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Transcript of GEOTECH - ARQ

ARQ GEOTECH (Pty) Ltd Capability Statement – Geotechnical - June 2017
Geotechnics has been one of ARQ’s key areas of specialisation from the outset and the past 24 years have
seen us active in a number of countries in fields as diverse as arch dam foundations, deep excavations for
high-rise buildings and piled foundations for large bridges.
Providing independent services to a significant diversity of clients in the private and public sector, ARQ’s
geotechnical engineers also offer support internally to our bridges, dams and structures divisions.
Heading up our Geotechnical Division are directors David Cameron-Ellis and Thomas O’Brien, supported
by 8 geotechnical engineers and engineering geologists. Alan Parock, though recently retired, works in
close cooperation with the geotechnical department and provides expert input where needed.
The Geotechnical Division makes use of advance 2D and 3D numerical analysis software packages, thus
ensuring we offer our clients cutting edge solutions. However, we have found hand-calculations or simple
spreadsheet models most appropriate to develop judgment in respect of certain geotechnical problems,
and will continue to use these to check and balance the more advanced methods.
Design-compliance field measurement is regularly applied to complete the investigation-analysis-design
loop or simply for the purpose of quality assurance, while the requirements of OHS and environmental
legislation are rigorously observed during investigations to ensure safety and minimum disturbance to
the environment.
We are well geared to handle geotechnical investigations ranging in size from small residential
investigations all the way through to the large scale and highly advanced investigations required for
unique and sensitive infrastructure.
Interesting or unique projects recently completed are touched on in the trailing sections.
The proposed Nwamitwa Dam is located at the
confluence of the Groot Letaba and Nwanedzi
Rivers, some 40 km east of Tzaneen, Limpopo
Province, South Africa. The dam will increase the
assurance of yield and enhance the water
resources of the Groot Letaba River in serving
the needs of the Mopani District of eastern
Limpopo Province.
above river elevation and will comprise the
following components: NWAMITWA DAM
• A central concrete section of 350 m length and a
maximum height above lowest foundation of 43.5 m,
• Zoned earthfill embankments on the left and right flanks,
1 200m and 1 900m in length respectively, with a
maximum height of 24 m,
• An outlet works with a dry well and a maximum
discharge capacity of 18 m3/s,
• A spillway with a maximum discharge capacity of
approximately 6800 m3/s,
concrete spillway is created with wrap-around
The underlying geology in the area of The Nwamitwa Dam
is Mesoaechean granitoid gneisses, specifically the Groot
Letaba Gneiss (previously Goudplaats Gneiss), which has been intruded by younger diabase dykes. ARQ
Consulting Engineers was sub-contracted to undertake the tender geotechnical investigations and to
prepare the tender design for the new Nwamitwa Dam. ARQ’s Geotechnical department was responsible
for co-ordinating and conducting the geotechnical investigation for the dam footprint and sourcing
applicable material for the construction of the dam as well as the access roads. This investigation
• Field mapping,
Super Heavy (DPSH) testing,
Penetration Testing),
• Test pitting,
the water table,
• Laboratory testing, and
• Seismic hazard assessment.
Data gained from this investigation was then used to design the various dam components. The
Geotechnical department worked in tandem with the Dams & Hydro department to develop the earth
embankment of the dam. RocScience’s RS2 finite element software was utilised to ensure the
embankment exhibited adequate margins of safety while producing a cost-effective and practical
implemented by the Department of Water and
Sanitation (DWS) in tandem with the
construction of the new Nwamitwa Dam. This
forms part of the Groot Letaba River Water
Development Project aimed at increasing the
assurance of yield of the Groot Letaba River in
serving the needs of the Mopani District of
eastern Limpopo Province. ARQ Consulting
Engineers was sub-contracted to undertake the
tender geotechnical investigations and to
prepare the tender design for the Tzaneen Dam
embankment non-overspill crest. The latter is
likely to be undertaken by means of engineered
fill and thus compatibility with the existing clay
core must be assured.
• Rotary core drilling with Standard Penetration
Tests (SPTs) on the existing embankment,
• Continuous SPT testing on the upstream slope
of the earth embankment,
for suitable construction material, and
• Comprehensive laboratory testing (including
foundation indicators, moisture-density tests,
triaxial testing and permeability testing).
This was done with the aim of confirming conditions within the existing embankment as well as sourcing
material suitable for construction of the raising
Using information gathered from the above
investigation, the Geotechnical department then
worked in conjunction with ARQ’s Dams &
Hydros department to develop a safe
configuration for the raised embankment
structure. The stability of the current
embankment was also evaluated. Design
configurations were assessed using hand
calculations, limit equilibrium and finite element
RocScience’s RS2 software was used for the finite element stability evaluation of the existing structure
embankment as well as the raising. Representative cross sections of the embankment were selected and
analysed under plane strain conditions. The use of this software ensured that acceptable strains and
stresses were calculated within the embankment and raising. It was also confirmed that factors of safety
adhering to current standards and norms were achieved.
Northam Platinum’s Booysendal Mine is situated near the town of Burgersfort in the Limpopo Province,
South Africa. As part of the mine’s recent endeavours to extend its mining activities into the Steelpoort
Valley, a new area was earmarked for the development of its Central Complex.
The Central Complex Development boasts a 30 m high, 140 m wide box-cut high wall at the foot of the
mountain in which seven portals, each 10 m wide, will be constructed to provide access to the sought-
after UG2 and Merensky platinum reefs. ARQ was responsible for the stability of the high wall and a
lateral support design was conducted based on data from a combination of 39 vertical and inclined
rotary-core-drilled boreholes. These boreholes were positioned strategically to intercept the main joints
in the rock mass, as well as important zones of weathering that will potentially influence the stability of
both the high wall (outside face) and the seven portals, the livelihood of the whole operation. The locality
of the boreholes is indicated on the pictures below.
The lateral support design for the high wall, comprising two benches, was conducted using advanced
finite element computer software to provide a life-of-mine solution. It comprises multiple rows of
Threadbar 500 soil nails and a steel-mesh-reinforced shotcrete layer. Various cross-sections of the high
wall were utilised in the design to provide a cost-effective soil nail layout. The diagram below indicates a
model of one of the cross-sections used for design purposes.
Designs were conducted for both serviceability (SLS) and ultimate limit state (ULS) load conditions.
The development also comprises a wide variety of supporting surface infrastructure on terraces which
includes a 4 000 ton raw ore silo, 8 Ml water reservoir, a crusher plant, a pollution control dam, a 140
person change house and smaller structures like offices and boardrooms. ARQ was also responsible for
the investigation and design of the foundations for these structures.
The remote Central Complex is situated some 7 km from the current functioning Booysendal North plant
and will be connected via an access road traversing the mountainous terrain. The optimised route
alignment includes eight cuttings up to 10.5 m high and 250 m long. The photographs below depict only
two of these cuttings under construction.
Further developments as part of the Booysendal Central Complex include a rope-conveyor system that
will transport raw ore material from the 4 000 ton silo at Booysendal Central to a newly acquired
processing plant some 4 km across the valley and the Groot Dwars River. This state-of-the-art system will
comprise 12 pylons up to 60 m high and will dramatically increase the raw ore processing capacity of the
new development. ARQ was responsible for the foundation investigation and design recommendations of
these structures, focussing on deep clay strata beneath the foundations coupled with high horizontal
structural forces and accompanying moments.
ARQ was appointed as geotechnical specialists in assessing and overcoming various challenges faced on the site of a large residential development in Pretoria. The two main areas of focus were:
2. Dynamic Compaction Large areas of the site were historically backfilled with reject brick rubble, ash and other uncontrolled materials. ARQ was appointed to investigate these fill locations and to derive a means to allow construction of housing units in the affected areas. After an extensive investigation, including percussion drilling, Continuous Surface Wave (CSW) testing, test pits and in-situ density testing, Rapid Impact Compaction (RIC) was determined to be the most effective means of densification. ARQ undertook an RIC compaction design using purpose programed spreadsheets and supplied the client with an RIC specification which could be used for tender. With these measures implemented, the problematic fill areas of the site could be used for construction.
1. Slope Stability
The development is located on the site of an old quarry, where a 40 m high cutting face exists. Before housing units can be constructed on top of the quarry high wall, the stability required assessment, in this case, improvement via the installation of piles. The piles interrupt the potential failure plane and thus stabilise the slope allowing construction of residential units in close proximity to the face. The design was undertaken using RocScience’s RS2 finite element analysis software and checked using various other methods. Ultimately, 30 m long, 750mm diameter reinforced concrete piles spaced at 5 m c/c were utilised to bring the probability of failure and reliability index within allowable limits.
A section of the Port Louis Ring Road (PLRR) from chainage 3 520 to 3 450, comprising a Reinforced
Earth structure of some 15 m in height, collapsed circa March 2014. Investigative drilling under the
control of ARQ of RSA was undertaken and supervised by an ARQ geotechnical engineer. The ARQ
engineer logged the core and undisturbed samples were retrieved ensuring that their integrity was
maintained. These were carefully packed in wooden boxes and sent via airfreight to the SANS accredited
Geostrada Laboratory in Pretoria, South Africa. There they were extruded, with little or no disturbance
and tested under SCU conditions. Some samples were sent to the accredited Geolab and tested under SCD
conditions such that Poisson’s ratio for the material was derived. Meticulous analysis of the test results
was undertaken in generating effective stress Mohr Coulomb and Duncan Chang hyperbolic parameters.
The situation, after removal of some of the failed area, was modelled by extracting sections from the
surveyed data within the Autocad CAD computer package and these were input to the RocScience’s RS2
version 8/9 finite element software. Remedial measures in the form of a vertical secant piled wall (1.08
m φ), supported by 7-strand, 3-level, high strength ground anchors, coupled with a geosynthetically
reinforced fill section behind the reconstructed RE wall, were incrementally modelled by the software in
up to 16 stages, mirroring actual construction practice.
Strength Reduction Factor (SRF) values in both the
serviceability (SLS) and ultimate limit state (ULS)
condition for each section were determined as
satisfying internationally accepted norms. These
values, coupled with very high reliability
predictions and numerous redundancy paths via the
chosen layout, indicated an extremely robust
The secant pile wall comprising 93 piles and the
ground anchors comprising 128 anchors with a total of 4 000 linear metres have been installed.
Monitoring of the wall movement is done on a weekly basis and the system is performing very well.
Backfilling at the anchors and the road embankment on top is currently in progress and will be completed
within the next two months.
Graphics below depict a) failed road b) finite element
model used in analysis and c) the secant pile wall and
ground anchors prior to backfilling.
Physical Address: 6 Daventry Street, Lynnwood Manor, 0081, Pretoria, South Africa
Postal Address: PO Box 76379, Lynnwood Ridge, 0040, Pretoria, South Africa
Office: +27 (12) 348 6668
Fax: +27 (12) 348 6669
Email: [email protected]