Geotechnical Assignment Case Study

7/25/2019 Geotechnical Assignment Case Study

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Table of Contents

TABLE OF CONTENTS """""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" #

LIST OF TABLES AND FIGURES """""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" ##

1.0 INTRODUCTION"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" $

1.1 BACKGROUND ######################################################################################################################################################### $1.2 PURPOSE AND SCOPE OF WORK #################################################################################################################### $

2.0 SITE STRATIGRAPHY """"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" %

2.1 TYPE OF CLAY AND GENERAL GEOTECHNICAL CHARACTERISTICS################################################# %2.2 GEOTECHNICAL PROPERTIES USED IN MODELLING ############################################################################### %

3.0 STABILITY ANALYSIS """""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" &

3.1 MODEL ####################################################################################################################################################################### &3.2 CALIBRATION OF MODEL FOR SHEAR STRENGTH #################################################################################### &3.3 DESIGN STABILITY IMPROVEMENTS ############################################################################################################### &3.4 TOE BERM STABILITY IMPROVEMENTS ######################################################################################################### &3.5 UPPER BANK OFFLOADING ################################################################################################################################ &

4.0 CONCLUSIONS"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" '

5.0 RECOMMENDATIONS""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" (

APPENDIX A: OUTPUT DATA """""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" )

REFERENCES """""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" $&


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List of Tables and Figures

TABLE 1:SUMMARY OF STABILITY CHARACTERISTICS ......................................................................................... 6

FIGURE 1:PLAN AND SECTION VIEW FROMASSIGNMENT ..................................................................................... 1

FIGURE 2:BACKANALYSIS -SENSITIVITY FSVS RESIDUAL PHI............................................................................ 7

FIGURE 3:SENSITIVITY -FSVS.OFFLOADING ..................................................................................................... 7

FIGURE 4:SENSITIVITY -FSVS.BERM HEIGHT ................................................................................................... 8

FIGURE 5:STARTING POINT (SS1) ..................................................................................................................... 9

FIGURE 6:STARTING POINT (SS2)..................................................................................................................... 9

FIGURE 7:OFFLOADING EMBANKMENT (1M) ....................................................................................................... 9

FIGURE 8:OFFLOADING EMBANKMENT (2M) ..................................................................................................... 10

FIGURE 9:OFFLOADING EMBANKMENT (3M) ..................................................................................................... 10

FIGURE 10:ROCKFILL BERM (1M,SS1) ........................................................................................................... 10FIGURE 11:ROCKFILL BERM (1M,SS2) ........................................................................................................... 11

FIGURE 12:ROCKFILL BERM (3M,SS1) ........................................................................................................... 11





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1.0 Introduction

1.1

Background

In August, 2009, the west bank of the Red River near a bridge to St. Adolphe was in a statefailure and in need of emergency engineering. SU3 had its piles sheared off and were

floating on the bank at a gradually increasing angle. Multiple retrogressive head scarps

immediately upslope of Pier SU 3 and beginning on the top of the bank were alarms of a

failure surface up to the west abutment. The lower banks were fully saturated. SU 3 shifted

horizontally and sunk about 2.5 m at the time of arrival of the engineer. The two spans

connecting at SU 3 were severely damaged and in danger of collapse. Emergency action

was taken in order to increase the factor of safety (FS) of the failing slopes.

1.2

Purpose and Scope of Work

The first line of action was to offload the approach embankment and the upper bank,

approximately 2 m x 25m x 60m long, for the best chance at saving SU 2 and the WestAbutment from failure. This task would allow for safe access through a newly constructed

access road to the riverbank to begin emergency rock fill berm lifts. The added lifts were

crucial for the stabilization of SS1, and to also increase the overall stability of all the slip

surfaces by 20-30%. A plan and section view layout of the site is shown below in Figure 1.

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2.0 Site Stratigraphy

2.1

Type of Clay and General Geotechnical Characteristics

Based on existing 1974 stratigraphic profiles the major type of clay deposit encountered for

this assignment is high plasticity glacio-lacustrine silty clay which is typically found below theUpper Complex Zone (approximately 3 m below surface). The glaciolacustrine silty clay

typically is 9 to 12 m thick, and can range from 18 to 21 m thick (Kjartanson et al, 1983). The

upper 1.5 to 4.5 m is typically weathered to a brown or grey-brown color clay with a stiff

consistency. Below this is grey clay with a firm to stiff consistency which becomes soft with

depth as it approaches the till at an approximate elevation of 218 m. The upper brown clay is

normally highly fissured with less fissure frequency as lower depths are reached to the grey

clay and finally the underlying till. There may be numerous silt clasts and pockets in both

clays, and rock fragments ranging in sizes from gravel to boulder sizes are usually found in

only usually the lower portions of grey clay and not typically the brown clay. Randomly

occurring white gypsum pockets and veins can be throughout the upper portion of the brown

clay and are often filled in fissures.

The glaciolacustrine silty clays generally have a transition zone between the weathered

oxidized brown clays and the underlying grey clays located between 4.5 to 9 m below the

ground surface. Silty interlayers within the brown clays which can be usually highly fissured

in the upper sections. The grey clays have numerous silt and rock clasts with only

occasional fissures. Moisture contents vary between 40 to 60 percent, and in areas that

depth to till is below 6 m the moisture content is typically lower then 40 to 45 percent and is

usually entirely weathered to a brown to brownish-grey color in these locations (Kjartanson

et al, 1983). The brown clay typically is more plastic then the grey clay, and has liquid limits

ranging from 80 to 110, and the plastic index from 60 to 70. The grey clay liquid limit ranges

are from 65 to 90, and the plastic index is from 40 to 65. Typically, the liquidity index is less

then 0.5 in the brown clay. In grey clays at greater depths the liquidity index may approach

or exceed 0.5. Near the clay/till contact the liquidity indexes can be near 1.0. The lower and

upper bound of the residual angle of internal friction for Winnipeg clays is from 8 to 12degrees, respectively. The peak angle of friction is difficult to define as the peak strength

envelope is curved due to different parameters related to different loading conditions. The

shear strength of the clay is strongly anisotropic.

2.2 Geotechnical Properties Used in Modelling

For this assignment, the geotechnical properties used are high plasticity, a bulk unit weight

of the both intact and residual clay, and rock fill of 18 kN/m3. Cohesion of intact clay of 5

kPa, and residual clay of a constant 2 kPa. Friction angle of intact clay of 14 degrees and

back analyzed residual friction angle of 8 degrees and friction angle of rock fill as 35

degrees. The site was observed to have the bank at saturation and this was modelled asshown in the Appendix for the piezometric line.


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3.0 Stability Analysis

3.1

Model

For this assignment, rotational failure was analyzed using the method of slices based on

Limit Equilibrium Method (LEM) and the Morgenstern-Price Method in SLOPE/W. It should

be noted that the methods used for modelling offer a measure of the average stress

mobilized in the slope and in reality this may not be exactly the actual shear stresses.

3.2 Calibration of Model for Shear Strength

Initially, the angle of friction was unknown for the residual soil of the failing slope. In the

tables contained in the following sections a sensitivity plot of the factor of safety vs residual

angle of friction for SS1 was plotted for three angles (5, 10 and 15 degrees) and from the

approximately linear solution the angle of friction associated with a factor of safety equal to

one was interpolated to be 8 degrees.

3.3

Design Stability Improvements

The first objective of this assignment was to increase the overall stability of SS2, up to the

West Abutment, by 20% so that construction activities could occur on the riverbank. This

was performed by offloading the top of bank behind the bridge in 1 m layers up to 3 m depth.

Initially, the emergency offloading was extended approximately 10 m to the back of the

abutment. Secondly, a rock fill berm was placed on both sides of the bridge between SU3

and SU4 in one meter lifts, up to 5 m height and connecting in the middle under the bridge.

This was for the second emergency procedure aimed at improving the stability of SS1 by

30%. The following plan and section view from the assignment are show below.

3.4

Toe Berm Stability Improvements

a. 1.0 m berm 2% and 11% increase in FS for SS1 and SS2, respectively.

b. 3.0 m berm 2% and 22% increase in FS for SS1 and SS2, respectively.

c. 5.0 m berm 13% and 36% increase in FS for SS1 and SS2, respectively.

3.5

Upper Bank Offloading

a. 1.0 m offloading 4% increase in FS for SS1.

b. 2.0 m offloading 14% increase in FS for SS1.

c. 3.0 m offloading 32% increase in FS for SS1.


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4.0 Conclusions

Given that SS1 was assumed initially to have a FS of 1 and through back analysis the residual friction

angle of the soil was calibrated to be 8 degrees. Moving forward using the governing residual friction

angle determined the first emergency action was to offload the approach embankment and upper bank in

1 m layers to reach a 20% improvement in FS which through interpolation shown in Table 1. Theestimated depth of 2.4 m is recommended for the contractor to remove to satisfy safety conditions and

working near the river bank. The second emergency action was to provide a rock fill berm at the toe of the

failure surface to provide a 30% increase in FS. The estimated depth of 5m toe berm for this problem

solution satisfied only a 13% increase the FS of SS1, and a 36% increase to the FS in SS2.


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5.0 Recommendations

It is recommended that the berm offloading occur in 1 m layers initially extending approximately 10 m

back of the abutment. This offloading of the abutment and the upper bank should be completed up to at

least 2.4 m, and at least 5 m of rock fill should be placed to increase the overall stability of the failure

surfaces by at least 30% to accommodate the next stage of demolishing the damaged portion of thebridge deck, and installing a shear key to further improve the FS to at least 1.5 for long term.


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Tables

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Appendix A: Output Data

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References

[1] Kenyon, Rob, University of Manitoba. CIVL 4230: Geotechnical Engineering Course Notes.

Winter 2016.

[2] Kjartanson, B., Baracos, A., & Shields, D. (1983). Geological engineering report for urbandevelopment of Winnipeg. Winnipeg, Man.: Dept. of Geological Engineering, University of

Manitoba.

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Geotechnical Assignment Case Study

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