The technical literature presents numerous classification systems for excavation shoring types. One of the more comprehensive classifications is that proposed by SiemiĆska-Lewandowska (2016), which has been expanded to include modular shoring systems. Among the methods used for supporting excavation walls, the following are distinguished:
- diaphragm walls,
- Berlin walls (soldier pile and lagging systems),
- steel sheet pile walls (interlocking sheet piling),
- bored pile walls (e.g. CFA piles or micro-piles),
- jet grouting column walls,
- soil-nailed walls,
- hybrid/composite methods,
- modular shoring systems.
The individual solutions are briefly described below.
A diaphragm wall is a wall constructed of concrete or reinforced concrete, formed within a trench excavated in the ground. Concrete is placed through a tremie pipe under the protection of a bentonite slurry, which braces the trench walls during construction. Once the structure is built, the diaphragm wall typically forms a permanent part of the completed building (e.g. underground car parks, basements, metro station walls). In general construction, such walls are typically installed to depths of 12â18 m; in transport infrastructure, to 22â25 m, although depths of up to approximately 60 m are technically achievable.
Fig. 1. Excavation support using diaphragm wall technology.
The Berlin wall (soldier pile and lagging system) is a retaining structure used for temporary excavation support, consisting of vertical elements, typically steel H-sections (wide-flange profiles), installed in the ground at regular intervals of several metres, with the gaps between them subsequently filled with lagging of timber, steel, or precast concrete. The Berlin wall is a relatively flexible structure and is therefore subject to limitations on its application. For excavations deeper than 4 m, it requires strutting at multiple levels. Anchors, waling beams, struts, and rakers also form part of the Berlin wall system. The Berlin wall exploits the soil arching effect, which allows the use of lagging with lower structural resistance. A significant advantage of this system is its suitability for supporting long, deep, single-sided excavations, with the option of leaving the structure permanently in the ground after completion of the works. Well established over many decades, having originated in the 1920s during the construction of the Berlin metro, hence the name, the Berlin wall is straightforward to design due to the wide availability of affordable calculation software.
Fig. 2. Soil arching effect, after "Trenching and Shoring Manual", California Department of Transportation, 2011.
Fig. 3. Excavation support using the Berlin wall technique with corner strut.
In some cases, pile extraction can cause ground loosening and settlement of the surrounding soil, and for the protection of nearby structures, the sheet piling is therefore left fully or partially in place, this is the so-called sacrificial or lost support.
Structurally, the sheet pile wall acts as a wall fixed in the ground, or as a propped, supported, or anchored fixed wall. The most widely used profile in Poland is the Larssen interlock section, which, once sealed, resists groundwater ingress. U- and Z-profiles are also used, as well as box sheet piles formed from H-section profiles, flat sheet piles, and tubular piles.
Fig. 4. Deep excavation support using a braced steel sheet pile wall (Larssen profiles).
Bored pile walls (e.g. CFA piles or micro-piles) are constructed to depths of up to 20 m, with diameters most commonly 0.6 m and 0.8 m. The advantages of this excavation support type are: low cost of wall construction; relatively high wall stiffness; absence of vibrations during pile installation; in the case of CFA (continuous flight auger) piles, prevention of ground loosening; applicability in a wide range of geotechnical conditions.
A potential disadvantage of bored pile walls is their lack of watertightness. In addition, when used as permanent support, the cost of additional works associated with wall surface finishing can be significant.
Fig. 5. Excavation support using reinforced concrete bored pile wall technique.
Jet grouting column walls represent the most recent of the methods described here for retaining wall construction. The method involves fluidising the in-situ soil using a high- pressure liquid jet (at 30â70 MPa) and mixing it with a cement grout. Depending on the fluidisation technique, single-fluid (water only), double-fluid (water and air), and triple- fluid (water, air, and cement grout) jetting systems are distinguished. These walls are typically used as cantilever structures, although struts or anchors attached to waling beams are frequently employed. The columns can be reinforced with steel I-sections. Column diameters are typically in the range of 60 to 70 cm.
Rys. 6. Fig. 6. Jet grouting column wall (source: www.tolos.pl).
Another technique for supporting excavation walls is soil nailing. Soil nailing involves reinforcing the ground with soil nails of 4 to 8 m in length, also referred to as passive anchors. A disadvantage of this type of construction is the permanent nature of the nails â they cannot be removed after the wall is completed, which may hinder construction works on adjacent plots. The stability protection provided by such a retaining wall is considered permanent. It is not a temporary excavation support system.
Fig. 7. Slope stabilisation using soil nails (source: www.cforcivil.com).
Hybrid/composite technologies are simply a combination of two or more excavation support methods, for example, a diaphragm wall combined with a Berlin wall, or a Berlin wall combined with micro-piles. Figure 8 below illustrates examples of combined Berlin wall and modular shoring systems.
Fig. 8. Composite excavation support using modular shoring system components combined with Berlin wall elements.
Modular Shoring Systems. From the range of excavation support types discussed above,
it is evident that some must be regarded as permanent structures that will remain in the
ground. These include diaphragm walls, bored pile or micro-pile walls, soil-nailed walls,
and in some cases steel sheet pile walls. A decisive advantage over these types of
excavation wall support is offered by reusable modular shoring systems, such as the
KOPRAS system analysed in the present doctoral thesis.
Fig. 9. Example of modular shoring system application as an alternative to steel sheet pile walls or bored pile walls.
Modular shoring systems can be used to support the walls of both linear trenches and excavation chambers of various sizes. They are straightforward and safe to use, and, most importantly, installation is rapid. Upon completion of the works, the shoring is fully recovered and can be reused.
Fig. 10. Modular shoring system for a point/shaft excavation.
Modular shoring systems are discussed below using solutions developed and manufactured by KOPRAS as representative examples.
Fig. 11. Excavation shoring using the KOPRAS OWS7 system.
Modular shoring systems for temporary excavations are factory-produced structures consisting of prefabricated components that are assembled on site, either prior to or during the process of supporting the excavation walls. The primary purpose of these systems is to retain the vertical walls of temporary excavations during the installation, construction, or reconstruction of underground utilities, foundations, or tanks. Temporary excavation shoring is designed to protect personnel working within the excavation as well as adjacent structures, buildings, vehicles, and bystanders in the vicinity of the excavation.
The development history of these systems is closely linked to the capabilities of hydraulic excavators for installing and extracting shoring components. Heavy shoring elements, sometimes weighing several tonnes, can be efficiently and rapidly positioned within the excavation. In the past, when excavations were carried out by hand, the installation of heavy shoring was not feasible, and excavation walls were supported with timber planks. In later years, when cable-operated excavators were used for excavation, their structural characteristics, limited precision, and inability to press shoring into the ground restricted the development of modular systems. During that period, temporary excavation support relied primarily on interlocking steel sheet piles and Berlin wall
systems.
The use of modular shoring systems is characterised by great versatility and repeatability of operations, which represents a significant advantage. The type of shoring selected, and its installation and removal procedure, depend on the following factors:
1) type of shoring system;
2) site conditions;
3) design of the foundation or pipeline structure;
4) location, and the consequent need to protect nearby structures;
5) available plant and equipment;
6) number and location of potential underground service conflicts.
Shoring components are most commonly installed in the excavation by means of excavators, sometimes with the assistance of a crane, or as a last resort, by hand (in the case of aluminium shoring systems, which are characterised by their low self-weight).
Two installation methods are distinguished: the “cut and lower” (push-down) technique and placement into a pre-excavated trench. The first method ensures the safety of personnel within the excavation and guarantees the stability of the ground around the trench within the failure wedge zone. It protects against the collapse or sliding of nearby buildings, as well as damage to equipment, roads, railway lines, and utilities adjacent to the excavation. The sequence of this procedure, in six steps is illustrated in Figure 12.2.
Fig. 12. Shoring installation procedure using the “cut and lower” (push-down) method.
The second method involves positioning the shoring within a pre-excavated trench. It protects personnel working in the trench but does not ensure the stability of the ground outside the shoring perimeter. This method is rapid and economical, as it enables the completed shoring assembly to be dragged along the trench (Fig. 13).
Fig. 13. Wall support method in a pre-excavated trench.
In addition to installation method, temporary modular shoring systems are also
classified according to:
1) material of construction;
2) type of interaction between individual components;
3) support and bracing method;
4) displacement of the shoring wall assembly;
5) types of struts/spreaders;
6) types of infill panels filling the space between guide rails or struts.
The most widely used material for manufacturing modular shoring systems at present is steel. It is employed in all construction types due to its numerous advantages: strength, cost-effectiveness, ease of welding of pre-formed plates and profiles, ease of repair, durability, and impact resistance. Timber, plastic, and aluminium are also used for the production of panel elements. Of these three, aluminium is the most frequently used, owing to its significant advantages: corrosion resistance, strength, and low self-weight. Cast iron is used partially in the production of struts, and rubber is used for manufacturing buffer stops that limit strut deflection.
Fig. 14. Rubber buffer stop of a modular shoring system.
The classification by type of interaction between components is significant, as it determines the structural static scheme and consequently the installation procedure. Basic box-type elements are assembled prior to placement in the trench using 2 to 5 struts. The extension element is assembled using 2 to 4 struts, then lowered onto the basic element already positioned in the excavation and connected using coupling devices.
Fig. 15. KOPRAS modular box shoring system with extension unit.
Post-and-plate shoring systems consist of guide rails â posts (1), struts (3), roller trolleys (4), locking pins (5), and panels (2) (Fig. 16).
Fig. 16. Post-and-plate shoring system.
The space between the guide rails, together with permanently fixed or sliding struts, may be filled either by a monolithic panel (as shown in the shoring above) or by a segmental wall constructed from vertical trench sheets of various profiles.
Fig. 17. Segmental wall with modular profile (trench sheet) infill. Components: 1 â bottom beam, 2 â top guide beam, 3 â KOPRAS steel trench sheet, 4 â tension rod, 5 â tension rod hook, 6 â lifting eye.
Shoring systems for point/shaft excavation support are based on components familiar from post-type construction, but must be treated separately due to the different structural static scheme and the different type of structural displacement involved. When corner posts extending to excavation formation level are used, the analysis focuses on retaining wall displacement limited only to the deflection of panels between the posts.
Fig. 18. Shaft (point) chamber composed of corner posts and fin panel system.
In the case of composite constructions, the removal of struts and the use of a sacrificial base strut at formation level, combined with a horizontal waling beam providing upper support to the posts, results in improved redistribution of bending moments in the post and reduced deflection. This thereby allows the elimination of the transverse strut from the system and, consequently, provides a large unobstructed working volume for installation operations..
Fig. 19. Post-and-plate construction with horizontal waling beam and bottom strut.
According to the structural static scheme, the following configurations are distinguished: a) braced using struts placed directly in the panels at various heights, at the mid-point or at the panel edges, such a panel is subjected to biaxial bending and its construction requires reinforcement at strut connection points (Fig. 20); b) panels are fixed in guide rails and act primarily in uniaxial bending, supported along two opposing edges.
Fig. 20. Braced construction (panel subjected to biaxial bending).
With regard to guide rail support, the following configurations are distinguished: systems supported by struts placed at various heights, and systems supported along their entire length by perpendicular panels positioned in corner posts.ach naroĆŒnikowych.
Fig. 21. a) System supported along its full length; b) system supported by struts.
The support configurations described above influence the displacement of the retaining wall and, consequently, the magnitude and distribution of the anticipated earth pressure, which is determined using various methods and formulae.
Fig. 22. Post-and-plate shoring system with earth pressure diagram and distribution of resultant forces acting on individual panels according to Rankine’s theory.
Shoring systems are classified by strut type into those with struts permanently fixed in the panel structure or temporarily in guide rails; within the latter, sliding (friction) or roller-type movable struts are distinguished.
Fig. 23. Roller-type strut.
Depending on the adjustment method, struts are classified as fixed or variable-length, the latter adjusted by means of a screw thread, hydraulic cylinder, or additional inserts. According to structural behaviour and static function, a distinction is made between struts acting axially and transferring compressive forces only, and struts capable of transferring both compressive and tensile forces as well as bending moments. Hydraulic struts form a separate category, they transfer compressive forces and are currently used in aluminium shoring systems.
Fig. 24. Panel connecting elements struts: a) continuously adjustable strut; b) fixed strut.
Modular shoring systems use panels of various designs, differing primarily in their ribbing configuration and the presence of additional elements such as a cutting shoe (knife edge) at the bottom or an upper lifting reinforcement for transportation purposes.
Fig. 25. Summary of the most commonly used panel cross-sections.
Segmental walls form a separate category of construction. They resemble interlocking sheet pile walls but are not embedded in the ground below formation level. The structural behaviour and static scheme of this type of construction differ from those of monolithic panels. However, they are an integral component of modular shoring systems, used particularly at locations where underground service crossings are negotiated or where pipelines are brought out of a chamber using trenchless technology methods.
In accordance with the applicable standard PN-EN 13331 Part 1 “Trench Lining Systems, Product Descriptions” and Part 2 “Trench Lining Systems, Evaluation by Calculation or Test”, three main structural components subject to design verification are distinguished: the strut, the post/slide rail, and the panel.
Screw-type struts require strength verification in a certified testing laboratory. They are designed for compression, buckling, and lateral torsional buckling, and their resistance is expressed in [kN]. Depending on the type of strut, specific design standards apply to threaded, bolted, and welded connections, or in the case of hydraulic struts to the relevant principles and standards for designing such equipment.
The post is designed for bending, and its resistance shall be expressed in [kNm]. Panel resistance is expressed in [kPa] and is calculated for biaxial bending.
REFERENCES
[1] California Department of Transportation (2011): Trenching and Shoring Manual. Caltrans, Sacramento, CA.
[2] Jarominiak, A. (1999): Light Retaining Structures [Lekkie konstrukcje oporowe]. Transport and Communication Publishers (WKiĆ), Warsaw. [In Polish]
[3] SiemiĆska-Lewandowska, A. (2016): Deep Excavations â Design and Construction [GĆÄbokie wykopy â projektowanie i wykonawstwo]. Transport and Communication Publishers (WKiĆ), 1st edition, Warsaw. [In Polish]
[4] Kopras, M. (2021): Verification of Design Methods and Structural Calculations for Temporary Trench Shoring Panels [Weryfikacja metod projektowania i obliczeĆ konstrukcji pĆyt obudowy wykopĂłw tymczasowych]. PoznaĆ University of Life Sciences, PoznaĆ. [In Polish]
[5] KOPRAS Product Catalogue (2022). KOPRAS Sp. z o.o., PoznaĆ. Available at: Catalogue