Aim

This investigation aimed to assess the structural integrity of stopbanks. Several geophysical techniques were used to identify stratigraphic changes along the length, culverts underneath and potential seepage zones within the stopbanks.

Approach

The 10km of stopbanks were investigated using several geophysical techniques:
• 2D MASW (Multi-Channel Analysis of Surface Waves)
• 250MHz GPR (Ground Penetrating Radar)
• Vs30 (Shear Wave Velocity to 30m depth)
• ERT (Electrical Resistivity Tomography)

The subsequent results from these investigations were collated to produce a 2-dimensional horizontal cross-section of the stopbanks, which allowed for an engineering analysis of any soft spots, areas of interest or utilities within its structure.

Results

In total across the sites, there were 33 2D MASW lines, 96 Vs-30 profiles, 106 GPR lines and 19 ERT profiles collected. Overall, the stopbanks showed lateral and vertical heterogeneity with a range of shear wave velocities between 100 & 250m/s. Several areas were highlighted to be of interest, with water table depths in some locations also being interpretable. Man-made features such as culverts were easily identified within the processed data, which gave further confidence to the interpreted results, even with the absence of invasive testing.

Outputs & Value

Obtaining 2-dimensional profiles of the stopbanks allows councils and engineers to better understand how the stopbanks were constructed, areas which may be compromised or more susceptible to erosion. Such information can be useful for future flood protection planning.

The geophysical investigation of the stopbanks is a cost-effective way of obtaining accurate imaging of the stopbank construction along its entire length. This provides details of problematic areas that will require further investigation while saving valuable project time and reducing overall costs.

Contact our Geophysics Team
geophysics@coco.co.nz
Wellington (+64) 04 472 7282
Christchurch (+64) 03 365 5960