Discussion Paper No. 3 Geotechnical Considerations

Transcription

1 Eastern Hillsides Area Plan 2011 Discussion Paper No. 3 Geotechnical Considerations The Eastern Hillsides of Chilliwack is situated on the north slopes of the mountain ridge that separates Chilliwack River Valley from the flat agricultural land of the Fraser River Valley. It displays a relatively steep topography and faces access and servicing challenges. In some locations, slope instability is a concern. Creeks are fast flowing and deeply incised into the hillsides, resulting in a steep profile. Geotechnical issues are critical considerations in the development application process, and strong geotechnical report guidelines (with specific levels of safety) are necessary to ensure sufficient information for the evaluation of site capability and capacity. 1. Area Description The Eastern Hillsides is approximately 6 km long and 2 km wide. It has a north to northwest aspect and rises from the Fraser River lowland (El. 13 m) to mountains along the drainage divide with the Chilliwack River valley to the south. The highest levels of the Eastern Hillsides are about El. 650 m. To the south of the study area, the slopes rise sharply, culminating in four peaks, namely, Lookout Ridge (El. 706 m), Elk Mountain (El. 1,400 m), Mount Thurston (El. 1,639 m) and an unnamed peak (El. 1,582 m). The area is drained by a number of creeks in sharply chiselled channels, such as (from west to east) Calkin, Elk and Ford Creeks. They fan out into a significant number of tributaries, and not all of them are recorded on the base map of this report. 2. Bedrock Geology The bedrock in the study area comprises Cascade Range units rarely encountered in the Lower Mainland. (Mathews 1986.) They include meta-siltstone/sandstone and limestone of the Cultus Formation (Monger 1989), and shale, metasiltstone/sandstone, schist, meta-conglomerate and meta-volcanic rock that are layered to massive hard limestone of the Chilliwack Group. It is difficult to differentiate the units in these two formations because of their common elements and the tendency for the weaker rocks (typically shale) to weather recessively, thus limiting exposures.

2 Bedding surfaces documented by Monger (Monger1989) show bedding planes dipping ENE at 10 to 40. Flat, weathered shale beds and northward dipping hard schistose have been observed. Monger identifies several faults in the area but none are mapped or known in any detail. The Vedder Fault, below the Fraser Valley floor and north of the study area, may be capable of movement. Aerial photograph interpretation has located possible slope fissures, landslides (possible ancient, large bedrock landslides) and slide debris on the bedrock slopes above the southern study area limits. However, these interpreted occurrences have not been verified on the ground or by helicopter reconnaissance. In its study of the Eastern Hillsides Hazard areas, Thurber Engineering summarizes the evaluation of Golder Associates studies in 1993 and 1994 on a drawing for general reference (Thurber Engineering Ltd. 2003, Drawing ). (Please see the references at the end.) 3. Soil Conditions Detailed surficial geology conditions in the study area are not well known due to the limited site investigation by drill holes or test pits. Ancient debris flow deposits are speculated to have occurred along lower Ford Road (now part of Nixon Road). In addition, several apparently ancient alluvial fans have been found along the east side of middle Elk Creek. An unusually large alluvial and debris flood fan complex has also been identified below the gullied mountain-sides at the east end of the area. Wind-blown and glacial lake silts and clays are relatively abundant. They are highly erodible and may be subject to soil piping and caving. 4. Groundwater Conditions Little information is available on the groundwater conditions in the Eastern Hillsides. However, groundwater seepage areas have been noticed on the lower slopes and a perched water table should be expected to exist above the surface of the relatively impermeable glacial soils or bedrocks. The area typically has many springs at or near surface and they usually appear through excavation. 2

3 Figure 1 Slope Analysis 3

4 5. Geotechnical Issues 5.1 Panorama No-Build Area The Panorama No-Build Area is bounded by Patterson Road, Gilkins Creek and Patterson Brook West. The southern boundary has not been fully explored, which may extend further uphill and beyond the tentative boundary (at El. 300 m contour) suggested by Thurber Engineering. (TEL 2003, Dwg ) The No-Build Area was established in 2002 after earth movements had been detected and verified by GPS hubs. The No-Build Area is enforced through a moratorium on development within its limits and the situation remains under monitoring. 5.2 Top of Slope Hazard Setbacks A second issue concerns the potential hazard of building on top of a steep slope. A geotechnical setback is required for any building to locate near the top of a slope steeper than 50%. A detailed geotechnical study should be submitted as part of the development application to assess the risk and recommend an appropriate setback and other safety measures. Where creek channels and riparian zones are affected, the Riparian Area Regulations (RAR) would apply to setbacks from the top of the bank to all buildings, structures and pools. 5.3 Small Landslides Sixty percent or steeper slopes are susceptible to small slides, which are unpredictable be they natural or man-made events. Hence, no part of the Eastern Hillsides with such gradients (especially slopes of thick silty soils) can be considered free from slide hazard: they should be excluded from development. 5.4 Large Bedrock Landslide Hazards Large bedrock landslides and slope deformation features have been identified on steep mountain slopes above the study area by Golder Associates (GAL 1993). Many of these high slopes are unexplored and it is likely that more sites of landslides and slope deformations will be found. The nature, age, triggering mechanism and probability of reactivation of these ancient landslides are unknown. Data is also needed on the residual strength of the debris that may control its run-out to some extent. 4

5 Thurber Engineering (TEL 2001, Dwg ) shows a preliminary, conservative down-slope limit for the run-out of the landslide debris. Below that limit, residential development within the proposed potential low-risk areas may be permitted subject to further site specific study of large bedrock landslide hazards where warranted. In 2004 Thurber Engineering reviewed the preliminary limit for a development on Marble Hill Road 1 and concluded that the run-out limit should be moved uphill by at least 250 m. While this may be valid, its application to other areas in the Eastern Hillsides should await a thorough investigation of the ancient slide area 2 with helicopter reconnaissance and detailed field work. 5.5 Erodible Soil Highly erodible, silty to sandy, glacial and windblown soils occur in the study area and are susceptible to surface and sub-surface erosion. Erosion control and sediment catchment techniques will apply to developments under those soil conditions. 5.6 In-ground Storm Water Disposal The feasibility of in-ground disposal of storm water by means of subsurface drains or soak-away pits needs careful study of soil and slope stability conditions. Given the relative impermeability of shallow bedrock and silt-rich soils in the Eastern Hillsides, it is doubtful that in-ground disposal would be feasible over much of the study area. Further drainage study is necessary in order to adequately address this issue. 6. Current Development Requirements The City s Official Community Plan has designated the Eastern Hillsides as part of the Development Permit Area No. 2 for the protection of: Fish, wildlife and their habitats; and Existing and future developments from natural hazards such as flooding, debris torrents, channel bank erosion, and potential instability of adjacent slopes. All development proposals in this D.P. Area have to identify those habitats and hazards, and to submit plans to address any potential environmental and geo- 1 At 7195 Marble Hill Road. 2 This is city staff s opinion. 5

6 technical issues. Detailed information requirements are laid out in the City brochure for Development Permit Area 2, including: (i) (ii) (iii) (iv) (v) (vi) A site plan to illustrate D.P. Area 2 compliance A concept plan of the proposed development A Geo-hazard Assessment and Investigation report on: precautions and protocols of working with hazards during and after construction; soil profiles and depth of water table; setbacks from significant slopes and watercourses; and an on-site cut-and-fill scheme. A report/plan to address potential development impact on all watercourses, such as: proposed alterations or diversions of watercourses; drainage and silt abatement during construction; and drainage design showing retention/detention systems and connections with off-site storm drainage. Any diversion or work within and/or adjacent to a watercourse also requires approval from the Province of B.C. Ministry of Environment (Environmental Stewardship Division, Fish and Wildlife Branch) and the Federal Department of Fisheries and Oceans. Where fish-bearing streams or wildlife habitats are affected, the proposal must also be supported by an Environmental Consultant s report/plan that provides: a detailed inventory of fish and blue-listed wildlife species; the locations of the protected habitats; and guidelines for setback and meeting other Provincial conservation requirements; If a property owner wishes to clear land or harvest trees for commercial purposes, the development permit application shall include a report from a B.C. Licenced Professional Forester that : reviews the extent of trees to be harvested; addresses geotechnical, drainage, and habitat protection concerns; sets out a re-planting and land stabilization plan; and specifies a Tree and Green Space Management Plan. (vii) A Hazardous Tree Assessment by a Certified Arborist or Landscape Architect may also be required if there is imminent danger to the development site along the forest interface. The Assessment will identify trees to be retained and the location of new plantings for the tree density stipulated by the City. 6

7 (viii) A Fire Risk Assessment shall be submitted to the Fire Chief for new development in an identified moderate to extreme fire hazard area; 7. Opportunities and Constraints 7.1 Opportunities (i) (ii) (iii) Areas with 30% or less gradients slopes total to 55% of the Eastern Hillsides; however, some of these slopes are handicapped by other constraints as noted in this report. Thirteen potential low-risk areas have been identified for a total area of 519 ha; again, they may need to be scaled down in the future as detailed geotechnical studies become available. (See Figure 3.) Some basic utility (community water and sewer) and road systems and a small suburban settlement are in place. In brief, the basic ground work for the development of a suburban community has been laid down and the slope analysis suggests a number of opportunities for development. However, this initial impression has to be tempered with the on/off site constraints and other planning considerations. Slope Analysis of Eastern Hillsides Area Slope Gradient % of Planning Area Ha 0-10% 8.50% % 24.70% % 21.80% % 15.50% % 29.50% 369 All % 1,250 Note: Analysis by AECOM 7.2 Constraints (i) Slope with 30% or steeper gradients are considered unsuitable for development purposes according to the City s Hillside Development Guidelines, and at least 45% of the planning area falls into this category. Site planning proposals involving small-scale steep slopes (within the Potential low-risk areas) will be evaluated in the context of that development and the best practices as recommended by the Hillside Development Guidelines. 7

8 (ii) (iii) (iv) Streamside setbacks are to be in compliance with the provincial Riparian Area Regulation, and should not be closer than 15 m from the top the banks; wider setbacks may be warranted by exceptional ecological and slope stability conditions. Bedrock landslide areas have been identified on the upper slopes above the existing Eastern Hillsides developments. The debris run-out limits of those areas should be recognized as a potential geological hazard and no development should be permitted. This precaution should prevail if the runout limits and the Potential low-risk areas overlap. Nevertheless, some flexibility may be exercised if a site-specific geotechnical study can verify that the actual run-out limit lies beyond the proposed development site and that effective mitigation measures will be taken. Several creeks are known for their debris flows and represent significant hazards to future developments. Mitigation measures, including wider than standard setbacks from the stream banks, may be introduced to ensure sufficient protection of residents and properties. 8. Recommendations (i) (ii) (iii) Fully serviced hillside developments should be confined to potential lowrisk areas where major geological hazards are not known to exist, or are low and can be mitigated to a satisfactory level. However, these areas, or cells in Figure 3, need to be screened by future site-specific geotechnical studies, and economic feasibility and financial analyses in terms of servicing, road/highway improvement, and other on/off-site improvements. The actual developable areas will be significantly less than the potential low-risk areas as outlined in this report. Outside the potential low-risk areas, the existing acreage developments will continue but new acreage (10-acre plus lot) subdivisions will be limited in light of their low prospects of future road upgrade and municipal servicing (water and sewers). Developments within the potential low-risk areas must be supported by detailed geotechnical studies that identify localized geological hazards, and how they will be addressed through best practises in site planning and geotechnical engineering to minimize their environmental impact and to protect lives and properties. All development applications must comply with the requirements proposed in this report and other applicable municipal regulations, including the OCP and Development Permit Area No. 2, Hillside Development Guidelines and Tree Management (Land Development) Bylaw

9 (iv) The geological study of the Eastern Hillsides area should continue, in a comprehensive manner, in order to more accurately define or verify the hazard boundaries and the magnitude of the geotechnical risks to the existing and future developments. 9

10 Figure 2 Geotechnical Constraints 10

11 Figure 3 Constraint Overlays and Potential Development Cells 11

12 References: Major references presenting geotechnical and geological information for the Eastern Hillsides are listed below, generally in chronological order. This list does not included geotechnical reports pertaining to development of small parcels of land. Furthermore, the list is unlikely to be complete since some landowners or private developers may not have submitted their reports to the City. Also excluded from the list are numerous technical and expert opinion reports prepared for the Panorama Subdivision (identified as the Patterson Road and Stam Subdivision in reports prepared by Klohn Crippen for Dr. Morgenstern of the University of Alberta). The identification number proceeding the reference is used in this paper. 1. Mathews, W.H., Physiography of the Canadian Cordillera. Geological Survey of Canada map 1701A. 2. Monger, J.W.H., Geology of the Hope and Ashcroft Map areas, BC. Maps and :250,000 scale. 3. Cascade Engineering Ltd., Geotechnical Report Phase 1 Golf Course and Residential Development at Highway 1 and Annis Road, Chilliwack, BC. Report to Blackburn Developments Ltd. 4. Numerous other reports prepared by Cascade Engineering Ltd. for Blackburn Developments Ltd. between 1994 and 1999 on specific aspects of the development. 5. Golder Associates Ltd. (GAL), Preliminary Geotechnical Assessment Proposed Gold Spring Development Site Lookout Ridge Area near Hinkley Road, Chilliwack, BC. Report to Gold Spring Mountain Estates Ltd. 6. Golder Associates Ltd, (GAL), Geotechnical Hazard Evaluation of Proposed Marble Hill Subdivision, District of Chilliwack, BC. Report to Adriana Pacific Group. 7. Thurber Engineering Ltd. (TEL), Geotechnical Hazard Evaluation Eastern Hillsides, Chilliwack, BC. Report to Urban Systems Ltd. 8. Thurber Engineering Ltd. (TEL), Geotechnical Investigation of 7274 Marble Hill Development, Chilliwack, BC. Report to Suncor Development Corporation. 9. Thurber Engineering Ltd. (TEL),, Preliminary Engineering Infrastructure Evaluation Plan Eastern Hillsides Geotechnical Assessment. Report to UMA Engineering Ltd. 10. Thurber Engineering Ltd. (TEL),, Eastern Hillsides Hazard Areas, Chilliwack Geotechnical Assessment. Report to City of Chilliwack. 12