MWH was responsible for the tailing storage facility reclamation design and construction quality assurance (CQA) of an inactive copper mine in Arizona. MWH worked closely with the client to develop innovative strategies for the reclamation of two tailing storage facilities at the project site. The overall objective of the project was to create and sustain a life-cycle habitat that could be tested and monitored over time with the hope that design features used could be implemented on similar future projects. The client has labeled the site a test reclamation facility and encouraged MWH to implement value engineering during the design and CQA for the purpose of converting innovative concepts into immediate cost-savings and potential future savings.
The design objectives include:
• Improve ecosystem functionality by topographic integration;
• Demonstrate methodologies for attenuation of stormwater on and from the surface of the reclaimed facilities;
• Construct and evaluate options for self-sustaining growth of native plant species;
• Design and construct low-maintenance surface water drainage controls that reduce erosion and sedimentation;
• Satisfy applicable stormwater discharge standards;
• Reduce the quantity of cover material required;
• Reduce infiltration into the tailing impoundments;
• Geotechnical stability; and
• Create a testable design.
The project entailed capping the tailing impoundments with unimpacted soils and directing stormwater flows from the reclaimed tailing facilities to armor-lined channels placed on the top surfaces and outslopes of the impoundments. Stormwater from the reclaimed tailing facilities was directed away from the impoundments through armor-lined toe channels and into infiltration basins for onsite stormwater retention. Innovative implementation included construction of attenuation basins atop the impoundments that regulate stormwater flow through weir outlets. The attenuation of stormwater flow allowed downstream stormwater facilities to be smaller with reduced armor and project costs. The project site encompasses approximately 710 acres of reclaimed land, five attenuation basins, and several sideslope channels, toe channels, and downdrains.
Preliminary designs revealed the high magnitude of design flows for the site leading to costly erosion protection facilities required to safely convey stormwater from the project area. The client was instrumental in communicating with MWH that this project was not intended to be a “walk-away” closure project, but a demonstration facility that would be tested, monitored and repaired, as necessary, over time. This understanding allowed the design team to think outside-the-box regarding key design components leading to multiple discussions between the client and MWH focused on innovative and complex engineering concepts. MWH completed various studies and alternative evaluations that created construction cost-savings and mitigated risks. The following value engineering opportunities were investigated by the client and MWH and were eventually executed during the project.
- Method Specification Procedures – Method specification tests were developed by the project team early in the project to determine effective placement procedures and rolling patterns that could be used for tailing and cover placement. MWH developed and observed the test procedures which consisted of constructing test plots out of soil material placed to the desired lift thickness to be used during construction, monitoring the number of passes traversing the pad by the selected compaction equipment, and documenting settlement and compaction after each pass. The tests were carried out until diminishing settlement and compaction results were evident. The results of the method specification tests were used to establish an adequate lift thickness, the approved placement/compaction equipment, and required number of passes and moisture content ranges.
The method specification procedures helped significantly advance soil placement activities over the course of the project, thus reducing overall construction costs.
- Attenuation Basins – MWH designed the attenuation basins atop the tailing facilities to retain/detain stormwater. A total of five lined attenuation basins were constructed for the purpose of storing or attenuating stormwater flows that discharge the top surfaces. The attenuation of stormwater led to significantly reduced flows discharging the top surface through sideslope downdrains, resulting in manageable flows for designing the downdrain and downstream channel erosion protection systems, thus reducing overall construction costs for the project.
- Life-Cycle Habitat – One of the client’s main objectives was to create a site condition accommodating the several wildlife species in areas surrounding the project site. Innovative technologies used during design and construction included designing a reclaimed mine site that blends in functionally and aesthetically with surrounding natural topography, while still providing erosion control and reduced long-term active maintenance.
During the wet season or after storm events, stormwater-filled ponds provide a location for waterfowl and other wildlife to gather. Brush packs consisting of purposefully stockpiled wood materials on the top surface of the reclaimed impoundments aid with topsoil nutrient capture which is mobilized across the surface and also provide habitats for wildlife while vegetation is establishing.
The client has frequently commented on the several wildlife species observed on the site and atop the reclaimed facilities since construction has been completed.
- Stormwater Throttle – A borrow pit adjacent to and spanning almost the entire east side of the site was used as a source for capping material during reclamation of the tailing storage facilities. Discussions between the client and MWH led to a plan to use the borrow pit as a stormwater conveyance corridor in lieu of constructing robust main channels along the toe of the tailing facilities. For the remainder of the project, the borrow pit was excavated to maintain a positive slope so that it could freely drain stormwater.
The concept of using the borrow pit for stormwater conveyance led to the design team’s decision to combine flows from areas adjacent to the site with flows from the reclaimed stockpiles. The addition of stormwater flows from these areas increases the required capacity for downstream channels. Routing the increased flows through the borrow pit utilized the broad base and gradual longitudinal slope of the pit, which helped maintain shallow flow depths, reduced velocities, and manageable riprap sizes and quantities. However, controlling this flow in narrower downstream channels with steeper slopes required sizeable quantities of robust channel armor.
To reduce the relatively large flows of the order of 2,000-cubic feet per second (cfs) in the downstream channels, MWH evaluated throttling stormwater through the borrow pit. This concept entailed attenuating flow through a culvert pipe and weir constructed in an earthen embankment within the pit. Throttling flow through the borrow pit reduced downstream flows by more than 90-percent and considerably reduced construction costs for the channels downstream.
- Caliche as Channel Armor – During construction, shallow caliche beds were discovered on the west side of the property. The caliche surface withstood flows from multiple rain events during construction leading to the design team’s decision to utilize this natural feature in place of riprap for flows along the west. The use of caliche resulted in significant reductions in riprap quantities and associated project costs.
The irregular cross-section and profile slope of the caliche beds along the west toe alignment led to MWH’s complex analysis of the west side hydraulics. MWH evaluated cross-sections for each distinct cross-section along the approximate 2-mile alignment to determine the design stormwater flow depths projecting onto the toe of the reclaimed tailing facilities, which would require armoring. Approximately two hundred cross-sections were evaluated by MWH to develop the stormwater flow surface profile along the west side. This effort led to a several million dollar construction cost savings.
- South Deflection Berms – Early in the project, the client performed pothole excavations to search for a borrow source meeting the cover material specifications. The potholing revealed subsurface caliche beds that extended into areas south (downstream) of the site. The presence of caliche in these areas prompted the project team to evaluate potential cost saving alternatives versus risk for implementing sheet flow downstream of the impoundments.
The project team adopted the concept to allow erosion in the natural downstream portions of the site, as long as stormwater and sediment was maintained on site. As an alternative to constructing channel systems, the south stormwater deflection berm concept was developed as a means to control stormwater in these downstream areas. With the presence of armored berms and underlying caliche beds, the expectation is that the site will reach physical equilibrium over time.
The stormwater deflection berms have been designed to direct stormwater from the east, west, and south areas of the site towards and into two cascading borrow pits in the southern portion of the site. Sedimentation from erosion will also be collected in the borrow pits and will be cleaned out by the client over time, as necessary. The berms have a staggered design and provide a 2-foot freeboard from the design storm as insurance should erosion alter the course of flow. Furthermore, each berm was armored with riprap that was toed into natural ground, extending the performance time of the berms and giving the client fair warning of required maintenance without damaging the functionality and structural condition of the berms.
- Infiltration Basins – Two borrow pits in the south portion of the site were developed during construction in a manner to capture stormwater and sediment from the site, maximizing onsite retention. The infiltration basins were designed to cut-off stormwater flows from the entire site with assistance from the deflection berms.
The cascading borrow pits were designed with four overflowing cells (two cells within each pit) where overflow from the north borrow pit spills into the south borrow pit through a riprap lined spillway.
Separating the two cells in each pit is an earthen embankment which protects a known fault below. Stormwater will enter the borrow pits on the west side of the embankments. During larger storm events, stormwater fills the borrow pits’ west cells and spills over the in-pit embankments to the east cells through riprap-lined spillways.
The borrow pits have the combined capacity to contain stormwater from the design storm event. Downstream of the borrow pits is another borrow area which provides additional site containment should it be needed.
The decision to retain stormwater onsite meant that costly improvements to downstream offsite facilities were eliminated.
Overall, MWH estimates the total construction cost savings for the project due to value engineering was approximately 60% of the original budget, saving the client more than $7M in capital costs.
The scope of CQA services included observations, testing, and documentation of materials and methods used to complete construction of the project. In general, CQA activities included:
- Monitoring of clearing and grubbing operations;
- Monitoring of test plots for method specification procedures;
- Monitoring of placement and testing of subgrade materials;
- Monitoring of material stockpiling and processing;
- Monitoring of cover fill placement including density, moisture content, and soil texture classification tests;
- Conformance testing of soil and riprap materials;
- Monitoring of riprap placement procedures;
- Monitoring of pipe installation;
- Observation and approval of liner subgrade preparation and final surfaces;
- Observation of installation, review of third party testing, and documentation of geosynthetics; and,
- Addressing requests for information (RFIs).
- Providing construction support.
- Documentation of construction activities including production of a Final Construction Completion Report and Record Drawings.
MWH focused on observing that the various project elements were constructed in accordance with the plans and specifications. Early in the construction process, the MWH CQA team held discussions with the client, contractors, and the quality control (QC) team to review and address construction procedures. The MWH CQA team and the QC subcontractors were coached by MWH so that the project team was aware of MWH’s technical requirements for compliance and quality testing. This led to development of the QA/QC matrix, which summarized the testing requirements and frequencies of the CQA plan and project specifications. The CQA and QC teams were encouraged to carry a copy of the QA/QC matrix with them in the field as a guide during construction activities. This example of communication between the project teams continued throughout the duration of the project and played a key role in the execution of the project scope while meeting the construction schedule.
- Arizona, United States
- 34.0489281, -111.09373110000001