Earthworks

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.

road construction

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.

Excavation

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:

E
Easy digging - Loose free running soils eg sands, fine gravels.
M
Medium - Denser cohesive soils eg clayey gravel, low PI clays
M-H
Medium to Hard - eg broken rock, wet heavy clay, gravel with boulders
H
Hard - material requiring blasting and hard high PI clays

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.

Table 1 - Soil Properties

MaterialBulk Density
Mg/m3
Bulking
Factor
Shrinkage
Factor
Diggability
Clay (Low PI)1.651.30-M
Clay (High PI)2.101.400.90M-H
Clay and Gravel1.801.35-M-H
Sand2.001.050.89E
Sand & Gravel1.951.15-E
Gravel2.101.050.97E
Chalk1.851.500.97E
Shales2.351.501.33M-H
Limestone2.601.631.36M-H
Sandstone (Porous)2.501.60-M
Sandstone (cemented)2.651.611.34M-H
Basalt2.951.641.36H
Granite2.411.721.33H

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.

Sands/Gravels
Free Draining. Tend to have few problems.
High PI Clays
Low permeability will prevent water ingress so the surface becomes dangerous but not in the long term.
Silts/Low PI Clays
These cause the most problems. Permeability allows ingress which softens the soils thus weakening them.

Earthworks Balance

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.

Calculation of Cross Sectional Area

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.

Cumulative Volumes

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.

Mass Haul Diagram

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:

Earthmoving Equipment

This section will highlight some of the principal earthmoving equipment and their areas of application.

Problems

Question 1

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.

ChainageGround Level
(mAD)
Formation Level
(mAD)
ChainageGround Level
(mAD)
Formation Level
(mAD)
02835800411
100293290038
2003229100025
30035261100-52
40030231200-52
50019201300105
60011171400158
70071415002311
  1. Construct a Mass haul diagram for the project given the following
    • Bulking Factor = 1.1
    • Shrinkage Factor = 0.8
  2. A River breaks up the project at chainage 1160m. Calculate the volumes of waste material and borrow for two scenarios:
    1. Material Cannot be moved across the river
    2. A Bailey bridge is constructed allowing material to be transported across the river.

ANSWERS

  1. Borrow=68,000 m3 Waste=89,500m3
  2. Borrow=0m3 Waste=21,500m3

Worked Solution


References

1) Department of Transport, Manual of Contract Documents for Highway Works, Volume 1, Specification of Highway Works, Series 600 - Earthworks. 1993

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e-mail: D.G.Toll@Durham.ac.uk.

Last Updated:25 February 1997