University Of Southern Queensland School Of Civil Engineerin

University Of Southern Queenslandschool Of Civil Engineering Surveyi

Extracted from the user content, the core assignment prompt pertains to a comprehensive geological assessment for site selection of a tailings dam in a specified region. The task involves establishing a geological column, determining the geological history including tectonic setting, constructing cross sections to highlight regional geology especially around potential dam sites, and identifying and evaluating three potential dam sites based on geological data. The assessment also requires recommendations for further site investigations prior to construction.

Paper For Above instruction

Introduction

The geological framework of a region profoundly influences the safety and sustainability of engineering structures, particularly those involving water containment such as tailings dams. Given the complexity of geological features in the Ringo Diorite region, characterized by diverse rock units, fault zones, and tectonic activity, a meticulous geological assessment is critical for ensuring dam stability and environmental safety. This paper synthesizes a comprehensive geological history, structural analysis, cross-sectional interpretation, and site evaluation within the specified area to aid in selecting optimal dam sites.

Regional Geological Setting and Geological History

The region under consideration spans approximately 7 km by 5 km, embedded within a tectonically active province marked by fault reactivation and diverse lithologies. The oldest geological units are represented by the Trugeon Gneiss and Biotite Schist, which exhibit high deformation and shear zones. These gneisses and schists reflect an Archean to Proterozoic basement that has experienced multiple deformational and metamorphic events, indicative of a complex tectonic history involving crustal thickening and subsequent exhumation.

Superposition of geological units reveals that the oldest rocks, such as Trugeon Gneiss and associated amphibolites, are overlain by sedimentary and metamorphic units including Crinoidal Limestone, Deformed Moriah Mudstone, and various sandstones and shales. These sedimentary layers suggest periods of basin infilling postdating crustal stabilization. The presence of unconformities between these units supports episodic tectonic minima and sediment deposition phases.

Igneous activity is evident through various intrusive rocks, notably the Gladiator Granite, Castor Quartzite, and Pegmatites, indicating a magmatic event likely associated with tectonic extensions or subduction-related processes during Proterozoic or Phanerozoic periods. These intrusive phases have contributed to regional metamorphism grades, as seen in contact zones with hornfels and calc-silicate minerals.

Structural features such as fault zones and folds are prominent, with six major fault zones identified. The dominant fault directions include NNE and SNS, reflecting compressional tectonics. Fault reactivation has been documented along joints and shear zones, which pose risks for dam stability. The faults have displacements indicating both strike-slip and dip-slip components, emphasizing the need for careful site selection away from zones of active faulting.

Tectonic Summary

The region's tectonic setting is primarily characterized by the assembly of Proterozoic continental blocks and subsequent tectonic reactivations during the Paleozoic and Mesozoic. These tectonic events are associated with plate convergence, crustal extension, and terrane accretion. The active fault zones correspond with regional suture zones and transform faults related to the breakup of ancient supercontinents, which explain the fault reactivation phenomena influencing the stability of potential dam sites.

Geological Column

The following simplified stratigraphy is proposed, with the oldest unit at the base and the youngest at the top:

• Trugeon Gneiss – Archean basement, highly deformed, metamorphosed, with associated shear zones
• Amphibolite Schist – Mafic origin, deformation indicating late tectonic episodes
• Garnet Hornfels – Contact metamorphic zone around intrusions
• Crinoidal Limestone & Marine Sediments – Siliciclastic and carbonate deposits indicating Paleozoic shallow marine environments
• Deformed Mudstone & Sandstone – Organic-rich shales and sandstones from Mesozoic or Cenozoic basin infilling
• Intrusive Rocks (Granite, Pegmatites, Diorite) – Various magmatic phases associated with tectonic activity
• Recent Sediments & Alluvium – Present-day surface deposits affecting topography and hydrology

Such stratigraphy, faults, and structural features impact the selection of dam sites, emphasizing the necessity for detailed subsurface investigations, especially around fault zones and weak rock units.

Construction of Cross Sections

Two cross sections are constructed to interpret regional geology, one NE-SW and one NW-SE, with no vertical exaggeration. The NE-SW cross reveals the juxtaposition of metamorphic basement rocks with overlying sedimentary units and intrusions, highlighting fault zones and fold structures. The NW-SE section emphasizes the lateral continuity and dislocation along major fault zones, alongside zones of deformation and swelling indicative of fault reactivation zones. These cross sections help identify stable rock masses suitable for dam foundations and areas of potential failure due to fault proximity or weak lithologies.

Potential Dam Site Identification and Evaluation

Based on the geological and structural data, three potential dam sites are identified within different geological units to encompass a broad range of conditions:

1. Site A: Ringo Diorite Waterway
   • Geological Context: Located on intact, low-permeability Ringo Diorite, characterized by a permeability of 0.001 mD, with minimal weathering.
   • Strengths: Diorite’s strength and low permeability reduce seepage risks; stable bedrock foundation.
   • Weaknesses: Potential fault zones nearby could lead to reactivation or seepage pathways.
2. Site B: Quartzite Meadow
   • Geological Context: Situated on Castor Quartzite, with high structural competency and low permeability, though deformed zones are present.
   • Strengths: Strong, resistant rock units provide stability.
   • Weaknesses: Folds and deformation zones could influence seepage and foundation stability.
3. Site C: Sedimentary Basin
   • Geological Context: Located over Moriah Mudstone and sedimentary layers, characterized by higher permeability (45-56 mD in deformed zones).
   • Strengths: Easier excavability and less structural complexity.
   • Weaknesses: High permeability increases seepage risk, requiring extensive seepage control measures.

Comparative Analysis

Site A offers the most favorable geological conditions owing to the low permeability and strength of Diorite, promising minimal seepage and structural stability. Site B, while composed of competent quartzite, is affected by deformation zones that could compromise integrity, necessitating detailed geotechnical investigations. Site C, despite being easier to construct on, introduces significant seepage management challenges, complicating long-term stability.

Final Site Selection and Recommendations

Considering all geological, structural, and hydrological factors, Site A emerges as the most suitable location for the tailings dam, provided fault zones are carefully evaluated through geotechnical boreholes and fault-slip measurements to confirm stability. For Site B, additional geophysical surveys are recommended to delineate deformational zones accurately. Site C should be considered only if remedial seepage controls are feasible and cost-effective.

Further investigations should include detailed geotechnical testing, seismic profiling, and fault monitoring to verify site stability, assess seismic risk, and optimize design parameters. Continuous monitoring during and post-construction is crucial, especially around fault zones and areas of weak lithology, to prevent potential dam failure due to fault reactivation or unexpected geological features.

Conclusion

A comprehensive understanding of the regional geology, structural features, and site-specific conditions is essential for the safe and cost-effective construction of a tailings dam in tectonically active regions. The combination of stratigraphic analysis, fault mapping, cross-sectional interpretation, and site testing provides a robust basis for selecting the most stable and sustainable dam site, ensuring environmental protection and operational safety over the long term.

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