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GROUND INVESTIGATION AND GROUND IMPROVEMENT TECHNIQUES FOR
DEVELOPING MINING POND LANDS ECONOMICALLY
Nathan Narendranathan1 and Bandula Samarasinghe2
1.Managing Director, Infra Tech Pty Ltd, 2 Jones Street, OConnor, WA 6163, Australia. ([email protected])
2.
Group General Manager (Corporate Sustainability and R&D), Infra Tech Pty Ltd, 2 Jones Street, OConnor, WA6163, Australia. ([email protected])
ABSTRACT
Brown field sites such as mining tailings ponds, old quarries and old landfills are common in most countries. The lack
of cost effective and technically sound approaches have meant that the developers had only one choice of ground
improvements, which is piling for all structures to be erected on such sites with deep loose or soft deposits. This paper
provides an overview of how mining pond land can be assessed rapidly and strengthened economically to enable
building of low rise, medium rise or even high rise structures that will not experience large settlement and differential
settlements. Case histories from Malaysia and Australia will be presented on site investigations using shear wave
techniques and Controlled Dynamic Compaction and other ground improvement techniques for effective ground
strengthening.
1. INTRODUCTION
Mining for earths resources such as metals, precious stones, construction materials and salt has been in progress for
thousands of years. Processes relating to mining involve excavation, separations, chemical or physical processing and
finally the storage of waste products. These waste products could be solids or soft slimes. The soft wet slimes or pastes
are termed as tailings and the storage locations of those are called tailings ponds or tailings dams. Tailings ponds
normally contain very soft slimes and sediments and remain in this condition for decades after the mine closure. This
not only poses a hazard but also creates impediments in developing these locations after mining has ceased. Several
countries have requirements for mining pond or tailing pond closures before a mining company can be released from
their obligations. However, in the past such regulations did not exist and hence in most countries there is a legacy of old
disused mining ponds. This paper provides an overview of the various geotechnical issues associated with developing
such sites and some of the ongoing research being undertaken in the authors organisation.
2. SITE INVESTIGATIONS AND POINTS TO NOTE
The first step in the process of tailings pond rehabilitation is the characterisation of the spatial distribution of the
material depths, variability in depths, depth to hard layer, and assessing the physical and chemical properties of the
sediments or slimes. Properties of interest to geotechnical engineers are the strength, solids content or moisture content,
permeability and compressibility, to name a few.
Also, resolving the practical issues such as access into the slime pond is vital and need to be addressed early in the
developments. Floating access platforms made of bamboo poles and geosynthetics are possible options. Floating
Marsh Buggies and air cushion vehicles such as hovercraft can be used for very soft areas. Figure 1 shows an air
cushion supported work platform operating in a slimy environment as associated with tailings ponds.
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Figure 1: Air cushion supported work platform
The authors recommend the use of non invasive geophysical investigations as an economical means of obtaining a
preliminary assessment of the sub soil strata. Echo sounding, seismic wave measurements, electromagnetics are some
techniques that have been used with success as they can provide inidcation of sediments and hard layer profiles. Figures2, 3 and 4 provides information relating to different geophysical techniques that may be used in investigations.
Ground Penetrating
radar
Figure 2: Typical Ground Penetrating Radar (GPR) profile
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Figure 3: Typical seismic shear wave velocity profile
Figure 4: Clay mine pond filled with slurry investigated by Electro Magnetics
The geophysical investigative techniques will provide a rapid and cost effective means of looking at depth variations,
layering, and changes in strengths. Seismic shear wave technique has been very promising in the above approach.
Geophysical techniques can be complemented by relatively inexpensive Mackintosh Probing. These methods can
precede traditional drilling and sampling or probing by Dutch Cone Penetrometers.
Since the engineering properties will be governed by the geochemistry of the tailings, it is very important to carry out
chemical and geochemical analysis of the tailings to assess the components in the tailings. This can be done by X ray
diffraction, scanning electron microscopy along with more conventional mass spectrometry techniques. The authors
recommend the use of assessing the specific surface area and mineral types by Methylene Blue Test (MBT), which is a
rapid and inexpensive filed test. Correlations can be developed between engineering properties and MBT values basedon the exchangeable ions determination.
An adequate investigation is a pre requisite for an in-depth understanding of the tailings properties and hence will
influence the selection and design of economical ground improvement processes.
While using investigative techniques such as Dutch Cone Penetrometer for probing to measure the cone resistance and
sleeve friction is very useful it should also be borne in mind that the empirical equations correlating the cone resistance
and sleeve friction to various engineering properties such as bearing capacity, elastic modulus etc. were established for
soils and not mining pond slimes. These tests could be used to get a relative assessment of the ground conditions which
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should be followed by some form of sampling and testing, especially if local experience or knowledge base is not
available.
3. RHEOLOGY AND ENGINEERING PROPERTIES OF MINING SLIMES
Though mine tailings and slimes are derivatives of soils and rocks, it should be borne in mind that the mining and mineprocessing operations and any chemicals used in the processing might have rendered the tailings with different
properties to soft clays or silts. The geochemistry and electro chemistry of the slimes could be changed thereby
affecting the application of traditional engineering analytical equations or even ground improvement techniques. In
general many of the tailings will tend to have a large negative charge on them. Furthermore the amount of
exchangeable ions (cat ions) will determine how stable the tailings will be after strengthening and will in fact have a
major influence on the strengthening process. Chemical or other additives used in the processing of the mineral may
also be present in the slimes and this need to be factored into. The grading of the fines and the types of minerals present
in the slimes will have an effect on the engineering properties.
Depending on the age of the tailings the solid content could vary from 15% to 50%. The sedimentation process is often
very slow due to the fact that the fine particles possess large negative charge which prevents rapid agglomeration and
setting. Sampling of these slimes and carrying out tests such as one dimensional consolidation test is quite common in
the geotechnical industry. The consolidation test data are used to assess compressibility, prediction of settlements etc
using conventional one dimensional small strain consolidation theory.
It should be noted that conventional consolidation theory will not give representative predictions of settlements,
consolidation times etc. since this approach does not account for the change in permeability of the tailings as
consolidations proceed. In addition the coupling of the soil particle settlement and fluid movement is also not
accounted for. It is necessary to use large strain consolidation theory which accounts for the change in permeability of
the setting sediments or tailings. In the 1960s and 1970s, treatment of the mathematics associated with large strain
consolidation theory was difficult and hence simplified finite difference modelling was used. However with the
development of powerful computers it is now possible to use 2D and 3D consolidation analysis using large strain
consolidation theory.
In a technical and contractual sense application of inappropriate predictive tools can not only lead to uncertainties in
settlement prediction but also contractual disputes between clients and ground improvement specialists.
4. TRADITIONAL APPROACHES FOR BUILDING ON BROWN FIELD SITES
4.1 LEAVE AS IS WITH SURCHARGE FOR SEDIMENTATION
In the past the view has been to leave the site to sediment for decades with some amount of surcharge fill placed in
stages. This will seldom work since the low permeability of most tailings will require several decades before
appreciable improvements can be seen to the strength and compressibility. Relying on evaporation also is not reliable
since a dry crust forms during intense evaporation and this crust reduces further long term evaporation of the underling
slimes. Figure 5 shows a coal mine tailing pond after 20 years of closure.
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Figure 5: Coal mine tailing pond after 20 years
4.2 PILING
There is a view that piling of any structures built on tailings ponds will prevent settlement. This is partially correct and
may not always be the most functional solution. See Figure 6, showing differential settlement. Most tailings ponds are
filled up to create a platform that is above the flood levels or to match surrounding developments and roads. The newly
placed fill will trigger settlements in the underlying soft slimes. This settlement (unless accelerated) will occur over 10
to 25 years depending on the permeability of the slimes. Piles supporting any structures will then be subjected to down
drag forces (called negative friction) and hence will be overloaded. If the piles are designed to withstand these negative
friction forces then the cost of piling tends to be expensive. Furthermore settlement of the fill platform will cause
breakage and differential settlement between piled structures and other un piled pipelines, roads, aprons etc that are
supported directly on the fill.
Figure 6: Photo of differential settlement behind piled bridge
Thus it is apparent that ground improvement of mine tailings ponds will be required if sustainable and economical infra
structure developments are to be undertaken in such areas. The significance of this for a country like Malaysia can be
appreciated when it is realised that many of the mining areas have been located very close to large cities and towns as
shown in the mine location plan in Figure 7.
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Figure 7: Mining areas in Peninsula Malaysia (From Mineral Distribution Map of peninsula Malaysia 6th
edition 1973)
5. GROUND IMPROVEMENT SOLUTIONS
The approaches and technologies available for ground strengthening are constantly evolving. These techniques must
consider the following factors.
Type of soils Type and magnitude of loads Time available for ground improvement Constraints placed by surrounding developments Availability of skilled geotechnical personnel Cost
Some of these technologies applied in Malaysia and Australia are discussed in this paper. Figure 8 shows a slime pond
with fresh slime being pumped in for storage.
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Figure 8: A slime pond with fresh slime being pumped in for storage
5.1 HIGH IMPACT ENERGY DYNAMIC COMPACTION (HIEDYC)
There are many techniques that use high impact dynamic energy for the compaction of ground. The methods of
delivering dynamic compaction energy to the ground include dropping heavy weights on the ground from a height
traditionally called Dynamic Compaction (DC), imparting dynamic energy on a compaction plate placed on the ground
by varied means and imparting energy on the ground by rolling a non circular drum on the ground to be compacted,
referred to as HIEDYC in this paper. The method of compaction adopted depends on many factors, such as the type of
soil, depth to water table, strength requirement and the depth of improvement required.
A heavy drum with a square cross section has been pulled using a prime mover so that the roller continues to thump the
ground imparting the energy as it advances. Compactors with various cross sections have been adapted such as
triangular and pentagonal. Over the years, these shapes have been refined to maximise the ground compaction depth and
minimise vibration when being towed at appropriate speeds.
The depth of improvement and strengths achieved in different compaction techniques depend on the type and condition
of the soil, size and shape of the compactor and the travel speed. Based on these factors, the cost and time required for
compaction vary as well.
HIEDYC compaction has been applied to sands, silts, silty sands, sandy clays and clays at moisture contents not
exceeding OMC +4%. In general, it was assumed that HIEDYC process is applicable to sands only. However, HIEDYC
has been applied successfully for the compaction of clay fills according to authors past project experience.
Figure 9 shows the application of HIEDYC in clay fills at Senai Desaru Expressway in Malaysia, a 77km long highway
in the Malaysian road network (Infra Tech Pty Ltd 2008).
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Figure 9: Application of HIEDYC on clay fills at Senai Desaru Expressway in Malaysia and Cowal Gold Mine, New
South Wales
A summary of typical soil properties from some projects where the successful compaction achieved using HIEDYC
deep compaction is presented in Table 1.
Table 1: Summary of typical soil properties from some ITPL projects where the compaction was successfully achieved
using HIEDYC deep compaction.
Project type of soilPlastic
limit
Liquid
limitPI
%
finesOMC MDD
Compacted
moisture
content
Remarks
Senai Desaru
Expressway,
Malaysia
Light
yellowish
red motled
white sandy
silty clay
35.97 51.10 15.13 49.8 15.0 1.81 17% to 21% HIEDYC Penta
mostly compacted
in wetter than
OMC
AMC
Henderson,
WA,
Australia
GW
CL
ML
NP NP NP 7 10.5 2.03 4% to 6% HIEDYC Penta
mostly compacted
very dry22 35 13 95 NT NT
25 32 7 9 NT NT
Cowal Gold
Mine, NSW,
Australia
Orange
brown silty
clay, tracegravel
14 35 21 Natural
moisture
HIEDYC Penta
mostly compacted
in wet state
Mermaid
Marine
Supply Base,
Dampier,
WA,
Australia
Sandy clay
Gravelly
sand
Silty clay
18 31 13 62 NT NT Natural
moisture
HIEDYC Tria
mostly compacted
in dry stateNP NP NP 5 10.1 1.922
18 52 34 99 NT NT
NP Non plastic
NT Not tested
Source: Project geotechnical reports of Infra Tech Group
The data in Table 1 demonstrates the applicability of HIEDYC compaction for sands, clays and silts provided the
appropriate HIEDYC module is used. It was also noted that the soils were compacted at moisture contents ranging
between 5% wetter to very dry natural moisture state. In addition to soils, HIEDYC has also been used to compact old
landfills filled with building rubble (Infra Tech Pty Ltd 2009-2).as well as ponds with slimes (Infra Tech Pty Ltd 2004
and Infra Tech Pty Ltd 2009-3).
To decide on the most optimum high impact energy ground improvement solutions, it is necessary to have not only anunderstanding of the operational aspect of HIEDYC rollers, but also a sound understanding of geotechnical engineering.
Due to the requirement of matching the equipment to the ground to be compacted to achieve performance criteria, there
are several configurations of HIEDYC modules that have been deployed in the projects that are used as case studies in
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this paper. Authors organisation operates these equipment under the trade mark HIEDYC and has three basic
modules, namely Tria, Qadra and Penta. Table 2 provides basic information of these modules with an overview of their
capabilities and applications. Photographs of these three modules; Penta, Qadra and Tria are shown in Figure 10.
Table 2: HIEDYC modules, capabilities and applications
ModuleNumber of
sides
Mass
(tonnes)
Mining engineering applications Civil engineering applications
Tria 3 17 Pit floors/rock crushing, haul roads, soil
dumps, rock dumps, tailings
consolidation and strengthening.
Subgrade compaction, earthwork
compaction, coarse sand and silt
compaction and clay soils or slime
ponds in conjunction with Prefabricated
Vertical Drains
Qadra 4 14 Haul roads, soil dumps, tailings
consolidation and strengthening.
Subgrade compaction for road
pavements, earthwork compaction and
sand and silt compaction not greater
than 1.5m
Penta 5 16 Pit floors/rock crushing, haul roads, soil
dumps, rock dumps, tailings
consolidation and strengthening.
Subgrade compaction, earthwork
compaction, coarse sand and silt
compaction and clay soils or slime
ponds in conjunction with Prefabricated
Vertical Drains
Figure 10: Photographs of HIEDYC modules, (L to R) Tria, Qadra and Penta
Application of high impact energy compaction when used in combination with other techniques can be effective andalso the application of this technique can be made much broader, facilitating the compaction of varied types of soils.
One such application is the use of prefabricated vertical drains in conjunction with HIEDYC for deep compaction of
clay soils as shown in Figure 11.
Figure 11: Installation of PVD in old tine mine pond in Kepong and HIEDYC deep compaction over PVD
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5.2 CONTROLLED DYNAMIC COMPACTION (CDYC)
Another form of deep dynamic compaction suitable for mining pond improvements is Controlled Dynamic Compaction,
(CDYC).
CDYC is the acronym for Controlled Dynamic Compaction technique adapted by Infra Tech Pty Ltd. CDYC has been
found to achieve deeper compaction than that achieved by HIEDYC and was developed by the authors organisation tocater for deep ground improvements.
The CDYC technique consists of dropping a weight by hydraulics on a steel impact plate varying in diameter between
1.0m and 1.5m. The hammer weight and the drop height can be changed. In general 10 to 30 blows a minute can be
imparted on the steel plate by the drop hammer. The greater efficiency of energy transfer (compared to traditional free
fall Dynamic compaction) is due to the fact that the impact plate is constantly in contact with the ground. Figure 12
shows the assembly and the CDYC operation in progress, while the Figure 13 shows the stages of CDYC compaction
from a slimy pond to a developable land.
Figure 12: CDYC equipment being assembled and CDYC ground improvement work in progress
Figure 13: CDYC work stages in a swampy site transforming a slimy pond in to a developable land
The ground strength after CDYC can be assessed by Electric Friction Cone Penetrometer (EFCPT) .or seismic wave
velocity profiling. Figure 14 show the ground being pegged for testing by EFCPT and the EFCPT equipment
conducting testing.
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Figure 14: Pegging of EFCPT locations and Tests in Progress after CDYC deep compaction and EFCPT in progress
5.3 ELECTRO OSMOSIS
Electro osmotic (EO) consolidation means the consolidation of soft clays by the application of electric current. It wasstudied and applied for the first time by Leo Casagrande (1948). It is inherent that fine grained clay particles with large
interfacial surface will consolidate and generate significant settlement when loaded. Electro osmosis was originally
developed as a means of dewatering fine grained soils for the consolidation and strengthening of soft saturated clayey
soils. Electro osmotic dewatering essentially involves applying an electric potential across the sediment layer. It is the
process where in positively charged ions move from anode to cathode. I.e. Water moves from anode to cathode where it
can be collected and pumped out of soil. Electro osmotic flow depends on nature of soil, water content, and pH value
and on ionic type concentration in the pore water. Due to the applied electric potential, the electrolysis of water occurs
at the electrodes. Electro osmotic transfer of water through clay is a result of diffuse double layer cations in the clay
pores being attracted to a negatively charged electrode or cathode.
When electrodes are placed across saturated clay mass and direct current is applied, water in the clay porespace is transported towards cathode by electro osmosis.
In addition frictional drag is created by the motion of ions as they move through the clay pores helping totransport additional water.
The flow generated by the electric gradient is called electro osmotic flow.The ability to consolidate soft sediments without the need for high surcharge fill makes Electro osmosis desirable
especially when working on very soft ground where surcharge weight cannot be supported. Figure 15 shows electro
osmosis in progress for soft clay deposits.
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Figure 15: Electro osmosis in progress for soft clay deposits
6. SUMMARY
To determine economical and sustainable solutions for the development of mining pond lands, it is necessary to
understand the soil types through adequate investigations, understand that tailings rheology and consolidation
mechanisms. Each site will require site specific solutions. HIEDYC, CDYC and EO techniques offer good options that
can achieve the desired outcome of transforming the mining ponds into developable lands economically and
sustainably.
REFERENCES
Infra Tech Pty Ltd (2004), Closure report for Westport Container Terminal CT4, Malaysia (unpublished).
Infra Tech Pty Ltd (2008), Closure report for Senai Desaru Expressway Project, Malaysia for Randhill Engineers.
(unpublished).
Infra Tech Pty Ltd, (2009 -1), Closure Report for Digesters 6 & 7, Beenyup, WA for Thiess, SKM Consulting and
W2WA Alliance. (unpublished).
Infra Tech Pty Ltd, (2009 -2), Closure Report for Lake Coogee Stage 1 Development for VDM Consulting and DM
Civil. (unpublished).
Infra Tech Pty Ltd (2009-3), Closure Report for Mermaid Marine, Dampier, WA for Ertech Contractors. (unpublished).