soil
drainage
CE2004
What Influencing well cover
Wheres
the water?
Where does it come from?
Why is it important?
Managing site water
Dewatering
(shallow, intermediate deep)
Slow seepage on cut faces
Water in the Three-Phase Soil Model
Volumes
Soil
Column
air
Va
AIR
Masses
air
Ma
Ma = 0
voids
Total
volume
Vv
water
Vw
V
WATER
water
Mw
Total mass
M
solids
Vs
SOLIDS
solids
Ms
Soil water catena e.g.
slope
Surface
Pond
Groundwater
table
Well drained
Poorly drained
Poorly drained
Well drained
Soil water catena e.g.
soil
.
Groundwater
table
Why is soil water important?
Pore water pressure support for soil
skeleton
Water resource
Hazard
= + u
/compaction
Surface
seepage and .
Equipment gets trapped
..
Dewatering
Done when soil is
e.g.
(5 x 10-5mm sec-1)
Especially important if excavation open for a
.. time
Various .
Shallow/deep
Gravity/pumped
Shallow dewatering
Drainage (..) ditch
Ditch
.
May need to maintain .. within ditch
Flow
to a low point
Install . and pump
After: McCarthy (2007, fig 7.1)
Intermediate depth dewatering
(may be many around site)
May
be close together (few metres apart)
Borehole drilled
Permeable/screened
. lining inserted
OR pumping tubes directly inserted by
water .
Water pumped out via riser pipe
Can apply to this deep dewatering holes and pumps
are bigger (and is greater!)
Dewatering of clay and silt rich soil
Permeability very .. so water input
to well-points
May take long time to dewater site
in start of and
structure construction
Increase in project .
Electro-osmosis
(<25% of soil is <0.002mm)
Soil water includes dissolved minerals
These
are (positive charge)
Induce DC electric current through soil
Anode
(+) within site
Cathode (-) at well-point
Soil water (with cations) to
cathode (away from excavation area)
Very . demanding!
drainage
causes rise in and creates ue
ue decays to new us through loss of
Causes loss of total .
Filters on seeping slopes
Seepage
=
= erosion
Initial
groundwater
table
Pumping
Surface +
subsurface
flow
= +pore water
pressure
= potential
What do you do with the extracted
water ?
(may be millions of litres or more)
Summary
Dewatering is straightforward in theory
Different techniques can be applied
depending on the groundwater and soil
context of the project
If you do extract the water what do you
do with it?
References
McCarthy, D. F. 2007. Essentials of soil
mechanics and foundations : basic geotechnics.
Pearson. London. Chapter 10.
Tomlinson, M.J. Foundation design and
construction 7th ed. Pearson. London. Chapter
11.