Geoscience Reference
In-Depth Information
8.17 Time required for drainage of deposits
•
The drainage time depends on the coefficient of consolidation, and the drainage
path
•
t
90
- time for 90% consolidation to occur
Table 8.17
Time required for drainage.
Material
Approximate
Approx. time for consolidation based on drainage path length (m)
coefficient of
consolidation,
0.3
1
3
10
C
v
(m
2
/yr)
Sands & Gravels
100,000
1hr
1 hr
1 to 10 hrs
10 to 100 hrs
<
<
Sands
10,000
1 hr
1 to 10 hrs
10 to 100 hrs
1 to 10 days
<
Clayey sands
1000
3 to 30 hours
10 to 100 hrs
3 to 30 days
1 to 10 mths
Silts
100
10 to 100 hours
3 to 30 days
1 to 10 mths
10 to 100 mths
CL clays
10
10 to 100 days
1 to 10 months
1 to 10 yrs
10 to 100 yrs
CH clays
1
3 to 30 months
1 to 10 yrs
30 to 100 yrs
100 to 1000 yrs
•
Silt and sand lensing in clays influence the drainage path length.
•
Vertical drains with silt and sand lensing can significantly reduce the drainage
paths and hence times for consolidation.
•
Conversely without some lensing wick drains are likely to be ineffective for thick
layers, with smearing of the wicks during installation, and possibly reducing the
permeability.
8.18 Estimation of permeability of rock
•
The primary permeability of rock (intact) condition is several orders less than in
situ permeability.
•
The secondary permeability is governed by discontinuity frequency, openness and
infilling.
Table 8.18
Estimation of secondary permeability from discontinuity frequency (Bell, 1992).
Rock mass description
Term
Permeability (m/s)
Very closely to extremely closely spaced discontinuities
Highly permeable
10
−
2
-1
Closely to moderately widely spaced discontinuities
Moderately permeable
10
−
5
-10
−
2
Widely to very widely spaced discontinuities
Slightly permeable
10
−
9
-10
−
5
No discontinuities
Effectively impermeable
10
−
9
<
8.19 Effect of joints on rock permeability
•
The width of joints,
its openness, and the joint sets determine the overall
permeability.
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