A geotechnical investigation was carried out on the proposed replacement area of the decommissioned North Plant at Tubatse Alloys which is located some 16km west of Burgersfort on the R37 to Polokwane.
The replacement plant comprises a crushing and screening plant, conveyors, daybins, a master control centre, a furnace, a water tower and coolers.
The site is located on the northern slope of Cheetah Mountain which had been terraced into five levels to accommodate the various structures of the decommissioned North Plant as well as the ring road to the raw material stockpile located in the southern portion of the terrain. The site is well drained and control of surface run-off comprises a number of cut-off trenches and concrete-lined V-drains.
The site investigation included a walk-over survey, a borehole census, the excavation, profiling and sampling of 10 test pits with a 65KW powered tractor-loader-backhoe. The soil samples were analysed by SGS Matrolab (Pty) Ltd based in Silverton, Pretoria.
In situ testing comprised 10 dynamic penetration tests (DPSH) and a single Marchetti Dilatometer probing (DMT). These tests were carried out adjacent to the test pits.
No seepage was recorded in any of the test pits, the DPSH probing was also done in ‘dry’ soils and no groundwater seeps were observed within the confines of the proposed plant layout area.
Four groundwater monitoring boreholes were recorded on site and their static water levels were measured, indicating that the groundwater level is some 18 – 24m below natural ground level with the general hydraulic gradient towards the northeast.
The North Plant site is blanketed by a thin layer of crushed slag and crusher dust underlain by sugary textured, coarse silty sand with a medium dense to dense consistency derived from in situ decomposed norite bedrock. Note that the topsoil and most of the residuum had been cut to spoil and material in deeper cuts upslope from the two baghouses comprises moderately weathered bedrock at shallow depth. Although outcrop is absent, decomposed to highly weathered bedrock was exposed in the access road cutting between the North Plant’s secondary crusher and the raw material stockpile area near the southern boundary.
Most of the structures are heavy and foundation pressures range from 150KPa for the master control centre, 650KPa for the furnace to 1375KPa for the daybins. The primary and secondary crushers were assumed to have foundation pressures of 650Kpa as well.
An analysis of the expected settlements was carried out, using the DMT data, building dimensions and the provisional foundation dimensions and pressures as provided by Tirisano Engineers.
The preliminary analysis for the various sites indicates settlements in the order of approximately: –
- MCC Building 3mm (Anticipate <=5mm)
- Daybins 17mm (Anticipate >=25mm)
- Furnace 11mm (Anticipate <=15mm)
The settlement analyses are based on limited information and are only indicative, providing a guide and first estimate to anticipated settlement. They have been based on the constrained modulus determined from the single DMT test.
The daybins, furnace and crushing and screening plant founding in the cover soils are expected to have too high a settlement to ensure a stable structure and therefore should be founded on bedrock. The MCC building has an estimated settlement of ~3mm and normal foundation construction methods will apply to this lightly loaded, single storey structure. The conveyor trestle foundations have low bearing pressures (~50KPa) and normal construction methods apply.
Note however that grubbing and clearing of the site will result in top 500 – 1000mm of the soil profile being disturbed and foundation trenches within these materials should be wetted up to optimum moisture content and compacted to at least 93% Modified AASTHO density.
The imported slag and crusher dust layer, residuum and decomposed bedrock are regarded as soft excavatable. Moderately to slightly weathered bedrock ranging from intermediate to hard excavation with very heavy ripping and some blasting may be required to excavate service trenches and foundations to refusal depth.
The silty sandy residuum complies with G10 Class pavement construction material suitable for backfill and subgrade only; amelioration of the silty sand can be done by adding a slag/crusher dust mix on a 1:1 basis which should upgrade the poor material quality to at least G7 class pavement material.
Good site drainage is required and ponding of surface run-off must be prevented.
The structures located on the middle terrace – that is the crushing and screening plant, daybins and water cooler can found on competent bedrock, some 1.5m below ngl.
The EFT plant, water tower and furnace structures are spread out over the two lower-most terraces and are underlain at an average depth of 4.8m below ngl by competent bedrock. Highly weathered bedrock with an estimated bearing capacity of at least 300KPa is present at an average depth of 2.0m.
The DPSH and DMT tests both indicated a similar dense profile and that a shallow layer of loose material was found immediately above the bedrock (assumed to be probe refusal). It is recommended that this contact zone be investigated with 3 or 4 shallow diamond core boreholes (6-12m) where structures sensitive to settlement are involved.