NWT RESEARCH DATABASE

Tags: physical sciences, infrastructure planning, engineering, geotechnology

Objective(s): To determine the best proposed bridge location and road route; and to provide data to the bridge designer and the road designer.

Project Description: The goals of the hydrotechnical research are to: - learn if there are possible problem areas due to water or ice along the proposed road; - choose the best proposed bridge location so that a new bridge won't wash out like the temporary one does; and - decide on length of bridge so that abutment locations can be chosen for later geotechnical drilling sites. The goals of the geotechnical research: - to find out what kinds of soils there are at the proposed bridge site; - to learn if there is a lot of ice in the soil; - to decide what kind of foundation is best for the bridge; and - to provide data to the bridge designer and the road designer so that they can make drawings to be reviewed by the Environmental Impact Screening Committee (EISC) and Gwich’in Tribal Council (GTC) and the other regulatory agencies. In early June, at the time of spring break-up (high water), a hydrotechnical engineer will take a helicopter flight over the road alignment to take note of streams and other water bodies that affect the proposed road. He may also stop at selected locations to make observations on the ground, if there are areas that could present challenges to design or construction of the road. There will also be a site-specific hydrotechnical evaluation at the site of the proposed bridge. The site topography will be surveyed by land surveyors, and water depths will be taken at the proposed bridge location area, with the aid of a Zodiac and a sounder. The hydrotechnical engineer will be on site at the same time, making observations of the site characteristics. He hopes also to tie in the survey data to the Water Survey of Canada station on the Peel Channel. On the basis of his review of available Water Survey of Canada data from the nearest station in the Peel Channel, the topographic and hydrographic survey data, and his site observations, he will choose the preferred bridge abutment locations and bridge length. The gathered data will be used to determine the required height of the bridge. Nehtruh-EBA / Kiggiak-EBA proposes to drill a minimum of two boreholes at the proposed bridge site, one on each side of the crossing, at the proposed abutments, and possibly two additional shallower boreholes for the road approaches, again one on each side of the crossing. Drilling is likely to be in the riparian zone because the entire area is on the Mackenzie Delta; however, the two closest proposed boreholes are intended to be for the abutments which we intend to be at least as far from the stream as for a clear- span crossing according to the Department of Fisheries and Ocean’s definition. That is, the borehole locations are expected to at least be located above the ordinary high-water mark (1 in 2 year return). Furthermore, the presently-proposed bridge length of 32 m is anticipated to be adequate for a 1 in 50 year return, based on the Hamlet of Aklavik’s observations, so in this case the preferred borehole locations would be located above the 1 in 50 year return. The hydrotechnical evaluation will help pin-point the exact borehole locations. Based on the current estimate of the bridge span at 32 m, the boreholes would be located about 16 m either side of the centreline of the stream. These locations will be fine-tuned before drilling on the basis of the hydrotechnical evaluation, likely during an earlier site visit. The boreholes will be about 15 m deep. The boreholes will be approximately 15 cm in diameter, meaning that there will be some volume of soil and/or ice cuttings to be disposed of after samples are taken. Two additional shallow boreholes will be considered for the approach fills, with depths to be determined depending upon whether soils are consistent with the deeper holes. Suitable locations will be sought just off the main trail so that the boreholes will not obstruct the trail itself, but will still be representative of the best apparent bridge location. The boreholes will be logged, with changes in soil type and estimated ice content recorded during drilling. Samples of the soil and ice cuttings will be obtained from the boreholes for testing later in the laboratory. The test results will help in determining the engineering properties of the soils at the site. Nehtruh-EBA / Kiggiak-EBA plans to install PVC pipes with thermistor cables in each of the boreholes to determine what the temperature profile is at the bridge site. Knowing what the soil/ice temperatures are will help Nehtruh-EBA / Kiggiak-EBA to determine what type of foundation would be the most appropriate at the site, and will also permit potential climate change impacts on the foundation to be considered. Once the PVC pipes are installed and the boreholes backfilled, the borehole locations will be marked with lumber posts and help prevent them from being accidently run over. For the cuttings with low ice content, it is possible that some of this soil may be suitable for backfilling in the boreholes around the PVC pipes. But, because it will be difficult to compact, some of the material may have to be stockpiled and surrounded with silt fence to mitigate possible silty runoff in spring. These remaining soil cuttings may be suitable for use in the future bridge approaches, or they can saved for use as landscape fill once the bridge approaches are done. Cuttings that consist of ice or mostly ice could be spread over the site, or distributed carefully in the nearby brush in order to take advantage of the natural filtration properties of the brush. These icy cuttings could also be surrounded by silt fence if they cannot be effectively separated from the other cuttings. If no suitable solution can be found to deal with the cuttings at the site, they will be removed. If the cuttings are placed on a tarp over a sling net, they can be slung out by helicopter. Once the most suitable cuttings materials are used up in backfilling the boreholes, imported sand (slung in by helicopter) will be used to finish backfilling the boreholes up to ground level. Any leftover material will be stockpiled on site in case there is some settling of the backfill material and the boreholes need to be topped up. Leftover material could also be used later in construction. The driller will be required to have a site-specific emergency response/spill contingency plan. A copy of the plan will be provided to the GTC, EISC and other interested parties prior to the initiation of the site investigation activities. Hydrotechnical and geotechnical reports will be prepared, and drawings for the bridge and road design will be produced for input into a Project Description Report. With each phase, further consultation will take place with the regulatory authorities and community agencies. A brief summary of the findings will be prepared for general community use (estimated at 2 pages, plain language). This may also be associated with posters that announce interim consultation sessions. The Project Description Report will provide the main venue through the EISC and the GTC for community information and consultation.