Environmental Engineering Reference
In-Depth Information
Figure 2.37.
Sugarloaf Dam, geological plan showing dipslopes. (c) axes of minor plunging folds;
(d) dipslope, slightly disrupted; (e) scar of old landslide; (f) dipslope, very disrupted; (g) out-
line of embankment.
Figure 2.37 shows the layout of the embankment which extends across the valley of
Sugarloaf Creek and then for 500 m along the crest of a ridge. No other site was accept-
able topographically.
During the feasibility stage, detailed geological mapping of surface features and
trenches dug by bulldozer and ripper were the main investigation activities.
After initial excavation the floors of the trenches were cleaned carefully by small back-
hoe and hand tools to ensure that all minor defects such as open and clay-filled joints and
thin crushed seams were clearly exposed.
This work showed that much of the foundation area on both banks was formed by dip-
slopes; thinly interbedded and weathered siltstone and sandstone were dipping parallel to the
ground surface (Figure 2.37). Dip and slope angles ranged from 12° to 30°. On the right bank
a suspected old landslide was confirmed by the trenching to be underlain by deep, disturbed
soil. With the exception of this feature the valley slopes in the foundation area showed no
topographic evidence of past landsliding or of significant undercutting of the dipslopes.
Trenches cut straight down the dipslopes showed the bedding surfaces to be very smooth
and that slickensided seams of low plasticity clay 2 mm to 20 mm thick were present along
them at spacings of 1 m to 2 m. The seams were highly dispersive. Diamond core drilling
showed that the clay seams were present to depths of more than 1 m and that they were the
weathered equivalents of bedding surface faults (crushed seams, see Chapter 3, Figure 3.19
and Section 3.5.2), which were present at about the same spacings, at greater depths.
Present in some parts of the trenches were near-vertical features of the kind shown on
Figure 2.38
. These were mainly joints which were open or infilled with gravel or up to
20 mm of high plasticity clay. Also present near the base of a nearby dipslope was a large
but ancient “dropfold” structure where upper beds had clearly collapsed into a gaping
slot in a lower bed. Figure 5.10 in Chapter 5 shows this feature.
It is judged that these near-vertical and “dropfold” features had been formed by exten-
sion of the near-surface beds during downslope sliding movements along weathered bed-
ding surface seams. Cross-cutting defects near the base of the slope and in some places
