Teretek® | Foundation subsidence and movement between floors
Two buildings in a multi-storey commercial precinct: Building A - 4 storeys, Building B - single storey | Melbourne, Victoria | 2017
Two commercial buildings that are part of a complex in the outer Melbourne suburb of Caroline Springs were suffering from significant foundation damage. The two structures, Building A and Building B, were both showing signs of subsidence.
Building A is a four-storey commercial building supported on perimeter footing beams. The ground floor is an infill concrete slab laid on ground independent of the main structure. Several years of use had caused the building to become unstable, floors to become unlevel and the ground floor began moving independently from the rest of the building. The resulting compression between floors caused some of the ground floor windows to shatter.
Building B is a single-story structure located in the same complex and built on a conventional raft slab with a deepened edge beam. The ground floor was out of level and a significant void had formed under one side of the building.
The body corporate arranged a geotechnical site investigation, which revealed that the foundations of both buildings were built predominantly on a mixture of fill material and highly reactive clay. Water ingress had caused the clay to heave, resulting in building movement and damage to the structures.
Mainmark was contracted to remediate the problem using a combination of JOG Computer-Controlled Grouting (JOG) and Teretek® resin injection.
Injectors and androids located on the exterior of Building B
The scope of works comprised of the design, supply, installation and survey verification of JOG and Teretek® to improve ground conditions, fill voids and, where possible, re-level the commercial properties known as Buildings A and B. Due to excessive water ingress, the scope of work did not include soil stabilisation to the infill slabs as the owner’s corporation was required to let the ground dry and subside naturally.
Before commencing the drilling and injection works, a levelling survey was conducted by Mainmark to quantify the settlement across each building footprint and set targets for the two buildings. For Building A, settlement of up to 19mm was found across the ground floor, relative to the structure above. The main objective for Building A was ground improvement and to strengthen foundations between the bedrock and the slab. The works were to take place predominantly inside the building footprint with the aim to prevent further movement of the structure.
For Building B, the levelling survey determined that there had been some heave to the centre of the slab with a central high spot of 30mm. A void was also discovered under one side of the building. The key objectives for Building B were re-levelling and void filling. A target was set to lift the edge beams of the building towards level within a tolerance of +/-20mm. Unlike Building A, for Building B all the injection points were to be located on the exterior of the building.
Mainmark primarily used JOG, a non-invasive, highly precise system of grout injection, to remediate both buildings. This technology was originally developed in Japan and has been used extensively to re-level large, complex (and seismically settled) structures, including apartment blocks and office buildings up to 12-storeys. It works by delivering a mix of high mobility cementitious grout to targeted areas of the structure’s footings customised for the conditions presented on site. The injection process is fast and efficient, with no excavation required, allowing the existing floor slab to remain in place, with minimal disruption to the building’s occupants and owners.
The cementitious grout is applied through narrow injection ports, typically 25mm-40mm in diameter, and controlled using a central computer that can monitor up to 128 points at any one time. The process is carried out as one continuous operation, automatically circulating the grout to each point as required, allowing the structure to be raised evenly and gradually without causing further stress to the structure.
Although similar in nature, the two buildings were treated as separate projects. Building A was the first to undergo repairs. Mainmark installed 74 JOG injection points, 40mm in diameter, at varying depths of up to 2.7m in some locations. Injections points were made by hand using specialised drills to drill through to the foundations of the underlying structure. As injections this deep are rare, extra-long drill bits were custom made for the project.
As most of the injection points were inside the building, it was necessary to carefully select the injection location points, to minimise disturbance to the commercial tenants. Each point was connected to a pump via hoses leading to the building exterior. Mainmark used cable ramps and tape to minimise the visible and physical obstructions while the works were carried out. This allowed the occupants to conduct their business without significant disruption.
Two computer-control systems were used to gradually and sequentially lift the building while limiting the stress on the structure. The works at Building A were completed over six days with a maximum of 16mm lift achieved at an average of 3.5mm over the building’s footprint.
For Building B, a total of 32 injection points were installed with varying diameters of 16mm, 25mm and 40mm. These were drilled at varying depths, the deepest being 3m. All points were injected externally, targeting the building’s perimeter strip footings. A maximum of 32mm lift was achieved at an average of 18mm over the footprint of the structure. Once the JOG re-levelling works were completed, Teretek® engineered resin injection was used to fill the voids under the outer area of the building. All injection works at Building B were completed over the course of eight days.
Overall, the planning and execution of this project was completed successfully with both buildings responding well. Due to the complexity of the project, there were several challenges that the Mainmark team successfully navigated due to the unexpected thickness and extreme drilling depths required for some of the injection points. The Mainmark team of seven worked six days per week to successfully complete the entire project in less than five weeks. Nightworks were also required on one occasion.
We chose to work with Mainmark as they are widely regarded as leaders in building rectification works. While JOG is a proven technology for lifting buildings, for this project it was all about the outcomes and not the product. Collaboration and working together as a team is essential on projects like this one and that’s where Mainmark’s strength was. They had a great team and they were able to do the work with minimal interference to the occupants.
The works on both buildings met the client’s expectations. According to building consultant, Dan Wood from Sherwood Constructions