The effect of groundwaterflows on hydrostatic and earth pressures

If the base of the sheet pile wall is not embedded in an impermeable stratum, groundwater can flow under the sheet piling structure. Proper planning and design of sheet pile walls located in groundwaterflows calls for a knowledge of the effects of the flowing groundwater.

As the groundwaterflows from regions of high hydraulic head to regions with a lower head, the hydrodynamic pressure is directed downwards on the excess hydrostatic pressure side and upwards on the opposite side. This means that the hydrostatic pressure on the excess hydrostatic pressure side is lower and that on the opposite side higher than the hydrostatic pressure.

The hydrodynamic pressure also acts on the granular structure of the soil: it increases the effective particle-to-particle stresses on the excess hydrostatic pressure side and decreases them on the opposite side. This means that the active earth pressure on the excess hydrostatic pressure side is increased, and the passive earth pressure on the opposite side is decreased.

Taking into account the groundwater flows has a beneficial effect on the excess hydrostatic pressure and a detrimental effect on the passive earth pressure. Whether on the whole a more favourable or less favourable in fluence prevails, must be investigated in each individual case. Generally, there are three ways of considering the hydrostatic pressure on a wall in flowing groundwater:

  1. Ignore the flow and assume the excess hydrostatic pressure according to section 4.2.
  2. Perform calculations with the help of a flow net.
  3. Perform calculations with the help of an approximation method assuming modified unit

In the majority of cases it is sufficient to ignore the groundwater flow and assume the excess hydrostatic pressure according to section 4.2. If high excess hydrostatic pressures are present, then more accurate flow net calculations are advisable in the case of strati fied soils with different permeabilities. In addition, an accurate investigation of the flow conditions is necessary for verifying resistance to hydraulic ground failure, especially in the case of large water level differences and strata with low permeability near the surface on the passive earth pressure side.

Corrosion and service life

The service life of a sheet piling structure is to a large extent dependent on the natural process of corrosion. Corrosion is the reaction of the steel to oxygen and the associated formation of iron oxide. Therefore, a continuous weakening of the sheet piling cross-section necessary for the stability of the wall takes place over several years. This weakening must be taken into account when analysing the serviceability and the ultimate load capacity. For corrosion in the atmosphere, i.e. without the effects of water or splashing water, a corrosion rate of approx. 0.01 mm/a is low. Also very low is the corrosion rate (on both sides) of sheet pile walls embedded in natural soils, which is also approx. 0.01 mm/a. The reason for this is the exclusion of oxygen. The same corrosion rate can be expected on sheet pile walls backfilled with sand. However, in this case it must be ensured that the troughs of the sections are filled completely with sand. A coating with a high protective effect forms in calcareous water and soils with a calcium carbonate content. Aggressive soils, e.g. humus, or aggressive groundwater should not be allowed to come into contact with the surface of a sheet pile wall. Furthermore, corrosion of the sheet piling can be promoted by bacteria in the soil. Considerably more severe corrosion can be expected in hydraulic structures, which is, however, not evenly distributed over the full height of the structure. The greatest weakening of the wall thickness and hence the resistance of the component takes place in the low water zone. When designing a sheet pile wall, care should be taken to ensure that the maximum bending moments do not occur at the same level as the main corrosion zones.

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EAU 2004 includes diagrams in section (R 35) with which the weakening of the wall thickness due to corrosion can be calculated. Using these diagrams, sheet pile walls can be designed for the mean and maximum losses in wall thickness if no wall thickness mea- surements are available from neighbouring structures. The areas shaded grey in the diagrams represent the scatter for structures investigated hitherto. To avoid uneconomic forms of con- struction, EAU 2004 recommends using the measurements above the regression curves only when local experience renders this necessary. For structures located in briny water, i.e. in areas in which freshwater mixes with seawater, the reduction in wall thickness can be interpolated from the diagrams for seawater and freshwater.

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According to current knowledge, adding a coating to the sheet piles can delay the onset of corrosion by more than 20 years. One way of virtually eliminating corrosion below the waterline is to employ an electrolytic method in the form of a sacri ficial anode. Another way of achieving protection against corrosion is to overdesign the sections, but in this case an economic analysis must be carried out first.

Walls with different support conditions at the base and more than one row of anchors

Walls with more than one row of anchors can be calculated as described above by using iden- tical boundary conditions. Establishing the embedment depth is carried out via the force or deformation boundary condition at the base of the wall according to section 6.5.

It should be pointed out that owing to the static indeterminacy , the analytical solution involves considerably more work when more than one row of anchors is employed. Nomograms for calculating both simply supported and fully fixed walls with two rows of anchors can be found in the literature (H OFFMANN, 1977) together with accompanying explanations.

It is worthwhile employing a computer for sheet piling structures with more than one row of anchors. Design programs speci fically for foundations calculate the required embedment length automatically depending on the chosen support conditions for the section. Any frame program can be used to calculate the embedment length by means of iteration.

For the purposes of preliminary design, several rows of anchors can be approximated to one row.