Vertical Pit Development

Dewatering and Cleanup

When the Company returned to the Koidu Kimberlite Project Mining Lease Area in 2002, No. 1 Pipe was flooded and was being used by the local community for bathing and washing clothes. After dewatering of the 30m deep excavation made by SLST/NDMC in the 1970s and 1980s, removal of the remaining silt and mud commenced. Between March 2003 and November 2003, the No. 1 Pipe was dewatered, cleaned and prepared for the conventional open pit mining phase.


Flooded No. 1 Pipe January 2002	Dewatering of No. 1 Pipe March 2003	Exposing the historical workings on No. 1 Pipe	Removing silt and mud September 2003

No. 1 Pipe was mined by conventional methods from the previous pit floor at approximately 349m amsl down to 339m amsl, from January 2004 to May 2004, after which mining shifted to No. 2 Pipe while establishment of the vertical pit collar and infrastructure got underway. By May 2005, the vertical pit was established to such a point that mining could resume in No. 1 Pipe making use of temporary ramps for access into the pit. After the hoist and winder were commissioned in November 2005, the temporary ramps were removed and the waste rock and kimberlite ore lifted to surface in large buckets known as kibbles.


Lowering the ramp into No. 1 Pipe - November 2003	Development of conventional open pit at No. 1 Pipe - April 2004	Final depth of conventional open pit at 339m amsl - April 2005	Commissioning of A-frame headgear and winder - November 2005

Vertical Pit Rationale and Methodology

Due to the disruption to the community and the impact of blasting at No. 1 Pipe, the Company reviewed the mining method and adopted the new vertical pit mining method.

Several factors contributed to the decision to establish a vertical pit at Koidu No. 1 Pipe:

Proximity of Koidu Town and presence of houses very close to the perimeter of the pipe;
The environmental and social impact would be a fraction of that created by an open pit;
Due to the small size of the pipe, the maximum depth of mining by open pit methods would have been only 80m before the stripping ratio became too high thus making the deposit uneconomical to mine;
Insufficient information regarding grade and diamond value was known to justify establishing an underground mining operation from the outset.

Vertical pit headgear

Vertical pit floor

Vertical pit mining is a relatively new mining technique, which has been successfully implemented only once before, at the Nyala chrome deposit in Zimbabwe in the late 1990s. By supporting the vertical sidewalls of the pit by means of cable anchors, rock bolts, wire mesh and wetcrete, a large open air shaft roughly the size of the ore body is being established.

Although vertical pit mining is technically challenging and substantially more expensive than conventional open pit mining, the environmental and social impacts are significantly less. The floor area of the vertical pit is approximately 4,388m², just over half the size of a soccer field. Blasting in the vertical pit, by necessity, is well controlled in order to limit damage to the vertical sidewalls, and the amount of fly rock is considerably reduced. The fact that the earthmoving equipment operating in the vertical pit remain inside the pit during blasts, illustrates how well controlled blasting in the vertical pit has to be.

An A-Frame is used as a headgear. The foundations for the headgear are cast on the edge of the vertical pit, with the hoist positioned parallel to the edge approximately 20 meters away from the headgear. The tipping arrangement for the kibble is a conventional 'lazy chain' system, and trucks haul the kimberlite to the plant stockpile. The sidewall support recommended by SRK Consulting, who originally masterminded the vertical pit concept, is a combination of 20m and 40m long, high tension cable anchors, and wire mesh, rockbolts and wetcrete (a special type of cement). The rockbolts are 3m long, fully grouted and hold the wire mesh, which is then covered with a thick layer of wetcrete.