Our January 2022 newsletter is out. Take a read and see what we accomplished around the world last year, and what we look forward to this coming year.
At the request of the Government of Western Australia Public Transport Authority (PTA), we carried out a comprehensive condition assessment of three Perth metro rail lines using geophysical and geotechnical test methods. During the investigation over 120km of rail track was tested using multi-frequency Ground Penetrating Radar (GPR) supplemented with test pits made at 2km intervals.
The GPR acquisition system was mobilised on a rail trolley with the GPR antennas housed in sleds suspended over the track centre line and on either side of the rail sleeper. Positioning was achieved using a GPS receiver linked to the GPR acquisition computers and digital odometer running on the rail track.
The acquired data was analysed to assess the condition of both the rail ballast and underlying formation material along the Perth heritage lines. Original construction of these lines involved laying track and ballast directly on scraped formation material rather than on engineered formation as is the current practice. Consequently, sections of the original line may exhibit track pumping and formation subsidence leading to increased likelihood of track and rail wear and ongoing maintenance costs.
The GPR dataset calibrated with the test pit logs provided information pertinent to the ballast condition including definition of thickness, moisture content and fouling; and formation condition including thickness, interlayer condition, identified defects and material type. The results of the investigation enabled the demarcation of the original formation type with sections that have since been remediated to consist of engineered formation material
The results of the investigation were provided in AutoCAD, GIS and spreadsheet formats showing the ballast and formation layer thicknesses and condition together with an assessment of track sections defined as being of high, moderate and low priority for future remediation.
As a requirement of Western Australia’s State Coastal Planning Policy, local councils are formulating Coastal Hazard Risk Management and Adaptation Plans (CHRMAP) in order to determine sections of coastline with a high vulnerability for coastal erosion and inundation due to predicted sea-level rise and increase in frequency and destructive force of storm events.
Geomorphic modelling carried out during the CHRMAP process often assumes a ‘sandy coastline’ where there is no evidence of surface rock along the shoreline or coastal dune system. Such an assumption can prove to be erroneous in predicting whether existing coastal infrastructure is in fact at risk, whilst also prohibiting new coastal land development. With the use of geophysical methods, the potential existence of rock within the subsurface which can act as hard natural barriers to slow down the rate of erosion can be ascertained.
We were commissioned by a local council within the Perth metro region to carry out a geophysical investigation over a 3.8km section of coastline as part of the CHRMAP process. During the investigation over 5km of geophysical profiling using Seismic Refraction and Multi-channel Analysis of Surface Waves (MASW) was carried out as a series of transects parallel and perpendicular to the coastline.
The geophysical datasets were processed and inverted to provide cross-sections showing variations in the seismic wave velocity of the subsurface material. The velocity sections were demarcated into velocity ranges for the generation of interpreted geological sections showing the modelled depth to top of rock relative to Mean Sea Level. The top of rock as modelled along the seismic transects were gridded to generate a Digital Terrain Model (DTM) of the underlying rock profile.
The results of the geophysical investigation proved that certain sections of coastline did in fact have competent limestone at shallow depths sufficient to act as a barrier against erosion. Furthermore, the results showed sections of vulnerable coastline where rock was not observed, enabling the council to determine which assets and infrastructure were most at risk.
Concerns were held of a possible transport mechanism for saline contaminants detected in a freshwater stream north of a water storage dam on the power station site. GBG Australia was contracted to undertake a multi-discipline geophysical investigation.
Localised boreholes provided limited information on the stratigraphy of the area. A broader understanding of the greater area was necessary to understand how fluid may potentially be transported toward the fresh water stream. To gain this broader understanding, two geophysical techniques were employed, Frequency-Domain Electromagnetics (FDEM) and Electrical Resistivity Tomography (ERT).
Prior to the geophysical investigation these sand and gravel lenses were thought to have offered the permeability required for fluid transport and therefore were deemed to likely be the transport mechanism.
The requirements of the geophysical investigation were to identify the location and extent of sedimentary layers and to aid in the definition of more permeable layers. The data would then be used to generate a groundwater conceptual model. GBG Australia carried out FDEM to cover the areas broadly between the water storage dam and a freshwater stream to the north. ERT was conducted concurrently. Three ERT profiles were positioned perpendicular to the expected direction of flow towards the freshwater stream, and at equal distances moving away from the power stations water storage dam. The locations the survey are shown in Figure 1. The rationale to these positions was to attempt to intersect any sediment lenses or migration paths, as well as to understand how they may change between the power station and the freshwater stream.
The FDEM results revealed a high conductivity response that appeared to be originating and migrating from the water storage dam in question. The results showed the response decreasing in intensity towards the freshwater stream. This was interpreted as either a deepening migration path to the north and being beyond the penetration of the instrument or some sort of barrier between the power station site and property to the north separated by an unsealed road.
The ERT results show small lenses of low resistivity, or conversely, higher conductivity, that deepen towards the freshwater stream. This corroborated the theory that a possible migration path is deepening towards the North.
GBG was asked to investigate a mineral processing plant overseas to test the ground beneath the processing plant.
Concern about the integrity and life of the structures meant that remediation or replacement was required for the plant infrastructure. The material beneath the plant has a risk of being eroded by the acidic contents of the tank. Access below the tanks was limited to tunnels lined with corrugated iron or concrete lined rooms. Concrete cores were taken using a drill to test the concrete and soil conditions.
Determining the size and location of areas that had been eroded chemically or physically required a geophysical approach. Areas of low density or voiding would impede the velocity of seismic waves. The geophones were placed at regular intervals along the length of the access tunnels – one row in the ceiling and another in the floor. Hammer blows were struck around the circumference of the tank at the surface. The travel times from the surface to each geophone were analysed and measured in order to determine variations.
By mapping the raypaths and comparing their known distances and the time for the wave to travel, the velocity was determined. Each raypath was then inverted to generate a model of the subsurface in GeoTomCG software. Because the elevation varied between the geophones, as well as the shot points, a three dimensional model could be generated. Areas of overlapping low bulk velocity raypaths were able to be modelled as discrete anomalies.
The geometry of this survey was unusual and required specific processing that can be applied to other jobs where traditional seismic surveys would be unable to generate an appropriate model.