Swedish National Road & Transport Research Institute (VTI) Investigate Subgrade Deformation

As part of a project funded by the Swedish Transport Administration to investigate subgrade deformation, a dynamic triaxial apparatus with suction control was identified as a key element to assist in fulfilling the project goals, which seeks to better understand fine-grained subgrade compliance of Heavy Goods Vehicle (HGV) traffic.

Subgrades are often located in unsaturated zones, alternating between more saturated and dry states throughout its life cycle. A dynamic triaxial apparatus equipped with an unsaturated soil testing upgrade could recreate specimen conditions to specific suction values and moisture content, referring to the alternating states mentioned above. It would also allow control over a testing material, for those parameters, while running Repeated Loading Tests (RLT) to assess both its resilient and plastic responses. The apparatus chosen was GDS Instruments (GDS) Enterprise Level Dynamic Triaxial Testing System (ELDYN) with Method A UNSAT Testing Upgrade.

A testing schedule on fine-grained soils used in subgrades in Sweden was proposed. Initially, we planned to identify the Soil Water Retention Curve and then perform RLT with the aim to develop models that capture their mechanical characteristics, with suction used as one of the variables of interest, along with the applied stresses. Such models are necessary to enhance the mechanistic-empirical design method, so that it is possible to factor the weight of the subgrade into the calculations as opposed to using a design table.

The standard GDS ELDYN apparatus required some customisation, this was carried out by GDS during its manufacturing. The installation and training took place in February 2022, by GDS’ support team. This included operating the ELDYN and of all the control subsystems, unsaturated testing methods and training on their software GDSLAB. We are currently running the first trials and getting familiar with the equipment to perform a full test programme.

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