Earthworks are the most common product of civil engineering operations. Nothing can be built without some excavation and transfer of soil (or rock) from one part of a site to another. Earthworks (or earth structures) are constructed where it is required to alter the existing topography. Most commonly they occur for highway works as cuttings and embankments, but also for site levelling ahead of development. Whatever the type of work, the principles are the same.
It was not until relatively recently that earthworks were designed, as once maintenance engineering was considered an accepted part of earthworks; however, today better certainty is expected and required, and so careful design and execution of earthworks is essential. In practice, earthworks design is an iterative process where design decisions are often taken by various parties (employer, consultant, main contractor, sub-contractors and construction validation team) – See extracted Figure 1 BS:6031 Code of Practice for Earthworks below.
A carefully designed and specifically tailored site investigation is essential ahead of earthworks so that the formations and sub-formations for the assets are suitably understood in terms of how they will respond to a change in load, and also so that the materials to be used can be adequately characterised in terms of their compacted strength condition.
Characteristic material types are generally divided into coarse or fine materials along with specific tests to inform adequate compaction and durability. The Specification for Highway Works (Earthworks 600) separates nine out material classes, each with differing testing and reuse requirements.
Control of earthworks, in particular material grading, moisture content, and compactive effort, is essential to ensure performance and re-use at design stage are accommodated and occur during the construction phase. See extracted Figure 2.2 Nowak&Gilbert Earthworks; A Guide, below.
A range of specification types are commonly adopted (method, end-product, performance) each with varying degrees of controls and validation testing.
In circumstances where land raising occurs over unconsolidated or weak soils, the change in loading can result in unacceptable settlements. When the underlying soils are of low permeability, such as estuarine or river alluvium clays, the period for the settlement to occur can be significant, as the change in stress is initially supported by the pore water pressures, and then gradually releasing as the pore water slowly drains and the soil mass consolidates.
In these circumstances, it is common place to prepare the ground by implementing a program of pre-loading whereby an advance load, normally in excess of the development load, is applied – known as surcharging. At the end of the pre-loading stages the excess surcharge materials are removed. Where development timescales require, accelerated consolidation can be induced by the use of by vertical drains (commonly referred to as band-drains, or wick drains) which shorten the drainage paths to better permit the relief of excess pore water pressures. An example of the rig used to install the vertical drains is presented below.
Examples of recent ESP experience:
- Oldmixon, Weston Super Mare (Lovells): Major housing development requiring earthworks for landraising and surcharging with vertical drainage to manage consolidation periods to suit the development program.
- Pencoedtre High School (Bouygues): School redevelopment requiring cut and fill earthworks to prepare a level development footprint.
- Commonhead Land (Persimmon): Major housing development requiring construction of a major noise attenuation bund between the development area and the adjoining M4 motorway.