EXECUTIVE SUMMARY

Prototype in situ 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 the Heath Steele and Waite Amulet sites performed satisfactorily. The design, construction, and monitoring programs for these soil cover systems provided a unique opportunity to evaluate the performance of 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 is provided in Chapter 2. Several case histories of soil cover systems for various parts of the world are ix summarized in Chapter 3. The key laboratory tests and performance data for the Heath Steele and Waite Amulet soil cover systems are summarized in Chapter 4. A synopsis on the design and performance of these soil cover systems is provided in Chapter 5. Chapter 6 contains the conclusions and recommendations with respect to the design and analyses of soil cover systems.

An annotated literature review pertains to 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 provides a backdrop against which the design of covers can be developed.

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 be conducted into the characterization of the soil water characteristic curve for soil cover and mine wastes materials. Of particular interest are the methods of characterizing and estimating the soil water characteristic curve and the prediction of the effect that weathering processes may have on this property.

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 which consider innovations such as the development of multilayer performance with the use of one construction material, and the use of waste materials (tailings + waste rock) as potential cover 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.