chapter 274. simplified risk analysis chart to prevent slope failure of highway embankment on soft...

Simplified risk analysis chart to prevent slope failure of highwayembankment on soft Bangkok clays

A. Sawatparnich & J. SunitsakulBureau of Road Research and Development, Department of Highways, Ministry of Transport, Thailand

ABSTRACT: The soft Bangkok clay has been well known for high water content, low shear strength, highcompressibility, and high sensitivity to geotechnical engineers for several decades. Due to high compressibilityof the Soft Bangkok clay, the Prefabricated Vertical Drain (PVD) with preload embankment is introduced toaccelerate the primary consolidation settlement during the construction of the new Bangkok-Chonburi highway.During preloading, embankment failures and surface cracks occurred on several sections. To investigate theslope failures, the deterministic and probabilistic approaches are performed to determine the overall factor ofsafety and their associated probability of failure (Pf ). It is found that the overall factor of safety which is normallyused to check the stability of the slopes may not be the only key parameter for slope stability analysis especiallywhere wide range of estimated engineering properties is found. Based on reliability analysis, the so-called ‘‘riskanalysis chart for slope stability analysis’’ is proposed to prevent potential slope failure of embankment on softBangkok clays. The use of the proposed risk analysis chart is also presented.

1 INTRODUCTION

The soft Bangkok clay has been well known forhigh water content, low shear strength, and highcompressibility, to geotechnical engineers for severaldecades. For constructing highway embankment onthe soft Bangkok clay, the main geotechnical concernsare excess settlement and potential stability failure.Thus, Department of Highways (DOH), Thailand,applies several soil improvement techniques, deepmixing cement column and preload with and with-out vertical drain, in order to increase their highwayserviceability and lower their maintenance costs.

Tourist attractions, seaports, and industrial estatesare located in the east of Thailand. With the secondinternational airport as part of national logistics, thedemand for highways is increased dramatically in thearea. Thus, Department of Highway (DOH), Thailand,proposed the construction of the new Bangkok-Chonburi highway to connect the Bangkok and easternpart of Thailand and to alleviate the traffic congestionin the existing Bangna-Bangpakong highway. The newBangkok-Chonburi highway is fully-controlled accesswith flyovers at intersections. Almost entire highwayroute is found to be sitting on the well-known ‘‘softBangkok clay’’.

The soft Bangkok clay layer along highway routeis about 8 to 15 meters in thickness. Severe settle-ment of the highway embankment is anticipated if itis constructed on the unimproved Bangkok clay; thus,

the Prefabricated Vertical Drain (PVD) technique withpreload embankment was introduced to acceleratethe primary consolidation settlement.

Since their low shear strengths, stage of construc-tion of the preload embankment with preload timeis proposed for three loading stages. During the firstand second loading preload stages, there is no slideoccurring. However, during the third preload stage,stability failures of preload embankments occur onsome sections as shown in Figure 1.

On this study, an application of reliability analysesis introduced in order to be used as the pilot study of

Figure 1. Stability failures of the preload embankment dur-ing the construction of the new Bangkok-Chonburi highway(courtesy to Apimeteetamrong).

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© 2008 Taylor & Francis Group, London, UK

the reliability analysis for future highway constructionprojects in Department of Highways, Thailand.

2 SITE CONDITIONS AND IN-SITU TESTS

Soil profile along the new Bangkok-Chonburi high-way alignment can be classified to four layers asfollows: weathered crust, soft clay, medium stiff clay,and stiff clay, respectively. Summary of their basicproperties soil properties is shown in Table 1. In addi-tion, Atterberg limit results are plotted in Casagrande’ssoil classification chart; moreover, the Bangkok clayis classified as CL or CH as shown in Figure 2.

The Vane Shear Test (VST) is considered to be themost reliable tool to estimate undrained shear strength(Su) throughout the world. To perform the vane sheartest, a vane is pushed directly into soil layer and rotateduntil the soil fails. The torque required to fail the soilalong the vertical and horizontal edges of the vane isa relatively direct measurement of the shear strength.Since vane shear time loading is, a correction factoris required to evaluate the undrained shear strength.Bjerrum (1974) evaluated the undrained shear strength

Table 1. Basic engineering soil properties of the soil layers(after Lin, 1999).

Unit WaterDepth weight content

Soil layer (m) (kg/cm3) (%)

Weathered crust 0 to 1 1.6 to 1.8 40 to 60Soft clay 1 to 16 1.4 to 1.5 70 to 160Medium stiff clay 16 to 20 1.6 to 1.8 40 to 60Stiff clay 20 to 22 1.8 to 2.0 –

BKK ClayPI = 0.74LL - 10.75

R2 = 0.93

0

20

40

60

80

100

120

0 25 50 75 100 125 150Liquid Limit (%)

Plas

tic I

ndex

(%

) .

U-Line

A-Line

Figure 2. Atterberg limit test results of the soft Bangkokclay in soil classification chart (after Lin, 1999).

0

5

10

15

20

25

0 2 4 6 8 10Undrained Shear Strength (tsm)

Dep

th (

m) Before PVD

During PVD

Figure 3. Undrained shear strength of the soft Bangkok claybefore and during PVD (after Lin, 1999 and Apimeteetam-rong et al., 2007).

from case histories of embankment failures and pro-posed the correction factor versus plasticity index asshown in Equation 1 (as sited by Das, 2002). As partof another study in Road Research and DevelopmentCenter, vane shear tests during preloading were per-formed. Thus, the undrained shear strength by vaneshear tests before and during PVD is shown in Figure 3.

μ = 1.7 − 0.54 log(PI) (1)

3 PRELOADED EMBANKMENTCONSTRUCTION

Due to low undrained shear strength and high watercontent of the soft Bangkok clay beneath, the preloadembankment is constructed into three steps as fol-lows; fifty centimeter working platform sand blanket;two layers of four to six layers of the compacted fillmaterial. Total proposed preload time is one year;however, most of the actual preload time is over oneyear (Lin 1999). In addition, engineering propertyspecifications of the preload materials is indicated inTable 2. Further information on the backfill materialsare reported elsewhere in the final construction reportby Lin (1999).

4 EMBANKMENT FAILURES

All embankment sections performed well during thefirst and second preload stages without any slide orsurface crack (Lin, 1999). However, during the thirdpreload stage, stability failures and surface cracks

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Table 2. Engineering properties of the preloading materials(After Lin, 1999).

Material CBRtype Gradation Plasticity (%)

Drainage 100% finer than 9.5 mm Nonplastic; –Material and less than 6% finer free from

than 0.075 mm clay lumpsFill Material Max Particle Nonplastic; 10

size of 3/8 inch free fromclay lumps

occur in several sections of embankments occurs.Lin (1999) concluded that most of the embankmentsfailures and surface cracks occurred where the softBangkok clay with high water content and very thickclay layer. In some sections, stability failure occurswhere canal and shrimp farms existing beside the rightof way (as shown in Figure 1).

5 STABILITY ANALYSIS

One of the design criteria of highway embankmentdesign on soft clay is to evaluate the stability of thehighway embankment. For highway embankment con-struction of soft clay, the critical stage for the stabilityfailure is at the end of the embankment construction.

On this study, stability of the preload embankmentis followed the limit equilibrium method together withthe Bishop’s simplified method. The commercial com-puter program named SLOPE/W is used to performstability analyses.

The undrained shear strength of the Bangkok claybefore and after preloading is evaluated by the vaneshear test and the SHANSEP method by Ladd andFoott (1974) as Equation 2, respectively; where OCRis overconsolidation ratio; λ is material constant.Since the Bangkok clay along the constructed high-way before preloading is slightly overconsolidated, theBangkok clay beneath the preload embankment shouldbe normally consolidated during preloading. Terzaghiet al. (1996) indicates that the relationship between themobilized strength ratios for the stability analysis ofthe embankment is as indicated in Equation 3.

(SU

σVO

)OC

=(

SU

σVO

)NC

OCRλ (2)

SU(mob) = 0.22σ′VO (3)

6 RELIABILITY ANALYSIS

Geotechnical variability is complex and results fromvarious sources of uncertainties as also mentioned by

Fill Material

Clay 1

Clay 2

Clay 3

01.5

7.0

12.Clay 4

Depth (m)

Figure 4. Uncertainty in soil property estimates (afterKulhawy, 1992).

Terzaghi (Goodman, 1998) ‘‘Unfortunately, soils aremade by nature and not by man, and the products ofnature are always complex. . . Natural soil is never uni-form. Its properties change from point to point whileour knowledge of its properties are limited to thosefew spots at which the samples have been collected.In soil mechanics the accuracy of computed resultsnever exceeds that of a crude estimate, and the prin-cipal function of theory consists in teaching us whatand how to observe in the field.’’

The three main sources of uncertainty are: (a)inherent variability, (b) measurement errors, and (c)transformation uncertainties. The first source can beattributed to the natural geologic processes that areinvolved in soil formation. The second source isattributed to equipment, procedural/operator, and ran-dom testing effects. The third source is introducedwhen field or laboratory measurements are trans-formed into design soil properties using empiricalor other correlation models. The relative contribu-tion of these sources to the overall uncertainty in thedesign soil property clearly depends on the site condi-tion, degree of equipment and procedure control, andprecision of the correlation model.

Reliability analysis is a method that introducesuncertainties described by probabilities and proba-bility distributions into calculations of engineeringperformance. In geotechnical applications, reliabilityanalyses typically assign probabilities or probabilitydistributions to soil or rock engineering properties andpropagate these probabilities through calculation mod-els to obtain probabilities or probability distributionsof engineering performance.

On this study, the reliability analysis is performedthru the Monte Carlo simulation incorporated in theSLOPE/W. The number of repetitions for each sim-ulation is trialed till the uniformly outcome reached.The analyses yield the outcome in term of the proba-bility of failure of the slope failure. Furthermore, thereliability index can be related to the probability offailure as shown in table 3 (USACE, 1997).

Engineering properties with standard deviationsof the Bangkok clay used in stability analyses aresummarized in Table 4. The mean value of total unitweight along construction site is found in the range

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Table 3. Relationship between reliability index and proba-bility of failure (USACE, 1997).

Reliability Probability of failure Expectedindex (β) (pf = �(−β)) performance level

1.00 0.159 Hazardous1.25 0.1061.50 0.0668 Unsatisfactory1.75 0.04012.00 0.0228 Poor2.25 0.01222.50 0.00621 Below average2.75 0.002983.00 0.00135 Above average3.50 0.0002324.00 0.0000317 Good4.50 0.00000345.00 0.0000003 High

Table 4. Statistical Engineering properties of soils, beforepreloading, used in stability analysis.

Mean COVSoil type Soil Parameter Value (%) Unit

Fill Material Unit Weight 2000 – kg/m3

Clay 1 Unit Weight 1.50 1.5 kg/m3




Fill Material Friction Angle 35 – degreeClay 1 Shear Strength 1.50 30 kg/m2

Clay 2 Shear Strength 0.85 15 kg/m2



of 1.34 to 1.60 kg/m3 with COV in the range of 1.5to 5.5. The COV of unit weight presented publishedin USACE (1999) is in the range of 3 to 8. The meanvalue of undrained shear strength by field vane sheartests (Su,VST) at the construction site is found in therange of 0.85 to 1.95 kPa with COV in the range of 14to 35. The COV of Su,VST presented by Phoon et al.(1995) is in the range of 15 to 50.

7 STABILITY ANALYSIS RESULTS

The stability analyses will be performed following thecase histories indicated in Table 5. Example of thecritical surface from stability analysis is provided inFigure 5. Moreover, results of the reliability analysesof the highway embankment for the new Bangkok-Chonburi construction are presented in Table 6.

Table 5. Information of the case histories.

Counter Fill Pond orCase weight height Service canalnumber embankment (m) road besides

1 Yes 3.0 No No2 No 3.0 No No3 Yes 4.0 No No4 No 3.0 Yes No5 Yes 3.0 No Yes6 Yes 3.0 Yes Yes

Fill Material

Clay 1

Clay 2

Clay 3

01.5

7.0

12Clay 4

Depth (m)

Figure 5. Reliability analysis of slope stability of highwayembankment sitting on soft Bangkok clays of the case historynumber 3.

Table 6. Stability and reliability analysis results.

Case FS FSnumber FMLV β σF Pf (%) (Min) (Max)

1 1.65 4.18 0.156 0.00142 0.85 2.382 1.28 1.17 0.157 3.8 0.57 1.993 1.09 0.83 0.113 20.2 0.55 1.574 1.01 0.02 0.121 49.3 0.49 1.555 1.27 2.17 0.127 1.5 0.77 1.876 1.03 0.28 0.103 39.0 0.53 1.48

From Table 6, stability analysis with the applicationof the reliability analysis yields results coincided withcase histories notified in Lin (1999). Stability failureoccurred where the thick soft Bangkok clay existedas well as canal and shrimp farms existing beside theright of way. It is shown that some values of factorof safety are less than 1 in which led to the failuresof slope in some stations along the highway construc-tion. The distribution of the factor of safety for the casenumber 6 is shown in Figure 6. For case number 1, theprobability of failure (Pf ) of the highway construc-tion is 0.0000142 (with associated reliability index of4.18). Based on Table 3, it is implied that the expectedperformance level of the highway construction site is‘‘Good’’ due to the uncertainty of the estimated soilproperties (as shown in Table 4).

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0

0.2

0.4

0.6

0.8

1

0.6 0.8 1 1.2 1.4Factor of Safety

Prob

abili

ty o

f Fa

ilure

(Pf

) .

Figure 6. Distribution of Factor of Safety (F.S.) for casehistory number 6.

It is demonstrated herein that the reliability analysisfor slope stability of highway embankment is essentialto be preformed especially for highway embankmentsitting on soft Bangkok clays.

8 SIMPLIFIED RISK ANALYSIS CHART TOPREVENT EMBANKMENT SLOPE FAILURE

Simplified risk analysis chart to prevent embankmentslope failure presented herein is adopted from thephilosophy of risk analysis chart to prevent damageof building-foundation systems caused by adjacenttunneling in soils presented by Sawatparnich (2003).Since the failure surface of the highway embankmentis in the soft Bangkok clay layer, input soft Bangkoklayer is simplified to one layer with the undrained shearstrength of 0.8 ton/m2 and the unit weight of embank-ment of 2 t/m3. The risk analysis chart to preventembankment slope failure is then simply constructedby plotting the reliability indices versus the maximumheight of the embankment on the same range of COVof Su in which so-called ‘‘Su-line’’. The Su-line intro-duced herein are COVSu between 10% and 40%. Thesimplified risk analysis chart to prevent embankmentslope failure sitting on Bangkok clays is presented asshown in Figure 7 and Figure 8.

The proposed chart is the practical methodologyto take into account of sources of uncertainty associ-ated in geotechnical engineering to prevent potentialslope failure as the case. In the design procedure onhighway embankment, one can utilize the chart in theselection of maximum height by taking into accounton the uncertainty of estimated soil properties in morerational way as follows:

1. Determine the mean value of Su of existing groundalong the route of highway construction.

0

2

4

6

8

10

0.5 1 1.5 2 2.5 3

Embankment Height (m)

Rel

iabi

lity

Inde

x (

)

COV = 10%

COV = 20%

COV = 30%

COV = 40%

Figure 7. Risk analysis chart to prevent embankment slopefailure: New Bangkok-Chonburi highway project.

0

5

10

15

20

25

30

35

40

45

50

0.5 1 1.5 2 2.5 3


Prob

abili

ty o

f Fa

ilure

(%

)

COV = 10%

COV = 20%

COV = 30%

COV = 40%

Figure 8. Risk analysis chart to prevent embankment slopefailure.

2. Determine the Standard Deviation (S.D.) for fromsite investigation along the construction route.

3. Calculate COV of Su.4. Construct the risk analysis chart from probabilis-

tic slope stability analysis (e.g., SLOPE/W, etc) inwhich Y-axis is either probability of failure or reli-ability index. X-axis is embankment height. TheCOV of Su shall be categorized into COVSu =10%, COVSu = 20%, COVSu = 30%, COVSu =40%, respectively.

5. Define the target reliability or the allow probabil-ity of failure associated with the reliability leveldescribe in Table 3.

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0

2

4

6

8

10

0.5 1 1.5 2 2.5 3


Rel

iabi

lity

Inde

x (

) COV = 10%

COV = 20%

Figure 9. The use of risk analysis chart to determinethe maximum height of highway embankment with targetreliability of 2.5 with COVSu of 10% and 20%.

Table 7. Comparison between Deterministic Approach and‘‘Risk Analysis Chart’’ to Prevent Potential Slope Failure ofHighway Embankment.

Traditional Risk analysis chart for highwaydeterministic approach embankment

• Soil properties are • Uncertainty of soil propertiesadopted as are considered in the predictiondeterministic • The propagation ofquantities uncertainties in the soil(uncertainties are properties into the predictionneglected). of slope stability in considered

• No statistic explicitly in the highwayevaluation embankment designof estimated processsoil properties. • Embankment height (critical

• No explicit criteria height) can be adjusted toto assess the need achieve a specified targetfor ground reliability index for slopemodification or stability analysis. If the criticalsupport structure/ height of highway embankmentberms to reduce the is less than the required heightpotential slope failure from geometric design, groundof embankment. modification or additional

support structure/berms may beemployed to limit potentialslope failures along the highwayduring the construction

• For the case of preloadingconstruction technique, the riskassessment shall be performedsuch that the stage ofconstruction (each preloadingembankment height) will notexceed the critical height ateach stage of construction

6. Draw the horizontal line along the X-axis to theirCOVSu.

7. Draw the vertical line from their COVSu alongY-axis to determine the critical height on the chart.

For example, if COVSu is about 20% and the tar-get reliability index (in which can be selected fromTable 3) is 2.5 (corresponded Pf of 0.00621), from thechart in figure 7, the maximum height in which the Pfof the slope failure is not exceed the target probabilityof failure (Pf ) is 1.45 meters as shown in Figure 9. IfCovSu is equal to 10 percent, the critical height shallbe 2.15 meters. If the critical embankment height isless than the required embankment height from high-way geometric design, berms or ground modificationtechniques shall be considered for particular sectionfor highway embankment construction. For the case ofpreloading construction technique, the risk assessmentshall be performed such that the stage of construction(each preloading embankment height) will not exceedthe critical height at each stage of construction. Com-parison between deterministic approach and ‘‘RiskAnalysis Chart’’ to prevent potential slope failure ofhighway embankment are shown in Table 7.

9 CONCLUSIONS

In this study, reliability-based methodology can beused as the more rational and consistent approach instead of the deterministic analysis in the assessmentthe potential slope failure for highway embankment inBangkok clays.

Slope stability evaluation is based on method ofslices with Bishop method in this study. Six Casestudies during the construction of New Bangkok-Chonburi Motorway were evaluated with the MonteCarlo simulation method (MCS) in SLOPE/W in orderto demonstrate the capabilities of methodology. Inaddition, the critical slip surface is first determinedby deterministic analysis with the mean input valuesof engineering soil properties in SLOPE/W. MonteCarlo simulation is then performed on the predeter-mined critical slip surface. The six cases with differentvariables of geological conditions of preloaded high-way embankments on this project were performed.The illustration was shown that the risk analysis chartfor assessing slope failure on highway embankmentcould be represented as the tool for practical engineersto check the potential slope failure of the highwayembankment with their designs.

ACKNOWLEDGEMENTS

The authors would like to express the profound grat-itude to Dr. Pichit Jamnongpipatkul, the director of

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Bureau of Road Research and Development, De-partment of Highways for his guidance and supportthroughout the study and Mr. Satipong Apimetee-tamrong for providing some valuable background onthe stability failure of the preload embankment andengineering soil properties of the Bangkok clay alongthe constructed highway. Special thanks are extendedto Assistant Prof. Dr. Sompote Youwai, Departmentof Civil Engineering, King Mongkut’s University ofTechnology, for providing the commercial softwareSLOPE/W in this study.

REFERENCES

Apimeteetamrong S., Sunitsakul J. & Sawatparnich A. 2007.Engineering Soil Properties of the Soft Bangkok Clay byPiezoncone Tests, Department of Highways, Bangkok. InThai.

Das B.M. 2002. Principles of Geotechnical Engineering,Brooks/Cole, California.

Duncan J.M. 2000. Factors of Safety and Reliability inGeotechnical Engineering, Journal of Geotechnical andGeoenvironmental Engineering, 126(4): 307–316.

Goodman R. 1998. Karl Terzaghi: The Engineer As Artist,New York, ASCE.

Kulhawy, F.H., 1992. On Evaluation of Statistical SoilProperties. In: Stability and Performance of Slopes &Embankment II, GSP 31, ASCE, New York, 95–115.

Ladd C.C. & Foott R. 1974. New Design Procedures for Sta-bility of Soft Clays, Journal of Geotechnical EngineeringDivision, 100(7): 763–786.

Lin P. 1999. Final Report of Ground Improvement Workfor Construction Supervision of Bangkok-Chonburi NewHighway Project, Bangkok.

Phoon, K.K., Kulhawy, F.H. & Grigoriu, M.D. 1995. Relia-bility Based Design of Foundations for Transmission LineStructures. Report TR-105000, Electric Power ResearchInstitute, Palo Alto, 380p.

Sawatparnich, A. 2003. Deterministic and Reliability-BasedAssessment of Existing Building-Foundation SystemsAdjacent to Tunneling In Soils, Ph.D. Dissertation, CornellUniversity, Ithaca, 406p.

Terzaghi K., Peck R.B. & Mesri G. 1996. Soil Mechan-ics in Engineering Practice, John Wiley & Sons, Inc.,New York.

U.S. Army Corps of Engineers. 1997. Engineering andDesign, Introduction to Probability and Reliability Meth-ods for Use in Geotechnical Engineering, ETL 1110-2-547, Department of the Army, Washington D.C.

U.S. Army Corps of Engineers. 1999. Risk-Based Analysis inGeotechnical Engineering for Support of Planning StudiesEngineering and Design, ETL 1110-2-556, Departmentof the Army, Washington D.C.

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chapter 274. simplified risk analysis chart to prevent slope failure of highway embankment on soft...

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