7.7 Stability of temporary cuttings and open excavations

The overall stability of slopes for temporary cuttings and open excavations should be determined in accordance with the principles of 11.5.1 of BS EN 1997-1:2004 and the guidance given in 7.3.

The designer should select appropriate soil parameters for use in the design of temporary slopes. In some cases it may be reasonable to rely on the short term (undrained) parameters where the designer is satisfied that insufficient time is available for a significant rise in porewater pressure to take place. However, this decision must be carefully considered as the transition to partially drained conditions occurs relatively quickly in some fine grained soils in the UK.

NOTE The overall effect of excavation for a cutting is to temporarily increase the stability of the slope due to reduced porewater pressures. With time the reduced porewater pressures rise towards higher equilibrium values with a consequent reduction in the shearing resistance of the soil mass. Thus the most critical conditions for temporary slopes to cuttings and open excavations occur some time after the formation of the slope (see Figure 6). The rate at which the porewater pressures rise towards equilibrium depends primarily on the soil type; for low permeability soils the process of reaching equilibrium porewater pressures may take decades whereas the porewater pressures in a highly permeable soil can reach equilibrium immediately following excavation.

7.8 Trenches with sloping sides

When excavating a trench for which it is not intended to provide additional support, the following should be considered:

  • a) the nature of the ground which should be suitable so that the sides of the trench can stand up at a stable angle without support for the required time;
  • b) that dewatering of the ground and trench can be effectively carried out to prevent the sides slipping or the trench flooding;
  • c) that the permanent work can be installed safely in the trench and that the design of any pipe or structure to be constructed in the trench takes account of open trench conditions in its design;
  • d) that excavation plant and equipment selected is appropriate to open trench conditions.

7.9 Planning for construction

NOTE 1 Clause 9 contains further details of construction issues.

When designing earthworks, consideration should be given to how the job will actually be delivered in practice, which is likely to necessitate an assessment of some or all of:

  • a) earthworks buildability;
  • b) suitability of excavated material for re-use;
  • c) cut/fill balance, mass profile, mass haul;
  • d) utilizing surplus or unsuitable material on site (discussion on good practice e.g. HA 55/92 [51] which is an advice note now widely implemented as standard good practice);
  • e) disposal of surplus materials (all surplus material both unsuitable and suitable);
  • f) the earthworks programme;
  • g) bulking factors;
  • h) trafficability;
  • i) effect of weather;
  • j) management of potential impact on adjacent/existing structures;
  • k) monitoring requirements;
  • l) designing for earthworks safety;
  • m) maintenance practicalities, e.g. 1:1.5 slopes make grass cutting too difficult;
  • n) environmental impact, e.g. flora, fauna, discharge consents (see EIA agreements at planning approval);
  • o) emergency response in the event of a major instability.

NOTE 2 Planning earthworks is an essential part of the design and construction process, whether the task involves a small volume of fill over a few months or multiple cut to fill operations over two or more years. The most effective solutions are possible if designer and contractor work in conjunction prior to any operations commencing: some types of contract make this more achievable than others.

If such liaison is not possible, the designer should ensure that they are not carrying out a theoretical exercise, but are setting the framework within which major logistical work has to be executed; a poorly thought out, impractical solution being likely to adversely affect deliverability and, ultimately, the cost.

The planning issues raised and discussed in Clause 9 should be considered holistically by both designer and contractor: there is considerable interaction between them; it is likely that, whilst no individual issue will be dominant, priorities will become apparent as the process is planned in detail; these priorities are also likely to be different from site to site.

NOTE 3 On any type of project, the issue which is likely to present the longest delay is that of obtaining planning consent for any disposal, borrow pit or extraction operation. The timescale for obtaining consent is at least 12 months; greater if objections are sustained. It might be necessary to produce an Environmental Impact Statement to the satisfaction of the Environment Agency. The appointment of an independent planning consultant may be beneficial.

7.10 Vegetation

7.10.1 Use of vegetation to assist surface stabilization

The designer should be aware that vegetation plays an important role in stabilizing slope faces and also acts to reduce erosion (CIRIA C708 [52]). The roots provide a reinforcing action and their need for water will reduce the in situ moisture content of the soil. The effects are principally developed in the surface layers although larger plants such as trees can extend to considerable depths and research has been undertaken into the potential benefits of such techniques as willow poles (Hiller and Macneil [53]).

Grasses are the most common form of control for near-surface stability and details of the appropriate seed mix and sowing requirements are included within Appendix 6/8 of the SHW [1]. The establishment of grass should follow quickly after the spreading of the topsoil on the slopes and it may be necessary to adopt a proprietary retaining system on the steeper slopes to ensure that the topsoil remains in place while the grass establishes. A range of seed mixes may be used to reflect local environmental needs.

It should be noted, however, that there are potentially significant disadvantages to the use of vegetation which need to be taken into account, particularly when dealing with clay soils. Seasonal effects have a pronounced effect on such soils leading to significant movements as they dry out/shrink in the summer months and swell in the winter months and such movements may lead to distortion within the upper layers of embankments leading to unacceptable ride quality as has been noted on a number of railways. However, research indicates that trees and shrubs can delay the onset of progressive failure of clay slopes (O'Brien [54] For these reasons careful consideration of the long-term effects of vegetation, in particular trees and large shrubs, should be undertaken as part of the planting regime for the individual slopes.

Designers should avoid retaining or planting large trees near the crest of clay slopes. Large trees have adverse loading effects and tend to dry the soil. Trees placed at the base of slopes can, however, have beneficial effects. When considering any planting scheme the designer should seek advice on long term maintenance of soft estate. Due consideration should be given to the potential for vegetation interfering with the intended use of the earthworks, e.g. trees obscuring sight lines.

See Clause 11 for information on the effect of vegetation on existing earthworks.

7.10.2 Problematic vegetation

Designers should be aware that there are various invasive plant species that should be carefully managed (see CIRIA C679 [55]).

BS 6031:2009 Code of practice for earthworks