GEOTECHNICAL PEER REVIEW OF THE STRATIGRAPHY CONDITIONS ENCOUNTERED DURING CONSTRUCTION OF THE CAISSONS ALONG NORTH WALL – UNIVERSITY OF TORONTO

As part of this review, Sirati & Partners Consultants Ltd. did not conduct any additional geotechnical and/or hydrogeological investigation to cater for a detailed characterization and modeling of the groundwater regime along the north wall. Our opinion is solely based on previous geotechnical, hydrogeological and environmental investigations conducted by others and our experience of similar projects.

Upon review of available information including the previous reports, design/construction drawings as well as conditions prevailing at the site, the following observations are:

  • An important observation about the geotechnical investigation report is that it is not site specific as it provides no direction or guidance regarding the caisson installation in the fine-grained deposit below the ground water table. There was no interpretation of the site’s geology and how this could affect the ease of construction for the project. The only direction was to dewater the site.

  • The simplified soil profile reported consist of a surficial layer of old backfill that extends to about 2 m depth, overlying an extensive compact to very dense glacial till deposit that is present to the bedrock surface at approximately 27 m depth. The groundwater table was recorded at 5 to 8 m depth, approximately. The texture of the till deposit is fine grained and varies in both the horizontal and vertical plains, from sand, silty sand silt, sandy silt with some clay.

  • The reports discuss that the excavation for the foundation pile caps will extend below the water table and would require “positive dewatering” of the site prior to construction. According to the hydrogeological report the pumping system should be capable of handling flow rates of up to 200 l/day. This value is considered to be a high flow rate and a successful argument that the difficulties experienced were caused by “an excessive amount of ground water”, would require data with some sort of measurements to suggest that the flow rate was well in excess of 200 l/day.

  • As mentioned previously, the overburden consists of a glacial deposit comprising of a compact to very dense heterogeneous mixture of clay, silt, sand, and gravel in varying proportions. The deposit is not layered and the different soil types are contained in pockets or zones of unknown sizes. Where such pockets or zones contain permeable material, e.g., sand, silty sand, and sandy silt, large volumes of pressurized ground water are present. In these circumstances, dewatering by means of well-points across the site would be at a major disadvantage since its success would depend purely on the ensuring that the well-points are well inserted into the permeable zone(s). This type of formation creates problems for caisson and soldier pile installations. When a permeable zone is breached, the pressurized ground water is suddenly released causing difficulties such as overwhelming the process of caisson or soldier pile installation.

  • Notwithstanding above, the released ground water contains eroded soil from the pocket or zone and is accompanied by the development of a “quick” condition that results in a loss of soil strength, i.e., the soil appear as very soft. Consequently, there is a perception of excessive water conditions and soft soils.

  • The difficulties experienced during construction of the caissons could have been minimized, or at best even avoided, if the nature of the deposit was consistent and uniform across the site as implied in the reports. Some procedures that could have achieved this objective include installation of relief wells across the site well in advance of the caisson and soldier pile installation. This way, the pressure in the permeable pockets would be released in a controlled manner and allowed to “bleed” continuously until equilibrium is achieved. Also, ensuring that the temporary liner is sealed in the bedrock by reaming 150 to 300 mm into it without removing the soil from inside the casing or liner. In addition, localized dewatering could be employed by strategically locating deep sumps or well points in areas where the relief well are flowing vigorously.

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