tut1a A_Embankment Dam_Borrow Sources
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GEOL3003/5010 - Tutorial 1– Borrow Sources for Embankment Dam Construction
Question 1) Using the geological map sheet “Lissadell – First Edition 1968” from the Geological Survey of Western Australia, mark areas on the map where you may find the following material types.
During the desktop study phase, the engineering geologist reviews available geological and
geotechnical information. Published information includes geological maps and accompanying
explanatory notes available from the Geological Survey of Western Australia, and aerial
photography.
Prospective Borrow Pit Locations:
Sand for Filter Zone - river and creek beds in the northern parts of Australia are often
ephemeral with low flows during the dry season. When flows are low, extensive deposits of
sand and gravel can be accessed. The aerial image below shows the Ord River bed before the
Ord Dam was built. Borrow areas would have been established where the deposits contain the
most sand, and least gravel.
Source: https://maps.landgate.wa.gov.au/maps-landgate
Clay Zone - clayey soils suitable as impervious clay core can be hard to find. Prospective
areas can be low-lying areas where soils are deposited by slow flowing water or sheetwash
across a relatively flat landscape. Areas of Alluvial (Qa) along the edges of the Ord River
valley are prospective areas, although it’s likely the soils will not be pure clay but will have a
sand component. For the Ord Dam, material for the core was sourced from alluvial areas
(floodplain areas) upstream of the dam site.
The Black Soil (Czb) is a potential source of clayey soils. The explanatory notes mention
cracking clays. However, according to the map, this material is quite a long way from the
dam site.
Rock-Fill - of the locally available rock types, the massive quartz sandstone has the best
qualities. It is massive. Quartz is a hard, durable, erosion-resistant mineral. The interbedded
siltstone may be problematic and result in some wastage of material that fails to meet the
construction specification for rockfill). Shale and micaceous rocks tend to be foliated and
have closely-spaced planes of weakness. Slate and schist are also typically foliated. The Bow
River Granite is potentially a suitable source of rock fill but it does not outcrop close to the
dam site.
The actual quarry site can be seen immediately south of the dam in the image below. The
Lissadell Formation (Pcl) outcrops on both the north and south sides of the dam location,
both areas are prospective quarry sites. The southern site was chosen because it was the site
with sufficient quantity of material and located far enough away from the dam for aesthetic
reasons.
Source: Google Earth
Actual quarry site is in this area.
Another prospective area for a rock fill quarry.
Actual area used Prospective a
rea
for clayey soils.
Prospective area for clayey soils.
Prospective area for clayey soils.
Prospective area
for clayey soils.
There is clay in this area but it is a long away from the dam site.
Prospective area for sand
and gravel and river channel.
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Question 2) Calculate the approximate %, fines, % sand and % gravel for each site. Fraction
Borrow 1
Borrow 2
Borrow 3
Test Pit 1
Test Pit 2
Test Pit 3
% Fines
14
35
8
63
35
5
% Sand
29
12
80
37
55
92
% Gravel
57
40
12
0
10
3
Question 3) Using the laboratory results plot Atterberg limits for all 6 soils. Question 4) Classify all 6 soils from the borrow pits (B1-B3) and test pits (TP1-TP3) based on the USCS classification system outlined in the lecture.
Soil ID Answer 1) Two Letter Soil Classification Potential Geological Origin Borrow 1 GM ‐
silty sandy GRAVEL or Silty GRAVEL with some sand ‐
alluvial deposit (stream bed) Borrow 2 GM ‐
silty GRAVEL with trace sand ‐
glacial till Borrow 3 SW ‐
SAND with some silt and trace gravel ‐
slope colluvium Test Pit 1 CH ‐
sandy CLAY ‐
estuarine muds Test Pit 2 SC ‐
clayey SAND with trace gravel ‐
riverbank overflow / flood deposits Test Pit 3 SP ‐
sand ‐
sand dunes
Question 5) Calculate the Hydraulic Conductivity for Borrow 3 and Test Pit 3.
Borrow 3: K = 7.4 m/day (D10 = 0.09 mm, D50 = 0.6 mm, D60 = 0.8 mm)
Test Pit 3: K = 16.4 m/day (D10 = 0.2 mm, D50 = 0.8 mm, D60 = 1 mm)
Question 6) Examine laboratory results and discuss which borrow area would be best for each location.
Borrow 1 – Most Suitable for road fill because it is the only soil that meets all the client requirements for plasticity and it is the only soil whose PSD curve falls within the grading envelope requirements.
Borrow 2 – Unsuitable with significant treatment, too much silt and clay for road construction and sand
filter; too much sand and gravel for dam core. Borrow 3 – Slightly too much silt and clay and plasticity is too high for road fill. Almost suitable for Sand Filter but slightly too much gravel. Screening out the gravel would make it suitable for sand filter.
Test Pit 1 – Most suitable for core fill because of high silt and clay content and high plasticity. Not suitable for road or sand filter because too much silt and clay Test Pit 2 – Too much silt and clay for road and sand filter, too much silt and not enough plasticity for dam core.
Test Pit 3 – Most suitable for dam filters because it is a clean sand. Not enough fines for dam core or road construction (need fines in road construction to get maximum compaction and density). Question 7) Using your conclusions from Question 6, draw grading envelopes for the Road Construction soil (Table 1) and Sand Filter (Table 2) on the PSD curves for the borrow pit or test pit location you think are most suitable. This will help confirm you conclusions. See Answers in Grading Envelopes for Road Fill and Embankment Filter shown below confirming assumptions made in Answer 6.
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