[American Society of Civil Engineers Seventh International Symposium on Field Measurements in Geomechanics - Boston, Massachusetts, United States (September 24-27, 2007)] 7th FMGM

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ROLE OF INSTRUMENTATION IN ASSESSMENTOF COMPLEX GROUND CONDITIONS

Vishnu Diyaljee F. ASCE, P.Eng and Murthy Pariti, P.Eng

1 Managing Director, GAEA Engineering Ltd, 2408 -113 A St, Edmonton, Alberta, Canada, T6J 4Y2;[email protected] Senior Geotechnical Engineer, Stewart Weir and Company Limited, Edmonton, Alberta, Canada;[email protected]

ABSTRACT: Slope indicators are commonly used in the evaluation and assessmentof ground conditions affecting the highway infrastructure of roads and bridges whenhistoric evidence, airphoto interpretation or judgment suggest that the natural orexisting ground might develop instability problems through new construction ornatural processes. During the planning and design stages of the re-alignment of twomajor Primary Highway routes in the Southern Region of Alberta, office studies andsite reconnaissance indicated the possibility of slope instability concerns. Theseconcerns warranted the installation of slope indicators to determine their validity andmeasures, if any, to ensure the suitability of the proposed routes. The slope indicatormonitoring concurred with the projections of the site and office reviews allowingdecisions to be made to ensure construction of the bridges and highways in stableterrain. These two case studies demonstrate the importance of using instrumentation tovalidate site observations and opinions, and to allow for sound decision-making.

INTRODUCTION

Over the last thirty years, slope indicator installation has been used along AlbertaHighways for the investigation and monitoring of unstable ground conditions affectingthe highway infrastructure of roads and bridges.

For the management of the highway network, the Province is divided into FourRegions - Northern, Central, North Central and Southern. Traditionally, the NorthernRegion is the one where problems of instability are most likely to occur because of itsundulating topography, climate and geological disposition.

Most instability problems associated with the highway infrastructure in Albertaoccur in the Northern Region because of landsliding activity. Fewer problems of thissort are associated with the Southern Region where the climate is generally drier andthe terrain less undulating. Nonetheless, problems do occur in the Southern Regionresulting from instability of natural slopes adjacent to rivers and along highwaycorridors.

This paper presents two (2) case studies involving highway infrastructure located inthe Southern Region illustrating how the use of instrumentation was beneficial in

FMGM 2007: Seventh International Symposium on Field Measurements in Geomechanics © 2007 ASCE

Copyright ASCE 2007 Field Measurements in Geomechanics (FMGM 2007)

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confirming suspicions and speculations about site conditions. This allowed decisionsto ensure that the proposed infrastructure works would be satisfactorily undertaken.

Case Study 1 is associated with the re-alignment of a section of a 2-lane primaryhighway, while Case Study 2 is associated with the realignment of a section of a 4-lane primary highway and construction of two (2) bridges in excess of $14 milliondollars.

The Geotechnical Services Section (GSS) of Alberta Transportation and Utilities, aMinistry of the Government of the Province of Alberta, undertook these case studies.At the time, the GSS was under the technical and administrative direction of the firstauthor. The second author was responsible for the geotechnical work involved.

CASE STUDY 1 - HIGHWAY 22:14 (BRAGG CREEK)

General Description and Site Characteristics

Highway 22:14 runs through the Village of Bragg Creek, which is located about 60km west of Calgary, Alberta. The upgrading of this highway between the Jct of Hwy66 and Hwy 8 was planned in 1993 to improve existing substandard horizontal andvertical geometrics.

A 270 m section of the existing highway required improved horizontal geometrics.The preference for highway improvement was to the west of the existing highwaysince any improvements to the east would affect the Sarcee Indian Reserve.

The shift of the highway to the east was also further complicated by slumping of thebackslope of the existing highway. However, the shift to the west placed the highwaycloser to the south historic bank of the Bow River.

Figures 1 and 2 show the area of instability. The fence shown in Figure 2demarcates the boundary between the highway right-of-way and the Sarcee IndianReserve.

The backslope slump existed for a number of years during the life of the existinghighway. Since this slump was not in the highway right-of-way, and did not affect thehighway performance a geotechnical investigation was not warranted. This is theapproach often taken initially by many highway jurisdictions until the highway has tobe widened or re-aligned for better geometrics.

The proposed alignment, however, was not subject to a geotechnical evaluation andassessment during the planning stage. When a geotechnical review of the designgradeline, cross-sections and aerial photos was undertaken in July 1993, cuts in excessof 14 m were identified.

These large cuts and observations from the airphoto review raised a fewgeotechnical concerns, which resulted in site reviews consisting of site inspections andgeotechnical evaluation and assessments of the proposed alignment. Had it not beenfor the geotechnical concerns, these site reviews may not have been undertaken.

Site reviews were undertaken between July 1993 and June 1994 and includeddetailed site inspections, geotechnical investigations consisting of testpitting anddrilling, and slope indicator and piezometer installation and monitoring.



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FIG. 1. Aerial Photo Showing Alignment Area in Question

FIG. 2. Backslope Slumping in Sarcee Indian Reserve

Aerial Photo, Gradeline and Site Reviews

Figure 1, shows the slide area in the vicinity where the existing highway crosses thehistoric south bank of the Bow River on a skew. The formation of the historic southbank of the Bow River dates to the historic glaciation period. The narrow present BowRiver is a remnant river of the initial Bow River within the floodplain of the originalRiver.

This floodplain is approximately 400 m in width measured from its existing to itshistoric bank. The Village of Bragg Creek occupies this floodplain to the west andsouthwest of the highway while the Sarcee Indian Reserve occupies the area east andnortheast of the highway.

As noted in Figure 1, Hwy 22:14 travels eastward along the historic flood plain afterabout a 200 m. descent of its north–south leg to the floodplain. A geotechnical sitereview undertaken in July 1993 showed that the south bank of the historic Bow Riverwas almost at a1:1 slope and contained a mature tree cover.



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Figure 3 shows the tree cover along the historic south bank. As can be seen, thetrunks of the trees displayed a backward tilting often indicative of surficial slope creepor signs of deeper slope instability.

Concerns about these prevailing site features on the existing and future stability ofthe area in question led to the decision to install slope indicator instrumentation toinvestigate the stability of the site. This instrumentation would determine whetherwere movements with depth and what influence, if any, those movements may haveon the proposed design and construction operations.

FIG. 3. Tilting of Trees along Historic South Bank

The decision to instrument was not typical for sites in Southern Alberta. However,since the proposed highway was to be in a deep cut in the historic riverbank andresidences situated at the toe of the bank, a cautious approach was deemed necessary.

Testpitting in July 1993

In July 1993, the GSS undertook testpitting as part of the overall investigationprogram to determine whether seepage was the cause of the slumping of thebackslope. The testpit investigation consisted of digging five testpits with a backhoe toa maximum depth of 4 m in each pit. These pits were located along the west ditch ofthe existing highway opposite the backslope slump.

Within the depth of excavation there was no visible seepage observed. However,the moisture contents were generally within the range of 20 to 35%. This range ofmoisture content was wet of Standard Proctor optimum moisture for typical clay-tillmaterial. There were, however, observations of rust staining and sand lenses withinthe overall clay-till formation. These characteristic features are often generallyassociated with moisture movement.

Testhole Drilling and Instrumentation

In September 1993, the GSS installed three slope indicators (S1s #1, 2 and 3) andtwo standpipe piezometers in the shoulder area on either side of the existing highwayto determine possible slide zones and groundwater levels, respectively. SIs #1 and #3were installed flush with the existing road surface, while SI #2 was installed in theslumped backslope to the west of the existing highway within the highway right-of-way.



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The slope indicators were installed to depths varying from 10.7 to 13.7 m andterminating 2 to 2.5 m in shale bedrock. The stratigraphy encountered in the SI #1and#3 holes consisted of soft to firm clay till to a depth of 5 m overlying gravel varying inthickness from 2.5 to 3 m. Below the gravel was clay shale. At the SI #2 location, theclay-till was of similar thickness as in the other holes but the gravel thickness wasabout 5m. Further investigations done after showed the gravel thickness to be about15 m closer to the historic south bank.

Of these three slope indicators, only SI #2 showed a distinct plane of movement atabout 6 m below the existing ditch level at the contact of the overlying clay and adense to very dense gravel stratum below, Figure 4. This depth corresponded to El1313 and was about 1 m above the proposed gradeline. Shale bedrock was below thegravel.

FIG. 4. Slope Indicator Plot of SI 2 in the A direction

The other SIs showed small kinks at depths corresponding to El. 1313 in the case ofSI #1, and El 1310, 1312 and 1316 in the case of SI #3. This pattern of results shownby SIs #1 and #3 is often typical of natural slopes that do not any signs of instabilityshow from observations.

The movement in SI #2 at Ele.1313 raised concerns on whether this was associatedwith a slide zone within the slumped backslope or movement of the historic southbank. However, further slope indicator installation was not possible within thebackslope area of the existing highway because of right-of-way restrictions imposedby the Sarcee Indian Reserve.

As there were expensive residential houses at the base of the south bank, it was feltany problems created by the proposed construction activity could likely influencethese residences and result in litigation. Since only slope indicator SI # 2 showed very



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distinct movement, the GSS installed four additional (S1s # 4, 5, 6 and 7) in January1994 nearer the historic south bank and terminating in the shale bedrock.

This additional instrumentation revealed the shale bedrock dipping towards theflood plain and containing a larger overlying thickness of gravel. Monitoring of theprevious and new slope indicators indicated that movements were occurring within thegravel stratum and possibly along two distinct slide zones. The headscarp was at thetop of the backslope where visible signs of movements were obvious.

In July-August 1994, a further investigation was undertaken. This time, the GSSinstalled three additional slope indicators (SIs # 8, 9 and10) to ensure that themovements observed by the previous ones were realistic and to map out a direction ofmovement if possible. In this investigation, the GSS also installed pneumatic andstandpipe piezometers. These piezometers showed, in general, water levels coincidingwith the slide plane within the gravel. At that depth, the gravel was very silty andsaturated. As anticipated, the direction of movement was towards the historic southbank of the Bow River.

Figure 5 shows the location of the slope indicators installed in this study whileFigure 6 shows a cross-section illustrating movement zones inferred from the SImonitoring along with water levels from piezometer monitoring.

FIG. 5. Plan Showing Location of Slope Indicators

FIG. 6. Cross- Section Showing Inferred Slide Planes



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Design and Construction Recommendations

Based on variability in ground conditions, it was decided that the alignment wouldremain as designed and that perforated pipe trench drains would be installed in thebackslope area to intercept water from the higher ground that would likely reach thegravel layer in the backslope area. The intent was to minimize slope movements,which were likely promoted by seepage.

Further, the site would be monitored during construction and any changes to theproposed drainage measures based on site conditions would be made as warranted.The GSS also projected the contingency of a possible tied back retaining wall ifmovements occurring during construction warranted action to prevent landsliding thatwould influence the residences at the toe of slope. However, no instability problemswere reported during or after construction.

CASE STUDY 2 - HIGHWAY 3:08 (MONARCH)

General Description and Site Characteristics

Highway planning studies were initiated in 1991 for the Twinning of Highway 3:08near the Village of Monarch, 25 km north of the City of Lethbridge, Alberta. Thesestudies suggested the relocation of the existing two-lane highway and proposedadditional two-lanes approximately 1200 m downstream of the existing bridge acrossthe Oldman River. Two alignments P-2 and P-6 were initially projected with the P-6alignment being preferred from highway geometrics, land acquisition, and land useperspectives. These alignments are shown on the aerial photo, Figure 7.

From a bridge hydraulics point of view, the river crossing of the P-6 alignmentbeing on a bend of the river was less attractive than the P-2 alignment. However, theP-2 alignment was not preferred from the point of geometrics and the fact that thealignment would sever cultivable land in the flood plain on the east side of the river.

FIG. 7. Existing Highway and Alignment Alternatives



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During a field review in 1991, the GSS made several observations along thealignment near the river crossing. These observations generated concerns regardingthe suitability of the crossing and the alignment to the east of the crossing.

Of concern was the degradation of the east bank at the proposed crossing locationand the inclined bedding stratigraphy observed, as well as the instability along thebend to the north and south of the crossing. In addition, there was obvious bankslippage occurring to the east of the crossing where the alignment infringed the northbank of a historic meander of the river. Figures 8 and 9 illustrate some of theseobservations.

FIG. 8. Inclined Bedding - East Bank FIG. 9. Bank Slippage - North Bank

Slope Indicator Installation and Monitoring Results

Based on the observations, the GSS installed slope indicators to determine thepresence of any deep-seated movements at the bridge location. Ten (10) slopeindicators were installed in 1992 on the east side of the Oldman River, as shown inFigure 10.

FIG. 10. Location of Slope Indicators in Relation to P-6 Alignment



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Monitoring of the slope indicators over a period of one year showed small groundmovements at different elevations above and below the riverbed level, Figures 11 and12. As a result, in 1993, the P-7 alignment was chosen at a short distance north of theP-6 alignment. This alignment to east of the historic meander loop would be at thebase of the historic riverbank. Overall, this alignment was also chosen to minimize theseverance of the cultivable land in the flood plain area on the east side of the river.

SI 7-Cumulative Deflection - B SI 4-Cumulative Deflection - B

FIG. 11. Slope Indicator Plots showing movement below and above River Level

Stratigraphic Characteristics of Foundation Holes

Shale and sandstone encountered at different elevations exhibited inconsistency instrength and variability in stratigraphy in various boreholes. This led to theexamination of the ground conditions at about 1 km upstream of the site where theexisting bridge was founded on footings in near-surface shale bedrock. Thisinvestigation was aimed at confirming whether the foundation conditions were similaror different between the sites. Inclined bedding of the shale as shown in Figure 8 wasalso observed in the cores from some of the foundation holes of the new bridge.However, the cores taken close to the alignment of the existing bridge showed no suchbedding.

Based on stratigraphic information, further suspicions were raised about theproposed alignment since the evidence pointed toward the possible existence ofcomplex geologic conditions along the river. To ensure that the bridge locations alongP-7 alignment would be founded in satisfactory ground, it was decided to undertake anindependent Engineering Geology Assessment of the proposed alignments.



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Engineering Geology Site Assessment

This assessment was undertaken in 1994 by AGRA (Earth and Environmental)presently known as AMEC. The general geology of the area consists of UpperCretaceous sandstones and shales overlain by glacial and glacio-lacustrine soils. In theOldman River floodplain, alluvial sands and gravels overlie this sequence.

Geological mapping (Irish, 1967) showed that the bedrock geology is interrupted bya north-south trending fault, referred to as the Monarch Fault Zone. Figure 12 showsthat the Monarch Fault Zone extends north from the US border through the proposedrealignment area to 7.2 km north of the Oldman River. The fault area is exposed onthe south bank of the Oldman River about 0.8 km south of the proposed rivercrossings. Figure 13 shows this outcrop area.

FIG. 12. Monarch Fault Zone ( After Irish 1967)

FIG.13. Fault Zone Outcrop Area



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The determination of the Monarch Fault Zone confirmed the suspicions from thebeginning that the site appeared to present some puzzling features and behaviour. Theconclusion from this assessment was that the primary slope stability concern was therock slope movement related to faulting and west dipping bedding. The slope indicatorresults showed complex and relatively slow movements with less than 12 mm over a2.5-year period.

A possible failure surface was projected as shown in Figure 14 as well as a roughplan as shown in Figure 11 depicting a possible area of slope movements. Preliminaryinterpretation from this figure showed the sliding to be controlled by bedding, faultingand shearing through the weak shale rock mass. The following conclusions weredrawn from the Engineering Geology Site Assessment as follows:

1. Rock slope movements should not affect the P-2 alignment. If this route isreconsidered then there should be attention given to erosion control measures asthe alignment climbs up to the east bank of the river.

2. The P-6 alignment runs directly up through the rock slope movement area.Extensive stabilization measures would be required to prevent structural damageto the bridges.

3. The east bank river crossing of the preferred P-7 alignment is located close to thenorthern extent of the sliding movements as shown in Figure 10. However, thegeological interpretation, surface expression of slope movements and thetopography suggested that the east bank foundation is not within the slidingzone. However, the erosion control apron and bridge abutment fill beneath andout from the EBL bridge seemed to be in the area of movements. The P-7alignment may also be exposed to small-scale slope movements of loose siltyclay materials along the alignment just east of the east bank of the river alongthe ridge between the P-6 and P-7.

The Engineering Geology Assessment confirmed the prior suspicions and visualobservations, which resulting in the preliminary abandonment of the P-6 alignmentand choice of a P-7 alignment.

FIG.14. Cross-Section A-A in Fig. 10 Showing Slope MovementAlong P-6 Alignment



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Design and Construction Recommendations

Since the existing alignment did not experience any known problems because of thefault zone and there was no other choice from the point of view of acceptablegeometrics for the proposed four-lane divided Highway 3 alignment, the only practicalsolution was to minimize the risk to the east abutments of the proposed bridgestructures. From a judgment point of view considering all the factors, it was decided toshift the alignment by a further 38 metres north of the P-7 alignment. This would putthe alignment further away from steep hillside in the flood plain and minimize anyinfluence of slope instability.

OVERALL SUMMARY

This paper presents two case histories where geotechnical instrumentation played animportant and role in determining complex ground conditions and movementsoccurring at two locations in Southern Alberta. Both locations involved the re-alignment of existing highways.

A combination of desk studies and site visual observations showed concerns aboutthe suitability of certain sections of the alignments. These concerns led to the decisionof slope indicator instrumentation installation and monitoring. The results of themonitoring led to the implementation of drainage measures during construction andthe planning of slope stability remedial measures in the case of Case Study 1. In CaseStudy 2, the monitoring led to an Engineering Geology Site Assessment, whichresulted in the discovery of a major historic fault zone. This fault zone may not havebeen determined otherwise. This finding resulted in a better understanding of thesubsurface stratigraphy and avoidance of an undesirable location for two costly bridgestructures.

While it may be argued in hindsight that there was no need for such detailedgeotechnical investigations since no serious problems occurred, the information wouldbe of significant value should problems manifest during the life of the infrastructure.Nonetheless, these case studies demonstrate the importance of instrumentation and itsrole in supporting the observations, opinions, and hunches of geo-practitioners inmaking informed decisions when faced with puzzling site conditions.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the support provided by Alberta Transportationand Utilities at the time of this investigation. The opinions expressed are solely thoseof the authors and do not necessarily reflect those of any referenced organizations.

REFERENCESAGRA Earth & Environmental, (1994). “Engineering Geology Assessment of

Hwy 3 R-route Near Monarch, Alberta”. Report submitted to AlbertaTransportation and Utilities.

Irish, E.J.W. (1971). “Southern Plains”. Geological Survey of Canada, Department ofEnergy, Mines and Resources, Ottawa, Ontario.



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Documents

[American Society of Civil Engineers Seventh International Symposium on Field Measurements in Geomechanics - Boston, Massachusetts, United States (September 24-27, 2007)] 7th FMGM