Pulacayo Project Feasibility Study - Apogee Silver

Pulacayo 1 000 t/d Phase I Feasibility Study - NI 43-101 Technical Report
090644-3-0000-20-IFI-100
18.9.5 Foundation Preparation
The site is overlain by a very thin layer of topsoil, which will be removed from the dike foundations
as the first step in construction (topsoil within the impoundment and outside of the dike
foundations may be either removed or left in place at the owner’s election. The stripped topsoil
will be stockpiled for use in closure and reclamation. The topsoil layer is underlain by alluvial soil
in and near the bottom of the valley, and residual soils up slope. There are a few areas where
moraine deposits remain, but these are limited in area and generally thin. The entire site is
underlain by a sandstone basement rock.
The alluvial soils have been placed by the seasonal flow within the tailings valley, and (more
importantly) the seasonal flow of the adjacent Rio Negro, which has its confluence with the
tailings valley just downstream of the facility. Since the tailings valley is very near the top of the
watershed, with a very small catchment area (about 45 ha), the extent and depth of the alluvium
is very limited within the TSF area.
The alluvium consists of well-graded gravels and sands, generally dense. The residual soils are
those that have weathered in situ from the basement rock and are dense to very dense. These
soils are poorly graded silty gravels and sands with cobbles. The glacial moraines are generally
composed of silty gravels with some cobbles, and are very dense. The basement rock consists of
principally of competent, medium grained sandstone. The surface of the basement rock is lightly
weathered and fractured to a shallow depth, and it gains competency with increasing depth.
Except for the thin layer of topsoil, the site soils are considered suitable for use as structural fill
(Dikes 1 & 2), random or mass fill (Dikes 3 & 4) and as foundations for all of the dikes. Some
removal (less than 3,000 m 3 ) of inadequate soils will be required beneath the first dike,
emergency pond and subdrain monitoring ports area. This soil will be stockpiled with the topsoil.
18.9.6 Foundation Underdrains
A system of underdrains or subdrains will be installed to capture shallow subsurface water and
seepage from the tailings. This system will drain the entire TSF basin and will also act to monitor
and provide an early indication if there is detrimental impacts to water quality from the TSF. This
subdrainage system will consist of a network of perforated double wall polyethylene pipes
(generally 100 mm and 300 mm diameter), which will be installed in trapezoidal ditches nominally
700 mm deep and filled with drainage gravel (conventionally called French drains). These French
drains will slope at a minimum of 2% and the flow will be collected in the small monitoring ports
located below the emergency pond.
The preliminary location of the drainage network is shown on the design drawings (by Anddes),
but the final locations will be determined and fit in the field based on the observed ground
conditions at the time of construction. That is, the network may be adjusted or expanded to
accommodate actual as-built conditions. The discharge from the subdrains is expected to range
from negligible to a peak of between 1.0 and 2.2 m3/h (based on the peak storm event).
18.9.7 Containment System
The facility is located in a tight, closed basin underlain by shallow, tight sandstone. This basin
drains to a single outlet just below the TSF. The tailings will be discharged at high solids contents
TWP Sudamérica S.A. Av. Encalada 1257 Of. 801, Santiago de Surco Lima 33, Perú (51-1) 4377473
Page 196