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A Case Study on Estimating the Embankment Settlement from1Piezocone Penetration Test Data2

ABSTRACT34

The in-situ piezocone penetration test (PCPT) has been widely used by the geotechnical5engineering community for subsurface soil identification and classification, and for the6evaluation of many engineering soil properties, such as the consolidation parameters. The7PCPT-derived consolidation properties can be used to estimate the magnitude and time8rate of consolidation settlement of loaded soils. This paper presents a case study on9implementing the PCPT technology to evaluate the embankment settlement at the Juban10Road I12 Interchange Bridge in Louisiana. The soil underneath each embankment site11of the bridge was instrumented with a horizontal inclinometer and vertical magnet12extensometers. In each embankment site, PCPT tests were performed and the soundings13of cone tip resistance ( qc) were used to estimate the constrained modulus ( M ) profiles14using Abu-Farsakh et al. interpretation methods. Dissipation tests were also conducted at15

specified penetration depths and used to estimate the vertical coefficient of consolidation16 (cv) using Teh and Houlsby interpretation method. Shelby tube soil samples were17collected and used to carry out a laboratory testing program to evaluate the consolidation18

properties. The embankments consolidation settlements were monitored with time and19the field-measured values were compared with the magnitude and rate of settlements20estimated using parameters derived from PCPT data and laboratory consolidation tests.21The results of this study showed that the piezocone penetration and dissipation data22reasonably estimated the magnitude and rate of consolidation settlement of both23embankment sites. The back-calculated M and cv parameters from field measurements are24in good agreement with PCPT-derived values.25

26

KEY WORDS : Piezocone Penetration Test (PCPT), Piezocone dissipation Test,27Horizontal inclinometer, Vertical magnet extensometer, Embankment settlement,28Constrained modulus, Coefficient of consolidation, Back-calculation.29

30


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INTRODUCTION1

Saturated fine-grained soils, when loaded, can undergo large consolidation settlements2over a long period of time, which can pose potential damage to overlaying3infrastructures. The presence of this type of soil deposit is very common in southern4Louisiana. Therefore, the construction of embankments, bridge abutments, and other5structures on soft Louisiana soils requires a reasonable estimate of the magnitude and6time rate of consolidation settlement of the natural soil deposits in order to conduct a7rational and safe analysis and design of these structures. This requires better and more8accurate evaluation of the consolidation parameters of the subsurface soils.9

The consolidation settlement of the soft soil underneath the embankment can cause10excessive differential settlement between the approach slab and bridge deck, creating11

bump problems, faulting at the approach slab-pavement connection, and/or sudden12change in slope of the slab at the bridge deck. This can cause unsafe rideability, damage13to bridge decks, and costly frequent maintenance. In an attempt to solve this problem,14state Departments of Transportation (DOTs) usually preload the embankment site for a15certain period of time prior to the construction of approach slab and pavement. Additional16surcharge load and/or installation of vertical drains are sometimes used to accelerate the17settlement. The real challenge facing DOTs is to be able to reasonably estimate the18magnitude and time rate of consolidation settlement caused by embankment loading.19

The consolidation properties of cohesive soils can be estimated from either20laboratory or in-situ tests. Laboratory tests, such as the one-dimensional Oedometer21consolidation tests, are conducted on small samples recovered from the site at different22depths. However, almost all recovered samples are subjected to certain degrees of23disturbance, which makes the laboratory-derived parameters not truly representative of24the actual in-situ conditions. Moreover, laboratory testing on small samples for25interbedded or fissured soils can be misleading. Profiling the consolidation characteristics26from laboratory tests on small samples taken from different depths can easily miss27significant thin drainage layers (1).28

In-situ tests, such as the piezocone penetration tests (PCPT), can provide more29accurate and reliable results than laboratory tests in evaluating the actual strength and30consolidation properties of the soil under in-situ stress and drainage conditions. The31PCPT is a fast and economical in-situ test that can provide continuous soundings of32subsurface soil with depth. The piezocone penetrometer is capable of measuring the cone33tip resistance, q c, sleeve friction, f s, and pore pressures at different locations. These34measurements can be effectively used for soil identification and evaluation of different35soil properties such as the consolidation characteristics of soils.36

The magnitude of consolidation settlement of cohesive soils can be estimated using37the deformation or constrained modulus ( M ), while the time rate of settlement is38estimated using the coefficients of consolidation ( cv or ch). Different interpretation39methods have been proposed in the literature to estimate M from PCPT data (2, 3, 4, 5, 6,40and 7), derived based on direct correlation with the laboratory measured constrained41modulus. Many interpretation methods have also been developed to estimate the42horizontal coefficient of consolidation ( ch) of cohesive soils from analyzing the43

piezocone dissipation curves (e.g., 8 through 13). Some of these methods were based on44estimating the time for 50% dissipation ( t 50) (e.g., 8, 10, 12, and 13), some based on the45gradient of initial linear dissipation (e.g., 12), and others based on the rate of dissipation46


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at a given dissipation level (e.g., 10). The rigidity of the soil ( I r ) was included in some1methods (12, 13). The vertical coefficient of consolidation ( cv) can then be calculated2from ch using the relationship suggested by Levadoux and Baligh (11), which is based on3the ratio of vertical to horizontal coefficients of hydraulic conductivity ( k v /k h) of the soils.4

Several case studies were reported in the literature to estimate the consolidation5

settlement of subsurface soils using parameters derived from the PCPT data (e.g., 6, 13,6 14, and 15). Oakley (14) used the PCPT data to calculate the settlement of a chemically7stabilized landfill. He reported reasonable comparison between the calculated settlement8from PCPT data and the measured settlement, while the time rates of settlement were9within 50% of the actual field measurements. Crawford and Campanella (15) compared10the measured settlements of earth embankment with settlements calculated from the11laboratory consolidation test, PCPT test, and dilatometer test. Their findings showed12good agreement among the three methods, but the actual settlement was about 60%13greater than the average calculated value. The calculated rates of settlement were also14compared with the observed values. Kuo-Hsia et al. (16) compared the PCPT predicted15settlement with the measured settlement of an instrumented test embankment. They16

reported that the PCPT was the most valuable basis for evaluating the constrained moduli17 and hence calculating the total settlement of soft soils. Abu-Farsakh et al. (7) compared18the magnitude and time rate of consolidation settlements estimated using PCPT data and19laboratory consolidation parameters with the field measurements at three different sites.20They demonstrated that the PCPT can be used to reasonably predict the consolidation21settlement better than the laboratory calculated settlements.22

This paper presents a case study on evaluating the embankment settlements at the23Juban RoadI12 Interchange Bridge in Louisiana. The soils underneath the embankments24on both sides of the bridge were instrumented with horizontal inclinometer and vertical25extensometers. The embankments settlements were monitored with time, and the field26measurements were compared with the magnitude and rate of settlements estimated using27

parameters derived using the PCPT data and laboratory consolidation test parameters.28

ESTIMATING CONSOLIDATION SETTLEMENT FROM PCPT29

The total magnitude of consolidation settlement ( S c) of cohesive soils can be estimated30utilizing the PCPT data through evaluating the constrained modulus ( M ) using the31following equation (3):32

=iav

ic M

S

iH (1)33

where H i is the thickness of soil layer i, i is the induced stress in mid of layer i, M avi is34the average constrained modulus for stress range from voi to ivoi + estimated using35

equation 2 as suggested by Senneset et al. (3):36

voi

ivoiiavi M M

+=

2/ (2)37

The time rate of consolidation can be estimated using the coefficients of38consolidation ( cv or ch) that can be evaluated from analysis of the piezocone dissipation39test curves with time, as will be discussed in the following sections.40


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Interpretation of Constrained Modulus1

The compressibility of the soils can be expressed by the constrained modulus ( M ) that2can be evaluated from PCPT. Several correlations have been proposed to estimate the3constrained modulus ( M ) from either the cone tip resistance ( qc) or the corrected cone tip4resistance ( qt ). The corrected cone tip resistance ( qt ) is given by:5

qt = qc + u2 (1- a) (3)6

where u 2 is the pore water pressure measured behind the base, and a is the cone area ratio,7equal to 0.59 for both the 10 and 15 cm 2 piezocones used in this study. The general8relationship between ( qc or qt ) and M can be expressed as follows:9

M = . qc or M = . qt (4)10

Sanglerat (2) proposed a correlation between qc and M. He presented a11comprehensive array of values for different soil types with different qc values. Jones12and Rust (5) found that for South African alluvial clay, a value of = 2.75 0.55 can13

provide good correlation between M and qc. Senneset et al. (3) conducted correlation14 between M and qt . For clayey soils, they related the M modulus by a linear interpretation15of the net cone tip resistance ( qn = q t - vo). They proposed the following relation:16

M = . qn = . (qt - vo) (5)17

where = 105 for the pre-consolidation range and = 62 for the normally consolidated18range, and vo is the total overburden stress. Kulhawy and Mayne (4) also studied the19relationship between M and qn and found a good correlation with = 8.25.20

Even though these relations might correlate well in some cases, local experience is21still essential to develop a better correlation between qt and M that can reflect the local22soil types and variations. To examine the possibility for obtaining better correlations, a23comprehensive study (6) was conducted on data collected from seven sites to reasonably24estimate the M values needed to calculate the consolidation settlement of cohesive soils25in Louisiana. In this study, the correlations were developed based on comparison between26the PCPT data and the reference M values obtained from the laboratory consolidation27tests. The following linear correlation was obtained between M and qt :28

M = 3.15 qt (R 2 = 0.91) (6)29

and the following linear correlation was also obtained between M and ( qn):30

M = 3.58 qn = 3.58 ( qt vo) (R 2 = 0.88) (7)31

The latest two correlations (equations 6 and 7) will be used in this study to calculate32the consolidation settlement of Juban Road embankments from PCPT data.33

Interpretation of Coefficients of Consolidation34

The coefficients of consolidation ( cv or ch) that is used to evaluate the rate of35consolidation settlement for cohesive soils can be estimated from the piezocone36dissipation test curves. The PCPT dissipation test consists of stopping the cone37

penetration at pre-specified depths and recording the dissipation of excess pore pressure38( u) with time.39

Several interpretation methods were developed to evaluate the horizontal coefficient40of consolidation ( ch) of cohesive soils from analyzing the piezocone dissipation test data41


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curves, based on cavity expansion theories (e.g., 8, 9, 10), strain path method (11), and1the combination of the strain path method with the finite element technique (13). These2interpretation methods can be found in the corresponding references.3

The most well-known interpretation method for estimating ch utilizing the piezocone4dissipation test was developed by Teh and Houlsby (13). They proposed the following5

equation to estimate ch(piezo) :650

2*50 /)()( t I r T piezoc r oh = (8)7

where *50T is a modified time factor at 50% dissipation (*

50T = 0.118 for the u 1 piezocone8and 0.245 for the u 2 piezocone), I r = G/s u is the rigidity index, G is the shear modulus,9and su is the undrained shear strength.. The shear modulus at 50% of yield stress ( G50) is10usually used, which represents an average value of stress levels.11

Since the dissipation of pore pressure occurs during the recompression range rather12than in the normal consolidation range, Levadoux and Baligh (11) suggested that the13

predicted ch(piezo) = c h(overconsolidated) and they proposed the following relation to14transfer ch(piezo) to the normally consolidated condition ch(NC) :15

(piezo)cCR RRc hh(NC) )/(= (9)16

where RR and CR are the recompression and compression ratios, respectively. The17vertical coefficient of consolidation ( cv) can then be calculated using the ratio of vertical18to horizontal coefficients of hydraulic conductivity ( k v /k h) using the following expression19suggested by Levadoux and Baligh (11):20

h(NC)hvv(NC) ck k c )/(= (10)21

In this study, Teh and Houlsby (11) method was used to evaluate the ch values of the22soil layers. In this method, the proper estimation of ch depends on the selection of an23appropriate value of the I r = G/s u index, and hence the G and su values. In this study, the su 24values were estimated from the PCPT- q t data. The k o-anisotropic consolidated undrained25(Ck oU ) triaxial tests were conducted on selected Shelby tube samples obtained from soil26

borings to estimate the G values for the soil layers. The average su value corresponding to27the same depth the sample was taken for triaxial test was used to calculate the I r value28that represents each soil layer.29

LABORATORY TESTING PROGRAM30

Boreholes were drilled in each embankment site and high quality 7.6 cm (3 in) standard31Shelby tube samples were recovered at different depths for comprehensive laboratory32testing. The laboratory testing program included basic soil characterization tests such as33water content, unit weight, Atterberg limits, grain size distribution, and specific gravity.34Computerized automatic one-dimensional consolidation tests were also conducted to35evaluate the constrained modulus ( M ), the coefficients of consolidation ( cv and ch), the36OCR ratio, and the compression indices ( cc and c r ). Unconfined compression tests and k o-37consolidated undrained triaxial tests (C k oU) were also performed to estimate the38undrained shear strength ( su) and the shear modulus ( G) of the soils.39

40


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IN-SITU TESTING PROGRAM1

The in-situ testing program included performing both piezocone penetration tests (PCPT)2and piezocone dissipation tests. The PCPT tests were performed in each embankment3using the 10 and 15 cm 2, 60 o piezocone penetrometers. All PCPT tests were conducted at4a penetration rate of 2 cm/sec. The 10 cm 2 piezocone provided measurements of the cone5tip resistance ( qc), sleeve friction ( f s), and porewater pressure behind the base ( u2). While6the 15 cm 2 piezocone provided measurements of qc, f s, and porewater pressure at the cone7tip ( u1). The profile of the PCPT tests was used to classify the soil using the probabilistic8region estimation method (17), evaluate the undrained shear strength ( su), and evaluate9the constrained modulus ( M ) using Abu-Farsakh et al. (6, 7) interpretation methods.10

The penetration of the piezocone was stopped at specified penetration depths to11 perform dissipation tests with respect to time. The dissipation test curves were then used12to estimate the horizontal and vertical coefficients of consolidation ( ch and cv) based on13the Teh and Houlsby (11) interpretation method.14

DESCRIPTION OF EMBANKMENT SITES AND MONITORING PROGRAM15

The site includes both ramp embankments constructed for the Juban Road Interchange16Bridge at Interstate I-12, located east of Baton Rouge in Livingston Parish. This includes17the construction of two embankment approaches to the bridge, the north embankment,18and the south embankment. Figure 1 presents a typical roadway cross section of the19Juban Road embankments, which has a top width of 30.5 m (100 ft) and a varied bottom20width depending on the location from bridge end. The embankments on both sides of21

bridge were instrumented with horizontal inclinometers and vertical extensometers to22monitor the consolidation settlement with time.23

24Figure 1. Typical roadway embankment section at Juban Road I-12 Interchange25

The north embankment has a top width of 30.5 m (100 ft), an average bottom width26of 96.3 m (316 ft), and an average height of 8.96 m (29.4 ft). The south embankment has27a top width of 30.5 m (100 ft), an average bottom width of 83.52 m (274 ft) and an28average height of 8.08 m (26.5 ft). A surcharge height of 0.91 m (3 ft) and wick drains29with a 1.83-m (6-ft) triangular spacing and 12.5 m (41 ft) depth were used to accelerate30the consolidation settlement. The construction of embankments fill including surcharge31was completed after 180 and 140 days for the north and south embankments,32respectively. The surcharge on both embankments was maintained for six months. It33

30.5 m (100 ft)

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should be noted here that the instrumentations used for measuring the embankments1settlement were installed at around the average dimensions of the embankments.2

Two boreholes were drilled on each embankment site and high Shelby tube samples3were recovered for laboratory testing. The subsurface soil stratigraphy, as revealed from4the soil borings, showed that the site consists of a top soil layer of grey to brown lean5

clay down to about 11 to 12 m with occasional traces of organics and/or sand. The Soil6 below that consists mainly of silty and clayey sand interbedded with silty-clay layers7down to about 17 m at the south embankment site and 20 m at the north embankment site.8A layer of brown to grey stiff clay and silty clay down to 20 m exists at the south9embankment site. The groundwater level was about 2 m below the ground surface.10

The results of laboratory tests on samples taken from the soil borings showed that the11natural water content was close to the plastic limit with a mean value of 25%. The12undrained shear strength, s u, varied from 17 kPa to 177.5 kPa for the north embankment13site and from 40 kPa to 137 kPa for the south embankment site. The vertical coefficient14of consolidation ( cv) was in the range of 1.36 x 10 -4 cm 2/sec to 9.6 x 10 -3 cm 2/sec. High15OCR (20 to 22) was observed in the top layer down to the depth of 2 m. The description16of the soil profile for the north embankment showing soil log, Atterberg limits, undrained17shear strengths, constrained modulus, coefficient of consolidation, and OCR are18

presented in Figure 2. The soil profile and properties at the south embankment are close.19Four PCPT tests were conducted on the north embankment site and three PCPT tests20

were conducted on the south embankment site, prior to the construction, down to 20 m21using either u1 or u2 measurements. The profiles of PCPT test results and the22corresponding CPT soil classification (17) for the north embankments are presented in23Figure 3. The PCPT data at the south embankment site are close. The CPT soil24classification indicates that the soil profile consists of silty clay soils down to about 10.525m interbeded with thin sand layers. Two PCPT tests (with u1 measurement) were selected26to conduct dissipation tests on each embankment site. The depths of dissipation tests27were: 2.13, 4.02, 6.04, 7.80, 7.91, 9.83, 10.81, and 11.01 m below the ground surface for28the north embankment; and 2.13, 4.05, 6.03, 8.09, and 10.01 m for the south29embankment. Figure 4 depicts the results of dissipation tests.30

One horizontal inclinometer was installed in each embankment at a selected location31to monitor the profile of consolidation settlement with time of the soil deposit underneath32each embankment. In this study, a digital horizontal inclinometer system manufactured33

by RST Instruments Ltd. was used, which consists of inclinometer casing, a horizontal34 probe, control cable, pull cable, and a readout unit. A 0.61 m wide 0.61 m deep trench35was first excavated across each embankments width prior to the placement of any36embankment fill. The inclinometer casing of 85 mm (3.34 in) diameter and the return37

pipe were then installed along the trench (Figure 5). The trench was then filled with sand38and compacted by the contractor. Each end of the inclinometer casing was attached to39two wooden posts to secure its position. The settlement survey was conducted by drawing40the probe from one end of the casing to the other, halted in its travel at 2 ft intervals for41inclination measurements. The elevations of posts relative to a reference fixed point were42measured every time using a survey level device. The first survey was conducted after43sand compaction to establish the initial profile of the casing (as baseline survey). The44subsequent surveys revealed the changes in the profile due to embankment settlement.45


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RESULTFS AND ANALYSIS1

Comparison between PCPT and Laboratory Derived Parameters2

The profiles of PCPT soundings with depth and the results of piezocone dissipation tests3were used to calculate the consolidation parameters: constrained modulus ( M ) and4

coefficients of consolidation ( ch and cv) of subsurface soils for each embankment site.5

Constrained Modulus6The profiles of constrained modulus (M) was calculated from PCPT using Abu-Farsakh7et al. (6, 7) methods based on corrected qt or net qn cone tip resistance (equations 6 and87). The average values of total overburden pressure ( vo) needed for each soil layer to9compute the qn were estimated from the soil borings. The comparison of predicted M 10values from PCPT versus the laboratory measured M are presented in Figure 6 for both11north and south embankment sites. It is evident from the figure that the PCPT- M values12for both sites are greater than the laboratory estimated M values. Comparison with back-13calculated values from field measurements for the south embankment site will be14

presented later.15

Coefficient of Consolidation16The horizontal coefficients of consolidation ( ch) for the subsurface soils were calculated17from the piezocone dissipation tests presented in Figure 4 using the Teh and Houlsby (13)18interpretation method (equations 8 and 9). This method requires the evaluation of t 50 from19the dissipation test curves. The vertical coefficients of consolidation (c v) can then be20calculated from ch using the relationship proposed by Levadoux and Baligh (11) in21equation 10, which is based on the k v /k h ratio. However, in this study the cv /c h ratio was22estimated to be about 0.56 from the results of one-dimensional consolidation tests23conducted on samples oriented both vertically and horizontally. By investigating the24figures, the reader can distinguish two different types of dissipation curves: Monotonic25dissipation curves (A Type I) and dilatory dissipation curves (Type III) (18, 19). Type I26curve shows a gradual decrease of excess pore pressure with time, and is usually obtained27in normally and lightly over-consolidated soils (19). The Type III curve is usually28obtained for pore pressure measurements behind the tip ( u2 and u3) in overconsolidated29soils. This is mainly due to the dilatory behavior of overconsolidated soils (18), and30

partially due to the redistribution of excess pore pressure around the cone at early stages31of dissipation before it dissipates to the surrounding media. Sully et al. (19) evaluated the32Type III dissipation curve and suggested applying certain correction by evaluating the33time t p at peak as the new zero time and the corresponding pore pressure is taken as the34

peak initial excess pore pressure for the dissipation curve. This approach was used in this35

study. To calculate the rigidity index ( I r = G/s u), the undrained shear strength was36estimated from the cone tip resistance data using the cone tip factor, N k = 15, and the37shear modulus ( G) was determined from Ck oU triaxial tests.38

The plots of piezocone estimated versus laboratory calculated cv values are presented39in Figure 7 for the Juban road embankments. Although the figure do not show good40correlation between the PCPT and laboratory cv values, it is obvious that the subsurface41cohesive soil at Juban road site has a cv value ranges from 210 -2 to 410 -3 cm 2/sec, with42an average of 110 -3 cm 2/sec.43


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0 5 10 15 20 25 30 35 40Constrained Modulu s (MPa)

0

2

4

6

8

10

12

14

16

18

20

D e p

t h ( m )

0 50 100 150 200 250 300 350 400

Constrained Modulus (TSF)

Constraints Modulus (M)M = 3.58 (q t- vo )

Measured

60

50

40

30

20

10

0

SANDY LAYER

SANDY LAYER

0 5 10 15 20 25 30 35 40

Constrained Modulus (MPa)

0

2

4

6

8

10

12

14

16

18

20

Constraints Modulus (M)M = 3.15 q tMeasured

SANDY LAYER

SANDY LAYER

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D e p

t h ( f t )

1Figure 6a: PCPT versus laboratory measured profiles of M (Juban North)2

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Constraints Modulus (M)M = 3.58 (q t- vo )

Measured

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3Figure 6b: PCPT versus laboratory measured profiles of M (Juban South)4


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E q u i t

y L i n e

10 -5 10 -4 10 -3 10 -2 10 -1

Laboratory c v (cm 2/sec)

10 -5

10 -4

10 -3

10 -2

10 -1

P C P T c v

( c m

2 / s e c

)

North EmbankmentSouth Embankment

1Figure 7. Measured versus predicted c v for Juban Road Site2

Comparison with Horizontal Inclinometer Measurements3

The first horizontal inclinometer survey was conducted after compaction of the sand layer4inside the trench, before placing any embankment lift, to establish the initial baseline5

profile. The subsequent survey measurements taken at different times were used to6calculate the settlement profiles of the soil underneath the embankment. The settlement7

profiles were also calculated using the PCPT interpretation method (equation 1), as well8as using the laboratory-derived consolidation parameters. The subsurface soil properties9and the results of in-situ PCPT and dissipation tests were presented earlier. The applied10stress ( ) used to calculate the magnitude of settlement is due to embankment loading11

plus surcharge, which increased with construction time until it reached maximum height.12Due to the installation of wick drains, the excess pore water pressure drained both13vertically and radially, with the radial consolidation governs most of the settlement. In14this case, the average degree of consolidation (U) is given as follows:15

U = 1 - (1 U v)(1 U r ) (11)16

where, U v, U r are the average degree of consolidation due to vertical and radial (or17horizontal) drainage, respectively. In this study, the radial consolidation was estimated to18contribute to about 85%-88% of the total consolidation settlement of the embankments.19

The settlement profiles along the width of the embankment calculated using the20PCPT interpretation method, laboratory-estimated parameters, and the settlement21measured using the horizontal inclinometer are presented in Figures 8a and 8b for north22and south embankments, respectively. The comparison in the figures was made at a time23corresponds to about 95% of the estimated consolidation. For the north embankment site,24the PCPT over-estimated the measured (actual) settlement by about 12% and the25laboratory parameters under-estimated the measured settlement by about 13%. However,26for the south embankment site, the PCPT and the laboratory parameters over-estimated27the field measurements by about 50% and 20%, respectively. This is in agreement with28


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findings from previous studies (7, 13, 14, and 15), which demonstrated the difficulty of1 predicting the actual field settlements using either PCPT or laboratory derived2 parameters. However, being able to estimate the magnitude of settlement using the PCPT3data within the same range as the laboratory estimate will have a potential benefit in4speeding up the construction and avoiding delays. 5

6(a) North Embankment7

8(b) South Embankment9

Figure 8. Comparison of predicted settlement profiles with field measurements10The consolidation settlements with time predicted from the laboratory parameters11

and PCPT dissipation tests using the Teh and Houlsby (1988) interpretation method are12 presented in Figures 9a and 9b for the north and south embankments, respectively. Figure139a demonstrates that, for the north embankment site, both the PCPT and laboratory14estimated rates of consolidation (slope of the curve) match fairly well with the field15

0 40 80 120 160 200 240 280 320 360Distance (ft)

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t t l e m e n

t ( c m

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H = 32.4 ft

Settlements at 340 days

HI Measurements

Lab Estimate

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t ( I n c

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0246810121416182022

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H = 29.5 ft

Settlements at 260 days

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Lab Estimate

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monitoring. However, for the south embankment site, the rate of consolidation estimated1from the PCPT is lower than field monitoring, but a little better than the laboratory2estimated rate of consolidation. It is worth mentioning that geotechnical engineers are3generally more interested in estimating the rate of embankment settlement than the4magnitude of settlement for better planning the extent of the preloading period needed to5

overcome the majority of settlement.6

7North Embankment8

9(b) South Embankment10

Figure 9. Rate of consolidation settlement11

Back-calculation of Consolidation Parameters from Vertical Extensometer12

Measurements from the vertical magnet extensometer were used to back-calculate the13consolidation parameters ( M and cv) of the subsurface soil layers for the south14embankment site. As mentioned earlier, the vertical extensometer for the north site was15damaged during construction. By recording the relative movement of spider magnets, the16corresponding vertical settlement of each layer was calculated for each incremental stress17

0 30 60 90 120 150 180 210 240 270 300 330 360Days

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t ( i n c

h e s

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Settlement with Time

HI Measurements

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( i). The end of primary consolidation was estimated using the rectangular hyperbola1curve fitting method (RHM) as proposed by Sridharan et al. (20). The total consolidation2settlement for each layer was used to back-calculate its constrained modulus ( M ) and the3results are presented in Figure 10a. The figure also compares the PCPT, laboratory, and4

back-calculated M values with depth, which shows the PCPT estimated M values are in5

good agreement with back-calculated values. Statistical analysis performed on the6 collected data showed that values [M= 1 q t, and M= 2 (q t vo)] for the south7embankment site has means of 3.01 (compared to 3.15 in equation 6) and 3.16 (compared8to 3.58 in equation 7) and standard deviations of 0.59 and 0.65. Figure 10b presents the9comparison of back-calculated cv with PCPT predicted cv using Teh and Houlsby (1999)10and laboratory measured cv values. As seen in figure, there is scattering but most of the11values fall within a narrow band of order of magnitude of one log cycle. Most12importantly, the parameters predicted using the PCPT measurements are fairly well13within the range of the average measured coefficient of consolidation.14

15(a) Constrained modulus (b) Coefficient of consolidation16

Figure 10. Comparison between PCPT, laboratory and back-calculated constrained17modulus and coefficient of consolidation values18

CONCLUSIONS19This paper presents a case study on evaluating the embankment settlement at the Juban20Road I-12 Interchange Bridge in Louisiana. The soil underneath each embankment was21instrumented with a horizontal inclinometer and vertical magnet extensometers to22monitor the settlement with time. Piezocone penetration and dissipation tests were used23to calculate the consolidation parameters ( M and cv), which were then used to evaluate24the magnitude and time rate of embankment settlements. Laboratory tests were also25conducted to estimate the consolidation properties of soils from borings. Settlement26

0 2 4 6 8 10


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D e p

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Constraints Modulus (M)Back-calculatedLab Measured

M = 3.58(q t- vo )

M = 3.15 q t36

32

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1E-004 1E-003 1E-002 1E-001 1E+000

Coefficient of consoli dation C v (cm 2/sec)

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Cv (cm2/sec)

Back-CalculatedLab MeasuredTeh and Houlsby

1.0E-004 1.0E-003 1.0E-002 1.0E-001

Coefficient of consolidation C v (in 2/sec)

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calculated using PCPT and laboratory derived consolidation parameters were compared1with actual field measurements. Based on this study, the following conclusions can be2drawn:3 Actual field measurements from horizontal inclinometers were compared with the4

consolidation settlements estimated using the PCPT and laboratory derived5

parameters. The results, in general, showed that the piezocone penetration and6dissipation data can be used to estimate the magnitude and time rate of consolidation7settlement of embankments within the same range as of the laboratory calculations.8

The settlement predictions based on PCPT derived constrained modulus ( M ) using the9Abu-farsakh et al. (6, 7) method reasonably estimated the magnitude of consolidation10settlement of Juban Road embankments within the same range as of the laboratory11calculations. Performing the PCPT tests is much faster compared to the time-12consuming of sampling and subsequent laboratory testing of soil samples, thus the13use of PCPT will help in speeding up the field construction.14

The PCPT estimation of rate of consolidation with time from dissipation tests using15the Teh and Houlsby (13) interpretation method matched fairly well the field16monitoring of the north embankment site, while it under-estimated the rate of17consolidation of the south embankment site. However, the PCPT gave a little better18estimate of the rate of consolidation than the laboratory estimations.19

Back-calculated constrained modulus from vertical magnet extensometers were20compared with PCPT and laboratory derived M . The results showed that the PCPT21estimated M values are in good agreement with back-calculated values. The22laboratory estimated M values are lower.23

Comparison of the cv values estimated from dissipation tests using the Teh and24Houlsby (13) method with the back-calculated and laboratory measured cv values25showed reasonable agreement among all and that most cv values fall within a narrow26

range of an order of magnitude of one log cycle.27

ACKNOWLEDGMENT28

This research project is funded by the Louisiana Department of Transportation andDevelopment (State Project No. 736-00-0781) and the Louisiana Transportation ResearchCenter (LTRC Project No. 00-3GT). The comments and suggestions of Mark Morvant,Associate Director of Research at LTRC, are gratefully acknowledged.

REFERENCES29

1. Baligh, M. M., and Levadoux, J. N. Consolidation after Undrained Piezocone30Penetration. II: Interpretation, Journal of Geotechnical Engineering, ASCE , Vol.31112(7), 1986, pp. 727-745.32

2. Sanglerat, G. The Penetration and Soil exploration. Elsevier , Amsterdam, 1972, 46433 pp.34

3. Senneset, K., Sandven, R., and Janbu, N. The Evaluation of Soil Parameters from35Piezocone Tests, Transportation Research Record , No. 1235, 1989, pp. 24-37.36

4. Kulhawy, F. H. and Mayne, P. H. Manual on Estimating Soil Properties for37Foundation Design, Electric Power Research Institute, EPRI. 1990.38


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5. Jones, G. A., and Rust, E. Piezocone Settlement Prediction Parameters for1Embankments on Alluvium, Proceedings of the International Symposium on2

Penetration Testing, CPT 95 , Linkoping, Sweden, Vol. 2, 1995, pp. 501-508.36. Abu-Farsakh M. Y., Evaluation of Consolidation Characteristics of Cohesive Soils4

from Piezocone Penetration Tests , Report No. FHWA/LA.04/386, Louisiana5

Transportation Research Center, Baton Rouge, LA, 2004, 106 p.6 7. Abu-Farsakh, M. Y., Zhange, Z., and Gautreau, G., Evaluating the Deformation7Modulus of Cohesive Soils from PCPT for Consolidation Settlement Estimation, 8

Journal of the Transportation Research Board , No. 2004, Soil Mechanics, 2007, pp.949 - 59.10

8. Vesic, A. S. Expansion of Cavities in Infinite Soil Mass, Journal of Soil11 Mechanics , ASCE, Vol. 98, No. 3, 1972, pp. 265-290.12

9. Torstensson, B. A. The Pore Pressure Probe, Paper 34, Geotechnical Meeting ,13 Norwegian Geotechnical Society, Oslo, Norway, 1977, pp. 34.1-34.15.14

10. Senneset, K., Janbu, K. and Svano, G. Strength and Deformation Parameters from15Cone Penetration Tests, Proc. 2 nd European Symp. on Penetration Testing, ESOPT16

II , Amsterdam, The Netherlands, Vol. 2, 1982, pp. 863-870.17 11. Levadoux J. N, and Baligh, M. M. Consolidation after Undrained Piezocone18Penetration. II: Prediction. Journal of Geotechnical Engineering, ASCE , Vol.19112(7), 1986, pp. 707-726.20

12. Teh, C. I. An analytical Study of the Cone Penetration Test, D.Phil. Thesis, Oxford21University, 1987.22

13. Teh, C. I. and Houlsby, G. T. An Analytical Study of the Cone Penetration Test in23Clay, Geotechnique , Vol. 41, No. 1, 1991, pp. 17-34.24

14. Oakley, III, Richard E. Case History: Use of the Cone Penetrometer to Calculate the25Settlement of a Chemically Stabilized Landfill, Geotechnics of Waste Fills-Theory26and Practice , ASTM STP 1070, American Society for Testing and Materials,27Philadelphia, 1990, pp. 345 357.28

15. Crawford C. B. and Campanella R. G. Comparison of Field Consolidation with29Laboratory and In situ Tests, Canadian Geotechnical Journal , Vol. 28, 1991, pp.30103-112.31

16. Kuo-Hsia C., William D. K., and Ming-Jiun W. Comparison of Predicted and32Measured Settlement of a Test Embankment over Soft Soil, ASCE Geotechnical33Special Publication No. 40. Vertical and Horizontal Deformations of Foundations34and Embankments, Proceedings of Settlement '94 , College Station, Texas, 1994.35

17. Zhang, Z. and Tumay, M. T. Statistical to Fuzzy Approach Toward CPT Soil36Classification, Journal of Geotechnical and Environmental Engineering, ASCE , Vol.37125, No. 3, 1999, pp. 179-186.38

18. Burns, S. E., and Mayne, P. W., Monotonic and Dilatory Pore Pressure Decay39during Piezocone Tests in Clay, Canadian Geotechnical Journal , Vol. 35, No. 6,401998, pp. 1063 1073.41

19. Sully, J. P., Robertson, P. K., Campanella, R. G., and Woeller, D. J. An Approach to42Evaluation of Field CPTU Dissipation Data in Overconsolidated Fine-grained Soils,43Canadian Geotechnical Journal , Vol. 36, No. 2, 1999, pp. 369-381.44

20. Sridharan, A. and Sreepada Rao, A. Rectangular hyperbola fitting method for one-45dimensional consolidation, Geotechnical Testing Journal, 4(4), 1981, pp. 161168.46

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