Designing Flow-Through Rockfill Underdrains in Unpredictable Climates

March 1, 2023   |  

Papua New Guinea experiences high annual rainfall, has rugged topography, and is one of the most seismically active zones in the world. These conditions have presented challenges to the diversion of surface flow at mine sites in the region.

Flow-Through Rockfill Underdrains in PNG

KCB has been involved in the design of flow-through rockfill underdrains for mine sites across BC and in Papua New Guinea (PNG). Flow-through rockfill underdrains have been used at mines to pass flows beneath waste dumps, rather than constructing diversion channels around the dumps. Mines with flow-through rockfill underdrains are common in BC, but KCB has done considerable work over the years with flow-through rockfill underdrains at the Porgera, Hidden Valley and Ok Tedi mines in PNG. The stable dump at Porgera, known as the Kogai dump, has a rock underdrain that has been in operation for about 30 years; the Hidden Valley Western Sector waste dumps have underdrains in operation for over 10 years.

Underdrains are typically built from durable, non-acid generating rock from the open pit. Rock is either dumped from a high dump (> 20 m high) to segregate the rock as it falls, so that the coarsest rock forms the drain at the toe of the dump, or it is screened by size and placed by conventional methods. Underdrain rock must be comprised of strong, durable rock that can withstand the stresses imposed by the overlying waste dump, and resistant to mechanical breakage and chemical weathering.

Many of the dumps in PNG could not be built without the rock underdrain as it is not possible to divert the creek base flow or flood flow. Heavy rainfall needs to be diverted, and natural soil and rock, and waste rock is highly erodible. The rock underdrains act as diversions to allow the dump stability and then serve to pass the creek flow during the mine's operating life and closure.

Designing Flow-Through Rockfill Underdrains using Wilkins’ Formula

Design principles for flow-through rockfill underdrains have been developed from civil engineering applications, including Wilkins’ formula for estimating flow capacity for non-Darcy turbulent flow in rockfill:

Q = flow in m3/sec
W = Wilkins constant, ranging from 5.24 m0.5/sec for crushed gravel to 7.33 m0.5/sec for polished marbles
i = hydraulic gradient
e = void ratio
m = hydraulic mean radius of the rock voids (m)
= void ratio/surface area per unit volume
= e*d/8.4 for coarse angular rock where d is the rock diameter in metres
A = area of rockfill transverse to flow (m2)

KCB's design of flow-through rockfill underdrains uses Wilkins ’ formula with a factor of safety of 10 applied to the calculated area. The design allows for a period of ponding for storms (e.g. 24-hour 100-year return) at the upstream face of the dump where the drain inlet is located. Design flood volumes can then be discharged over a reasonable period, often 7 days.

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