Watertightness of interlock sealings
E.1 General considerations
If sheet pile walls are being used to retain the earth at the sides of deep excavations in which a low groundwater level has to be maintained, the hydrogeological situation can be adversely affected. In this case very often special requirements regarding the Watertightness of these sheet pile walls are demanded.
This implies that the sheet piles have to be driven to sufficient depth in an impervious soil stratum. However, this is not always sufficient as the leakage of water from an aquifer through the interlocks may still exceed the permissible groundwater flow.
Prevention of leakage through the interlocks is a difficult problem if the permissible flow is limited to very low values. In severe cases additional measures may therefore be needed to reduce leakage through the interlocks. Possibilities are:
- the application of special filling fluids or mastics in the interlock space;
- the application of water swelling sealing materials or elastomeric seals;
- welding of the interlocks, if possible;
- injection of the free interlocks with cement bentonite or swelling and hardening chemical fluids during the installation of the sheet piles;
- filling holes which are bored at the locations of the interlocks before driving with cement bentonite slurry;
- placing the steel sheet pile wall in a trench which is filled with cement bentonite slurry;
- installing a separate impervious screen behind the sheet pile wall;
- combination of two techniques as "special interlock filling" and "prebored holes filled with bentonite".
Treatment of the interlocks with a bituminous filler will normally be sufficient. In the case of severe permeability requirements, expanding fillers, an elastic, moulded seal or the combination of two methods, may be appropriate.
A unique method for the definition of the permeability of a steel sheet pile wall does not exist. This also applies to methods for testing the effectiveness of the sealing technique. It is recommended to describe the permeability in terms of the so called inverse resistance :
which is :
qz the discharge per unit of the joint length at level z in cubic meters per second per meter (m3/s/m);
Δp the pressure drop at level z in kilopascals (kPa);
p the inverse joint resistance in meter per second (m/s);
γw the unit weight of water in kilonewton per cubic meter (kN/m3).
The discharge per unit length of the joint can be established by testing a specimen of the sealed interlock in the laboratory or by tests in prototype situations in a specially equiped testpit.
The large scattering in the results of this type of permeability testing is taken into account by applying a factor of safety in the order of 10 to the average of the testing results.
E.2 A simple case of assessing the discharge through a steel sheet pile wall
- a excavation level
- b resulting water pressure
- c impermeable layer
In figure E.1 the data for the assessment of the discharge through the wall is shown. The sheet pile wall penetrates some distance into an impervious soil stratum.
Consequently the flow around the pile toe is neglected.
The resulting hydrostatic pressure diagram is drawn. The maximum pressure drop is at the bottom of the excavation and stays constant thereafter.
The maximum pressure drop is :
Δpmax = γwH
The total discharge through one joint is :
with a pressure drop of:
Δp = γwz for z ≤ H;
Δp = γwH for H < z ≤ H + h.
Thus the integral yields the area in the pressure diagram and the equation becomes:
Q1 = ρH (0,5H h)
The total number of interlocks in the sheet pile wall for the excavation is :
n = L/b
L the length of the perimeter of the excavation in meter; [m]
b system width of the sheetpiles in meter. [m]
The total discharge into the building pit is :
Q represents a safe approximation for the discharge as certain favourable aspects have been neglected, for example the influence of the flow pattern on the geometry of the water table in the vicinity of the wall.
E.3 Worked example
For an excavation with a circumference of 180 m, constructed of 600 mm wide sheet piles :
b = 0,6 m;
H = 5 m and h = 2 m.
The interlock with filler is fully described by the inverse joint resistance :
ρ = 5 × 10-10 m/s
The number of interlocks is :
n = 180/0,6 = 300
Discharge per interlock :
Q1 = 5 × 10-10 × 5 × (0,5 × 5 + 2) = 1,125 × 10-8 m3/s
And the total inflow into the building pit is :
Q = 300 × 1,125 × 10-8 = 3,375 × 10-6 m3/s or 12 l/h