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«2 Embankments 2.1 Stability The inherent stability of an embankment will normally be assured if it has been designed and constructed in accordance ...»

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Substructure: Design and construction. Stability

1 Purpose and scope

This chapter deals with the stability of embankments, earth cuttings, rock cuttings and other

structures and solutions.

The design and construction of the substructure must provide the superstructure with the stability

required by the regulations to ensure safe and regular traffic operations. The ballast bed must not be

subject to unacceptable settlement or deformations and, in accordance with stipulations, must be safe from ground failure or landslides.

These factors are partly determined by the requirements regarding the ballast bed and its composition and construction, but also to a large extent by its interaction with and adaptation to the subsoil and terrain. It is therefore essential to have thorough knowledge of the ground conditions along the route of the line, and geotechnical surveys and calculations must form a natural part of the project.

2 Embankments

2.1 Stability The inherent stability of an embankment will normally be assured if it has been designed and constructed in accordance with the guidelines provided in Underbygning/Prosjektering og bygging/Banelegeme. A stable slope gradient is a function of the type of material and embankment height. Guide values are provided in Tabell 1. In Tabell 1, the embankment height is considered to be its entire height from terrain to track, i.e. the embankment up to the formation, frost protection layer (if present), reinforcing layer and ballast.

Tabell 1: Guideline embankment geometry Max. embankment gradient Blasted rock, H (m) Gravel, sand, H (m) Clay/silt, H (m) 1 : 1.5 0–15 0–5 10 - 1:2 - 10 0–5 1 : 2.5 - - 5–10 1:3 - - 10 In Table 1 it is assumed that the subsoil has a satisfactory load-bearing capacity and does not present any stability problems.

As a rule, the total stability of the embankment will be determined by the ground conditions, and particularly by the ground's strength parameters. This may result in considerable limitations to the potential weight of the embankment and mean a change in the design conditions. Special measures will often be required to ensure that stability is satisfactory.

2.1.1 Stability calculations Railway design loads that must be used in stability calculations for embankments are provided in Underebygning/Prosjektering og bygging/Generelle tekniske krav.

Recognised calculation methods must be used to document that the embankment is safe from landslides (material coefficient). The selection of the material coeffici

–  –  –

2.1.2 Embankment toe/bed reinforcement When an embankment is constructed on terrain with a steep side slope, local stability at the embankment toe must be given particular attention. Good contact between the embankment and underlying terrain must be assured. When the terrain slope is steeper than 1:3, the bed must be reinforced in accordance with the principles shown in Figur 1. It may be necessary to abrade the rock in order to provide a key.

Figur 1: Bed reinforcement

2.2 Stabilisation measures

In principle, stabilisation measures may be divided into two main groups:

1. Measures to reduce stresses (shear stresses) in the ground. These can occur during the construction of berms or when the weight of embankments is reduced by the introduction of lightweight fill. The lightweight materials that may be utilised here must have the strength properties required to support the superstructure and traffic loads in the long term.

2. Measures to increase strength properties in the ground. This can be achieved using lime or cement stabilisation, electro-osmosis, salt diffusion, deep drainage, preloading, etc. It may also be necessary to construct supporting structures (e.g. piles and a pile cap) and supports (e.g. sheet piles anchored with stays).

Separate job specifications are normally prepared for any stabilisation measures that are required.

These regulations only cover the basic provisions for measures within the first main group.

2.2.1 Berms Berms are constructed with a crossfall of 1:20 out from the track, unless otherwise specified. See Figur 2.

Figur 2: Principles for berm construction The entire berm must be constructed before the railway embankment level exceeds the level of the berm. Materials used in the embankment must consist of ordinary 'heavy' soil materials. Blasted stone materials containing large stones should not be used in the bottom layer. Materials such as organic soil or light construction waste must not be used.

2.2.2 Lightweight aggregate and foam glass Tabell 3 shows parameters for the grading, density and unit weight of lightweight aggregate.

Tabell 3: Grading, density and unit weight of lightweight aggregate Design unit weight (kN/m3) Grading (mm) Dry density (kg/m3) above water under water 0–32 400 6 8

–  –  – Implementation In principle, when lightweight aggregate or foam glass is used, it must be deposited and distributed as shown in Figur 3.

Figur 3: Principle. Lightweight fill.

Lightweight fill is normally laid to a maximum of 0.60 m below FL. A reinforcing layer of stones is then laid, see Underbygning/Prosjektering og bygging/Banelegeme. The layer of lightweight fill must be entirely enclosed by a fibre membrane, of a minimum Class III. (Cf.

Underbygning/Prosjektering og bygging/Banelegeme). A cover layer must be laid on the side slopes. The cover layer must have a minimum thickness of 0.6 m, normally measured on the slope.

For high embankments (higher than 3.0 m), the thickness of the cover layer must be increased, and the embankment's internal stability must be specially assessed to determine whether any reinforcement measures are required.

Embankments higher than 5 m made of lightweight aggregate or foam glass must be approved by the 'Infrastruktur, Teknikk, Premiss og utvikling' department ('Infrastructure, Technology, Conditions and Development').

When lightweight aggregate or foam glass is used in parts of an embankment, the lightweight materials must be laid as low as possible in the embankment. Inspection

The following points must be inspected:

• acceptance check of material/grading supplied

• check that fibre membrane has been laid correctly

• layer thickness of the lightweight fills Compacting Lightweight aggregate and foam glass may be compacted using a tracked vehicle imposing a maximum load of 50 kN/m2. For areas adjacent to abutments, retaining walls, etc., a vibrating plate compactor with a maximum weight of 50–200 kg may be used.

2.2.3 Polystyrene In principle, polystyrene fill is to be laid in accordance with the guidelines provided in forms 482– 484, prepared by the Road Laboratory (Veglaboratoriet) regarding the use of this material in roads.

Other than the general instructions regarding the levelling, distribution and construction of embankments (which will be the same for roads and railways), points in this regulation apply specifically to railways. Stability Any EPS fill used must lie entirely above the groundwater level or highest floodwater level.

The use of an EPS layer thicker than 3.5 m in embankments is not recommended. Special assessments of the embankment's inherent stability must be performed if the embankment is asymmetric.

The risk of water pressure at the rear edge of the embankment must be given particular attention. Materials The material must be blocks of expanded polystyrene (EPS), with a minimum compressive strength of 200 kn/m2 (at 5% deformation) and a minimum density of 30 kg/m3. The outermost layer of blocks, if not the entire embankment, must be made of fire-retardant (self-extinguishing) material. Implementation The blockwork embankment must be constructed in courses, topped by a cast reinforced concrete slab with a thickness of 0.15 m, to the same width as the formation level (FL), with its upper edge a minimum of 0.30 m below the FL.

The principle for the construction of railway embankments using EPS is shown in Figur 4.

Figur 4: Principle for the use of expanded polystyrene blocks in railway embankments. Inspection Inspections must be carried out in accordance with the Road Laboratory's Form 484 regarding EPS

embankments. Some of the points are shown here:

• Acceptance checks of EPS material: Weight, strength and deformation

• Geometry and distribution of blocks, inspections of gaps between blocks and uniformity of base

• Blocks must be laid in courses (using inter-block connectors, etc.)

• Outer layers (if not the entire embankment) must be made of fire-retardant material

• Concrete layer at top of embankment 3 Earth cuttings

3.1 Stability The inherent stability of a cutting will normally be assured if it has been designed and constructed in accordance with the guidelines provided in Underbygning/Prosjektering og bygging/Banelegeme.

However, it is important to note that the gradient of a slope in soil must be suitable for the stability properties and erosion conditions of that soil type. If ground conditions are poor (soft clay and silt), with unfavourable terrain conditions, stability can quickly become critical, even in shallow cuttings.

The materials contained in earth cuttings should therefore be identified early in the planning process. If there are any doubts about stability, special geotechnical surveys and calculations must be performed. Since the stability of a cutting usually diminishes over time, analyses of long-term stability are of particular interest here.

3.2 Stabilisation measures Suitable stabilisation measures may be divided into two main groups described in Sikring mot dyperegående stabilitetsproblemer and Sikring mot overflateglidninger eller siginger/deformasjoner i de øvre sjikt i grunnen.

3.2.1 Protection against deeper stability problems As a rule, measures require thorough geotechnical surveys, and as mentioned in item 2, are principally based on reducing ground stresses or increasing ground strength. Figur 5 shows several examples of stabilisation in accordance with these principles. However, stabilisation measures of this type will not be described in detail in this regulation.

Figur 5: Stabilisation.

3.2.2 Protection against surface slides or subsidence/deformations in the upper ground layer Separate job specifications are normally prepared for any stabilisation measures that are required.

Sections– describe several basic provisions regarding the stabilisation of surfaces on slopes. Soil replacement in cutting slopes Soil replacement will be necessary in slopes in which soil stabilisation proves difficult. Alternative designs are shown in Figure 6. If climate conditions make it difficult to establish grass cover, alternatives b) and c) should be chosen. In locations where the soil type is considered to be particularly active in frost (susceptible to frost), measures in the form of soil replacement by heavy friction materials may also be considered in order to prevent later slides due to frost.

Figur 6: Soil replacement. Drainage of cutting slopes Refer to Underbygning/Prosjektering og bygging/Drenering. Stabilising slopes during excavation It may be necessary to stabilise slopes temporarily during the excavation process. This type of temporary stabilisation may be incorporated into permanent stabilisation measures. In most cases, it will be necessary to remove this temporary stabilisation before the installation is commissioned.

Suitable temporary stabilisation measures could be:

surface water diversion • plastic sheet over surface to prevent drying out • lowering groundwater level by means of ditches • well points • insulation, e.g. using winter mats to prevent capillary water absorption from freezing • 4 Rock cuttings

4.1 Stability For stability in cuttings, see Underbygning/Prosjektering og bygging/Banelegeme.

4.2 Stabilisation measures

There are several ways in which rock cuttings may be stabilised. The most appropriate methods are:

scaling • bolting • meshing • fibre-reinforced sprayed concrete • supporting blocks of rock • This chapter provides a brief description of each method. For further descriptions of stabilisation categories, see Tunneler/Prosjektering og bygging/Stabilitetssikring.

4.3 Scaling After rock has been blasted, cracks and fissures occur, even if the rock was previously solid and strong. It must always be meticulously scaled after any blasting work. In principle, this means that any loose stones that could present a hazard to safety on the line are removed if possible. This work should preferably be undertaken using scaling picks to prise out the stones.

Water in cracks and fissures has the effect of weakening connections between blocks of rock.

During the winter, the freeze-thaw cycle of water in cracks can cause blocks to break loose.

Particular attention must therefore be paid to any parts of cuttings where there is running water, or any parts of cuttings that are generally very damp.

4.3.1 Bolting Bolting is an alternative to removing rock by scaling. Systematic bolting may be necessary in sections that are very prone to landslides. The positioning of the bolts must ensure absorption of tensile forces in preference to shear forces. This work must be performed by experienced

professionals. Suitable bolt types are:

• grouted rebar bolts

• rebar bolts anchored by polyester resin 4.3.2 Meshing In sections prone to landslides, it may not be practical to stabilise every single block using bolts. In such instances, it is possible to lay a mesh across the rock.

The mesh may be attached using one of two methods. The mesh may be attached using bolts across the entire rock surface, or it may be attached using bolts at the top, thereby remaining loosely suspended over the section of rock that is prone to landslides. If the first method is used, any loose stones that are trapped by the mesh must be periodically removed. This is achieved by breaking open the mesh and 'sewing' it closed again. If the second method is used, an area must be allocated at the base of the rock face for the collection of stones. (See Figur 7).

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