Environmental Engineering Reference
In-Depth Information
Budget limitations required that detailed investigations using seismic refraction surveys
and test borings be limited to four of the more critical locations.
Evaluations and Treatments
Debris Avalanches
The debris avalanches were occurring along 80 km of roadway beginning near the low-
lands, where the dominant conditions are relatively soft volcanic rocks with a moderately
thick cover of residual soils, and in places, colluvium. The natural slopes usually were
inclined at about 35° or steeper. Initially, vegetation was removed and roadway cuts were
inclined at 53°, much steeper than the original slopes. Subsequently rainfall and seepage
resulted in the residual soils sliding along the fractured rock surface.
Unsupported cuts were made because the large number of cuts would cause support
with retaining walls to make roadway construction prohibitive. A more stable alternate
slope design could have included 10-m-high 45° cuts with benches to result in an overall
cut slope inclination of 38°. Construction costs would be increased but roadway slope fail-
ures and maintenance costs should be decreased.
In many cases where failure has occurred the residual soil has been removed and the
slope is now self-stabilized, although in some cases, there remains potentially unstable
material upslope of the failure scar. Future failures will be removed from the roadway
and disposed off in designated spoil areas, rather than dumping over the sides of the
roadway.
Downslope Roadway Drains
It was recommended that all roadway drains be relocated so that discharge downslope is
where erosion will not endanger the roadway. Drainage channels should be lined to pre-
vent erosion. The failure shown in
Figure 9.6
resulted from storm water discharge eroding
the slope below a shotcreted gabion wall that was supporting the roadway. Storm water
entered the catch basin shown on the upslope side of the roadway and discharged through
a pipe exiting on the downslope side. The failure in Figure 9.6 was corrected with the con-
struction of an anchored wall and relocating the storm water discharge point.
Large Slump Slide
At km 75 the roadway makes an abrupt switch-back as shown in
Figure 9.133.
At this loca-
tion is a large slump slide evidenced by tension cracks upslope and roadway movement.
When first visited during 1999 the portion of the roadway in the middle of the photo
(Figure 9.133) had dropped about 2 m; when visited the following year, the roadway had
dropped an additional 2 m and was almost impassable. This was considered a priority site
for remediation.
Explorations with seismic refraction surveys and test borings determined geologic con-
ditions to include about 15 m of colluvium overlying 5 to 20 m of fractured rock grading
to hard rock.
Treatments recommended for stabilization included:
1.
Surface drains constructed upslope to collect runoff and to discharge away from
the failing area
2.
Subhorizontal drains installed along the toes of the cut slopes areas shown on the
photo
3.
Upslope roadway cut slope to be reshaped with benches and covered with shot-
crete
