9.13 Earthworks balance and material suitability
The design of the earthworks should seek to achieve a cut-fill balance and thus minimize the volume of offsite disposal or the volume of imported fill. The contractor should maintain this balance. However, factors of which the contractor should be aware, and which can alter the cut-fill balance include:
- unforeseen, unsuitable material;
- unforeseen, contaminated material incapable of remediation;
- unforeseen factors affecting the execution of the works (e.g. traffic management constraints, land access constraints, or programme constraints, all of which can create local surpluses or shortfalls);
- inappropriate use of material with specific performance criteria (e.g. use of structural backfill for embankment construction);
- bulking which occurs when any material is excavated and re-compacted;
- poor management of surface and ground water or construction plant resulting in suitable fill becoming unsuitable.
The effect of pavement design on the earthworks balance should not be underestimated.
9.14 Programme and weather windows
In programming earthworks, the following should be considered.
- Adverse weather, which is probably the single greatest cause of delays to an earthworks programme.
- The Met Office (www.metoffice.gov.uk) provides historic weather records on a regional and seasonal basis, which should be consulted when programming earthworks.
- Freezing weather can be a limited opportunity to progress the works by moving materials along haul roads unusable due to softening by rainfall.
9.15 Physical constraints
When creating a mass haul diagram the following physical constraints to bulk movement of materials should be considered:
- significant watercourses;
- major roads (for which a plant crossing is not permitted);
- deep valleys too steep for efficient plant working (requiring a major structure to bridge);
- hills which are to be tunnelled;
- poor ground conditions, e.g. bog, or other ground requiring treatment before trafficking; and
- environmentally important sites (e.g. SSSIs).
Means of overcoming these constraints may include:
- delaying earthworks in the section affected until the structure bridging the constraint is able to take construction traffic;
- temporary bridging, e.g. Bailey bridge;
- transporting material across the constraint by conveyor belt or similar;
- moving material around the constraint by other means, e.g. road lorries;
- relocation of flora and fauna.
NOTE Each of these solutions has its benefits and drawbacks. The optimal solution will depend on the circumstances.
9.16 Interface with other project activities
Activities which might disrupt bulk earthmoving productivity should be considered carefully and with the full involvement of the earthworks management team.
If the critical path network of a project identifies work for which the related earthworks have to have reached a certain stage, the mass haul diagram should incorporate this.
NOTE The prime earthmoving season is a finite number of working hours; any other activity on a project which reduces these is potentially creating an unrecoverable loss of production. Earthworks subcontracts are let with definitive outputs which have to be achieved for the plant to be operating commercially and effectively.
9.17 Resolving material surplus
If, after all possible changes to vertical alignment have been input, a surplus of material remains, various options may be chosen.
- Stabilization to produce pavement foundation or base: this is doubly advantageous in reducing imported primary or recycled aggregates. Suitability of the material for stabilizing should be ascertained The likelihood of this should have been foreseen at design investigation stage and the appropriate testing carried out. (See HA 74/07 .)
- Modification to produce suitable fill: mixing with a lower percentage of lime than required for stabilizing, or blending with other materials (e.g. PFA) may be used to revert material that is too wet to be acceptable, which reduces the volumes of unsuitable material for disposal and import of suitable to replace.
- Synergy with other projects: the solution may depend on another project in terms of cost-effective road-haulage distance of material (timing is critical).
- Disposal alongside the project: in an urban environment this is unlikely to be available; in an agricultural situation, selection of location for a suitable landform may involve considerable research. However, the filling of valleys for agricultural betterment and the creation of false cuts for environmental screening are options.
NOTE These are likely to attract landfill tax if not incorporated in the works at design stage (i.e. land obtained by compulsory purchase order). If taken at a later date by a contractor, agreement of the landowner (including royalty) and planning consent will be required.
- Road haulage to offsite disposal facility: for inert material in a rural/agricultural project, this should be seen as the last resort and, as the most un-environmentally sensitive solution (unsatisfactory).
9.18 Resolving material shortfall
The options that may be chosen are likely to be as follows:
- synergy with another project with a surplus: as above, location and timing are critical;
- import from commercial sources: market rates will apply and the material is likely to be processed/recycled granular;
- borrow pit: as with disposal, selection of location will involve much research.
NOTE It is unusual for this solution to be carried out other than by the main works contractor. Landowner agreement and ascertaining that the material is, in fact, suitable, are essential. Planning consent is required. The essence of a borrow pit is for acceptable material to be dug and replaced with unacceptable. In reality this might not be feasible, at least in part. In any event, the final landform will be a condition of any consent granted.
9.19 Quarrying for aggregates and selected fills
Distinct from a borrow pit to supply general fill for earthworks, the opportunity may exist to extract locally available sands, gravels and rock by a quarrying operation contiguous in location and time to the project.
NOTE A planning application is unlikely to succeed unless the operation can be shown to provide significant environmental betterment over procurement from the existing marketplace. Any consent will take at least a year, much longer if objections are raised.
9.20 Design, construction and maintenance of haul roads
Earthmoving plant generally uses the haul route along the line of the project, the vertical alignment of which changes as cuts are excavated and embankments filled; provision should be made for plant to maintain this route, which, at a minimum, would be a motor grader to remove rutting before this reduces the efficiency of the plant.
In some circumstances, a dedicated, constructed haul road might be required; as failure of this haul road might significantly disrupt the earthmoving process, the design and construction should be substantially adequate in considering the volume of traffic it has to sustain. Maintenance of the haul road, e.g. patching should be carried out before deterioration results in structural failure.
9.21 Plant selection
Plant should be selected to suit both the desired rate of output and the nature of the fill to achieve desired excavation, transport and compaction of the fill. Earth moving plant manufacturers supply information on this topic (e.g. Caterpillar Performance Handbook ). It is important to recognize that heavy compaction plant can over-compact, raise pore water pressures, and consequently weaken certain fine-grained soils, which is a practice that should be avoided. The use of motor-graders and water bowsers to regularly maintain working areas and control dust during construction can have significant benefits that should be considered.
9.22 Geotechnical feedback
Geotechnical feedback should be provided, in proportion to the complexity of the earthworks (see 6.5).