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Soil & Groundwater - Yonge North Subway Extension

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Key Findings

  • Construction activities, including tunnel construction and excavations at the launch, extraction shafts, below grade stations and emergency exit buildings will extend below the groundwater table but construction methods will be selected to minimize construction dewatering.
  • Where groundwater is extracted, the groundwater will be treated, as required, to meet the relevant municipal sewer discharge by-laws.
  • The potential for groundwater impacts will be further reviewed and documented in the Soil and Groundwater Management Strategy to be developed prior to construction, and appropriate mitigation measures implemented during construction (see Potential Effects & Mitigation Measures).

crew conducting Geotechnical Investigations<br />

Figure 1: YNSE Geotechnical Investigations

  • As the tunnels, stations, and emergency exit buildings will be designed and constructed to resist pressures exerted by groundwater, permanent groundwater dewatering systems are not anticipated to be required for these structures. Accordingly, the structures are not anticipated to interrupt long-term existing groundwater migration pathways.
  • Without mitigation, there is potential for impacts to soil (e.g., ground movement and settlement as a result of excavation). The completion of geotechnical investigations  during detailed design, completion of soil management plans as planning progresses, and implementation of appropriate mitigation measures during construction are anticipated to minimize potential impacts (see Potential Effects & Mitigation Measures).

Potential Effects & Mitigation Measures

Potential Effects:

Construction Phase

  • Dewatering, if required, may cause local and temporary drawdown of the water table.
  • If extensive dewatering is required, for example at underground stations and some emergency exit buildings, drawdown of the water table has the potential to impact the recharge of local natural surface water features, if within the dewatering Zone of Influence (ZOI; the area where the groundwater level is lowered by the dewatering activity).
  • The potential for foundation settlement as a result of dewatering at the existing structures adjacent to the new alignment that have foundations built below the local water table. 
  • Displacement of soils as a result of construction activities may result in ground movement and settlement.
  • Construction activities (e.g., excavation) could expose and/or result in the spreading of contaminated materials.

Operational Phase

  • On-going dewatering is not currently anticipated during operations.
  • No impacts to soil are anticipated during operations.

Mitigation Measures:

Construction Phase

  • Prior to construction, conduct hydrogeologic assessments at dewatering locations to estimate/confirm groundwater flow rates, predict impacts (e.g., lowering groundwater table and features potentially impacted) within the dewatering ZOI, and evaluate treatment/discharge options.
  • A Groundwater Management and Dewatering Plan will be developed to guide the handling, management, and disposal of groundwater encountered during construction and in accordance with applicable regulatory requirements.
  • Best Management Practices (BMPs) will be implemented for managing groundwater, and contaminated groundwater will be managed in accordance with applicable legislation and regulations, such as O. Reg. 347 (as amended), O. Reg. 387/04, O. Reg. 63/16 and municipal discharge bylaws, in addition to the Project Groundwater Management and Dewatering Plan.
  • Complete a detailed settlement analysis as project planning progresses.
  • Employ excavation support systems as required and/or implement appropriate ground treatment to reduce the risk of ground loss during construction.
  • Develop management plans for handling, management and disposal of excavated material.
  • Develop and implement remedial action plans, risk assessment and risk mitigation plans for encountering contaminated soil.

Operational Phase

  • As on-going dewatering or impacts to soil are not currently anticipated during operations, no mitigation measures are required.

 

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