Caroline Street Private Residences
Differential Settlement Control Solution for a 23 Storey Residential Tower
Highrise condominium construction continues to be a red-hot trend across Ontario, and development is only increasing at an exponential rate. Hotbed areas of this activity include Toronto to the north/east, and well westward into communities like Newmarket, Milton, Hamilton, Waterloo, and London, to name a few. These areas and many more are at the forefront of a significant push for higher density development.
Typical components of tall buildings consist of a tower and a podium (base). The tower section of a building provides the vertical density and contributes to the majority of the building’s mass. The podium interfaces with the street level and its height governed by local tall building requirements to ensure a consistent feel, look and function within the surrounding neighbourhood. Often, its these types of buildings that tend to be shorter to better integrate into the surrounding space that tends to be shorter in height. There’s a very good light read on all of this in the Tall Building Guidelines, created by the Town of Milton.
Both the tower and base are of course supported by the structure’s foundation system whose responsibility it is to control the total settlement of the structure throughout its construction and service life and to ensure that differential movement between foundation elements is appropriately managed to within acceptable limits.
While most of the high-rise development along the lakeshore in Toronto has been blessed with bedrock within reach of their underground levels, other sites are not as lucky. Poor soil conditions can restrict building heights, reducing revenue for developers, unless they turn to expensive foundations options to gain height.
This was the case with the Caroline Street Private Residences in Waterloo, Ontario where the developer planned to construct a 23-storey tower with an adjoining three-storey podium. Both the tower and the podium were connected with one level of underground parking, and the proposed foundation design consisted of a raft foundation to support the tower and standard spread footings to support the podiums.
The tower’s raft foundation was heavily loaded, with service pressures ranging from 200 to 465 kPa (SLS). The podium, on the other hand, had column loads as light as 430 kN (SLS) and were designed for spread footing support with at a maximum service pressure of 180 kPa SLS.
The soil conditions below the basement level generally consisted of a compact to dense deposit of silts and sands followed by a very stiff to hard deposit of silty clay from 22.5 to 27 m below the underside of the raft. The geotechnical boreholes were terminated below the clay soils in a dense to very dense silt to silty sand. A variety of advanced geotechnical tests were conducted by the geotechnical engineer (SOLA Engineering) to confirm soils conditions due to the importance of getting the settlement control solution right. Pressuremeter (PMT) testing was conducted identifying direct E modulus values that could be used to calculated composite E modulus parameters for the improved soil, by layer.
During design development, the team identified that differential settlement between the podium’s spread footings and the tower’s raft foundation was going to be a challenge. Based on the available geotechnical information, as much as 100 – 150 mm (4 – 6 inches) of settlement was predicted for the tower’s raft, while minimal settlements were expected for the more lightly loaded podium. Two challenges arose from this: the differential settlement between the tower and the podiums was not tolerable and, the tower’s raft foundation would need extra thickness (greater than 3 m) and reinforcement to maintain its rigidity to better manage differential settlements within the raft itself.
The design team evaluated several foundation solutions to mitigate the risk of total and differential settlements for this project including putting everything on caissons or driven piles, and Geopier Rammed Aggregate Pier® ground improvement. The piling options were quickly dismissed due to the high costs associated with the deep foundation installation as well as the added costs associated with pile caps, grade beams, and structural slabs.
Ground improvement with simple raft and spread footing construction proved to be the most cost-effective. The Geopier Rammed Aggregate Pier® system was installed using the Impact method. This system was selected because it would provide outstanding settlement control and the ability to easily provide variable stiffness below the different building areas to match settlements across the structure. Additionally, the Geopier® system also represented the lowest cost sub-foundation option, with the easiest foundation system (raft and spread footing).
A differential settlement criterion of 19 mm between the spread footings and raft was required by the structural engineer. It was also determined that the differential settlement within the raft needed to be optimized to reduce the total thickness and cost of the raft. Detailed analysis using 3D settlement software and an iterative process with the structural engineer (Stephenson Engineering), afforded convergence on a customized ground improvement solution that tailored the improved ground stiffness to match the variable applied pressure from the raft.
The use of the Rammed Aggregate Pier® system was designed to provide variable composite stiffness below the raft by providing greater soil reinforcement where the slab pressures where the highest and tapering out the supporting lower pressure zones. This variable stiffness approach resulted in excellent differential settlement control and allowed for a significant reduction in the raft thickness from greater than 3.0 m to a much more manageable and affordable 1.5 m thickness, with significantly reduced reinforcing steel volumes. The podium footings rested on the native soil without the need for any additional improvements.
When combined with staged construction (constructing the tower ahead of the podium and using a delay strip in between), the podium-to-tower differential settlement of 19mm was met.
A full-scale modulus load test was conducted on site to verify that the design stiffness of the rammed aggregate pier system had been achieved. Deflection of the pier during loading was measured at the top of the pier as well as on tell-tales installed at the base of the pier to evaluate the behaviour and stiffness of the pier. The results of the test at the 100% load increment of 1,400 kPa showed displacements of 13 mm, and the load test was carried out to 150% of the design load to ensure that the settlement at design loads remains well within the elastic zone of the stress-strain curve and well within tolerable limits for the design.
Naturally, settlement of the elements themselves was not the whole story – lower zone settlements from the large areal raft were also calculated to ensure that total settlements (reinforced zone and unreinforced zone below) were captured in the analysis.
Cone Penetration Testing (CPT) was carried out between installed piers at 6 locations to determine the densification of the matrix soil due to the ground improvement. At all locations cone refusal was encountered at depths of 0.9 to 6.3 m, indicating that the densification achieved had surpassed the design assumptions.
Settlement monitoring of the tower raft slab began once the raft foundation was constructed. The maximum recorded settlement at the time of termination of measurements was 17 mm within the core of the raft and 15 mm along the raft edge. These measured settlements were roughly half of what was calculated in the design. It’s expected that the substantial improvement of the matrix soil proven from the CPT results, is largely responsible for this difference. The higher than anticipated installed pier stiffness and confining pressure of the raft likely also contributed to this result.
The Rammed Aggregate Pier ground improvement system was selected based on the low cost, ease of installation, and ability to provide a variable stiffness support solution that met the demanding variable load conditions present in the raft. Further, the simplification of the raft itself and the tidy differential settlement control provided between the raft and podium (supported on regular spread footings) provided great value for the project.
Very high contact pressures were supported on the Geopier® system for this project, much higher than what you can expect to achieve with other ground improvement systems. Proven and reliable Rammed Aggregate Pier systems can provide support for tall structures and large loads. An “aggregate pier” that isn’t a Rammed Aggregate Pier, simply can’t perform in this way.