Fill, an Ongoing Challenge

 In Feature Articles

Increased density and urbanization in Ontario has created the need to infill urban areas and construct new structures on sites that do not have adequate bearing capacity at founding level.  Inadequate bearing capacity is often caused on these infill sites by undocumented fill. In the past, there was an abundance of locations to build structures on native soils with adequate bearing capacity. Soil movement from site to site was much simpler, and excess soil generated during construction would often be placed on nearby sites in an uncontrolled manner. Since uncontrolled fill is rarely relied upon for any bearing capacity, some form of modification of the soil must be undertaken in order to provide reliable support for new structures.

Conventional means of dealing with uncontrolled fill consists of removal of the existing fill and replacement with engineered fill, the use of deep foundations and structural floor slabs, or a combination thereof. Each of these means for overcoming poor soil sites come with their own risks, which could cause an increase in the project schedule and/or overall budget.

When considering the removal and replacement of fill soils some potential issues to consider include; encountering soil types and debris not identified during the geotechnical investigation; encountering contaminated soil, and; the need to support and dewater the excavation. Each of these challenges has their own inherent risks. For example, removing contaminated fill can greatly increase the projects schedule and budget with increased costs for disposal, longer haul distances, and the need for analytical testing of the soil. Management of “clean fill” has been a concern in Ontario for the past 20-25 years, that being said, controls have tightened considerably in the last few years and the risk of finding dirty fill is significant.

The use of deep foundations to mitigate uncontrolled fill is also a common technique. Generally, deep foundations consist of either drilled caissons or driven piles. Deep Foundations can provide high capacity, yet can be very costly and time consuming to construct.  Often times, the depth to suitable bearing strata is variable across a project site causing the installation costs to increase when the deep foundation elements extend beyond the negotiated depths. The extensive structural components required for spanning across high capacity deep foundations can also add to the project cost and schedule.

An increasingly common alternative to the conventional means of dealing with undocumented fill, as mentioned above, is the use of various Ground Improvement techniques to mitigate the uncontrolled fill in-situ. Ground Improvement can usually be completed for uncontrolled fill in a costly and time efficient manner, generally proving bearing capacities the same or greater than engineered fill while being able to complete the work under an earthworks permit only. Two Ground Improvement techniques commonly used for fill soils are Rapid Impact Compaction (RIC) and Geopier Rammed Aggregate Pier® (RAP) soil reinforcement.

RIC is a Ground Improvement technique generally used on cohesionless (sand) soils that can provide bearing capacities at roughly 150 kPa at Serviceability Limit States (SLS) and 250 kPa at Ultimate Limit States (ULS).  The RIC process is completed by dropping a large weight onto a large diameter tamper foot at rate of 40 to 60 blows per minute. This process is completed in a grid pattern across the site densifying the soil to depths of typically up to 6 m or more in sand soil. The RIC ground Improvement method is a very quick and cost-effective process generally covering 600 to 1000m2 per day and can be completed prior to having foundation permits in place.

RAP ground improvement techniques can be used in all soil types, providing bearing capacities of 150 to 500 kPa at SLS and 225 to 750 kPa at ULS. RAP elements are constructed by creating a cavity using either an auger or a displacement mandrel, and then placing lifts of aggregate, which are compacted using a specially designed tamper head with vertical impact energy and crowd force. This tamper head densifies the aggregate vertically and drives the aggregate laterally into the cavity sidewalls. This installation method creates columns of stiff aggregate and increases the lateral stress in the surrounding matrix soil, which combines to provide an improved composite stiffness and shear strength greatly increasing bearing capacity. The RAP elements are installed directly under foundation elements (i.e. foundations, slabs, rafts, etc) allowing for an efficient design.  RAP installation is very quick (as quick or quicker than deep foundations), can be completed in most weather conditions, can be installed under groundwater, are delivered by many different installation methods, and are versatile for any soil condition, including and especially undocumented fill.

Uncontrolled fill is a common issue encountered in the construction industry. While conventional methods of dealing with fill continue to work, Ground Improvement is an increasingly common method to mitigate the risks associated with uncontrolled fill allowing projects to be constructed cheaper and faster.   Thousands of undocumented fill sites are supported on ground improvement in Canada and the US – We invite you to consider it as the viable third option to Dig-Replace and Deep Foundations.

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