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A Case Study on Estimating the Embankment Settlement from 2 Piezocone Penetration Test Data

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0 Abu-Farsakh, Pant, Gautreau, Yu, and Zhang
A Case Study on Estimating the Embankment Settlement from
1
Piezocone Penetration Test Data
2 Murad Y. Abu-Farsakh, Ph.D., P.E.
(Corresponding Author) 3 Research Associate Professor 4 Louisiana Transportation Research Center 5 Louisiana State University 6 4101 Gourrier Avenue 7 Baton Rouge, LA 70808 8 9 Rohit Pant 10 Former MS Graduate Student 11 Geotechnical Engineering, HNTB 12 Baton Rouge, LA 70809 13 14 Gavin Gautreau, P.E. 15 Geotechnical Engineering, LTRC 16 Louisiana Transportation Research Center 17 Louisiana State University 18 4101 Gourrier Avenue 19 Baton Rouge, LA 70808 20 21 Xinbao Yu, Ph.D. 22 Research Associate 23 Louisiana Transportation Research Center 24 Louisiana State University 25 4101 Gourrier Avenue 26 Baton Rouge, LA 70808 27 28 And 29 30 Zhongjie Zhang, Ph.D., P.E.
31 Geotechnical and Pavement Administrator 32 Louisiana Transportation Research Center 33 4101 Gourrier Avenue 34 Baton Rouge, LA 70808 35 36 Submitted to: 37 38 90
th
Transportation Research Board Annual Meeting 39 January 23-27, 2011 40 Washington, D.C. 41 42 43
TRB 2011 Annual Meeting
Paper revised from srcinal submittal.
1 Abu-Farsakh, Pant, Gautreau, Yu, and Zhang
A Case Study on Estimating the Embankment Settlement from
1
Piezocone Penetration Test Data
2
ABSTRACT
3 4 The in-situ piezocone penetration test (PCPT) has been widely used by the geotechnical 5 engineering community for subsurface soil identification and classification, and for the 6 evaluation of many engineering soil properties, such as the consolidation parameters. The 7 PCPT-derived consolidation properties can be used to estimate the magnitude and time 8 rate of consolidation settlement of loaded soils. This paper presents a case study on 9 implementing the PCPT technology to evaluate the embankment settlement at the Juban 10 Road – I12 Interchange Bridge in Louisiana. The soil underneath each embankment site 11 of the bridge was instrumented with a horizontal inclinometer and vertical magnet 12 extensometers. In each embankment site, PCPT tests were performed and the soundings 13 of cone tip resistance (
q
c
) were used to estimate the constrained modulus (
M
) profiles 14 using Abu-Farsakh et al. interpretation methods. Dissipation tests were also conducted at 15 specified penetration depths and used to estimate the vertical coefficient of consolidation 16 (
c
v
) using Teh and Houlsby interpretation method. Shelby tube soil samples were 17 collected and used to carry out a laboratory testing program to evaluate the consolidation 18 properties. The embankments’ consolidation settlements were monitored with time and 19 the field-measured values were compared with the magnitude and rate of settlements 20 estimated using parameters derived from PCPT data and laboratory consolidation tests. 21 The results of this study showed that the piezocone penetration and dissipation data 22 reasonably estimated the magnitude and rate of consolidation settlement of both 23 embankment sites. The back-calculated
M
and
c
v
parameters from field measurements are 24 in good agreement with PCPT-derived values. 25 26
KEY WORDS
: Piezocone Penetration Test (PCPT), Piezocone dissipation Test, 27 Horizontal inclinometer, Vertical magnet extensometer, Embankment settlement, 28 Constrained modulus, Coefficient of consolidation, Back-calculation. 29 30
TRB 2011 Annual Meeting
Paper revised from srcinal submittal.
2 Abu-Farsakh, Pant, Gautreau, Yu, and Zhang
INTRODUCTION
1 Saturated fine-grained soils, when loaded, can undergo large consolidation settlements 2 over a long period of time, which can pose potential damage to overlaying 3 infrastructures. The presence of this type of soil deposit is very common in southern 4 Louisiana. Therefore, the construction of embankments, bridge abutments, and other 5 structures on soft Louisiana soils requires a reasonable estimate of the magnitude and 6 time rate of consolidation settlement of the natural soil deposits in order to conduct a 7 rational and safe analysis and design of these structures. This requires better and more 8 accurate evaluation of the consolidation parameters of the subsurface soils. 9 The consolidation settlement of the soft soil underneath the embankment can cause 10 excessive differential settlement between the approach slab and bridge deck, creating 11 bump problems, faulting at the approach slab-pavement connection, and/or sudden 12 change in slope of the slab at the bridge deck. This can cause unsafe rideability, damage 13 to bridge decks, and costly frequent maintenance. In an attempt to solve this problem, 14 state Departments of Transportation (DOTs) usually preload the embankment site for a 15 certain period of time prior to the construction of approach slab and pavement. Additional 16 surcharge load and/or installation of vertical drains are sometimes used to accelerate the 17 settlement. The real challenge facing DOTs is to be able to reasonably estimate the 18 magnitude and time rate of consolidation settlement caused by embankment loading. 19 The consolidation properties of cohesive soils can be estimated from either 20 laboratory or in-situ tests. Laboratory tests, such as the one-dimensional Oedometer 21 consolidation tests, are conducted on small samples recovered from the site at different 22 depths. However, almost all recovered samples are subjected to certain degrees of 23 disturbance, which makes the laboratory-derived parameters not truly representative of 24 the actual in-situ conditions. Moreover, laboratory testing on small samples for 25 interbedded or fissured soils can be misleading. Profiling the consolidation characteristics 26 from laboratory tests on small samples taken from different depths can easily miss 27 significant thin drainage layers (1). 28 In-situ tests, such as the piezocone penetration tests (PCPT), can provide more 29 accurate and reliable results than laboratory tests in evaluating the actual strength and 30 consolidation properties of the soil under in-situ stress and drainage conditions. The 31 PCPT is a fast and economical in-situ test that can provide continuous soundings of 32 subsurface soil with depth. The piezocone penetrometer is capable of measuring the cone 33 tip resistance, q
c
, sleeve friction, f
s
, and pore pressures at different locations. These 34 measurements can be effectively used for soil identification and evaluation of different 35 soil properties such as the consolidation characteristics of soils. 36 The magnitude of consolidation settlement of cohesive soils can be estimated using 37 the deformation or constrained modulus (
M
), while the time rate of settlement is 38 estimated using the coefficients of consolidation (
c
v
or
c
h
). Different interpretation 39 methods have been proposed in the literature to estimate
M
from PCPT data (2, 3, 4, 5, 6, 40 and 7), derived based on direct correlation with the laboratory measured constrained 41 modulus. Many interpretation methods have also been developed to estimate the 42 horizontal coefficient of consolidation (
c
h
) of cohesive soils from analyzing the 43 piezocone dissipation curves (e.g., 8 through 13). Some of these methods were based on 44 estimating the time for 50% dissipation (
t
50
) (e.g., 8, 10, 12, and 13), some based on the 45 gradient of initial linear dissipation (e.g., 12), and others based on the rate of dissipation 46
TRB 2011 Annual Meeting
Paper revised from srcinal submittal.
3 Abu-Farsakh, Pant, Gautreau, Yu, and Zhang at a given dissipation level (e.g., 10). The rigidity of the soil (
I
r
) was included in some 1 methods (12, 13). The vertical coefficient of consolidation (
c
v
) can then be calculated 2 from
c
h
using the relationship suggested by Levadoux and Baligh (11), which is based on 3 the 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 consolidation 5 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 chemically 7 stabilized landfill. He reported reasonable comparison between the calculated settlement 8 from PCPT data and the measured settlement, while the time rates of settlement were 9 within ± 50% of the actual field measurements. Crawford and Campanella (15) compared 10 the measured settlements of earth embankment with settlements calculated from the 11 laboratory consolidation test, PCPT test, and dilatometer test. Their findings showed 12 good agreement among the three methods, but the actual settlement was about 60% 13 greater than the average calculated value. The calculated rates of settlement were also 14 compared with the observed values. Kuo-Hsia et al. (16) compared the PCPT predicted 15 settlement with the measured settlement of an instrumented test embankment. They 16 reported that the PCPT was the most valuable basis for evaluating the constrained moduli 17 and hence calculating the total settlement of soft soils. Abu-Farsakh et al. (7) compared 18 the magnitude and time rate of consolidation settlements estimated using PCPT data and 19 laboratory consolidation parameters with the field measurements at three different sites. 20 They demonstrated that the PCPT can be used to reasonably predict the consolidation 21 settlement better than the laboratory calculated settlements. 22 This paper presents a case study on evaluating the embankment settlements at the 23 Juban Road–I12 Interchange Bridge in Louisiana. The soils underneath the embankments 24 on both sides of the bridge were instrumented with horizontal inclinometer and vertical 25 extensometers. The embankments’ settlements were monitored with time, and the field 26 measurements were compared with the magnitude and rate of settlements estimated using 27 parameters derived using the PCPT data and laboratory consolidation test parameters. 28
ESTIMATING CONSOLIDATION SETTLEMENT FROM PCPT
29 The total magnitude of consolidation settlement (
S
c
) of cohesive soils can be estimated 30 utilizing the PCPT data through evaluating the constrained modulus (
M
) using the 31 following equation (3): 32
∑
∆=
iavic
M S
σ
i
H (1) 33 where H
i
is the thickness of soil layer i,
∆
σ
i
is the induced stress in mid of layer i,
M
avi
is 34 the average constrained modulus for stress range from
voi
σ
′
to
ivoi
σ σ
∆+′
estimated using 35 equation 2 as suggested by Senneset et al. (3): 36
voiivoiiavi
M M
σ σ σ
′∆+′=
2/
(2) 37 The time rate of consolidation can be estimated using the coefficients of 38 consolidation (
c
v
or
c
h
) that can be evaluated from analysis of the piezocone dissipation 39 test curves with time, as will be discussed in the following sections. 40
TRB 2011 Annual Meeting
Paper revised from srcinal submittal.

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