Geotechnical Report of Impala Platinum Mine

Geotechnical Report of Impala Platinum Mine

Geotechnical Report

An engineering geological investigation was conducted on a proposed 25ML reservoir site at North 12 Shaft, Impala Platinum Mine, Rustenburg, North-West Province.

The fieldwork comprised a walk-over inspection followed by the excavation of ten test pits, profiling and sampling. In situ soil testing comprised Dynamic Penetration Super Heavy (DPSH) and Marchetti flat dilatometer (DMT) tests at selected positions.

The site is blanketed by imported mine discard sequentially underlain by black, highly active clays followed by residuum with norite bedrock at an average depth of 2m.

The imported mine discard, norite residuum and decomposed bedrock can be used for the construction of pavement layers and backfill whilst the black clay having no application should be cut to spoil.

The excavation profile indicated that the reservoir is to found on shallower bedrock in the northern portion of the site with the pump station in the southern portion where deeper bedrock occurs.

Backfill material should preferably comply with C2 class requirements.

The soil is slightly alkaline with a low corrosion potential but subsurface pipelines should preferably be coated in areas where groundwater seepage was encountered.


Geotechnical constraints encountered on-site include the following:  

  • A densely rooted profile in the top 1m of cover soils.
    • A shallow perched water table occurs at approximately 1.5m bngl.
    • The permeable cover soils are compressible and susceptible to consolidation under load.
    • Deep compaction may be inhibited by a shallow perched water table.
    • The absence of a shallow founding horizon for movement-sensitive structures.
    • The potential immediate collapse of excavations within very moist to wet material at depths in excess of 1.5m.
    • The site soils have barely adequate bearing capacity for lightly loaded structures, marginal bearing capacity for moderately loaded structures and inadequate bearing capacity for heavily loaded structures, implying that pre-treatment, the use of engineered fill and/or piling is expected.
    • There is an absence of good quality construction materials on-site for engineered fills.

Geotechnical Report: Waste Landfill Site


Geotechnical Report – EXECUTIVE SUMMARY

An investigation was carried out to assess the geotechnical constraints on three even with a combined size of 7.4 ha located on the western portion of the RBIDZ Phase 1 development in Alton, Richards Bay. The core of the proposed development comprises three gas-to-power turbines generating 400MW of electricity.

Fourteen test pits were excavated, profiled and sampled and a single geotechnical borehole was drilled in the centre of the site. Disturbed representative samples obtained during the test pitting phase and the drilling were submitted for analyses to obtain typical engineering parameters of the site soils from the surface to a depth of 29.5m. Using a DPSH, a Marchetti DMT and a Memocone CPTu, in situ probing was carried out to an average depth of 24m. The mean values of permeability, consistency, friction angle, shear modulus, undrained shear strength, OCR and unit weight were obtained. The DMT and CPTu material property values were compared and found to correlate well in the top 10m of the profile. Settlement and bearing capacities of selected structures were calculated based on the average values.

The site is blanketed by grey, loose cover soils underlain by clay-silt-sand mixes up to 29.5m. The soil profile is dominated by fine to medium-grained sand with subordinate clay and silt fractions. The 3.5m thick topsoil layer complies with G10 class pavement construction material suitable for backfill only; G5 – G8 class pavement construction material is not available on-site and will have to be imported. Two quarries are located in a radius of 50km from the site and various construction materials suitable for terracing, backfill and pavement construction materials can be obtained from these sources.

The site is located within the W12F quaternary catchment and surface run-off drains southwards.

Currently, the perched water table is at an elevated level of 1.5m bngl. The rebound of the site’s water table is attributed to a reduction in evapotranspiration caused by the felling of Eucalyptus trees, the removal of hydrophilic shrubs and grasses as well as a shortened grass cover.

The permeable sandy cover soils are underlain by an impermeable clay layer and unless properly drained, the site could become waterlogged during high rainfall periods.

The stand-up time of excavations within very moist to wet soil encountered at depths in excess of 1.5m bngl was very short and most of the test pit sidewalls collapsed whilst being excavated.  This is significant in terms of excavation stability.

The site is located on the Zululand coastal aquifer associated with unconsolidated sediments of the Maputuland Group, characterised by an intergranular aquifer with a potential yield ranging from 0.5 to 2l/s. The downward coarsening and partially concretized sand-silt layer encountered from 25.5m – 29.5m can possibly be exploited for a future raw water supply for the gas-to-power plant. Based on static water level records of a monitor borehole, six piezometers and fourteen test pit excavations, the groundwater appears to drain westwards at a shallow hydraulic gradient. This is attributed to a local variation of the ground flow regime on the site.

The shallow groundwater sources on-site can be classified as typical marine waters with similar deeper sources inclusive of Ca-HCO3 type waters associated with fresh, deeper-seated aquifers.

The hydrochemical characteristics of the groundwater should not affect concrete structures such as piles or basements negatively and unless treated, the corrosive tendency may impact negatively on industrial machinery. The groundwater is deemed unsuitable for human consumption and requires treatment.

The shear wave velocities recorded vary from a low value of 182m/s in the loose topsoil to the highest at 341m/s in the silt–sand mixes encountered from 22.1 – 23.9m, whilst the Shear Modulus fluctuated between 62MPa and 251MPa in the depth range 10.1 – 16.9m. The Richards Bay area is in a low seismically active area and therefore has a low liquefaction potential caused by natural seismic events. Other dilatometer properties were the Constrained Modulus (M) with an average of 46.9MPa and an average friction angle of Ø= 36° (lowest = 33°). A good correlation was evident between DMT and CPTu in the top 10m of the profile but the CPTu values were consistently higher in the deeper section of the profile – that is from 10 to 24m.


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