DESIGN OF A FULL-SCALE FIELD TEST FOR EVALUATING ROCKFILL COLUMN PERFORMANCE Kendall Thiessen, Marolo Alfaro, and James Blatz Civil Engineering – University of Manitoba, Winnipeg, Manitoba, Canada ABSTRACT This paper discusses the design of a full-scale field test of rock columns used for riverbank stabilization. It outlines the instrumentation design, site preparation and preliminary analysis. The 2.1m diameter columns will be pre-drilled, backfilled with crushed limestone and compacted with a vibratory lance. The testing of the stabilized riverbank will be done by surcharging the upper portion of the bank with clay fill. A rotational failure moving horizontally through the weak clay/till interface and passing through the rock columns is expected. The results will provide evidence of the shear strains required to mobilize the shearing resistance provided by the rockfill. RÉSUMÉ Cet article discute la conception d'un essai à l’échelle réelle sur des colonnes de roche utilisées pour la stabilisation des berges. Cet article décrit la conception d'instrumentation, la préparation d'emplacement et l'analyse préliminaire. Les colonnes seront perforées, remblayées avec de la pierre à chaux écrasée et rendues compactes avec une lance vibratoire. L'essai de la berge stabilisé sera fait en surchargeant la partie supérieure de la banque avec un remblai d'argile. Une défaillance par rotation qui se déplace horizontalement par l'interface faible d’argile/till et passant par les colonnes de roche est prévue. Les résultats fourniront l'évidence des contraintes de cisaillement exigées pour mobiliser la résistance au cisaillement fournie par le remblai de roche. 1 INTRODUCTION AND BACKGROUND Landslides along the riverbanks of Winnipeg have long been a topic of concern to The City of Winnipeg, businesses, and home owners alike. Current local stabilization practices include implementing retaining structures, re-grading, granular ribs, granular shear keys, and rockfill columns. In this application, rockfill columns are constructed by pre-drilling large diameter (1.8-3.6m) shafts into the underlying till and backfilling with compact limestone. Rockfill columns have been used successfully to protect infrastructure such as the Shoal Lake Aqueduct, bridge abutments, roadways, multi-storey buildings, homes and parklands from riverbank slope failures. Rockfill columns are desirable for a number of reasons. The stabilizing mechanism of rockfill columns is similar to that of granular shear keys; weak clay is replaced by compacted crushed limestone with higher shear strength. Construction of a shear key requires an excavation perpendicular to the direction of the potential landslide, temporarily reducing the factor of safety by excavating at the toe of the slide mass. Rock column construction avoids this because one hole is drilled at a time. Rockfill columns also improve the slope stability by acting as vertical drains aiding in the relief of excess pore water pressures.

Applications of rockfill columns have been successful in that there have been no cases of catastrophic failure in the City of Winnipeg, to the knowledge of the authors. Conversely, there are concerns over the behaviour of reinforced slopes both during and after construction. Tweedie et al. (2004), Yarechewski and Tallin (2003) and Goughnour et al. (1991) all report slope movements during and after the installation of rock column stabilization works. Horizontal displacements of over 200mm have been measured over the construction period, at rates of 70 to 2500 mm/yr. Displacement rates of up to 130mm/yr have been observed in the months immediately following construction but they slow dramatically with time to rates typically less than 5mm/yr at depth (Tallin 2006). Riverbank failures in Winnipeg are often triggered by fluctuations in groundwater conditions in fall and spring. As the river level drops following the spring runoff, higher pore water pressures remain in the banks after the stabilizing effect of the floodwater is gone. During summer, the Red River level is controlled by locks at the entrance to Lake Winnipeg, and in fall the river level is dropped to the winter ice level, approximately 2m below the normal summer river level with the same effect. The purpose of this project is to improve on the existing understanding of rock column performance and to refine and optimize local design procedures. To this end, investigation, analysis and design techniques follow

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normal practice except where they are meant to compliment conventional methods. The site investigation program, laboratory testing, instrumentation, analysis and design were all done to include and exceed the methods used locally for similar slope stabilization projects. This project is a continuation of work done by others at the University of Manitoba. Advanced modeling of rockfill column stabilized slopes and the testing of rockfill and composite rockfill-clay samples in a large scale direct shear machine have been completed. 2 PROJECT DEVELOPMENT 2.1 Site Selection The test site has been provided by the City of Winnipeg. It is located in River Road park, along River Road, west of Minnetonka Street. The test site is typical of many unstable riverbank locations in Winnipeg. It is located on the outside bend of the Red River. Figure 2 is a photograph of the site taken after the fall drawdown in 2006. The site has a history of slope movements and observations at adjacent sites suggest that slope movements were deep seated in nature, making it a suitable candidate for rockfill column remediation. The test site located a suitable distance from critical infrastructure and private development. The Red River is approximately 125m wide at the test site and the elevation of the channel bottom is controlled by the underlying glacial till. 2.2 Site Investigation A comprehensive site investigation program was carried out in the summer of 2006. A topographical survey was completed, including four river-bottom cross sections extending 45m beyond the shoreline. Figure 1 shows a typical cross section with some of the installed

instrumentation. During the week of August 8th, 2006 five test-holes (TH-1 to TH-5) were drilled. The test-hole locations are shown in Figure 3. The soil stratigraphy was logged and samples were collected for laboratory testing. Four of the test-holes were drilled using solid stem augers, and the fifth was drilled using hollow stem augers to collect continuous wire-line samples. All test-holes were drilled to practical auger refusal reached in the glacial till. The soil stratigraphy consists primarily of glacial-lacustrine silty clays, overlying glacial till and limestone bedrock. A silt layer varying in thickness between 0.3 and 0.6m was encountered in TH-1 and TH-5, and was also visible along the head scarp of the slope. The clays were brown to approximately 8m below prairie before turning gray with depth. Loose and wet silt till was encountered 13m below prairie elevation. The till became dense with depth. No alluvial soils were reported at this site. Spring depositions of fine grained soils were noted in 2006 and 2007 but seasonal erosion has prevented

Figure 1. Cross Section and instrumentation profile.

Figure 2. Research Site in October, 2006.

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notable accumulation. Slope inclinometer casings with diameter of 70mm were installed in TH-1, TH-2, TH-4 and TH-5. The inclinometer casing in TH-3 was slotted over the bottom 0.3m, and backfilled with sand to 1.5m from the bottom to act as a standpipe piezometer. A string of three vibrating wire piezometers was installed in TH-2 at depths of 3, 7.3 and 11m. The vibrating wire piezometers were backfilled with cement-bentonite grout following the method outlined by Mikkelson (2003). 2.3 Laboratory Testing and Soil Characterization The purpose of the laboratory testing program was to develop site specific soil properties to be used in the design and analysis of the field test. Shear strengths were of primary interest, and they were determined through direct shear and consolidated-undrained triaxial testing. All laboratory testing was carried out at the University of Manitoba’s geotechnical laboratories. Other completed laboratory tests include moisture contents, Atterberg Limits, oedometer testing, flexible walled permeameter tests. Winnipeg area lacustrine clays are often classified into two layers: an upper over consolidated and weathered layer of silty brown clay overlying a layer of softer gray silty clay. These conditions were also found on the research site, and the soil characterization has been done accordingly. The plasticity indices for the brown and gray clay are 66 and 52% respectively, with all

natural moisture contents within the plastic range. At 3m depth the brown clay was found to have an over-consolidation ratio (OCR) of 13, and at 10m depth in the gray clay layer the OCR was 3.2. Table 1 provides a summary of the soil parameters used in the numerical stability modeling. Table 1. Material properties used for numerical analysis. Material Φ c Density degrees kPa kN/m3

Brown Clay 19 30 17 Gray Clay 13 6 17 Glacial Till 42 0 23 Limestone Rockfill 1 56 0 20 Reinforced Brown Clay (averaged) 41 17 18

Reinforced Gray Clay (averaged) 39 0 18 1 after Kim (2007) 2.4 Site Monitoring Site monitoring prior to installation of the rock columns has been carried out to observe bank performance under a full year of groundwater and surface water conditions. Monitoring of the slope inclinometers has been carried out typically every two to three weeks, with a decreased interval during critical periods. No major deep seated movements have been observed, but smaller movements (less than 10mm) have identified a possible slip surface.

Figure 3. Plan view and instrumentation layout of rockfill column field test site.

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The vibrating wire piezometers in the clay, and the standpipe piezometer in the till have been monitored every one to two weeks during non-critical periods, and two to three times per week during and following the spring floods and fall river drawdown. 3 PROJECT DESIGN 3.1 Numerical Modeling To this juncture, two stages of modeling have been completed. Preliminary modeling was done at the outset of the project to determine whether the site would be suitable. The stratigraphy and groundwater conditions were based on the geotechnical investigation report from a nearby stabilization project. Typical material properties for Lake Agassiz lacustrine clay deposits were assumed. This modeling exercise provided results with factors of safety below unity due to conservatism in pore water pressures and shear strengths. The second phase of the modeling incorporates the stratigraphy determined from the site investigation. The material properties were established from the laboratory testing program, and the ground water conditions were based on observations since August 2006. Numerical modeling of rock columns is typically done using one of two methods, either using equivalent strips or averaged strength parameters (Barksdale and Bachus 1983). Both methods are based on the area replacement ratio which is the area of the rockfill columns divided by the horizontal projection of the improved area. The area replacement ratio, Ar can be calculated as shown in equation 1 for a triangular array (Goughnour et al. 1991):

°=

30cos42

2

S

dAr

π

[1]

where S is the center to center column spacing and d is the column diameter. Using 2.1m diameter columns and 3.1m spacing, the area replacement ratio is 0.42 The equivalent strip method converts the total rockfill volume of the columns and models it as an infinite continuous strip, suitable for two-dimensional plane strain stability analysis. Analysis using the equivalent strip method resulted in calculated factor of safety improvements of 53% using the layout shown in Figure 3. Using the equivalent strip method, it has been calculated that approximately 3.5 m of fill will be required to reduce the factor of safety back to unity. Figure 4 shows a cross section from the completed modeling including the placed fill. It is assumed that there will be no vertical stress concentration in the rockfill as there will not be additional surface loading in the improved area. The average shear strength method models the improved soil as a homogenous material by averaging the strength parameters of the rockfill and clay. Equations 2 and 3 show how the average Φ and c values were calculated:

)1(

tan)1(tan)(tan

rcrs

crcsrsavg

AA

AA

−+

−+=

γγ

φγφγφ

[2]

and

)1( rcavg Acc −=

[3]

Winter Ice Level at 221.4m

3.2H:1V

3m

Glacial Till

Grey Clay

Brown Clay

Rockfill Columns

-15 -10 -5 0 5 10 15 20 25 30 35 40 45 50 55 60210

215

220

225

230

235

240

Figure 4. Cross-section of model using equivalent strip method.

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where γs and γc are the wet unit weights and Фs and Фc are the friction angles of the rockfill and clay respectively. cc is the cohesion of the clay (Barksdale and Bachus 1983). The average shear strength method provides a calculated factor of safety improvement of 39%. A final stage of stability modeling will be conducted after the field test. The full-scale field test will afford the luxury of back calculating the reinforcing contribution of the rock columns by assuming the factor of safety to be unity. At this stage all modeling has been done using limit equilibrium methods. Future modeling will be done using the finite element method to numerically replicate the porewater pressure conditions and stress states in the columns and native clays. Geo-Studio from Geo-Slope International Ltd.1 was used for all modeling. 3.2 Instrumentation The instrumentation plan for the full-scale field test will build on the existing instrumentation. Slope inclinometers and piezometers will be the primary means of data collection. Slope inclinometers will be used to determine the rates, locations, and extents of differential movements. The profile of the slip surface will be determined by installing inclinometers upslope, in-front, in-between, behind, and down slope of the rockfill columns. These inclinometers will also provide important information on the relative rates of movement at various points and at selected elevations. Inclinometers will also be offset to both sides of the centerline to help determine the shape and extents of the slide mass. A string of in-place slope inclinometers will be installed in the casing immediately in front of the upper row of rock columns. This will provide real-time data on how the rock column behaves throughout the testing process. Slope inclinometer casing 85mm in diameter will be used in the installations with the in-place inclinometers to maximize data collection once slope movement is initiated. Vibrating wire piezometers were chosen because of their ability to rapidly respond to pore water pressure fluctuations in clay, and because they can be continuously monitored with relative ease. As mentioned, a vertical string of three piezometers was installed in August, 2006. Another vibrating wire piezometer will be installed in the vicinity of the rockfill columns at the elevation of the expected slip surface to record the pore pressure response to loading and shearing. The water level in the rock columns will be measured through a casing installed in a rock column. The rock columns will be anchored in the glacial till which has a relatively high hydraulic conductivity, and thus should reflect that water level.

1 GeoStudio 2004 (Version 6.21, Build 2007) Geo-Slope International Ltd.

Figure 5. Piling rig used for pre-boring of columns.

Figure 6. Vibrolance and compacted rockfill columns.

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4 INSTALLATION AND FIELD TEST 4.1 Installation and Compaction A piling rig similar to the one pictured in Figure 5 will be used to drill the 2.1m diameter caissons. Once the drilling is completed, the hole will be backfilled immediately with 100mm down crushed limestone to the surface. The Vibrolance is then advanced to the bottom of the borehole and then raised in 0.6m increments, pausing at each depth for 1 minute to complete compaction. Compaction of the rock columns will be done with a 235 kW model 400 HL Vibrolance from PTC pictured in Figure 6. The drop in the level of the limestone backfill will be measured as a means of estimating the compacted density of the backfill. It is assumed that the lateral displacement of the rockfill into the in-situ clay is minimal. A compaction test in a steel sleeved borehole will be conducted to confirm the achieved density obtained by the compaction effort. Water will be added to aid in the column compaction. In 2001 UMA Engineering Ltd. conducted a compaction test of rock columns using 100mm down crushed limestone. In a 10m deep, steel-sleeved hole, a density increase from 1712kg/m3 to 2219kg/m3 was achieved. Without the testhole being sleeved 3% more rockfill was required to achieve the same density (assumed). From this it can be inferred that the actual reinforced cross sectional area is 3% greater than the drilled diameter for a 2.1m diameter column in similar in-situ conditions (Tallin 2001). 4.2 Testing Procedure The rockfill columns will be installed in the fall of 2007, after the drawdown of the Red River. This will allow for a larger working area, even though the columns will be installed above the normal summer water level. The field test will take place in February-March of 2008. The performance of the rock columns will be closely monitored in the three to four month period in between column installation and testing. Deep seated slope movements are expected due to loading of the upper portions of the slide mass. Clay fill will be delivered by truck and spread and track packed with a small bulldozer. Care will be taken to limit vibrations and additional loading from construction equipment. Loading of the embankment will proceed at a rate of approximately one meter of height per day. This rate should be achievable without excessive equipment requirements, but will be rapid enough to prevent pore pressures from dissipating. The test will be terminated at the point where the slope inclinometers become blocked due to shearing or excessive bending. At the point of termination, the excavation equipment on standby will unload the bank to minimize further movements. Because the slope is relatively shallow, it is anticipated that the energy dissipation associated with a failure will prevent rapid movements.

To contain the slide mass laterally, additional rows of rock columns will be installed along the edges of the test site. To prevent shallow failures from occurring, partial depth columns will be installed upslope of the main rock columns to encourage a deep seated failure. Re-grading of the slope will be conducted to control the extents of the failure and minimize the effects of frozen soil conditions After the completion of the field, the site will be remediated fully as part of the City of Winnipeg’s riverbank stabilization program. 4.3 Monitoring The site will be monitored for surface movements, deep seated movements, pore water pressures and loading conditions throughout the field test. The in-place inclinometers and vibrating wire piezometers will be monitored using a data logger linked to Slope Indicator’s Argus Web-Based Monitoring system. Monitoring the numerous instruments by hand over the testing process would not be practical. The real-time monitoring instrumentation will be critical in understanding the subsurface conditions throughout the test. A traditional slope inclinometer and data collector will be used to monitor the remaining inclinometers. The site will be surveyed after each loading stage and each morning to monitor surface movements, and determine the placed fill quantities. The truck weights will be recorded to determine the weight of soil delivered to and placed at the research site. The University of Manitoba’s geotechnical group has a 6.4m aerostat with a camera mounted on it. The aerostat and camera setup will be used to take pictures at intervals during the testing process. Fixed reference points will be installed on the ground surface allowing for relative movements to be recorded. 5 CONCLUSION Rockfill columns have proven to be a worthy riverbank stabilization technique for many projects in Manitoba, and this project aims to answer some questions regarding the performance of the method. With only one opportunity to run the test, it is important to ensure its success through thoughtful design and comprehensive planning. This project will incorporate leading edge and unique monitoring technologies such as in-place slope inclinometers, real-time piezometer monitoring and aerial photography to make the most of the opportunity. It is expected that the results of this research project will contribute greatly to the understanding of rock column performance. It is anticipated that monitoring the absolute and differential movements of the slide mass and rock columns during and after construction will improve current understanding of how stresses develop in the columns. Through continuous pore pressure monitoring combined with a comprehensive laboratory program to determine the shear strengths of both the rock

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column material and the native soils, the ultimate strength of a reinforced slope will be determined. ACKNOWLEDGEMENTS The author’s would like to thank the project sponsors, including the City of Winnipeg, Subterranean (Manitoba) Ltd, UMA|AECOM Ltd., KGS Group Ltd., AMEC Earth and Environmental and NSERC for their financial support and technical guidance. NSERC Undergraduate Student Research Award winners David Kurz and Carly Delavau have also helped with various aspects of this research project. REFERENCES Barksdale, R.D., and Bachus, R.C. 1983. Design and

construction of stone columns, Report FHWA/RD-83/027, Georgia Institute of Technology, Atlanta.

Goughnour, R.R., Sung, J.T., and Ramsey, J.S. 1991. Slide correction by stone columns. Symposium on Deep Foundation Improvements: Design, Construction, and Testing, Jan 25 1990. Las Vegas, NV, USA. Publ by ASTM, Philadelphia, PA, USA, pp. 131-147.

Kim, C.S. 2007. Evaluating shear mobilization in rockfill columns used for riverbank stabilization. M.Sc., University of Manitoba, Winnipeg.

Mikkelson, P.E., and Green, G.E. 2003. Piezometers in Fully Grouted Boreholes. Symposium on Field Measurement in Geomechanics. Oslo, Norway. September, 2003, p. 10.

Tallin, J. 2006. Branch 1 aqueduct Seine River Siphon riverbank monitoring. Report, UMA Engineering Ltd., Winnipeg.

Tallin, J. 2001. Provencher Bridge: Rock Column Compaction Testing Program. Report 0727-002-01, UMA Engineering, Winnipeg.

Tallin, J. 2006. Branch 1 aqueduct Seine River Siphon riverbank monitoring. Report, UMA Engineering Ltd., Winnipeg.

Tweedie, R., Clementino, R., Papanicolas, D., Skirrow, R., and Moser, G. 2004. Stabilization of a highway embankment fill over an arch culvert using stone columns. 57th Canadian Geotechnical Conference. Quebec City, Quebec, Canada, pp. 24-31.

Yarechewski, D., and Tallin, J. 2003. Riverbank stabilization performance with rock-filled ribs/shear key and columns. 56th Canadian Geotechnical Conference 2003 NAGS. Winnipeg MB. on CD-ROM.

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