geonorth engineering ltd - prince...

GEONORTH ENGINEERING LTD.3975 18 Avenueth

Prince George, B.C., V2N 1B2

Phone 250-564-4304

Fax 250-564-9323

E-mail [email protected]

August 8, 2016

Ms. Ellen Davis File No. K-43692989 Barr RoadPrince George BC V2K 4J8

Dear Ms. Davis:

Re: Geotechnical Assessment of Slope Stability Conditions, 2989 Barr Road, Prince George, B.C.

1.0 Introduction

City of Prince George requires a geotechnical assessment of slope stability conditions aspart of a Rezoning and Official Community Plan Amendment Application you submitted for theproperty noted above. GeoNorth Engineering Ltd. carried out the assessment based on the scopeof work outlined in our proposal to you dated January 8, 2016. This letter presents the results ofthe assessment, discusses slope stability conditions at the property and providesrecommendations for construction setbacks.

The property is located in the Hart Highlands area of Prince George, northeast of theintersection of Austin Road East and Dawson Road. The southwest part of the property isproposed to be subdivided into six residential lots. The lots are located on relatively flat terrainwith about 1 m of relief, between geodetic elevations 731 and 732 m. The east boundaries of theproposed lots overlook an approximately 21 m high, moderate gradient slope, that leads down toOwl Lake. A plan showing the location of the site is on Drawing 4369-A1, attached.

2.0 Background Information

Geological Survey of Canada Map 1288A, “Surficial Geology, Prince George, BritishColumbia”, identifies the area as pitted glacial lake deposits. From our review of the geologicalinformation and air photos, the area was a delta that formed at the outlet of a large glacialmeltwater channel as it entered Glacial Lake Prince George. The delta and lake existed for up toseveral hundred years at the end of the last glaciation, about 12,000 years ago. The ridges andhollows that are common in the area, called kames and kettles respectively, are the result ofgrounded blocks of ice that were covered with the deltatic silt and sand deposits andsubsequently melted.

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GEONORTH ENGINEERING LTD.

Ms. Ellen Davis August 8, 2016

Geotechnical Assessment of Slope Stability Conditions,

2989 Barr Road, Prince George, B.C. File No. K-4369

Topography east of the lots is highly irregular with many kettles and kames. Aerialphotographs and LiDAR data from the City of Prince George website indicate the moderategradient slope is the side wall of a kettle which is currently occupied by Owl Lake. The slope isbenched, is crossed by several bladed trails and has an average gradient of about 18E. A bareearth image from the LiDAR data is shown on Drawing 4369-A2, attached.

A record of soil conditions encountered at a water well north of the property describessubsurface conditions as coarse sand to 15.2 m depth, over till to 15.4 m, over heavy sand to27.4 m depth, over gravel. This information is available from a website maintained by B.C.Ministry of Energy, Mines and Petroleum Resources.

3.0 Site Assessment

On April 21, 2016, I visited the site, observed soil and groundwater conditions in naturalexposures and shallow hand-dug test pits and surveyed slope profiles with a hip-chain andclinometer.

At the time of our visit the proposed subdivision lots were recently harvested. The lotshave about 1 m of relief and slope east at a gentle gradient down to the slope crest overlookingOwl Lake. Terrain below the crest has gentle to moderately steep gradients between 23% and62%, is irregular and is benched with several bladed trails crossing the slope. The bladed trailshave undermined parts of the uphill side of the slope, with nearly vertical cuts up to 2 m high. Profiles of the slope, taken at the locations shown on Drawing 4369-A1 and based on LiDARdata, are shown on Drawing 4369-A3, attached.

Soil conditions visible in natural exposures and in hand-dug test pits generally consist oflayered, non-plastic silt with some fine grained sand. Sandy gravel with a trace amount of fineswas encountered adjacent to the southwest corner of Owl Lake. Trees on the slopes below theproposed lots are typically straight, and are not significantly tilted or curved, which indicatesthere has been no recent slope movements due to erosion, slope creep or instability.

4.0 Slope Stability

The factor of safety against sliding typically required for new residential development is1.5 under static (non-seismic) conditions. The Association of Professional Engineers and1

Geoscientists of British Columbia, in Guidelines for Legislated Landslide Assessments forProposed Residential Developments in BC, indicate that stability conditions under seismicloading are adequate provided the factor of safety against sliding is greater than 1.0.

Dercole, F. The District of North Vancouver. Natural Hazard Risk Tolerance Criteria. Report Council,1

November 10, 2009.

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Ms. Ellen Davis August 8, 2016

Geotechnical Assessment of Slope Stability Conditions,

2989 Barr Road, Prince George, B.C. File No. K-4369

Slope instability occurs when driving forces, such as the weight of soil, groundwaterpressure and surface loads, exceed the resisting forces along a critical slip surface. The ratio ofresisting to driving forces is defined as the factor of safety against sliding. A value of 1.0 occursat impending slope failure. The factor of safety is reduced when weight is placed uphill of theneutral axis of the slide, when material is removed from the toe, and when groundwater levels inthe slope rise.

To assess stability conditions of the slope east of the site, we carried out slope stabilityanalyses using the program Slope/W by Geo-Slope International Ltd., version 2012. Theanalyses are to determine an adequate setback from the slope crest to meet current developmentstandards, discussed above. These analyses are based on limited information gathered from oursite visit, the slope geometry from LiDAR data and hip-chain and inclinometer measurements,and estimated soil strength parameters.

Our stability analyses show that the existing slope has a Factor of Safety (FoS) of about1.1. This is expected when slope gradients are near their angle of repose, about 28E for silt. Wethen checked several slip surfaces to determine the setback from the crest that would coincidewith the required FoS of 1.5. The results indicate that a setback of 5 m is required to achieve afactor of safety of 1.5 under static conditions.

5.0 Discussion and Recommendations

Changes to slope stability conditions could be affected by a rise in groundwater level anderosion. Development frequently increases groundwater levels due to removal of vegetation andthe addition of water from irrigation, as well as possible leaks from water and storm sewer mains,and swimming pools. Assuming similar water infiltration rates and no significant increases ingroundwater level, slope stability conditions are unlikely to change.

The natural silt with some fine grained sand has a relatively low permeability and ishighly susceptible to erosion. To reduce the potential for erosion we recommend not dischargingwater over the slope. We understand a storm sewer with grates and manholes is proposed nearthe crest of the slope to intercept surface water before it reaches the slope crest. This will reducethe potential for erosion and slope instability and we agree with its use.

We recommend that vegetation be retained on the slope, and that only those trees at thecrest be removed where they are a hazard to buildings. Fill placed over the slope crest will addload to the top of the slope and reduce its factor of safety against sliding. It will also disruptexisting vegetation. We therefore recommend that no fill be placed on the slope and, if thebuilding lots are raised with fill, that the toe of the fill be setback at least 5 m from the slope crestand placed at an angle no steeper than 3 horizontal to 1 vertical.

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Ms. Ellen Davis Geotechnical Assessment of Slope Stability Conditions, 2989 Barr Road, Prince George, B.C.

August 8, 2016

File No . K-4369

Based on the results of our stability analyses we recommend that permanent structures, ponds or swimming pools not be located within 5 m from the crest of the existing slope, as shown on Drawing 4369-A2 and A3. The setback does not account for unforseen erosion caused by an unexpected event such as a water main break or inadvertent redirection of water from a major storm event, for example, should they occur.

Provided the recommendations in this report are followed, the southwest part of the property identified as Lot 1, District Lot 4028 Plan 23398 is safe for the use intended.

6.0 Closure

This report was prepared by GeoNorth Engineering Ltd. for the use of Ms. Ellen Davis, City of Prince George and their consultants. It may be relied upon in considering a zoning amendment application to rezone the subject property from Agriculture and Forestry (AF) to single residential (RS2) as regulated by the City of Prince George Zoning Bylaw No. 7850, 2007. The material in this report reflects GeoNorth Engineering's judgement in light of the information available to us at the time of preparation. Any use which Third Parties make of this report, or any reliance on decisions to be made based on it, are the responsibility of such Third Parties. GeoNorth Engineering Ltd. accepts no responsibility for damages, if any, suffered by any third party as a result of decisions made or actions based on this report.

Please call the writers if any parts of this report need to be clarified or discussed in more detail.

Reviewed by, GeoNorth Engineering Ltd.

,QH~ Per: D.J. McDougall, M.Eng., P.Eng.

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GEONORTHENGINEERING LTD.

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Tel. 250-564-4304 Fax 250-564-9323

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