History of sheet piling

The history of sheet piling goes back to the beginning of the last century. The book Ein Produkt erobert die Welt – 100 Jahre Stahlspundwand aus Dortmund (A product conquers the world – 100 years of sheet pile walls from Dortmund) describes the success story of sheet piling. The story is closely linked with Tryggve Larssen, government building surveyor in Bremen, who invented the sheet pile wall made from rolled sections with a channel-shaped cross-section.In 1902 the so-called LARSSEN sheet piles – known as such from this date onwards – were used as a waterfront structure at Hohentorshafen in Bremen – and are still doing their job to this day! Since then, have been manufactured in the rolling mill of HOESCH Spundwand und ProfilGmbH.

Over the years, ongoing developments in steel grades, section shapes and driving techniques have led to a wide range of applications for sheet piling. The applications include securing ex- cavations, waterfront structures, foundations, bridge abutments, noise abatement walls, highway structures, cuttings, land fill and contaminated ground enclosures, and flood protection schemes. The main engineering advantages of sheet pile walls over other types of wall are:

  • the extremely favourable ratio of steel cross-section to moment of resistance,
  • their suitability for almost all soil types,
  • their suitability for use in water,
  • the fast progress on site,
  • the ability to carry loads immediately,
  • the option of extracting and reusing the sections,
  • their easy combination with other rolled sections,
  • the option of staggered embedment depths,
  • the low water permeability, if necessary using sealed interlocks, and
  • there is no need for excavations.

Three-dimensional earth pressure

Quay structures are frequently built as combined sheet pile walls consisting of loadbearing piles and infill piles. In this arrangement, the in fill piles are often not driven as deep as the loadbearing piles. The passive earth pressure in the region below the infill piles can only be mobilised by the loadbearing piles. Every one of these generates a three-dimensional earth pressurefigure which, depending on the spacing of the loadbearing piles, can remain separate or can overlap. In the extreme case, the overlapping is so great that the loadbearing piles can be calculated as a continuous wall. DIN 4085:2007 section 6.5.2 contains further information on calculating the three-dimensional passive earth pressure.

Hydraulic ground failure

If there are large differences in the water levels on the two sides of the sheet pile wall, e.g. in a dewatered excavation or a quay structure at low water, a limit state condition can occur due to the flow under the base of the sheet piling. An upward hydrodynamic pressure S then prevails on the passive earth pressure side. If this hydrodynamic pressure is greater than the effective self-weight G of the body of soil in front of the base of the sheet pile wall, a hydraulic ground failure takes place. In this situation, the soil swells up and a mixture of water and soil in filtrates into the excavation.

Trials have shown that the uplift of the soil for a wall with embedment depth t occurs over a width of approx. t/ 2 from the wall. Therefore, in order to determine the factor of safety against hydraulic ground failure, the vertical force equilibrium in a body of soil with di- mensions t · t/ 2 is considered on the passive earth pressure side. Hydraulic ground failure occurs when the weight of this body of soil is less than the vertical component of the hydrodynamic pressure in this area.

Sk · γH ≤ Gk · γG,stb

where

Sk = characteristic value of hydrodynamic pressure in the body of soil in which the flow occurs

γH = partial safety factor for hydrodynamic pressure (LS 1A, DIN 1054:2005, Tab. 2)

Gk = characteristic value of weight of the body of soil in which theflow occurs under buoyancy

γG,stb = partial safety factor for favourable permanent actions (LS 1A, DIN 1054: 2005, Tab. 2)

The hydrodynamic pressure can be calculated with the help of a flow net. To do this, the excess hydrostatic pressure prevailing over the underwater level wu = n · Δh · γw is first applied to the intersections with the equipotential lines at a horizontal joint starting at the base of the sheet pile wall. The average excess hydrostatic pressure wum over the width t/ 2 starting from the wall is now read off at depth t. This excess hydrostatic pressure must decrease within the area of the hydraulic ground failure up to the water level and generate the required hydrodynamic pressure

Sk = t/ 2 · wu

The hydrodynamic pressure can also be approximated using the equation Sk = t · t/ 2 · ip · γw, where ip is calculated with the approximation equation 4.9.

Special attention has to be given to the corners of excavations because this is where the flow from two sides is concentrated in a small area, and there is a higher risk of hydraulic ground failure. During the driving of sheet pile walls, care should be taken to ensure that declutching of the interlocks is avoided because this shortens theflow path and consequently increases the hydrodynamic pressure locally.

The factor of safety against hydraulic ground failure can be improved by increasing the embedment depth of the sheet pile wall, e.g. by driving it into an impermeable stratum.

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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 8.1.8.3 (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.

Earth pressure redistribution

The classic earth pressure distribution only occurs for the active earth pressure with a rotation of the wall about its base. In the case of unpropped cantilever retaining walls fixed in the ground, a classic pressure distribution is to be expected. In the case of stiffened or anchored walls, the stiffening elements and anchors act as supports that prevent free rotation. As a result of this, the earth pressure redistributes corresponding to the support points. On the passive earth pressure side, the classic distribution of the earth pressure occurs only in the case of a parallel displacement of the wall. When taking into account a redistribution of the active or passive earth pressure, the active or passive earth pressure determined in the classic way is redistributed according to the movement of the wall to be expected, whereby the total value of the resultant earth pressure normally remains the same.

DIN 4085:2007 provides guidance on the distribution of the active and passive earth pressure for various types of wall movement.

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EAB 2006 provides information on the earth pressure redistribution for anchored and stiffened excavation enclosures. In this case, the number and position of the stiffening elements are particularly important. The follow picture shows the redistributionfigures for sheet pile walls with one support.

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EAU 2004 contains earth pressure redistribution figures for anchored waterfront structures which also take into account whether the structure is built on land or in water. On land, the ground in front of the sheet pile wall is excavated so that the earth pressure redis- tributes towards the anchor position as the excavation proceeds. In water, the ground behind the wall is back filled in layers so that only a minimal redistribution of earth pressure takes place.

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Wall Friction Angle in Sheet Piling Structures

Generally, the wall is not completely smooth, which means that a wall friction angle δ ≠0 between the wall and the soil is established. This is mobilised when the wall and soil move in relation to each other (as the follow picture shows). Here, δ is the angle between the direction of application of the active or passive earth pressure and a line perpendicular to the surface of the wall.

Assuming a straight slip plane, the wall friction angle in sheet piling structures may be assumed to lie within the limits δ a/p = ±2/3ϕ on the active and passive sides. If a curved slip plane is assumed for the passive earth pressure, the wall friction angle must be increased to δ p = ±ϕ according to EAU 2004 section 8.2.4.2 . Normally, δ a ≥ 0 and δ p ≤ 0 because the active wedge of soil moves downwards with respect to the wall and the passive wedge of soil upwards.

It is easy to see that the wall friction angle can change the forces in the polygon of forces considerably. In particular, the passive earth pressure increases drastically in the case of a negative wall friction angle δ p ≤ 0.

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Driving sheet pile walls

Sheet pile walls can be threaded into precut trenches, or pressed, impact-driven or vibrated into position. Threading and pressing do not involve any knocks or shocks, which is a complete contrast to impact driving and vibration methods. In difficult soils, the driving can be eased by pre-drilling, water-jetting, pre-blasting or even by replacing the soil.

When driving sheet pile walls, it is possible for the sheet piles to start leaning forwards or backwards with respect to the direction of driving (as the follow picture shows). Forward lean is caused by friction in the interlocks and by compaction of the soil while driving the previous sheet pile. The driving force is transferred to the pile concentrically, but the reaction forces are distributed unevenly across the sheet pile. Backward lean can occur in dense soils if the previous sheet pile has loosened the soil. To prevent leaning of sheet piles, they should be held in a guide frame or trestle. Vertical alignment during driving can be impaired by obstacles in the soil or hard strata at unfavourable angles.

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Driven anchor piles

Various steel sections and precast concrete piles can be used as anchor piles. Anchor piles carry the tensile forces on their surface by way of skin friction. They are frequently encountered in quay wall structures in which high tensile forces occur (as the follow picture shows). In such cases, steel piles enable a straightforward welded connection between pile and retaining wall structure.

Driven piles at shallow angles are guided by leaders. Slow-action hammers are preferred to rapid-action devices (EAU 2004 section 9.5.2). In the case of raking anchor piles, settlement due to back filling, relieving excavations or the installation of further piles behind the sheet pile wall can lead to loads at an angle to the axis of the pile. The additional deformations cause an increase in the stresses in the pile which in some circumstances means that the maximum axial force may not occur at the head of the pile but instead behind the sheet pile wall (see M ARD- FELDT, 2006). This must be taken into account when designing the piles and the connection to the wall.

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General

Various design methods have proved worthwhile for thestructural analysis of sheet piling structures. There are methods based on classic active/passive earth pressure theory, idealisation of the subsoil through elastic-plastic spring models, and ultimate load approaches. Sheet pile walls belong to the class of wall-type retaining structures whose design is covered by section 10 of DIN 1054:2005-01. DIN 1054 is an overriding standard that provides a general format for all analyses. The establishment of actions, resistances, calculation procedures and construction is covered by the specialist standards and recommendations of the German Society for Geotechnics (DGGT).

In accordance with the current state of the art, sheet piling structures are calculated and dimen- sioned with the help of computers these days. It is nevertheless essential for the design engineer to have a sound knowledge of the various methods of calculation, either for the purpose of checking the computer calculations or for carrying out quick and simple preliminary designs.

Pressing

Pressing is used primarily when there are severe restrictions placed on noise and vibration. This is mostly the case in residential districts, very close to existing buildings and on embankments. In contrast to driving using impact hammers and vibration techniques, the sheet piles are simply forced into the ground using hydraulic pressure. Noise and vibration are therefore kept to a minimum. We distinguish between pressing plant supported from a crane, plant guided by a leader and plant supported on the heads of piles already driven.

In the first method, a crane lifts the pressing plant onto a group of piles which are then pressed into the ground by means of hydraulic cylinders (as the follow picture shows). To do this, the hydraulic cylinders are clamped to each individual sheet pile. At first, the self-weight of the pressing plant and the sheet piles themselves act as the reaction to the pressing force. As the sheet piles are driven further into the ground, it is increasingly the skin friction that provides the reaction. Both U- and Z-sections can be pressed, and the method can also be used to extract sheet piles.

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