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 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. is can i run it safe 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.
In this case, the design of the shoring system must be able to withstand earth pressures, hydrostatic pressures, bottom heave, equipment loads, applicable traffic and construction loads, and other surcharge loads, to allow safe construction without movement or settlement of the ground, to prevent damage or movement of adjacent structures, streets and utilities and the design be compatible with the geologic conditions and anticipated ground behaviour.
The stability of the excavation must also be maintained against sliding and bottom heave. As these walls are required to form part of the final wall system for the structure, the wall system has to be analysed for lateral pressure loading distribution on the final structure. The pile and shoring system has to also perform as a structural element for the finished structure. The analysis of a combined system has to be carried out in order to develop a computer model that would provide the anticipated behaviour of the system when subjected to the various loading and stresses during its construction as well as lifetime period.
The top down construction allows for the piles to be right on the borders or walls of adjacent properties and can usually be driven with minimal disturbances to the adjacent structures. The wall has to be designed to safely support all earth, water pressure, existing loads, permanent loads, traffic or construction loads, while protecting utilities without permitting undesirable wall deflections and ground settlements behind the wall. The secant piles can be installed in difficult ground conditions with more flexibility in the construction alignment.
The analysis of the combined wall system under the various conditions of loading and stresses that the structure would be subjected to as part of a load bearing structure was carried out. The lateral pressure distribution against the structural walls was estimated by applying certain factors known as lateral earth coefficients to the effective soil stresses and also adding water pressure to the lateral earth loads.
The tunnel and trench sections were analysed as a plane stress structure with the actual soil conditions simulated and reflected by the soil parameters recommended in the geotechnical report by the City of Calgary’s representatives. For the structural analysis of the combined wall system, all of the lateral loading was designed to be taken up the secant piles which can safely accommodate this. The interior walls of the combined wall system and tunnel sections were designed to carry the vertical loadings
However, structures must also be designed to ensure that deflections are within the acceptable limits for intended use. The deflection of the wall members depend on factors such as degree of cracking at specified load levels, cracking due to construction loads, creep and shrinkage characteristics of the concrete, modulus of elasticity and support conditions.
There is a high degree of variability of factors affecting the deformations and the procedures used for deflection computations provide only approximate results. Deflections must be calculated and compared with specified maximum permissible values. Excessive deflections in this case can present not only an aesthetic problem but a more serious functional problem such as tunnel leakage caused by possible movement of the waterproofing membrane. Immediate deflection as well as long term deflection due to creep and shrinkage for the wall system was considered for the analysis. The vertical stability of the pile wall is highly influenced by the complexity of the interaction between the different elements of the structure and the supported soil mass.
Aziah North is an experienced civil engineer with 16 years experience in the field of civil and structural engineering and project management, including 12 years experience managing public sector projects in Malaysia and United Kingdom, and 4 years experience managing transit and rail projects in Canada. Aziah is a member of the Canadian West Coast Chapter Project Management Institute.