Calgary The Western Light Rail Transit Line, which connects the city center with the south-west of Calgary, extends 8.2 km and includes six stations, two parks. two bus stations, seven traction power substations, the main interchange, pedestrian and road bridges and an assortment of related road works.
The guide consists of approximately 3.7 km in tie and ballast, 1.5 km of elevated road and 3.0 km of tunnels and trenches. The Light Rail Transit (LRT) transmission passes through a densely populated area with a multitude of related structures and utilities. In some places, the tunnel invert will be at a depth of about 10-12 meters below the existing ground level.
Tunnel sections usually consist of a double-box construction with outer walls on both sides of the track alignment, an inner partition wall, an overlapping structural roofing slab and a base plate. In some alignment segments where the intersection of the incoming and outgoing tracks occurs, the section will then be a separate box without an internal wall. Trench sections consist of a U-shaped trench with different thickness of external or retaining walls depending on the conditions of the subsurface layer.
Since the type of attachment system and wall depends on the actual stratification occurring at specific sites, various options have been evaluated to determine which one will have minimal distortion in adjacent structures, while maximizing the usable area for development. The LRT passes through a densely populated area, stretching from the 11th street SW in the city center to 73rd SW street.
To assist in the design of underground structures, a geotechnical study was carried out along the alignment to assess the existing surface and subsurface conditions. Studies have shown that subsurface conditions consist mainly of silt clay to sand and traces of gravel underlying bedrock. The lithology of the bedrock varied greatly and included layers of sandstone, silt, clay, and shale.
Based on the emerging surface conditions, the most practical and cost-effective solution for building a tunnel structure is a cut-and-cover method or excavation and backfill. In order to excavate in a safe manner and in accordance with the Alberta Safety and Hygiene Act, it is necessary to develop a mounting system to best suit the site conditions, soil type, and depth of excavation.
However, due to the densely populated area in which alignment is performed, standard excavation of an open section with a significant construction area is impossible in areas where excavations can penetrate private objects. It was believed that the shape of the fastening system, which forms the upper structure down, is more suitable to minimize inequalities to adjacent structures and properties.
In this method, the earth is excavated to the required depth with retaining walls supporting the soil on the sides. Upon completion of the excavation to the required depth, the base plate of the tunnel structure is cast at the lower level, and then the side walls. The casting of concrete continues until the roof of the tunnel structure is completed, and the earth is again filled and restored.
As one of the options, the cutting pile technology was considered, considered as a form of the upper construction system with the additional advantage of the fastening system walls that are part of the final structure. Another important aspect of securing piles is the minimum vibration and noise that the system provides. Second piles are drilling shafts that are blocked to form a solid wall. Walls are formed by creating intersecting reinforced concrete piles, each second or third pile usually reinforced with a wide flange steel section or frame of reinforcing steel. With proper waterproofing and finishing, this wall can be made in order to form part of the final tunnel or trench structure.
Some projects in Edmonton and Calgary used split walls of piles, and more often in Europe, such as the Heathrow couuferdam project, but not very common in Vancouver. From the geotechnical survey report, which was transmitted by an engineering consultant in the city of Calgary, relevant case histories were reviewed for the construction of an LRT in Edmonton, in which a similar fastening system was successfully used for the construction of two underground stations. established in an area where short-term and long-term land settlements are recorded.
For proper and economic construction of pile walls and in general any retaining wall, it is very important that complete information on all existing operating conditions that may affect the pile wall during its short-term and long-term conditions can be obtained.
In this case, the design of the support system must be able to withstand the ground pressure, hydraulic pressure, underturning, equipment load, operating transport and construction loads and others for an additional load to ensure the safety of the construction without moving or settling the ground to prevent damage or movement adjacent structures, streets and engineering networks, and the design should be compatible with geological conditions and expected behavior.
The stability of the excavation should also be maintained against slipping and lower impact. Since these walls should form part of the final wall system for the structure, it is necessary to use the wall system to distribute the load on the lateral pressure on the final structure. The pile and shore system should also act as a structural element for the finished structure. Analysis of the combined system should be performed in order to develop a computer model that would provide the expected behavior of the system when it is different loads and loads during its construction, as well as during its service life.
The top-down design allows piles to be directly on the boundaries or walls of adjacent properties and can usually be set in motion with minimal distortion to additional structures. The wall must be designed to safely support all the land, water pressure, existing loads, constant loads, transport or construction loads, as well as protect utilities, avoiding unwanted deflections of walls and soil settlements behind the wall. Welded piles can be installed in difficult ground conditions with greater flexibility in leveling the structure.
The system of combined walls was analyzed under different loading conditions and stresses, which the structure was subjected to as part of the supporting structure. The distribution of lateral pressure on the walls was estimated by applying certain factors, called lateral coefficients of the earth's crust, to the effective stresses of the soil, as well as adding water pressure to the lateral terrestrial loads.
The tunnel and trench sections were analyzed as a flat stress structure with real soil conditions, simulated and reflected by soil parameters recommended in a geotechnical report by representatives of the city of Calgary. For the structural analysis of the combined wall system, the entire lateral load was designed so that it could be captured by slashing piles that can safely place it. The interior walls of the combined wall system and tunnel sections were designed to withstand vertical loads.
However, structures should also be designed so that deviations are within acceptable limits for the intended use. The deflection of wall elements depends on factors such as the degree of cracking at certain load levels, cracking due to building loads, creep and shrinkage characteristics of the concrete, modulus of elasticity, and support conditions.
There is a high degree of variability of factors affecting strain, and the procedures used to calculate the deviation give only approximate results. Deflections should be calculated and compared with the specified maximum allowable values. Excessive deviations in this case can not only represent an aesthetic problem, but also a more serious functional problem, such as a tunnel leakage, caused by the possible movement of the waterproofing membrane. For analysis, an immediate deflection was considered, as well as a long-term deflection due to creep and shrinkage for the wall system. The vertical stability of the pile wall strongly depends on the complexity of the interaction between the various elements of the structure and the supported mass of the soil.
