The Process of earthworks is to excavate the existing land to a suitable level so that road construction may begin. The earthworks can take the form of either excavation in the form of cuts or the construction of embankments to carry an elevated highway. Normally in a road design project, both will be necessary and movement of earth from one part of the site to the next will be necessary. This should be done with as little waste created or as little extra material required as disposal or collection is expensive.
Also included under the topic of earthworks is the compaction of the road materials to the appropriate level. This however is not covered as it is more concerned with the actual construction of the road than the design of it.
This page is concerned solely with the design of the earthworks and not with the actual design of the embankments or cuts. If you wish to learn more about this then links to relevant pages are contained in the geotechnical section of the links page. A link to this can be found opposite.
Of the topics covered in this page, they can be split up into the design of the earthworks and the plant used in the construction.
The most important feature of the excavation is the material you are working with. This will be known from the Site Investigation. Poor information can lead to technical problems and to cost overruns.
There are many ways of classifying the soil in terms of it's ease of excavation including seismic techniques. The most common in the United Kingdom at present however is the Ease of Digging scale or diggability. This classifies the soil in one of four categories:
Typical diggability factors can be seen in Table 1 below.
Another important feature of rock is the amount of fissuring. There are two methods of assessing this, the percentage Rock Quality Data method and the Spacing of joints method. Each of these leads directly to an estimate of the uniaxial compressive strength and thus an indication of the excavation method. Both these can be found in the Manual of contracts document, Series 6001.
Excavation increases the volume of material. It is therefore necessary to use a bulking factor to determine the volume of material that will be created by excavation. Bulking factor is defined as:
Bulking Factor = Volume after Excavation/Volume before Excavation
Similarly a shrinkage factor is defined for the compaction of a soil at it's final destination.:
Shrinkage Factor = Volume after Compaction/Volume before Excavation
Typical values can be found in Table 1 below.
|Clay (Low PI)||1.65||1.30||-||M|
|Clay (High PI)||2.10||1.40||0.90||M-H|
|Clay and Gravel||1.80||1.35||-||M-H|
|Sand & Gravel||1.95||1.15||-||E|
In addition to the above properties, it is important to have some idea about the trafficability of the soils. This is because the excavation plant will need to drive over the soils without becoming bogged down. The trafficability of the soils is related to their drainage properties.
In order to minimise material waste or borrow, it is necessary to produce what is called a Mass Haul diagram. This is essentially a plot of cumulative volume of soil against distance along the road, often called the chainage. Cut volumes are taken to be positive and fill volumes to be negative.
The first stage in the production of the Mass Haul Diagram is the calculation of the cross sectional areas of cut or fill at different points along the road.
For a cut or fill on horizontal ground.
|Figure 1 - Typical Cut Cross Section|
Assuming a cut such as the one above, the cross sectional area is given by:
Area = h.2b + 2nh²/2 = h(2b + nh)
For a cut or fill on sloping ground
|Figure 2 - Typical Sloping Cut Cross Section|
Assuming a cut such as the one above, the cross sectional area is found firstly by calculating WL and WG:
WL = S(b+nh)/(S+n)
WG = S(b+nh)/(S-n)
Thus Area = ½(h + b/n)(WL + WG) - b²/n
For more complicated cross sections, simply combine the above. It should be noted that this is NOT part of the design process for the slope stability.
Once the cross sectional areas are known at various points along the road, it is possible to calculate the cumulative volume along the cut by interpolating between the different points.
The simplest way of doing this is to assume a straight line variation and use the prismatic rule. Other slightly more complicated methods involve using Simpsons rule or similar. Do not forget to take account of the bulking factor or shrinkage factor although care should be taken not to use them both as this will produce incorrect results. If you are using the shrinkage factor then changes in volume due to excavation is accounted for automatically. The same is true for the bulking factor.
The mass haul diagram is now simply a plot of cumulative volume against chainage. Areas below the line represent net fill and areas above the line represent net waste. The following points are worth noting:
This section will highlight some of the principal earthmoving equipment and their areas of application.
The table below shows ground levels and formation levels for a proposed road construction. Embankments are to be built with side slopes of 1:2.5 and cuttings with slopes of 1:3.0. The embankment crest width and cutting base width is 13m. It may be assumed that the ground is horizontal across the section.
1) Department of Transport, Manual of Contract Documents for Highway Works, Volume 1, Specification of Highway Works, Series 600 - Earthworks. 1993Return to top of page.
Last Updated:25 February 1997