News
New Technologies for Moisture Mapping
Why does moisture matter?
More and more clients are coming to us seeking a better understanding of subsurface moisture patterns because high moisture content can weaken materials and whole structures. It can cause variable swelling and heave, dissolution, hydraulic cracking, washout and void formation in foundation layers and can weaken earthworks such as embankments, dams and levees.
In response, Fugro Aperio have invested in developing a package of non-destructive methods to map moisture, and there are producing promising early results in terms of defining changes in moisture content in earthworks and within and below bound pavement layers.
Flooding causes extreme challenges – but underlying moisture can also weaken roads and cause long term damage
How does it work?
Key to the approach is the fact that water has a relatively high conductivity value compared to other pavement materials and this can be identified using geophysical survey techniques.
It is possible to identify changes in moisture content by analysing the frequency response of ground penetrating radar signals, but this approach is problematic because it relies on determination of the response of material boundaries – which is not always clear in a pavement subbase, especially in wet ground.
The latest method therefore involves a combined approach measuring a number of electromagnetic properties to identify spatial variations. Ground penetrating radar is used to determine the structure so the effects of concrete or asphalt can be modelled to isolate them leaving a residual response associated with wet ground.
The system is vehicle mounted and can cover tens of kilometres per day. Positioning is achieved by GPS and distance encoder enabling reliable referencing of results to clients’ networks. The approach is effective between depths of a few tens of cm to about 1.5 metres.
There are, however some conditions in which the approach looks set to struggle, namely below reinforced concrete and within materials that are extremely variable in terms of their response to electromagnetic signals, for example metalliferous slag or ash rich subbase materials.
Promising future
Dr Rod Eddies leads the moisture mapping development team and is excited about the potential “the methodology is becoming increasingly robust and we are building a greater understanding of it’s potential and limitations, we expect the best results to come where electromagnetic moisture mapping is used to target integrated investigations such as desk top study, trial holes and CCTV drain inspection”.
The work fits neatly alongside other innovative non-destructive site investigation ventures at Fugro Aperio, particularly the use of radiometric methods to identify contamination in slag based pavement sub-layers.
We are currently seeking further trial sites, particularly where clients are aware of moisture related problems but want a better understanding of their nature and distribution. Traffic management and targeted trial holes will be required.
To find out more contact M.Thomas@fugro-aperio.com or R.Eddies@fugro-aperio.com.
