The Townes of Richview
As demand for new housing grows, developers look to optimize land use and maximize the number of units they can make available for each new development. Getting the most out of your site often means placing structures as close to the property line as municipalities will allow. In some cases, when developing challenging soils sites this edge-to-edge strategy can lead to additional costs and schedule implications that the project team must consider.
This was exactly the case with a new development by National Homes near Eglinton and Kipling in Etobicoke. National Homes was looking to construct six new three-storey townhome blocks, each with 10-12 units. Three separate geotechnical investigations were completed on the site. The soil conditions generally consisted of sandy silt, clayey silt, sand and silt, or silt fill soils extending to depths of 3.1 m to 6.6 m, where very dense native soils were encountered.
These fill soils were determined to be in a very loose to compact state, and deemed unsuitable for the support of the proposed townhouses. Over-excavation and replacement of the fill soils with engineered fill was proposed. Removing the fill soils adjacent to the property boundary would require the installation of a shoring system to protect the public’s right-of-way.
Proceeding with a shored excavation would not only increase the project costs, but would also delay the start of the project by requiring a professional shoring design be reviewed by the city to allow for the issuing of permits, a process that can take several months or more to complete.
Through referral by WSP, the geotechnical consultant, National Homes contacted GeoSolv to determine the feasibility of a ground improvement approach for the project. Several options were considered including the use of the Rapid Impact Compaction (RIC) system.
The RIC system involves an excavator mounted hydraulic hammer applying energy to the soils through a 5-foot diameter tamper foot, at the rate of 40-60 blows per minute. This system has been shown to be successful in generally sandy soils, and provides an improved geotechnical resistance of 150 kPa at Serviceability Limit States (SLS) and 225 kPa and Ultimate Limit States (ULS) or greater. An onboard computer diagnostic system records the number of blows and the travel of the tamper foot at each location to understand the soil behavior with successive blows.
The RIC approach was shown to deliver the maximum amount of cost and schedule savings for the project. The elevated fines content (silts and clays) in areas of the site, identified through review of the soil report and visual observation of the fill soils, led the designers at GeoSolv to be cautious of moving forward without further site-specific testing to ensure the required densification performance could be met.
To verify the effectiveness of the RIC process, three (3) separate test sections were completed prior to completing the contract work. Each test section consisted of a 40 ft. by 40 ft. grid of tamping locations near the previous borehole investigations. The test sections were located in an area of deep fill, an area of clayey silt, and an area of predominantly sand soils.
Site specific vibration monitoring was also completed during the test sections to verify that the City of Toronto Vibration Guidelines were being met. Although all criteria was met at property lines, the contractor was instructed to install a vibration isolation trench along one side of the site, as a precaution. This narrow trench, in this case backfilled with a small amount of EPS, is a measure that can be successful when working in closer proximity to potential receptors on adjacent properties.
Following the completion of the RIC test section, three confirmatory boreholes were advanced and the results were analyzed alongside the computer feedback to determine the RIC effectiveness. Ultimately, the performance of the RIC method was deemed suitable and applied to the remainder of the site. Additional confirmation tested was completed using both Standard Penetration Tests (SPT) and Piezocone Penetration Testing (CPT) prior to certifying the site for construction.
The RIC system added significant value on this development. Jason Pantalone of National Homes said, “Sure, there were significant savings on the project by going with GeoSolv’s Ground Improvement System, but the big thing GeoSolv helped us with was improving our earthworks schedule and getting our project out of the ground.”