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Protecting the Base of an Excavation

In anticipation of the rebuild process, many property owners have elected to not backfill or partially backfill the open excavation following the removal of the damaged foundation. Ahead of the ground freezing, residents should be aware of the following:

Moisture

The Alberta Building Code (ABC) requires that excavations, more specifically soil under the foundation, are not compromised by excessive moisture. Accumulated water in open excavations can have a negative effect on the soil and could cause issues with the integrity of the foundation.

The ABC (Articles 4.2.4.4. and 4.2.5.5.) note that property owners – and their contractors – should monitor the soil constantly and control the water in excavations. If the soils have been affected by moisture, property owners should conduct insitu (on site) testing.

In other words, when excavations are left open, the moisture content of subgrade soils will need to be continually checked. Residents and their contractors are encouraged to remove any accumulated water immediately to prevent possible degradation of the soil at the foundation.

Frost

With cooler temperatures around the corner, residents are encouraged to protect the base of the excavation or proposed excavation elevation, which includes foundations that were not removed following the wildfire, new foundations, exposed excavations and partially backfilled excavations.

Most soils are susceptible to movement in frost, and with greater moisture in the soil, the greater the movement can be expected.

Base Protection

The ABC (Article 9.12.1.3.) requires excavations be protected from freezing throughout the construction process. Freezing of the ground under a building can negatively affect the structural integrity due to possible shrinking or exapansion of the soil under a building as a result of a change in temperature.

When subgrade soils eventually thaw out, the soil can soften and reduce the bearing capacity of what was, prior to construction, considered undisturbed soil. Should this occur, geo-technical testing will be required to confirm all of the frost has come out of the soils beneath the foundation and to confirm bearing capacity.

Summary

If the base of the excavation becomes saturated with water and/or freezes, ABC notes that residents will be required to remediate prior to receiving a permit to pour a new foundation, which could come with additional costs.

Prior to rebuilding, any soils with reduced bearing capacity will have to be removed under the supervision of a geo-technical engineer, meeting the minimum ABC requirements.

Property owners and contractors will be required to provide this confirmation at the foundation inspection before any authorization to proceed further is permitted. The geo-technical engineer may have alternative solutions to resolve the reduced bearing capacity issue. These solution would have to be submitted to the Building Safety Codes Officer for review for verification.

If a foundation is not removed or a new foundation is installed and construction is delayed or continues through the fall and winter, the building, or at least the foundation will need to be heated to prevent any freezing of the subgrade. Sites shall remain fenced until the floor is on the foundation and any service trenches are backfilled.

More Information

For questions, call the Safety Codes Branch at 780-743-7813 or email inspections@rmwb.ca.

Wildfire Rebuild  [Learn more]

Winter Maintenance Zones In Effect [Learn more]