Background
Freeport-McMoRan, a large copper mining company, tasked GSI with creating an impermeable cutoff wall / grout curtain at their Chino Mine facility to prevent runoff from traveling off site. Due to the on-site acidic environment (resulting from the use of Pregnant Leach Solution (PLS) during the copper extraction process) the cutoff wall / grout curtain needed to be resistant to deterioration from sulfuric acids with a pH as low as 1.7. After initially considering cementitious-based materials, it was decided that using an Acrylamide-based chemical grout to create a Grout Curtain would provide a better long-term solution.
Description of Work
A single row grout curtain, approximately 300 feet long and 40 feet deep, was constructed by using a Sonic SR-17 track mounted drill rig to drill 64 holes (spaced 5 feet apart) through the overburden and bedrock, install 3” slotted PVC casing, and then pressure grout each hole until refusal.
Significant Project Features:
- Approximately 64 holes were drilled, vertically and at inclinations up to 40 degrees, ranging in depth from 5 feet to 50 feet, for a total of 1,800 linear feet of drilling.
- Each hole was pressure grouted to refusal by using expandable grout packers and ported pipe to inject a low-viscosity chemical grout into the overburden and bedrock. Over 18,000 gallons of chemical grout was pumped under low pressure (2-5gpm), and varying set times, to create the grout curtain.
GSI’s scope of work on this project included additional efforts to protect the new underdrain system, including using the chemical grout material:
- As infill for the 18” HDPE pipe trench boundary between the grouted overliner base and the crusher fines flow fill.
- To create a surficial grout cap along the axis of the grout curtain which will provide additional seepage protection.
Project Challenges
- The original RFP asked the contractor to install a combination Grout Curtain (in Bedrock) and Self-Hardening Cutoff Wall (in Overburden), using cementitious-based materials. However, after considering the nature of the on-site acidic conditions, GSI worked with WSP (the Owner’s Engineer) and Avanti (the chemical grout manufacturer) to develop custom set of chemical grout mixes and procedures better suited to withstand the effects of sulfuric acid.
- Resulting from the decision to install the Grout Curtain in both bedrock and overburden (as opposed to the overburden section being a cutoff wall), the approach to pressure grouting the overburden material had to be fine-tuned in the field. The original plan was to pressure grout the overburden material by placing a packer at the bottom of the 5” sonic casing, pull up 2.5’ at a time, and grout each 2.5’ section. However, this often caused “short-circuiting”, wherein the grout would travel vertically up the sides of the casing and out of the hole instead of spreading out laterally. Ultimately, GSI’s decision to use slotted 3” PVC pipe, place the packer at the top of the hole, and pump the grout under low pressure, was successful.