Stabilizing Slides Using a Spider Excavator for Headscarp Removal and Installation of Remedial Drainage
Presented at the 1998 Annual Forest Site Rehabilitation Workshop, Kelowna BC
Eric L Kay, Rob Wilson.
INDEX
Part 1 - Spider Excavator
Part 2 - Headscarp removal
Part 3 - Multi-flow pipe
Part 4 - Contour Drains
Spider Excavator
The conventional track
excavator is a common workhorse in the logging industry of BC. On steeper ground the operator needs to
bench into the slope to gain access and help insure user safety. However, this is not always desirable when
we are concerned with stability of a slope since it may lead to costly problems
related to placement of the excavated material and other problems such as a
loss of toe support and over steepened cut and fill faces.
As an alternative, the
Spider Excavator is a machine that has the ability to walk, (like a spider),
with its 4 legs on steep slopes of up to 100% without the requirement to
construct large benches or create undue ground disturbance. The only effect is the ground compaction at
each of the pads on the base of the spiders "legs".
The Spider Excavator moves
by walking on the legs, or pushing and pulling two rear wheeled legs with the
hoe bucket and arm. It can also be
equipped with hydrostatically driven rubber tires for travel speeds up to 6kph
on flatter ground.
The spider legs can step
over shrubs and small trees and also over obstructions up to 2m high and wade
through water just as deep for instream remedial work (equipped with an
environmentally safe hydraulic oil substitute).
The larger Spider excavator
has an operating weight of 8,100kg; with the excavating performance of a
15,000kg conventional hoe. It's long
stick, when fully extended, gives the machine a reach of 26 feet (8000 mm) from
it's center. The boom, connected near
the Spider's center of gravity allows excellent leverage and torque. It can easily straddle a trench and using
its own reaction weight, optimize it's capability while digging a hole below
itself. Equipped with a thumb and a
twist wrist for the bucket adds versatility to the machine. Commonly it is also
equipped with a winch to assist its movement and provide safety. The winch can effectively be used to "yard"
large woody debris to the work area for remedial site enhancement.
The Spider Hoe's high
performance and light weight makes it an ideal candidate for helicopter
transport to remote sites, fully fueled and ready to work.
Headscarp removal
A typical feature of a
landslide is the headscarp that remains poised after the initial event. The material at the head of a slide is the
largest driving force to cause downslope movement. The headscarp is typically an unstable mass which has proved to
be an ongoing problem for effective landslide rehabilitation. Material from the continually failing
headscarp often destroys rehabilitation work in the body of the slide and is a
potential safety hazard to the rehabilitation workers. Subsequent failures of the headscarp are
retrogressive in nature and this can become a chronic problem which could
destroy a large land base of potentially valuable growing area, along with a
high potential for sediment generation to our waterways.
In the past, where site
access to the headscarp for conventional excavators is not possible, explosives
have been the only available tool to reduce or remove the headscarp. The explosives are used in combination with
Bio-engineering to stabilize the remaining material.
We can now use the Spider
Excavator as an effective tool that can climb up many slide tracks of up to
100% (One to one) slope gradients, to access the headscarp area. The Excavator can then safely and
effectively remove and re-contour the headscarp in a controlled manner.
Multi-flow pipe
Multi-flow pipe is a
flexible, subsurface drainage composite. It is made up of multiple small
"Big O" type perforated drainpipes, approximately 2.5cm in diameter,
fused together and laid flat, all of which are enclosed in a filter fabric.
It is available in 15cm, 30cm and 46cm
widths and comes in 45.75m long rolls.
These rolls can be easily carried or rolled uphill by 2 men, or slung
uphill by an excavator such as the Spider.
Installation can easily be done by lowering it into place from the
surface without the need for workers entering the trench.
Using Multiflow, a
performance and cost effective subsurface drainage system equivalent to drain
rock can be engineered without the need for producing and trucking granular
aggregate. This is especially useful
for sites which are difficult to access such as slide headscarps.
Contour Drains
An active earth slump can be
stabilized by drainage measures if the seepage control will reduce the weight
of water in the unstable mass of material or if it will reduce the seepage
force. Many of these active slides are
shallow seated, with drainage problems originating above the headscarp.
Contour drainage, upslope of
the headscarp of a slide can be an effective stabilization measure, utilizing
both surface and subsurface drainage systems.
A trench is dug to install a
groundwater collection system and when backfilled, a shallow sloping swale is
shaped overtop to intercept the surface run-off. Both of these measures redirect captured water to stable flanks
of the slide area, thus reducing the over-all weight acting on the slope and
minimizing the seepage force.
A drainage composite such as
the Multi-flow pipe is buried in the trench to collect groundwater
seepage. A seepage barrier such as a
sheet of 6 mil polyethylene can also be buried in the trench to block
seepage. The work crew need not enter
the trench if the drainage composite is secured to the barrier sheet (i.e.
using duct tape). The assembly can be
lowered in a relatively safe manner into the trench and the barrier sheet can
be pinned in place with wooden stakes to the top of the trench walls.
After backfilling the trench
with the natural soils, the surface is swaled into a shallow ditch with woody
debris spread over top. If required, a geosynthetic erosion mat can be placed
in the shallow swale to minimize erosion.
The exposed mineral soils should be seeded to finalize the contour
drainage rehabilitation measures.
The spider excavator allows
good access on the steep slopes with negligible site disturbance and is
efficient for digging shallow trenches from 2 to 4 meters deep. There is little
need for benching as personnel do not need to enter the trench, thereby
minimizing site disturbance and resulting in a safe work site relative to
conventional track excavation work.
Contour drainage using a spider
excavator can be a cost effective and relatively safe means of stabilizing
shallow retrogressive slides with shallow groundwater seepage problems.
Eric L Kay E Mail Forest Road Consultant
