Environmental Engineering Reference
In-Depth Information
Pingos (11) are conical mounds of buried ice up to 40 m high and 600 m in diameter.
Pingos, which have developed by the freezing of upward-moving groundwater, have
caused updoming of the surrounding sediments. Melting of the ice results in the dough-
nut-shaped surface features shown on
Figure 3.55
.
Paterson (1971) provides a useful account of periglacial and other effects of glaciation fea-
tures at Parangana Dam. Core-drilling into the steep sided buried pre-glacial river channels at
the site (
Figures 3.44
and
3.45
)
showed the lower part of each to be filled mainly by angular
rock fragments up to 500 mm across set in a sandy clay to clayey sand matrix. The ratio of
rock to matrix was roughly 60 : 40. The rock fragments were of quartzite and schist, clearly
derived from the bedrock immediately upslope in each case. These deposits were judged to
have been formed by “solifluction debris avalanches” from the steep valley sides, under
periglacial conditions during the glacial advance period.
In the upstream channel the avalanche material contained a discontinuous bed of clay
believed to have been deposited in a lake formed upstream from one of the slides. Also
present in the slide deposits were lenticular beds of sand and gravel, inferred by Paterson
to be stream deposits in channels eroded through the slide dams. The distribution of sand
and gravel beds suggested that at least five major landslides had occurred.
Locally derived talus covers most of the ground surface at the site and extends to about
10 m depth in the foundation area (Figures 3.44c and 3.45). This material is believed to
be a periglacial solifluction product formed during the final retreat stage of the glacier.
3.12.5
Glacial environment - list of questions
Only the most significant glacier-related questions are listed here; these and other features
which are usually of less significance in dam engineering are shown on
Figures 3.51
and
3.55
.
- Buried valleys?
- Bedrock surface or boulder?
- Bedrock disrupted near upper surface?
-Wide variety of till types?
- Materials unsorted - clay to boulder sizes?
- Slickensides in clay-rich till?
-Variable compaction and cementation?
- High permeability sands and gravels?
- Loess?
- Landslipped deposits?
-Creeping landslides?
