ARQ GEOTECH (PTY) LTD COMPANY PROFILE - Consulting Engineers
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
ARQ GEOTECH (PTY) LTD
COMPANY PROFILE
ARQ GEOTECH (PTY) LTD
SUBSIDIARY OF ARQ CONSULTING ENGINEERS (PTY) LTD
A BIT ABOUT OURSELVES
Capability Statement – Geotechnical
Geotechnics is one of ARQ’s key areas of specialisation. ARQ Geotech is active in a wide range of
geotechnical fields both within the borders of South Africa and internationally, providing professional
engineering services to a wide range of clients in the private and public sector.
We undertake and oversee geotechnical investigations which range from basic to highly advanced – each,
tailor made for site conditions, proposed land use and our clients need. We specialise in various forms of
analyses and designs including Slope Stability & Seepage, Lateral Support, Piled and Shallow Foundation
Design, Ground Improvement, and more; both within South Africa and internationally.
While hand-calculations and simple spreadsheet models are used to develop judgment in respect of
geotechnical problems, our sophisticated analytical methods and software allow us to design ambitious
and innovative geotechnical systems ensuring that our client receives the optimization allowed by the
latest technological developments. 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 to ensure safety and minimum
disturbance to the environment.
Innovative and cost-saving solutions, coupled with detailed attention to each client’s specific needs, have
established us as one of the preferred Geotechnical firms in the industry.
OUR CAPABILITIES
THE GEOTECHNICAL “A-TEAM”
Directors
Thomas O’Brien Yokazi Mkalali
Pr. Eng. BSc Geology
Geotechnical
Engineers
Coert van Dyk Hannes Taljaard Katy O’Brien Werner Britz
B. Eng. Hons B. Eng. Civil B. Eng. Hons. B. Eng. Civil
Brendon Blok Paul le Roux
B. Eng. Civil B. Eng. Hons.
Engineering
Geologists
Steve Mandumba Madaleen Booysen
Pr. Sci. Nat. BSc Geology
Drafting
PROJECTS OF INTEREST
PILING AND LATERAL SUPPORT DESIGN SERVICES
ARQ Geotech works closely with Terra Strata Construction and is responsible for undertaking tender
design, detailed design, input during construction as well approval of quality and monitoring
documentation from construction works.
Piling projects typically entail the installation of
foundation piles for various structures including
multi-storey buildings, bridges, silos, cranes,
warehouses, reservoirs etc. Pile types employed
include self-drilling anchor (SDA) micropiles,
percussion bored micropiles, large diameter ODEX
piles, auger piles (open hole, underslurry and cased)
and continuous flight auger (CFA) piles.
Lateral support projects typically entail providing
support to retain soil and/or rock in excavations for
various purpose including road cuttings, basement
excavations, large foundation excavations, bridge
widenings, bulk earthworks platforms, pits, box cuts,
highwalls etc. Lateral support systems employed
include passive systems utilizing soil nails and rock
bolts with a reinforced shotcrete facing, cantilevered
pile walls as well as active systems utilizing multiple
rows of post-tensioned ground anchors often installed in conjunction with a pile wall (secant or soldier pile
wall). Temporary as well as permanent lateral support solutions are provided depending on the project
specific needs.
TASEZ DEVELOPMENT PHASE 1A
ARQ Geotech (Pty) Ltd was requested by Zutari
(Pty) Ltd to provide geotechnical services for the
bulk earthworks design of the site as well as for
foundation recommendations for various proposed
structures (retaining walls, buildings). The
geotechnical services consisted of geotechnical
investigations, evaluations, and inspections in
Watloo, Pretoria.
An initial geotechnical investigation was completed
by ARQ in 2019 providing a preliminary insight into
the geology and geotechnical conditions on site. The
outcome of this report was used as an input in the
preliminary design of the bulk earthworks by Zutari. Between 2020 and 2021, ARQ conducted a desktop
study and evaluation to support Zutari’s design by providing typical foundation recommendations.
Furthermore, two new additional geotechnical investigations were conducted by ARQ to provide detailed
geotechnical information to support the design bulk earthworks and foundation designs for various
structures.
Lastly, ARQ conducted regular site visits/inspections to support construction and provide valuable
geotechnical inputs.
Some geotechnical constraints dealt with in the project consisted of:
• Complex geology with numerous diabase intrusions into the Silverton Shale. This resulted in highly
variable geological conditions at different areas of the site,
• The abundant shale rock on site proved inadequate for use in engineered fill due to lower classification
(G9-G8) than the required G7 fill. Large quantities of import material were required as well as a
selectively used shale rock fill material,
• Abundant shallow perched groundwater occurring, generally in the norther part of the site. Various
temporary and permanent groundwater drainage solutions were proposed to facilitate construction
and protect the structures.
WATERKLOOF MARINA – UNCONTROLLED FILL IMPROVEMENT AND RISK
MITIGATION
Central Developments constructed a large residential development (including multi-level structures) on a
site in Monument Park situated partially on a historic clay quarry backfilled in places with uncontrolled
brick and other rubble. The brick fill is up to 20m thick in places and highly variable in composition. This
naturally posed a risk to structures founded upon it and required investigation and settlement mitigation
measures. ARQ was tasked with, amongst others, the geotechnical investigation of uncontrolled fills,
characterisation of materials and assistance in derivation of appropriate measures to afford safe
construction of structures on this material (as removal and replacement was not economically viable).
After extensive investigations, Dynamic compaction (DC) was conducted on the north western platform of
the development to improve fill density and uniformity. See Figure 1 depicting the platform after primary
DC.
The efficacy of the DC was then evaluated through the following mechanisms to quantify risk and to serve
as input to appropriate foundation design for residential structures:
• Analysis of DC trial penetration data
• Test pits (post DC)
• Geophysical testing (multi-channel analysis of surface waves – MASW), and
• Before and after ground surface survey to assess effect of densification
Amongst the other investigative measures set out above, multi-channel analysis of surface waves (MASW)
geophysical testing was conducted as the primary mechanism to assess the quality and efficacy of DC, and
investigate the fill for anomalies or voiding. The MASW method entails measurement of velocity of shear
waves which are emitted by a
North-western platform after primary DC source, and propagate through the
subsurface. MASW was selected
over seismic refraction as it is more
apt at providing accurate readings of
possible soft zones overlain by
denser/stiffer zones and is also not
affected by the water table.
Typical MAWS profile in brick rubble after DC
This testing had the added benefit of providing information to depths beyond the brick fill and establishing
probable insitu shale rock level. Tests were conducted on five (5) traverses, almost 100m in length. Refer
to Figure 2 below for a typical MASW profile and to Figure 3 for “before DC and after DC” test pit wall
comparison showing densification effects.
Before and after DC view of test pit side
MASW testing indicated wallno major low-density (soft) zones, though variable subsurface conditions were
still present as expected. The results of the investigation were used to quantify likely long-term settlements
and differential settlements and were used as input into the design of appropriately stiffened reinforced
concrete rafts for founding of structures.
The structures have subsequently been constructed and are preforming satisfactorily.
FAR WEST GOLD RECOVERIES REGIONAL TAILINGS FACILITY
In the interests of re-mining and consolidating tailings on the West Rand for environmental and economic
benefits, the concept of the Far West Gold Recoveries (FWGR) Regional Tailings Facility (RTF) was
developed. The conceptual design of the RTF presents a 4th Generation upstream on-wall cyclone ring dyke
dam with a pumped decant and expected Rate of Rise (RoR) of 4-6m per year.
If the RTF is implemented as designed, it should be possible over the next 3 to 4 decades, to re-move and
re-locate virtually every existing gold tailings dam on the Far West Rand to the RTF – a total of 1,3Bt,
thereby generating beneficial economic activity for the period and substantially reducing the current
impact footprint.
The final RTF will have an ovoid footprint covering 960Ha, a top surface area of 550Ha, a height
of 135m and side slopes of 1:4.
FWGR
RTF
Geotechnical profiles and material parameters were obtained for the foundation through geotechnical investigations at the site of the proposed RTF. Tailings material parameters were derived via lab and insitu testing conducted on tailings at the Brakpan TSF – which is deemed representative of the proposed RTF. Limit Equilibrium, 2D and 3D Finite Element modelling was undertaken to examine the stability of the proposed facility under predefined Mohr Coulomb strength conditions, as well as by employing the NorSand constitutive model. Stability was found to be satisfactory for drained and peak-undrained conditions, but marginal when residual undrained conditions are assumed. A 2D and more detailed 3D dynamic analysis, where representative seismic time histories were developed and simulated indicate that meaningful liquefaction is not expected for a 1 in 5 000-year seismic event. BRAKPAN TAILINGS STORAGEACILITY STABILITY REVIEW The Brakpan Tailings Storage Facility (TSF) has been operational since 1985, with a footprint of 800Ha, perimeter of 11km and maximum height of about 100m, it holds approximately 496 million cubic metres of tailings material. An investigation into the Brakpan TSF was conducted by a team of professionals aiming to investigate and validate various stability aspects, not only for Brakpan TSF itself, but upstream cyclone facilities in South Africa in general. The evaluation involved a site investigation comprising insitu testing including CPTu, seismic cone and pressure meter testing. Samples were collected during this investigation and submitted for laboratory testing, which included conventional triaxial testing as well as critical state triaxial testing. All available information, including current and historic aerial surveys, piezometric data, findings of previous and the current geotechnical investigations, was interrogated to simulate conditions of the facility as accurately as possible. In addition to this, a probabilistic seismic hazard assessment was conducted and used to develop representative time history events for analysis. A stability assessment was conducted in 2D and 3D, utilising Limit Equilibrium and Finite Element software packages. The commonly used Mohr Coulomb material model as well as the more complex critical state NorSand constitutive model were utilised to assess the stability of the facility and potential for liquefaction. For the latter, various triggering methods were considered, including the effects of seismic occurrences.
For this, 3D models of the facility were submitted to dynamic seismic loading and stresses, strains and
displacements assess throughout the event.
ARQ Geotech was responsible for supervision and monitoring of the geotechnical investigation,
interpretation of insitu and laboratory test results and derivation of geotechnical profiles and material
parameters for analysis.
With this completed, ARQ conducted Limit Equilibrium, 2D and 3D Finite Element modelling and analysis
to examine the stability of the facility under predefined Mohr
Coulomb strength conditions, as well as by employing the
NorSand constitutive model. Staged construction simulation
of the facility allowed for stresses to be ‘built in’ to the
models and the potential for static liquefaction assessed.
Furthermore, dynamic analyses were conducted to simulate
seismic time histories, allow to assessment of potential to
cyclic liquefaction.
Model of a portion of the facility
Analysis results, showing failure mode for the Western portion of the facility
under peak undrained conditions.
ABEL ERASMUS PASS
A small portion of the embankment on the R36 (provincial road), between Ohrigstad and Hoedspruit in the
Limpopo Province, South Africa, failed circa 2013. Photographs of the failure is provided in the figures
below.
ARQ conducted various site inspections and investigations to determine the possible cause of failure. ARQ
was subsequently appointed to conduct slope stability (using finite element methods) and hydrology
analyses in order to provide remedial solutions. These analyses were focussed on improving the site
drainage characteristic and reinstating the failed embankment and roadway.
Improving site drainage would comprise constructing various channels, stone pitching, gabions and
improved inlet and outlet structures at the existing culverts to control runoff water. The failed slope was
stabilised by installing 21 No. vertical and 21 No. raked (at 45° from vertical into the embankment) self-
drilling anchor piles. Re-instatement of the embankment comprises G7 material reinforced with a 50kN bi-
axial strength geogrid every 5th layer. This provides additional stiffness against movements in the direction
perpendicular to the roadway in which the
failure took place previously.
The figure overleaf depicts the model of the
slope used in the finite element analysis with the
self-drilling anchor piles and geogrids installed.
Implementation of this design commenced in
March 2017 with the completion date set at
November 2017.
ARQ’s site supervision team consists of a
professional structural engineer as the resident
engineer (RE) and a geotechnical engineer as
assistant to the RE. The technical team to the
project comprises Mr Alan Parrock with over 40 years’ experience in geotechnical engineering, as well as
the director of ARQ’s Structural division and director of ARQ’s Geotechnical division.
KHUMA SPORTS COMPLEX
Geotechnical investigation and dolomite stability
evaluation for the sports complex. The development
comprises a hall, pavilion, sport field and a parking lot.
ARQ conducted a geotechnical investigation consisting of
a gravity survey, seven (7) test pits and four (4)
percussion boreholes. The test pits were excavated to a
maximum of 3.2m within the in-situ material and the
percussion boreholes were drilled at least 6 m into
competent dolomite bedrock.
Soil samples were collected and delivered to Pretoria for
laboratory tests.
The field and laboratory results were analysed at the office to
determine the potential presence of problem soils (heaving,
collapsible, erodible, dispersive…), the excavatability of the
material, its suitability for use in various engineered layerworksand fills and its adequacy as foundation material. Furthermore a dolomite stability assessment was
conducted and the Inherent Hazard Class of the site was allocated.
A report was submitted to the client provide inputs on some key geotechnical aspects of the site as listed
above and a foundation recommendation was provided
VOSLOORUS LICENSE DEPARTMENT DRIVE-THROUGH
Geotechnical investigation and dolomite stability evaluation for the license department drive-through
station. The development will include a new single storey concrete building.
ARQ conducted a geotechnical investigation consisting of a gravity survey, four (4) test pits and five (5)
percussion boreholes. The test pits were excavated to a maximum of 3.0m within the in-situ material and
the percussion boreholes were drilled at least 6 m into competent dolomite bedrock.
Soil samples were collected and delivered to Pretoria for laboratory tests.
The field and laboratory results were analysed at the office to determine the potential presence of problem
soils (heaving, collapsible, erodible, dispersive…), the excavatability of the material, its suitability for use
in various engineered layerworks and fills and its adequacy as foundation material. Furthermore a
dolomite stability assessment was conducted and the Inherent Hazard Class of the site was allocated.
A report was submitted to the client provide inputs on some key geotechnical aspects of the site as listed
above and a foundation recommendation was provided.
EXCELSIOR MASTS FOUNDATIONS
ARQ was requested, by Goldwind (Pty) Ltd, to conduct a geotechnical evaluation of the probable cause of
the movement observed on several of the intermediate monopole towers on the Vryheid-Excelsior
overhead distribution line for the Excelsior Wind Farm.
Furthermore, ARQ was to witness the proposed remedial work and provide input as to the integrity of the
foundation material at each tower during the remedial works.
Movement at the base of mast 48. Assessment of foundation conditions for
remedial worksAdditionally, ARQ assisted Goldwind in confirming the condition of the fills at the straining towers. Prior to removal of backfill material from around the monopoles, DCP tests and limited Troxler tests (for straining towers) were conducted on the backfill material, and the material was sampled in places for laboratory tests to investigate its adequacy and compliance to design specifications. After removal of backfill material a visual inspection and further DCP testing was done into the in-situ material in the base of the excavation to verify the integrity of the founding material. The field and laboratory results were analysed and presented in a report to the client. The report also detailed the issues encountered at specific towers/masts that needed additional attention during remedial works. RAMADIEPA DAM As part of an expansion project, the Ramadiepa Dam was selected for an upgrade that would consist in an increase in height and possible of the footprint as well. The geotechnical investigation at Ramadiepa Dam was aimed at determining the geotechnical properties of the construction materials used for the embankment dam. The investigation looked at the materials on the embankment slopes as well as in the dam’s clay core. Additionally, the investigation was conducted to obtain properties of the discontinuities of the rock mass at the floor of the Dam’s spillway. The works conducted consisted of the following: a) 8 Test pits on the dam, b) 2 Rotary core boreholes, and c) Dam foundation’s discontinuity mapping. The test pits were excavated to a maximum of 3m within the in-situ material and boreholes were drilled to 7m. Soil and rock samples were collected and sent to a SANAS accredited laboratory tests. The discontinuity mapping consisted of multiple joint line surveys attempting to identify and record geotechnical properties of the joints and the rock. Discontinuities were represented in terms of their dip and dip direction and illustrated by means of a stereographic projection. The field and laboratory results were analysed and presented in a report with limited interpretation. A report was submitted to the client provide inputs on some key geotechnical aspects of the site as described above.
SEK – KIPOI MINE EXPANSION
Geotechnical investigation for various structures forming part of
the Stage 2 expansion of the mine. The expansion includes the
assessment of the stability of a soil retaining structure (gabion
wall) as well as the construction of the following new structures:
a) Crushing and scrubbing circuits and ancillaries,
b) Copper cobalt Axillary Plant (CCAP), and
c) Water Treatment Plant.
ARQ conducted a geotechnical investigation consisting of six test
pits and 12 Dynamic Cone Penetrometer (DCP) tests. The test pits
were excavated to a
maximum of 5m within
the in-situ material and
DCP tests were conducted
on surface as well as at
some depth within the
test pits. Soil samples were collected and shipped to South Africa
for laboratory tests. Furthermore, a visual assessment of the
gabion wall conducted, and feedback of the slightly bulging wall
sent to the client. However, no further geotechnical analysis was
conducted for the wall. The field and laboratory results were
analysed at the office to determine the potential presence of
problem soils (heaving, collapsible, erodible, dispersive…), the
excavatability of the material, its suitability for use in various
engineered layerworks and fills and its adequacy as foundation
material. A report was submitted to the client provide inputs on
some key geotechnical aspects of the site as listed above and a
foundation recommendation was provided.
TELKOM HIGHVELD AND ERASMIA
ARQ Geotech (Pty) Ltd was appointed by Mr Chris du Plooy of Bidvest Facility Management, hence forth
known as the Client, to conduct a dolomite stability investigation for an existing Telkom Tower located at
Highveld exchange. The investigation comprised of a deep sub-surface investigation, in the form of
percussion boreholes, constituting a dolomite stability investigation.
Six (6) percussion boreholes were drilled by Hennie Erwee drilling contractors on site as close to the tower
as practically possible given the existing services on the site. The boreholes were also placed in areas of
suspected subsidence or where gravity lows were measured. The boreholes were drilled until 6m of
competent bedrock was encountered. This equated to depths varying between 10m to 37m. Chip samples
collected during the drilling process were profiled at ARQ’s offices by an ARQ geologist according to current
standards (Brink and Bruin, 2002).The dolomite stability investigation provided information in terms of:
• Materials occurring at depth on the site,
• Potential low-density zones and voids, and
• Water table.
This information was used in the dolomite stability assessment to provide a dolomite site classification of
IHC 6 to 8. According to the SANS 1936-1 the DAD is D4, based on the IHC classifications. The area is
currently used for “commercial and miscellaneous non-residential development category C3 (Tower of
55m, >3 storeys)”. This will necessitate that the development will have to comply with D4 precautionary
measures as outlined in SANS 1936-3. ARQ recommended a foundation solution/ remedial measures to
stabilise the communication tower and span the required 15m loss of support
BOOYSENDAL CENTRAL – CHANGE HOUSE & MANAGEMENT OFFICE
Northam Platinum’s Booysendal Mine
is situated about 50km North West of
Mashishing (formerly Lydenburg) on
the border of the Mpumalanga and
Limpopo Province.
ARQ Geotech was appointed in 2015
by DRA Global, the mine’s main EPCM
contractor, as Geotechnical Consultant
for the expansion project.
The mine extracts platinum and other
platinum group minerals from
deposits in the eastern limb of the
Bushveld Igneous Complex (BIC). Northam plans to increase its current production through construction
of several large infrastructure components at its new “Central Mine” complex.
As part of the infrastructure required on the platform at the Central Box-cut a state-of-the-art change
house, boasting a solar driven heating system that can produce hot water for over 2 000 people, and an
office building controlling underground activities remotely, are presently under construction.
ARQ’s initial geotechnical investigation on the platform in the area earmarked for the Change House and
Office Building comprised test pits. The investigation identified that the ground conditions below the
footprint of these buildings (single and double storey structures) was of concern – consisting of rock
fragments in a granular matrix which was described as loose in places. The platform fill layer varied in
thickness from 0 to 3m thick. This layer could be susceptible to movement resulting from the re-
arrangement of particles into denser formation, which would be triggered by the additional load imposed
by the structures and an increase in moisture content.
The risk was addressed via in-situ treatment of the platform material utilising Rapid Impact Compaction
(RIC). RIC if a form of dynamic compaction which entails densification of substrate through energy
imparted by a mass of 9-12ton falling through a distance of 1-1.5m at a rate of approximately 40-60 blows
per minute. Energy is imparted to the fill material through the machine’s foot/pad, which is moved over
the full extent of the site in a grid arrangement – typically 3-3.5m centre/centre with “one in the middle”.RIC was determined as the best-fit for the specific site as
the method possesses sufficient energy to achieve the
required “depth of influence” while minimising vibrations
which may otherwise detrimentally affect existing
structures and services.
ARQ undertook an RIC design and recommended
compaction imprints on a grid spacing of 3.2m “with one in
the middle”. A maximum of 35 blows per pad with the 9-
ton hammer were recommended, with blow count being
stopped short of this maximum figure should penetration
of the foot into the fill slow to an acceptance criterion.
After the completion of RIC in-situ treatment, quality assurance was undertaken via Continuous Surface
Wave (CSW) testing. CSW testing is a geophysical method which is used to calculate the in-situ stiffness of
soil or rock through measuring the propagation velocity of Raleigh waves (one of two types of surface
waves) through the media. The stiffness of the fill post-RIC was then utilised to calculate likely total and
differential settlements of the structures.
ARQ undertook close supervision of the foundation excavations to ensure that any residual variability
observed is addressed on its own merit. The supervision consisted of visual assessments as well as a review
of Dynamic Cone Penetration (DCP) and Troxler density data.
Through the application of a well suited in-situ ground improvement method, and appropriate quality
assurance methods, the founding of the Office Block and Change house was allowed to proceed with
appropriately reduced risk.
BOOYSENDAL CENTRAL BOX-CULVERT
Northam Platinum’s Booysendal Mine is situated
about 50km north west of Mashishing (formerly
Lydenburg) on the border of the Mpumalanga and
Limpopo Provinces. ARQ Geotech was appointed in
2015 by DRA Global, the mine’s main EPCM
contractor, as Geotechnical Consultant for the
expansion project.
The mine extracts platinum and other platinum group
minerals from deposits in the eastern limb of the
Bushveld Igneous Complex (BIC). Northam plans to
increase its current production through construction
of several large infrastructure components at its new “Central Mine” complex. Several box-cuts in the valley
surrounding the Groot-Dwars River will serve as portals, bringing ore to surface. The Booysendal Central
Box-cut is the first of these structures. ARQ Geotech oversaw the geotechnical investigation on the
proposed footprint area for the box-cut and together with DRA’s engineers set the base level of the box-cut
such that the portals would be situated in competent rock – facilitating stability of portal entries. With the
overall box-cut geometry set, ARQ Geotech proceeded to develop a 3D geotechnical model of the materials,
and derived strength and stiffness parameters.Conceptual design resulted in selection of a 70˚ highwall, stabilised in terms of global stability with soil
nails and rock bolts up to 9.5m in length. Local stability and durability of the highwall excavation face was
achieved by the application of a steel mesh reinforced, sprayed concrete facing. Detailed design was
undertaken using advanced finite element analysis (FEA) of representative cross sections, backed up by
simpler software and hand calculation methods, and vetted by years of experience.
Construction of the box-cut commenced in mid-
2016 with ARQ Geotech providing part time site
supervision to endure that the design was
implemented correctly, and that quality was up to
standard. The box-cut highwall stabilization was
completed in late-2017 and culminated in a high
wall standing proud at a height of 31m. Materials
excavated were utilised to construct a large
platform which will accommodate a wide range of
associated surface infrastructure.
Seven decline portals were driven into the
competent rock exposed at the base of the
highwall. Portals were developed “on reef” from word-go, thus aiding in the cost-efficacy of the
operation. To date the highwall lateral support is performing as predicted and is effectively retaining the
mountainside and allowing safe entry to the reef below.
SANI PASS – UPGRADING OF MAIN ROAD P318 FROM TROUTBACK TO SANI
TOP PHASE
The project involves the upgrade of the Sani Pass (P318)
Main Road. The scope of works was divided into 2 contracts,
namely a structures and roads contract, which were
awarded to Chris Africa Civils and Leomat Construction
respectively. Chris Africa were responsible for the
construction of 7 cast insitu box culverts, one incorporated
in a large valley fill cladded with Terramesh baskets and
one bridge. Leomat are responsible for mass earthworks,construction of ancillary drainage (v-drains etc.), installation of precast concrete pipe culverts,
construction of lateral support structures in the forms of concrete retaining walls, Terramesh and gabions
walls and shotcrete and soil nail walls.
PORT LOUIS RING ROAD – POST FAILURE STABILISATION FOR A
MECHANICALLY STABILISED EARTH WALL
The Ring Road Phase 1 project in Mauritius was
Figure 1
constructed during from 2010–2013, and
comprises a 4.9km long dual carriageway. Early
2014, cracks appeared on the northbound
carriageway, followed by the collapse of a 15m
high MESW portion of the fill (Figure 1).
Observations pointed to a textbook slope failure
with “slip at the lip” and “bulge at the toe”,
indicating that deep seated movement occurred.
ARQ was appointed to conduct a geotechnical
investigation to identify the mechanisms of
failure, to design remedial measures and to
supervise the implementation of the remedial
measures.
The investigation comprised rotary core drilling (triple tube to maximize core recovery and minimize
sample disturbance) with SPTs, undisturbed sampling and subsequent laboratory testing including
numerous triaxial tests.
Using results from the investigation ARQ conducted Finite Element Analyses (FEA) to identify the failure
mechanism and to design the remedial measures. A deep seated failure mechanism was identified (Figure
2). Remedial measures were designed which comprised inter alia a vertical secant piled wall supported by
7-strand, 3-level, high strength ground anchors, coupled with a geosynthetic reinforced fill section behind
the reconstructed MSE wall (Figure 3).
Figure 2 Figure 3Remedial work to the Port Louis Ring Road was completed mid-2017 (Figure 4) after the resolution of
several design and construction challenges. At the end of
Figure 4 the day all challenges were overcome and the road was
reinstated to its original state by implementing state of
the art geotechnical solutions.
NWAMITWA DAM
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:
• 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 of:
• 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.
RAISING OF THE TZANEEN DAM
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,• 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.
BOOYSENDAL CENTRAL DEVELOPMENT
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 lateralsupport 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. RESIDENTUAL DEVELOPMENT – PRETORIA EAST 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: 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. 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.
PORT LOUIS RING ROAD SLOP STABILISATION 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.
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.zaYou can also read
