The use of geophysical testing for Mainmark technologies

When it comes to the use of Mainmark’s technologies, a common question we encounter is “how do you validate your work?”. As ground engineering solutions do not always have visible results, validation testing is an integral part of any project.

Geophysical testing provides a suite of tools used in geotechnical engineering to identify the soil types present and to analyse their expected behaviour. Mainmark utilises these tests to determine project suitability and design. By comparing the ground investigation results before and after the completion of a project, we can determine if the targets set out in the design were met.

Re-levelling is a Mainmark solution that uses cementitious grout or an expanding resin to accurately lift and re-level a structure. A positive side-effect of these re-levelling approaches is ground improvement of the soils near the injection locations. While other mechanical lifting techniques can sometimes create voids beneath the foundation, our methods have a densification effect on the surrounding soil, which in turn can increase the strength of the bearing soils.

The primary measure of validation for a re-level project is before and after floor level surveys. However, a ground investigation tool called a Dynamic Cone Penetration (DCP) test is also conducted to demonstrate that the re-level process has not had a negative impact and has subsequently increased the strength of the bearing layer. Like a floor level survey, DCP tests are conducted before and after the re-level of the structure. A comparison of the before and after results from this test allow us to measure and define the increase in the bearing capacity of the soil as a result of the injection works.

Mainmark also offers shallow bearing capacity improvement as a stand-alone solution for structures with weak bearing soils. These projects specifically target the densification mechanism to achieve the increase in bearing capacity for depths less than 3 m. DCP tests are used prior to the initiation of a project to provide the pre-improvement bearing capacity (kPa) of the soil along with the target level of improvement required for the project. Post-improvement DCP tests validate if the target increase in bearing capacity was reached.

Another Mainmark solution that requires the use of geophysical testing is Terefirm® ground improvement. Terefirm is a technique used for bearing capacity increase (depths more than 3 m) and liquefaction mitigation. Ground improvement projects completed to date have treated soils as deep as 12 m below ground level, and therefore required geophysical testing tools that can be completed to greater depths. These projects also require an extensive understanding of the soil types present at the site and the expected behaviour of the soils during a seismic event.

Initial geophysical tests are required to determine if Mainmark’s ground improvement technique is a viable solution for the soil conditions present. Following this initial assessment, target design strengths and the parameters that will be used to measure improvement are set. There are many possible parameters than can be used to measure improvement and these need to be outlined prior to the commencement of a project, as these will determine the geophysical testing requirements. However, two ground investigations techniques are consistent.

On every Terefirm ground improvement project, Cone Penetrometer Tests (CPT) are conducted. This test can determine the soil type, soil strength, liquefaction triggering potential, and predicted settlement for a design earthquake. The targets for improvement are given as a percentage increase or set target values for these parameters. The injection design is then based on the targets, and post-injection CPT results are compared with the initial tests to determine if the targets were met.

Another test used on liquefaction mitigation projects is Direct-Push Cross-Hole (DPCH). This test determines the shear wave velocity of the soil which is used to determine the soil stiffness. It is similar to carrying out a CPT, with results compared before and after injection to determine the level of improvement.

All post-improvement tests are also analysed together to compare the overall performance of a structure before and after Terefirm ground improvement has been implemented.

Photo: Georgia Crosby Civil Engineer Mainmark Ground Engineering (NZ)

By Georgia Crosby

Georgia Crosby is Mainmark NZ’s Technical Manager. During her studies at the University of Canterbury she took an interest in Geotechnical Engineering and spent two summers interning at Mainmark before joining full time as a Graduate Engineer. Georgia’s current areas of focus are in the research and development of Mainmark’s solutions and internal systems, as well as, providing technical support for commercial projects.