The Calgary West Light Rail Transit Line which links the downtown area to southwest Calgary, extends 8.2 km and includes six stations, two park ‘n’ ride facilities, two bus terminal facilities, seven traction power substations, a major interchange, pedestrian and road bridges and an assortment of associated road works.
The guideway consists of approximately 3.7 km of at grade tie and ballast, 1.5 km of elevated track and 3.0 km of tunnel and trench sections. The Light Rail Transit (LRT) alignment runs through a densely populated area with many adjacent structures and utilities involved. In some locations, the tunnel invert will be at a depth of approximately 10 to 12 metres below the existing ground level.
The tunnel sections typically consist of a twin box shaped structure with external walls on either side of the track alignment, an internal dividing wall, an overlying structural roof slab and a base slab. In some segments of the alignment where there is a cross-over of the inbound and outbound tracks, the section would then be a single box without the internal wall. The trench sections consist of a U shaped trench with varying thicknesses of external or retaining walls depending on the sub-surface conditions.
Since the type of shoring and wall system is dependent on the actual stratification encountered at the particular sites, various options available were evaluated to determine one which will have minimal disturbances to adjacent structures while maximizing the usable area for development. The LRT runs through a densely populated area extending from 11th Street SW in the downtown area to 73rd Street SW.
To assist with the design of the underground structures, a geotechnical investigation was carried out along the alignment for an assessment of the existing surface and sub-surface conditions. The investigations indicated that the sub-surface conditions comprise primarily silty clay till with some sand and traces of gravel overlying bedrock. The bedrock lithology was highly variable and included layers of sandstone, siltstone, claystone, and shale.
Based on the sub-surface conditions encountered, the most practical and cost effective solution to constructing the tunnel structure is by “cut and cover” method or excavate and backfill. In order to do the excavation in a safe manner and in compliance with the Alberta Health and Safety Act, a shoring system has to be developed to best suit the site conditions, soil type and excavation depth.
However, due to the densely populated area geotechnical engineering that the alignment is on, a standard open cut excavation with significant construction area is not possible in areas where the excavation could be encroaching into private properties. A form of shoring system which forms a top down construction was considered to be more appropriate in order to minimize disturbances to the adjacent structures and properties.
In this method, the earth is excavated to the required depth with retaining walls supporting the soil at the sides. Upon completion of the excavation to the required depth, the base slab of the tunnel structure is cast at the bottom most level, followed by the side walls. Casting of concrete progresses upwards until the roof of the tunnel structure is completed and ground is then back-filled and reinstated.
The secant pile technology considered as a form of top down construction system with the added benefit of the shoring system walls forming part of the final structure was looked at as one option. Another important aspect of secant piles is the minimum vibration and noise that the system provides. Secant piles are drilled shafts that interlock to form a continuous wall. The walls are formed by constructing intersecting reinforced concrete piles, with every second or third pile typically reinforced with a wide flanged steel section or a reinforcing steel cage. With proper waterproofing and finishing, this wall can then be made to form part of the final structure for the tunnel or trench.
Secant pile walls have been used in some projects in Edmonton and Calgary and more commonly in Europe such as the Heathrow Express cofferdam project, but are not very common in Vancouver. From the geotechnical investigation report that was carried out by the City of Calgary’s engineering consultant, relevant case histories for the construction of the LRT in Edmonton were highlighted in which a similar shoring system was successfully used for building two of the underground stations. The disturbance to the surrounding structures were found to be minimal as proven by the measurement of extensometers installed in the area which recorded the short term and long term ground settlements.
For proper and economical design of pile walls and generally of any retaining wall, it is very important also that complete information on all prevailing site conditions that may affect the pile wall during its short term and long term conditions be obtained.