GEOTECH

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.

The dam indicates a maximum height of 43 m

above river elevation and will comprise the

following components: NWAMITWA DAM

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,

• The interface between the embankments and the central

concrete spillway is created with wrap-around

embankments.

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

comprised:

• Desk study of available geological information,

• Field mapping,

• Dynamic Cone Penetrometer (DCP) and Dynamic Probe

Super Heavy (DPSH) testing,

• Geophysical surveys,

• Additional rotary core drilling (incorporating Standard

Penetration Testing),

• Test pitting,

• Water pressure (Lugeon) testing and measurement of

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

configuration.

The raising of the Tzaneen Dam is being

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

raising.

The proposed raising will add an additional 3 m

to the spillway and approximately 1.5 m to the

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.

The geotechnical investigation, conducted by

ARQ’s Geotechnical Department, comprised:

• Rotary core drilling with Standard Penetration

Tests (SPTs) on the existing embankment,

RAISING OF THE TZANEEN DAM

RAISING OF THE TZANEEN DAM

• Continuous SPT testing on the upstream slope

of the earth embankment,

• Test pit investigation in the surrounding area

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

methods.

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.

BOOYSENDAL CENTRAL DEVELOPMENT BOOYSENDAL CENTRAL DEVELOPMENT

BOOYSENDAL CENTRAL DEVELOPMENT

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.

RESIDENTAL DEVELOPMENT- PRETORIA EAST

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

solution.

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.

PORT LOUIS RING ROAD SLOPE STABILISATION

WWW.ARQ.CO.ZA

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: arq@arq.co.za