Environmental Engineering Reference
In-Depth Information
by the overlying ice, has a well developed fissured fabric parallel to the ice flow direction
and is commonly highly deformed ( Figure 3.46 ). It usually has very low permeability, but
may be rendered locally permeable in the mass by thin glaciofluvial sand layers deposited
in channels of meltwater streams.
The till near the downstream toe at Parangana Dam ( Figure 3.45 ) ranged from “toughly
compacted' to weak rock and is considered by Paterson (1971) to be lodgement till.
Drumlins (2) are elongated dune-like mounds on the surface of the lodgement till,
apparently moulded to this shape by the moving ice.
Ice-cores (3) are masses of ice left behind by a retreating glacier and buried in till.
Melting of ice-cores causes the development of sinkhole-like features known as kettles (4)
and deposition of basal melt-out till (5) which is usually crudely stratified.
Flowed till (6) is till which has been reworked and deposited by mudflows, the scars of
which can be seen extending from the lateral moraine ridge through its adjacent kame ter-
races. The flowed till may be stratified due to redistribution of fines during flow. The kame
terraces here are underlain by complexly interbedded sediments as shown on Figure 3.49 .
Supraglacial morainic till (7) deposited at the snout, as shown on Figure 3.47 , contains
a large range of gravel and larger fragment sizes and is usually deficient in fines. It is
known also as supraglacial melt-out till or ablation till. The younger glacial deposit at the
dam axis area at Parangana Dam ( Figure 3.44 ) is believed to be of this type. It comprises
up to 70% of gravel to boulder sizes in a matrix of sand, clay and silt. The largest boul-
ders are up to 3 m diameter.
3.12.2.1 Properties of till materials
It is clear that basal melt-out till, flowed till and supraglacial morainic till will be poorly con-
solidated unless they become desiccated or covered by new sediment or ice at a later time in
their history. The latter appears to be the case at Parangana Dam where the supraglacial
morainic till is well compacted, apparently due to being overridden by ice during subsequent
glacier advances (Paterson, 1971). Experience has shown that most tills of all types have low
permeabilities (less than 10 10 m/s). However, till deposits may contain or be next to bodies
of permeable sands and/or gravels of glaciofluvial origin, filling old channels. For example
the various outwash deposits shown on Figure 3.48 would normally have relatively high per-
meabilities. At Parangana Dam the younger till beneath the valley floor contained a layer of
glaciofluvial sands several metres thick with measured permeability of 10 6 m/s. To allow
for this layer the core cutoff trench was excavated down into underlying materials of low
permeability (Figure 3.44). The younger till was left in place beneath the downstream shoul-
der, but was covered with a filter blanket before placement of the rockfill.
Walberg et al. (1985) describe remedial works required at the 23 m high Smithfield
Dam in Missouri, USA, after seepages and high piezometric pressures were recorded dur-
ing first filling. Exploratory drilling using 152 mm diameter cable tool tube sampling
showed that glacial outwash sands and gravels beneath the left abutment were more con-
tinuous and permeable than assumed from the pre-construction drilling and sampling.
At Cow Green Dam in Britain most of the material filling a buried channel beneath the
left side of the valley was found to be lodgement till, described as “stiff, dark brown,
poorly-sorted, unstratified, silty, sandy clay of medium plasticity containing subangular to
rounded gravel, cobbles and boulders..…”. The boulders ranged up to 2 m in mean diam-
eter (Money, 1985).
Sladen and Wrigley (1985) describe further generalisations which can be made about
the geotechnical properties of lodgement tills.
3.12.2.2 Disrupted bedrock surface beneath glaciers
Knill (1968) describes the open-fractured nature of bedrock beneath glacial materials at
several sites and concludes that gaping or infilled joints near-parallel to the rock surface
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