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.

20150923 a

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.

20150923 b

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.

20150923 c

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.

20150922

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.

20150921

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.

20150918

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.

20150915

Structural Systems

The basis of the structural calculations is a realistic, idealised representation of the system. Owing to the complex soil-structure interaction, the loading on the sheet pile wall is directly dependent on the deformation behaviour of those two components. The deformation behaviour of the wall depends, on the one hand, on the support conditions at the base of the wall, and, on the other,  on possible struts or anchors supporting the wall above the founding level (W EISSENBACH, 1985).

In terms of the support conditionsat the theoretical base of the wall, we distinguish between simply supported, partially fixed and fully fixed walls.

In terms of possible support, besides unsupported walls, those with single or multiple supports may need to be considered.

Generally, it can be said that for an equal depth of excavation and an identical number of struts or anchors, greater embedment depths are necessary for fully fixed walls when compared with simply supported walls, but that this results in lower internal forces, wall deformations and an- chor forces. Walls with partial fixity at the base lie somewhere between the simply supported and fully fixed forms with respect to the stresses and strains. The decision concerning the sup- port condition at the base of the wall is made by the design engineer based on the requirements of the respective construction project.

The deformation behaviour of simply supported and fixed walls is fundamentally different. For afixed wall, a rotation about its theoretical base is assumed, whereas for a simply sup- ported wall, a parallel displacement of the base of the wall is assumed. The follow picture shows the displacements on which the design is based and their corresponding stress distributions.

20150902

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.

Impact driving

Impact driving involves driving the sheet piles into the ground with a succession of hammer-blows (as the follow picture shows). A timber driving cap is usually placed between the hammer and the sheet pile. We distinguish between slow- and rapid-action systems. Slow-action plant such as drop hammers and diesel hammers is primarily used in cohesive soils so that the ensuing pore water pressure has time to dissipate between the individual blows. In a drop hammer, a weight is lifted mechanically and then allowed to fall from a height h. Modern drop hammers operate hydraulically. The number of blows can be set as required between 24 and 32 blows per minute. The drop height of a diesel hammer is determined by the explosion of a diesel fuel/air mixture in a cylinder. Depending on the type of hammer, the weight is either allowed to drop freely onto the driving cap or instead the weight can be braked on its upward travel by an air buffer and then accelerated on its downward travel by a spring. Using this latter technique, 60–100 blows per minute are possible, whereas with the non-accelerated hammer the figure is only 36–60 blows per minute. Rapid-action hammers are characterised by their high number of blows per minute: between 100 and 400. However, the driving weight is correspondingly lighter. Rapid-action hammers are driven by compressed air and the weight is accelerated as it falls.

The head of the sheet pile can be overstressed during impact driving if the hammer is too small or the resistance of the ground is too great. Possible remedies are to strengthen the head or use a larger hammer. In the case of a high ground resistance, excessive driving force or an incorrectly attached driving cap, the pile can buckle below the point of impact. To avoid this, use thicker sections or loosen the ground beforehand.

20150826

Modelling the sheet pile wall

The HOESCH 1605 sheet pile wall section is discretised with 3-node beam elements assuming a quadratic displacement. A linear elastic behaviour is assumed for the sheet pile wall. Using the section properties from appendix A, we get the following system parameters:

E = 2.1 · 108 kN/m2
A = 1.363 · 10-2 m2/m
I = 2.8 · 10-4 m4/m
E = 1.05 kN/m/m

=> EA = 2 862 300 kN/m; EI = 58 800kNm2/m

The sheet pile/soil boundary surface is discretised with interface elements. The wall friction angle for the steel/soil boundary surface is given as δ =2ϕ/3. In order to achieve a realistic bond between base of wall and body of soil, the interface elements are extended 2 m into the body of soil. However, δ = ϕ applies for these interface elements.