Review
of Soil Cover Technologies for Acid Mine Drainage - A Peer Review
of the Waite Amulet and Heath Steele Soil Covers
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Report
2.21.3
July 1997
Executive
Summary
Prototype insitu
covers were constructed at two mine sites, Heath Steele and Waite
Amulet, as a part of the Mine Environmental Neutral Drainage (MEND)
program. An independent study was undertaken by the Unsaturated
Soils Group at the University of Saskatchewan to critically review
the sites with respect to selection of soil materials, design, construction,
instrumentation, performance, and economics. This work was funded
by Cominco Ltd. The main objectives of the study were:
1. to undertake
a literature review of fundamental processes and methods of analyses
required for the evaluation and design of dry covers;
2. to identify
the key aspects of the design of soil cover systems;
3. to complete
an inventory of other "dry cover" sites and their performance;
4. to undertake
a critical scientific review of the Heath Steele and Waite Amulet
sites including the evaluation of the quality and completeness
of the following:
(a) field
and laboratory tests;
(b) site
instrumentation systems and monitoring results; and
(c) data
analysis and interpretation.
5. to analyze
alternative means that identify the essential components of soil
cover systems, while at the same time, minimize the construction
costs; and
6. to provide
recommendations as to critical research needs in the areas of
design, construction, monitoring, and analysis of soil cover systems.
The design
objectives for the Waite Amulet and Heath Steele soil cover systems
were to provide a low hydraulic conductivity barrier to minimize
the influx of water and provide an oxygen diffusion barrier to minimize
the influx of oxygen. The design of the two soil cover systems utilized
the capillary barrier concept. Both these soil cover systems were
extensively evaluated through laboratory, numerical modelling, and
field studies. The results of the research indicated that the capillary
barrier concept was attainable under field conditions and will result
in a reduction of the influx of infiltration and oxygen, thereby
reducing the potential for acid generation.
The soil cover
systems installed at Heath Steele and Waite Amulet site performed
satisfactorily. The design, construction, monitoring programs on
these soil cover systems provided a unique opportunity to evaluate
the performance of the engineered soil covers in Canadian climates.
In this report,
apart from providing an independent review of Heath Steele and Waite
Amulet projects, a brief state-of-the-art literature review was
provided in Chapter 2. Several case histories of soil cover systems
in various parts of the world were summarized in Chapter 3. The
key laboratory tests and performance data of Heath Steele and Waite
Amulet soil cover systems are summarized in Chapter 4. A synopsis
on the design of Heath Steele and Waite Amulet soil cover systems
are summarized in Chapter 5. Chapter 6 provides the summary and
recommendations with respect to design and analyses of soil cover
systems.
An annotated
literature review about soil cover systems, liners, and related
topics are provided in Appendix A. A methodology for numerical modelling
of soil cover performance as prepared by Swanson (1995) is provided
as Appendix B.
Over the course
of this review it became apparent that a template for a rationale
method of cover design for acid generating mine waste was emerging.
The basic elements of this rationale method is outlined and a backdrop
against which the design of various covers is provided. The lessons
learned from these experiences are catalogued.
The basic elements
of this design approach can be summarized as follows:
- Design
objectives/philosophy;
- Design
principles;
- Methods
of characterization;
- Methods
of analyses;
- Monitoring;
and
- General
issues including economics, construction, long term performance.
Each case history
was reviewed in terms of each of these design components. Based
on the review of the existing literature, the various case histories
and a detailed review of the Heath Steele and Waite Amulet sites
a number of key recommendations were proposed:
Recommendation
#1: That research can be conducted into the
characterization of the soil water characteristic curve for soil
cover and mine waste materials. Of particular interest are methods
of characterization, and estimation and the effects that weathering
processes have on these properties.
Recommendation
#2: That research be continued on the inter-relationships
between micro-climatic conditions, vegetation, and soil as it
pertains to cover performance.
Recommendation
#3: That current methods of analyses for dry covers
be reviewed including benchmark testing against documented case
histories and that a methodology for analyses be developed which
maximizes the capabilities of the various models.
Recommendation
#4: That further work be encouraged to expand the capabilities
of current models to include environmental coupling. Additional
capability is required in the prediction of moisture movement
during winter months, the influence of vegetation, multi-dimensional
effects, and erosion.
Recommendation
#5: That work be undertaken for the development and
testing of a reliable method of measuring suction within covers
and waste rock.
Recommendation
#6: That a review of field lysimeter design and performance
be undertaken with a view to the development of design procedures
which take into account the complexities of unsaturated flow on
field lysimeter performance.
Recommendation
#7: That further work be undertaken on the development
techniques for monitoring soil/environmental responses such as
erosion, runoff and vegetation factors relevant to cover performance.
Recommendation
#8: That design philosophy and objectives be encouraged
which focuses on the development of long-term engineered cover
systems which are fully coupled and integrated with the natural
environment.
Recommendation
#9: That full scale, long term, field case studies
be developed and documented to increase the level of confidence
in design principles, and methods of characterization, analyses
and monitoring.
Recommendation
#10: That studies be initiated to look at issues related
to the ecological stability of covers. These studies will include
a transfer of technology from the forestry, reclamation, and agricultural
areas to the area of cover design and will provide a basis for
predictions of long term cover performance.
Recommendation
#11: Ongoing work should be supported into the use
of economic design methods (1 material - multi-layer) and materials.
Recommendation
#12: That a state-of-the-art manual or handbook be
developed which incorporates a rationale design method supported
by documented case histories. This handbook should be prepared
in an open-binder format so that it can be maintained as a current
and dynamic summary of cover design methodology.
Each of these
recommendations are more fully discussed in Chapter 6.
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