Advanced Ground Investigation Techniques to Help Limit Risk or Examine Failure. Advanced Subsurface Investigations

Advanced Ground Investigation Techniques to Help Limit Risk or Examine Failure

Overview Introduction What is geophysics? Why use it? Common Methods Seismic Ground Radar Electrical Case Studies Conclusion

Introduction What is Geophysics? A Section of Earth Sciences that Employs the Principles of Physics Large scale The Universe Small Scale Your sites!

Why use it? Imaging subsurface Geotechnical information Cover large areas rapidly / cost effectively

Imaging of Subsurface Features Layer profiles Bedrock delineation Overburden calculations / volume Location of buried objects Location of geo-hazards

Determination of Geotechnical Parameters Profiling of bedrock depth and material hardness for rippability, tunnelling and piling hardness. Assessment of layer stiffness, elastic moduli, liquefaction potential. In-situ electrical properties for earthing design soil layer resistivity.

Common Methods Seismic MASW Refraction Ground Radar / Geo Radar / GPR Electrical methods EM Conductivity Resistivity

MASW Multi channel Analysis of Surface Waves (MASW) 1. Reasonably new technique (6 7 yrs old) 2. Gaining recognition and acceptance as a Geotechnical tool 3. Can be collected as a continuous method 4. Provides information of layers and layer stiffness as 1d or 2d Vs profiles 5. Can use to calculate Poisson ratio and densities of layers 6. Is only good for investigation to approximately 30m

Time to learn something!!

1d Vs profiles

2d Vs profiles

CPT correlation using MASW

example from Pilbara Rail line Culvert Failure

Ground Penetrating Radar What is it? A shallow geophysical investigation method some 40yrs old. Is as it sounds! Radio waves or EM energy of specific frequencies are pulsed into the ground 1000 s of times per second. The energy transmits through the ground and is reflected back to a receiver on the surface when there is a change in the electrical properties of the material eg: soil to metal or plastic pipe.

Impulse Radar (GPR) GPR Trace GPR Antenna Energy -1 0 1 Ground surface Tx Rx Time

Depth and resolution of subsurface targets is frequency dependant. Lower the frequency the better the penetration but the larger the target has to be to observe a reflection from and vis versa. GPR is a continuous scanning method where as the antenna travels over the ground surface scans are recorded at set intervals of between 10mm to 500mm depending on what subsurface information is being sought. Data is digitally recorded at a minimum 16 bit resolution, very large files often result from individual profiles. Targets are identified by virtue of their shape, amplitude and phase as recorded reflections. Depth to targets is calculated from the time taken for the energy to travel too, and be reflected back from the target. This time is multiplied by the velocity of propagation of the radio energy through the material between antenna and target and divided by two to give a distance below the antenna.

Darwin East Arm Wharf

Issues! Sheet pile structure with back fill local sands Failure of surface pavement Voids occurring around buried infrastructure Possible lack of compaction of material during construction 250m section to investigate Used Ground Radar and MASW

Ground Radar Profiles

Results

MASW profiling

MASW Profile

Conclusions There is a defined low compaction layer directly under the pavement and it is this that is moving around and appears in some locations to be extending deeper under the structure. This suggests a possible issue with material loss below the water line (tide range here is 7m) and movement through the compacted back fill behind the steel sheet piles. Alternatively, the back fill material may not have achieved compaction during the construction and has since settled and created voiding that has allowed further movement.

Sid Enfield Drive Reinforced Earth wall supporting major road Concrete interlocking panels containing compacted sand backfill supporting road pavement, Sand leaking from behind panels How big are possible voids?

Vertical GPR Profile Noticeable increase in amplitude and return reflections increased voiding

Results

Conclusions The sand movement is occurring within the first 400mm behind the concrete panels and is not effecting the competency of the reinforced earth straps. The sand / fill material deeper in the structure is still compacted and supporting the pavement above. The issue appears to be legacy from construction where inadequate compaction was achieved right up against the rear face of the panels. Over time vibration from the heavy traffic above has loosed the sand which is finding its way out through perished joints.

Dam Site Spillway, Qld

Issues

Construction

Local Geology

Investigation with Ground Radar Area of spillway was 125m x 80m = 10,000m 2 Slopes 1 in 10 and 1in 2 Temperature +36 Cores limited across the site Need to locate voids and undertake investigation of weep drain pipes with CCTV. Provide interpreted report and conclusions

CCTV

Results

Conclusions Weep drainage poorly designed Main long drains sitting above cross drains so do not continuously drain. Water sits in drains and sediment not flushed Soft friable sandstone / mudstone bedrock possibly dissolving under the gravel drainage layers evident in drains. Removal of the bedrock allows gravel to settle as uncontained causing voids under slabs and movement of clay pipe sections with open joints. Voiding not excessive at present and does appear to be predominantly in the 1 in 2 sloped section.

EM Conductivity Testing Method induces current into the ground through the use of an electromagnetic field at set frequencies. Measures the variation in the recorded current caused by the effect of the ground coupling and decay of conductive response. Variations in the sub surface materials poor spaces say from loose gravels, sands to clays will effect a recordable response. Can be used to map large areas quickly to look for: buried material landfill, drums. Geomorphology karstic topo / sink holes Changes in lithology sands and gravels with clays use multiple frequency system to create a profile from different depths to a maximum of 10 15m

Rockhampton Levee Investigation

Ground Conductivity for soil assessment along Levee, Design Route

Conclusions Technological advances in computing and modern mobile electronics have never made it easier of cheaper to undertake investigations in the field and get good repeatable information recorded digitally for analysis and interpretation. Geophysical testing methods can provide a much bigger picture investigation technique which, when tied with physical testing for cross correlation is able to fill in the gaps and provide more confident interpretation of subsurface issues. The methods discussed here are rapid and cost effective solutions that can be applied to very large or very small scale investigations.

Questions?