What is void? The evolution of voids beneath roads and quay lead to major pavement failures resulting to huge financial loss, man-hour loss, accident to human and equipment damage.
Voids mostly develop due to collapse and erosion of the base and subgrade materials. Void-related road and quay problems have often occurred in areas that are prone to water seepage, either from the natural waterbodies or water pipes. Water seepage and pipe leaks allow sand to be carried away, resulting in foundation erosion and formation of weak areas, which eventually become voids. Voids continue to increase in size until the load carrying capacity of the road or quay is compromised.
The hazards associated with the development of voids cannot be over emphasized hence early detection is an important aspect of infrastructure and asset management. Early detection of hidden subsurface voids under a roadway or quay structure is vital to preventing major failures from happening.
An important geophysical tool for void detection is the ground penetrating radar (GPR) technique. GPR is a simple tool that consist of three main parts namely the control unit, the antenna, and the survey cart.
The control unit is a small PC is used to select electromagnetic pulses which are transmitted through specialized antenna into the soil/ground. These pulses travel through the subsurface and are reflected. The reflected pulses are gathered, analysed for its derived electrical properties and subsurface characteristics. The survey wheel is responsible for movability and distance measurement during survey.
Voids can be either air or water filled; each type of void has a very different GPR signature. GPR technology is very effective in identifying areas with changes in physical properties, including potentially hazardous voids.
Void identified using GPR at a quay site in Lagos Nigeria