Abstract

Due to the inherent uncertainties in geological, geotechnical, and hydrogeological models, unexpected conditions may be encountered during open pit excavation despite best efforts to collect robust data during the pre-mining stages. An operation’s ability to respond quickly to unexpected changes is critical to project success, and this response often involves collaboration across multiple teams, including geologists, geotechnical engineers, hydrogeologists, mine planners, blast engineers, mine technicians, and operators. 

This paper presents a case study on the processes followed and the measures implemented to manage unexpected ground conditions encountered during excavation of the Diavik Diamond Mine A21 pit in the Northwest Territories of Canada. The A21 kimberlite pipe is located beneath Lac de Gras, which required constructing a water retention dike around the mining area to allow the isolated area to be dewatered prior to commencing open pit mining operations. The host rock in the A21 project site generally consists of fresh, strong to very strong, predominantly good quality granitoid (primarily tonalite) rock mass. However, during pit excavation, an area of highly altered tonalite was encountered, consisting of localized zones of very weak soil-like material that was prone to erosion from surface runoff and susceptible to degradation during mining. This zone of highly altered material was not identified during pre-mining investigations and was uncharacteristic of the project site based on the ground conditions encountered during excavation of nearby pits (A154 and A418).  

The presence of this weak material, in an area that had previously been interpreted to consist of competent rock mass, required immediate implementation of slope design changes and operational controls. These measures included targeted field investigations to update the geological, geotechnical, and hydrogeological models, including design input parameters; modifications to the pit slope design, including shallowing the inter-ramp angle in the altered tonalite zone and evaluation of opportunities for steepening slope angles in the surrounding competent tonalite areas; implementation of controls to manage surface water and seepage, particularly during the freshet season; modifications to blast design specifications for blasting near the altered zone; and replacement of the altered material intersected by the main access ramp with competent rockfill. Multiple technical teams collaborated to develop, integrate, and implement these change management strategies within a short timeframe, which allowed excavation of the pit to proceed successfully and with minimal impact to the mining schedule. 

The highly altered tonalite slope, which has a maximum inter-ramp height of approximately 40 m and a lateral extent of approximately 120 m along strike, has exhibited adequate slope performance since mining of the final wall was completed in 2022.