| To establish the infiltration rate of the soil excavate a
soakage trial pit to the same depth as anticipated in the full
size soakaway. For run-off from 100 square metres this will
be 1 to 1.5m below the invert level of the drain discharging
to the soakaway.
The trial pit should be 0.3 to 1m wide and 1 to 3m long.
It should have vertical sides trimmed square and, if necessary
for stability, should be filled with granular material.
When granular material is used, a full-height, perforated,
vertical observation tube should be positioned in the pit
so that water levels can be monitored with a dip tape.
Calculate the soil infiltration rate from the time taken
for the water level to fall from 75% to 25% effective storage
depth in the pit, using the lowest f value of three test results
for the design:
Soil infiltration rate, f = (Vp75 - 25) / (ap50 x tp75
- 25)
Where:
Vp75 - 25 = the effective storage volume of water in the
trial pit between 75% and 25% effective depth;
ap50 = the internal surface area of the trial pit up to
50% effective depth and including the base area;
tp75 - 25 = the time for the water level to fall from
75% to 25% effective depth.
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