FOUNDATION FAILURE

A L.K.Q.S.S. COMPLIANCE & REGULATORY RULING

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FOUNDATION FAILURE

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No Preload - Built on Organic Soils

REPRINT FROM THE INTEGRAL NEWSLETTER

The landslide a few years ago in North Vancouver which resulted in a fatality sent a clear message to municipalities that they are accountable. So much so that a preload bylaw has started coming to the forefront for residential housing. The commercial bylaw has been in existence for some time now.

More recently than North Vancouver, was the Ptarmigan landslide in Mission where two homes had to be destroyed immediately. Eleven more homes were eventually evacuated and were demolished. The saving grace is that no one lost their life or was injured.

In the case of the Mission slide, it was determined that these homes were built on top of land clearing debris, such as tree stumps, logs, brush, etc. with some soil on top, creating a very unstable fill. Just from osmosis, voids begin to develop a few years after their construction, and continue to grow over the years. These voids are reservoirs for natural underground water drainage especially when formed along an embankment.

These are not the only conditions that can cause unstable circumstances. Especially in the Lower Mainland where lands are flood plains and/or below sea level, surface deposits are sandy, silty soil with a loose clay base that is very unstable. Along with this there is organic soil that is mainly composed of organic matter in various stages of decomposition. Organic soils are common in fens and bogs. The Burns Bog is a prime example. The profiles of these soils have an obvious absence of mineral soil particles commonly referred to as the sub strata.

These peat, silt and other organic soils present an unstable base for construction. Buildings, parking lots and other improvements on such unprepared soils will settle unevenly, thereby causing structural instability and, eventually, costly site remediation or loss of the foundation. Unstable soils must be preloaded, excavated, and replaced, or pilings must be installed to provide a stable base for foundation. Depending upon the depth of the unstable soils, removal of undesirable materials and replacement with structural fill, or the installation of pilings may be uneconomical alternatives. Up until now proper staged preloading was the only viable alternative in relatively deep, unstable soils. Piering can also now be considered.

In Canada, firms have developed an expertise in preloading and stabilizing fragile land unable to support the weight of commercial buildings and reclaiming it for profitable development. Preloading is a process whereby unstable soils are consolidated and settled by loading the site in stages with structural fill sometimes yards deep and weighing thousands of tons in a predetermined manner. This compacts and consolidates the underlying soils over a period of months and possibly years. When the short-term settlement has been exhausted, the preload that is, the amount of material above finished grade is removed, leaving the site with a stable foundation for construction.

Preloading is not a new technology; in fact, it has been in use for many years. In recent years, however, a niche has developed where contractors have capitalized on the need for sites (landfills) to receive excess structural fill generated because of multilevel underground developments. Through the relocation of this structural fill, and with the revenue generated by tipping fees charged, sites which were once not economically viable to develop are afforded the opportunity to proceed.

The process will typically begin with a review of the site’s soils by a geotechnical engineer, including soil composition, depth and other characteristics that will describe its behavior and viability for its intended use.

Along with the geotechnical consultant, the contractor will develop a staged plan to fill and preload prospective sites. Next, the subject site would be cleared, and instrumentation installed in preparation for monitoring the site elevation changes and any possible effects on surrounding properties or structures. Structural landfill materials would be sourced through local projects and excavation contractors, with tipping fees being negotiated. Initial fill depths would be designed to allow for broad access over the site, while inducing some of the dramatic, primary settlement. Over a period of months and even years, additional layers of structural fill will be added as fill, and ultimately a preload to prepare the location of any structures to be built.

The rate of settlement will be monitored over the ensuing months using surveyors to gather data from the instrumentation installed prior to preloading. The geotechnical consultant is responsible for determining the array of instrumentation to be installed, the frequency of data collection, and interpretation of the data. The rate of settling or lateral movement is typically greatest in the early months, and declines over time. The performance of the site as proven in the data will determine adjustments to the proposed plan of site filling and preloading. This will be done in consultation with the geotechnical engineer.

This sounds like a simple process. However, the key to the whole process is time. Not months but years and today time is money. Preload sites do fail. --- Why? ----Not enough time was allowed to lapse for the preload to settle properly. Not enough money was spent in preloading the site due to budget restraints.

Now that residential development has entered this process, time will be an important factor because with this type of development, time is of the essence. The quicker the complex or subdivision can be built, the larger the profit for the developer. The longer it takes and the more money that is spent on it the preload puts the cost of the property out of reach to most consumers.

Municipalities now have a preload bylaw on their books that addresses low lying lands or flood plains. So, for those owning buildings in these areas, having bylaw coverage on the insurance policy is more important than ever. The average building owner cannot afford to rebuild with no bylaw coverage in place.

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