Ryzuk Geotechnical Engineering Assessment - Christie Point Water Main

INTRODUCTION

As requested, we have carried out an investigation of the subsurface soil conditions and existing underground services within the alignment of the proposed water main along Craigowan Road on Christie Point. The following summarizes the results of our investigation and provides our recommendations relating to the geotechnical aspects of the proposed new water main, particularly excavation considerations, groundwater issues, sewer bedding requirements, and road structure. Our work has been carried out in accordance with, and is subject to, the attached Terms of Engagement.

Based on the information and preliminary drawings provided to us, we understand that the proposed 250 mm diameter water main would be roughly 750 m long and would be installed below the paved portion of Craigowan Road, at a depth of 1.4 m. The proposed water main would tie into an existing water meter box in front of 2812 Shoreline Drive, and extend northward along Craigowan Road where it would terminate at the northern tip of Christie Point. There is an existing gas line and water main extending the length of Craigowan Road with associated property connections as well as some sporadic drainage culverts which we understand drain directly into the neighboring Gorge waterway (ie. no existing underground storm main). We understand the proposed water main is being installed only to service the existing fire hydrants along Craigowan Road and that the existing water main will be disconnected from the fire hydrants but remain to provide residential water service. An overview of the project area as well as the approximate locations of the existing gas and water main is provided on the attached Location Plan drawings, dwg. No.’s 8-6932-1-1 and 8-6932-1-2.

INVESTIGATION

Our investigation initially consisted of a review of historical air photos and topographical aerial imagery with a site visit on March 12th to complete a detailed visual assessment and geophysical survey of the subsurface conditions of the proposed water main alignment. The objective of our investigation was to gather information on the soil/rock conditions as well as existing underground services in the vicinity of the proposed water main alignment. The geophysical survey was completed utilizing ground penetrating radar (GPR) and electromagnetic tracer equipment in conjunction with a Tromino micro tremor instrument.

SURFACE & SUBSURFACE CONDITIONS

Notable observations of the surficial and subsurficial conditions along the proposed water main alignment include:

• Prominent bedrock exposures to the west and east of Cedar complex which extend down to Spruce building. We noted evidence of rock blasting to the rear of Cedar complex, along the eastern edge of Craigowan Road directly across from the northernmost unit within Cedar, and also at the southern end of Spruce building.
• Bedrock exposures showing evidence of blasting were also noted at the extreme northern tip of Christie Point in the vicinity of Oak and Maple complexes. Based on our review of historical air photos it appears this portion of Christie Point once had several large bedrock outcrops and undulating terrain which we suspect was leveled with a combination of blasting and filling.
• Both the Tromino micro tremor as well as the ground penetrating radar did not detect bedrock within a 2 m depth of the road surface along the proposed water main alignment.
• Based on the gentle topography elsewhere and our observations along the marine shoreline adjacent to Craigowan Road, we expect the soils within the remainder of the proposed alignment to be comprised of near surface stiff native marine clay and/or dense silty sand with some gravel (glacial till), with varying depths of miscellaneous fill above.

GEOTECHNICAL ASSESSMENT & RECOMMENDATIONS

Excavation

As mentioned previously, we understand that the excavation depth will remain constant at roughly 1.4 m for the entire water main alignment. Based on the results of our geophysical survey, we don’t expect bedrock to be encountered anywhere along the alignment within 1.4 m of the surface; however, the bedrock surface in the greater Victoria region is highly erratic and can rise and fall over very short distances. While we are quite confident with the GPR and Tromino results regarding the potential for bedrock within the proposed water main trench, they are not entirely definitive and the only way to be 100% sure is to excavate the trench. Such data is typically correlated with some sort of subsurface probing, drilling or test pitting in select areas to enhance our confidence in the results. Accordingly, if further confidence in the presence of bedrock is desired, we would recommend a few hours of supplementary probing in select areas to confirm our GPR and Tromino results. At this time, we consider there to be a remote chance that bedrock could be encountered within excavations in the following three locations (also shown on our attached Location Plans):

• At the southern end of the parking stalls directly across from the Cedar complex; we also understand that this is the desired location for the proposed new meter box.
• Along the eastern edge of Craigowan Road adjacent the visible rock cut just south of Spruce building.
• At the extreme northern end of Craigowan Road in the northeastern portion of the cul-de-sac.

Given the remaining uncertainty surrounding the presence of bedrock within the proposed excavation trench, we would recommend that you request unit rates for bedrock removal (lineal meter and cubic meter) in the case that bedrock is encountered during excavations.

Groundwater Seepage

Given the proposed trench excavations will be roughly 1.4 m deep, we expect groundwater seepage will be very minor throughout the entirety of the proposed alignment. We further believe that there exists minor potential of encountering entrapped groundwater within voids/loose regions of fill; however, we would expect these regions would locally dewater during excavation and the associated seepage would cease shortly thereafter. We understand that the lowest portions of the proposed trench excavation will still be at least 1 or 2 m above the elevation of the marine waters in adjacent Portage Inlet; accordingly, we do not believe the excavations will be influenced by tidal water influx.

Pipe Bedding & Trench Backfill

In general, bedding for the water main should consist of well-compacted 19 mm minus crushed gravel (road base). In our experience, in-place density testing of pipe bedding is typically required for gravity services; however, depending on the sensitivity of the proposed water main pipes to settlement within the bedding, it may be prudent to complete such on the proposed water main bedding prior to pipe installation. Compacted 19 mm minus crushed gravel (road base) could be used as backfill right up to the underside of asphalt grade; however, if preferred, a zone of more coarse-grained granular fill could be used above a level 300 mm above the top of the pipe, to a point 150 mm below the underside of asphalt, above which 19 mm road base should again be used. The intervening zone could consist of an imported 75 mm minus crushed rock product. Any backfill material should be placed in maximum 300 mm thick lifts and well compacted with a ‘hoe pack’, heavy diesel plate tamper, or a jumping jack compactor; if less powerful compaction equipment is utilized the maximum allowable lift thickness will be reduced. Given the proposed water main is below Craigowan Road, we recommend all trench backfill be compacted to the equivalent of 95% of Modified Proctor maximum dry density (MPMDD), and confirmatory density testing and/or proof roll testing should be carried out periodically during placement and on the finished backfill prior to paving. If such is desired, we can provide inspection and testing services during construction.

CLOSURE

In summary, given the results of our visual assessment and geophysical survey of the proposed water main alignment, we do not foresee any atypical geotechnical considerations and expect that very little to no bedrock or groundwater seepage will be encountered within the proposed excavation trench. However, if further confidence in the bedrock depth is desired, we recommend rock probing in select areas to confirm our GPR and Tromino results.

We trust the preceding is suitable for your purposes at present. Please don’t hesitate to contact us if you have any questions or require anything further.

Sincerely,
Ryzuk Geotechnical

Patrick Sails, EIT
Project Engineer

Shane Moore, P.Geo. (B.C., AB)
Review Geoscientist / Principal

Attachments: Terms of Engagement
Location Plan (South and North), dwg. No.’s 8-6932-1-1 and 8-6932-1-2