Draft Study Plan Vol 1 (PDF) - Alaska Power and Telephone Company

DRAFT DOCUMENT
The following are the resource studies we propose to conduct in 2013:
1. Cultural Resource Survey
2. Complete any unfinished surveys from 2012
Study Descriptions:
1. Geotechnical Survey
GLH will hire a qualified geology firm to evaluate the geology and dam design.
Such studies would include geologic mapping in the dam and powerhouse areas,
geophysical surveys of the powerhouse, spillway, and dam abutment areas, and office
studies to provide preliminary design parameters for the project structures.
Seismic Refraction: Seismic refraction studies will be conducted in 2012 to
better define the bedrock surface in the dam and powerhouse locations. Seismic
refraction studies involve: 1) placing a string of geophones along straight lines of interest,
2) setting off small explosive charges at certain points along the line, 3) recording the
time that the energy waves from the explosions reach each geophone, 4) measuring the
elevations along the line at each geophone, and 5) analyzing the results to determine
characteristics of the subsurface layers. A brief description of the mathematics involved
and the location of charge placement is included in Appendix D; as well as the 2011
Geotechnical Survey report.
Each seismic line is generally a few hundred feet long, with 24 geophones
uniformly spaced along the line (230 feet long with 10’ spacing, 345 feet long with 15’
spacing, etc.). Each geophone consists of a metal probe about 6 inches long and a fistsized
head. The probe is inserted into the ground so that it makes firm contact with soil
or rock, which may require a small amount of digging in loose or peaty soils or where
there are tree roots. Undergrowth is cleared as required along the line for ease of access.
On average, a crew can complete 1-2 complete profiles each day.
The explosive charges are generally 1/3 lb ANFO (ammonium nitrate fuel oil),
but in some deep soil conditions 1 lb charges are used. All charges are handled strictly in
conformance with applicable state and Federal laws by certified technicians. Each line
requires 5-7 separate shots (at each end of the line, the middle of the line, an off-end shot
at each end, and possibly shots at the ¼ and ¾ points along the line). Each charge is set
off in a hole 2-3 feet deep wedged into the ground, repacked with suitable soil material so
that as much energy as possible is directed into the ground. Depending on the ground
conditions and size of charges used, each shot disturbs a circular area from 1 to 5 feet in
diameter, with the disturbance ranging from just a small hump in the ground to a shallow
crater. In rocky or peaty soils, some material may be ejected up to 50 feet away from the
shot location.
The proposed studies for the Connelly Lake Project will include the 10 seismic
refraction profiles listed below. Because of the expected shallow bedrock conditions, the
Applicant expects that 1/3 lb charges will be used to minimize ground disturbance, with
the possible exception of lines at the powerhouse location that may be on deep alluvial
soils.
Draft Study Plan – June 2012 p. 42 Connelly Lake Hydroelectric Project