Understanding Earthwork in Construction

Introduction

Earthwork refers to the process of excavating the surface of the earth, transporting the waste material and compacting the resultant surface. Earthwork comprises of four main processes, namely excavation, transportation, unloading, and compaction. Earthwork is normally done in the early stages of construction. Once earthwork is done to some point, construction of the rest of the structure can continue. Minor earthwork activities are carried out as the construction continues. It is very crucial to plan earthworks before the commencement of the project. Such planning enables the contractor gain much profit in the long run. A careful analysis should be done on the extent of earthworks required. This will help determine the type and number of equipment to bring to site. Sometimes, the site may be largely inaccessible due to lack of roads to the site or poor road conditions. The first step in such a scenario would be to make the site accessible. This may involve cutting out of roads to connect the site to the nearby access road. For structures such as buildings, excavation needs to be done in order to lay the foundation. For roads and railways, the top soil needs to be scraped off because of its poor engineering qualities. Roads and rails need a stable and strong foundation material to rest on so as to avoid undue settlement. Before excavation begins, site clearance should be done to pave way for the excavation equipment.

Site Clearance

Site clearance prepares the site for construction activities. Generally, a piece of land in its natural setting is filled with shrubs, trees, and vegetation, which must be removed before construction begins. For trees, it is important to consult the local authorities for approval. A good approach is to first cut the branches and leave a tall stump that is easier to uproot. If possible, the trees can be uprooted using specialised equipment and transplanted elsewhere. The general aim should be to leave a 30 feet perimeter of clear land all round the structure. Other areas have piled rubbish which also need to be removed to pave way for construction. In case the new development is coming up on a piece of land that once had structures, demolition must be done. After clearing vegetation and removing demolition waste, about 200 - 300 mm of the loose top is scraped. The idea is to get to the bottom of the grassroot zone. The top soil is known to be unstable and filled with roots and other decaying materials. In this process, the contractor should consider recycling any recyclable materials. For instance, the top soil can be used elsewhere to fill parts of the site that need their level raised up. After removal of the topsoil, the new surface is graded to a constant fall. Care should also be taken when handling contaminated waste or hazardous material such as asbestos which causes lung diseases.

Precautionary measure to consider during earthworks

Earthworks may at times large areas of exposed soil, over which no structure will be built. Such should be stabilised through grassing. Before site clearance is carried out, appropriate measures should be taken to prevent erosion and sedimentation. A good approach is first to gravel the access road to the site. This way, sediments will not be spread by the vehicles by attachment to the wheels. Unnecessary ground disturbance should be minimised so as to prevent erosion through runoff. Steep slopes should adequately be protected. Watercourses should also be protected from the site waste. Stock piles should be built to only a reasonable height as they are known to generate a lot of dust and sediment. A general guideline is to keep the stockpiles below 2.5 m height with a maximum batter slope of 2:1. As such, they should also be sheltered from wind and any running water. It is possible that the site may have underlying utility lines such as fibre optic cables, water pipes, telephone lines, and gas pipes. Overhead power lines may also be in the vicinity of the site. The contractor must protect these at all costs so as not to cut off customers from the services. In the case that there are no permanent boundary fences to adjacent properties, it will be good to provide temporary fencing to protect the adjacent properties. Caution tapes should also be tied around all pits to prevent falls of the construction workers, as well as passersby.

Excavations

The basic goal of excavation is to get rid of poor soils so that the structure can rest on a stable subgrade. Concrete structures cannot be built over soft spots, debris or organic materials, otherwise they will experience settlement and may eventually cave in. These should all be dug out. Some sites may have rocks and boulders protruding above the surface. These may affect movement of construction equipment and therefore must be removed. The contractor should analyze the excavated material to categorize them according to their properties. Materials with similar properties should be piled together for future use. For instance, topsoil should not be mixed with lateritic gravel. In some instances, the contractor may encounter large rock outcrops. Itis not a must that these be removed. They can be preserved and later carved into beautiful landscape features.

Disposal of surplus soils

As earlier mentioned, not all material from excavations should be transported off the site. The resultant material can be recycled within the site. The material can be used for uniform widening of embankments. Another option is to use the material to uniformly flatten fill batters. Also, some sections can be selected for further flattening with the excavation material. If the site has no place to dump surplus material, the material should be transported to designated dumping locations. The material should be stock-piled in these locations and leveled so that vehicles can move on top of the previously piled material, as well as provide an aesthetic finish.

Cut and fill excavation

As regards excavation, cut and fill involves using the excavated material from one section of the site to build up embankments on another section of the site. This saves a lot of labor and costs. It would be absurd to transport all the excavated material away from the site and later purchase material for filling up embankments. Fill refers to earth that is brought in. Cut refers to the earth that is removed. To know the respective quantities of cuts and fills required, engineers draw Cut and Fill Diagrams. The site location is shown on the x axis. The positive range of the y-axis shows the fill. The negative range on the y-axis represents the cut. This is illustrated in figure 1 below.

Determining earthworks quantities

In every construction project, the quantity of earthworks should be accurately determined for both planning purposes and to enable the processing of payments. There are two main methods used to compute earthwork quantities. The first method entails preparation of cross sections to define the quantities of earthwork involved. Most projects require the use of this method. The second method involves the use of contour lines. Before a detailed discussion of these two methods, it would be good to define the units of measurement used. The Cubic Yard is used for calculation of eartwork quantities. It bears three variations: The Bank Cubic Yard is used for material that is in its natural state before being dug up. The Loose Cubic Yard is used for material that has been disturbed. The compacted Cubic Yard is used for material that has been compacted.

The following formulae are important in these calculations:

Swell (%) = (Bank/density loose -1) x 100

Load factor = Loose density/bank

Bank Volume = Loose Volume x Load Factor

Shrinkage refers to the reduction in the volume of soil when compacted. The formula is as given below:

Shrinkage (%) = (1-Bank Density)/Compacted Density x 100

Shrinkage factor = 1 - Shrinkage

Compacted volume = Bank volume x Shrinkage Factor

Estimating earthwork for trenches and foundations

Two basic methods are used to calculate the earthwork quantities for trenches and foundations. These are:

End area method

Contour Line / Grid Method

End area method

This is specifically used for sites where the length is far much greater than the width. A good example is road construction. Roads normally extend significantly lengthwise and have only a narrow width. The first step in this method involves taking the cross-sections at predetermined intervals, say 100 m intervals. Second, calculate the cross-section end areas. Third, compute the volume of earthwork between the sections by taking the average of the two end areas and multiplying by the distance between the two sections. Convert the final units into cubic yards.

Below is a further breakdown of the contours.

For cross section A’-A, the area is calculated as follows:

Area = 193/1.8 x 2 = 173.7

For cross section B’-B, the area is calculated as follows:

Area = 90/1.5 x 2 = 67.5

The rest follow suit.

Next, a table is created for the accumulative earthwork quantities.

The contour line or grid method

This is mostly used for calculation of earthwork quantities for parking lots. It is aso used site levelling. It makes use of square grids, say 10’ x 10’ to 50’ x 50’. Larger terrain variances require only small grids, and the vice versa is true.

The contour line method follows 5 basic steps, as highlighted below:

Survey the site drawing visually and try to tell whether the net total points to an import (where fill is greater than cut) or an export (where fill is more than cat, or a blend (where cut and fill are equal.

Determine what grids size you will use.

Determine the height of ground at all corners of the grid.

Compute the cut and fill required in each grid.

Add the results from Step 4 together to obtain the total cut and total fill, as well as the export or import yardage needed for the job.

The figure below illustrates typical contour line computations.

In case we choose the grid size as 50’ by 50:

Average elevation = (87.6+88.5+87.6+88.6)/4 = 88.08

Change = 90-88.08 = 1.92

Cut = 177.77 CY

The computations go on repeating the above calculations for different grids. The values for the individual grids are then summed up.

Mass Haul Diagrams

By definition, a mass haul diagram is a graph showing the cumulative quantities of earthwork moved along the center. It also shows distances over which the excavated materials will be transported. In mass haul diagrams, the vertical axis represents excavations and embankment in cubic yards. The horizontal axis shows the stationing. Mass ordinates refer to the cumulative total of the excavation and embankment on the project. All lines with a positive gradient indicate cuts. Lines with negative gradients indicate fills. Therefore, a flat line is used to show that the cuts and fills are equal. The balance point refers to the point where the graph crosses the baseline. This shows that the cut and fill have balanced out. Mass diagrams are used to compute the average haul, free haul, and overhaul on a particular section of the road. Through a mass haul diagram, the contractor is able to know how much material will be moved, and how this can be done economically. Haul refers to the transportation of excavated material from the site to the dumping zone. Average haul is actually obtained from the mass diagram. It is basically the area of the mass diagram that shows how many cubic yards stations of haul are there between balance points divided by the ordinate of the mass.

Calculation of average haul or free haul

To calculate the average or free haul, you need know the area and the volume. These two values can be determined from the mass diagram. The area refers to the area under the curve. The volume refers to the sum of the peaks and valleys on the diagram.

For all values above the balance line, peaks should be added and valleys should be subtracted. For all values below the balance line, valleys should be added and peaks should be subtracted. The following formula is then used for the average or free haul: Average Haul = Area(sum of peaks/valleys) Area = CY-sta. Peaks/Valleys = Cy

The average or free haul value helps the contractor to know where and when to pay for the overhaul. When the average haul distance is exceeded, the contractor must start paying for overhaul. By definition, overhaul is the authorized hauling of excavation beyond the specified free-haul distance.

Machinery used for earthworks

Below is a list of some of the most basic equipment used for earthworks.

Tractor

Tractors are mainly used for: i) shallow excavation up to 300 mm deep either on level ground or sidehill cutting, ii) clearance of shrubs and small trees, iii) clearance of trees, iv) towing, v) pushing hand tool machines.

Scrapers

Scrapers are used for earth moving. The back part of the scraper features a vertically moveable hopper with a sharp horizontal front edge that you can raise or lower. It is the front edge that cuts into the soil and fills the hopper. Once the hopper fills up, it is raised and closed and the load transported

Graders

Graders are earthwork machines with a long blade that scrapes the soil surface to a preferred grade. The grade can be flat or angled, depending on what is being constructed. The most common model has the blade coming between the 2nd and third axles. Some graders also include rippers at the rear end.

Tractor shovels

Tractors shovels are used to scoop up loose earth in the bucket located at the front of the machine and dump the contents into a truck. The are normally driven towards the pile of loose earth. Some are wheeled while other models are tracked.

Excavators Machine

Excavators are used to dig into the earth and load the material into trucks by the use of a powerful boom arm and the excavating attachment. The excavator works through hydraulics principles. For movement, it uses a chin wheel system.

Face Shovel

The face shovel is used to excavate above its wheel level. They also make use of hydraulics to move the mechanical arm. They are suitable for areas with all types of soils. In case rock is encountered, it has to be loosened first by blasting.

Back Actor

The back actor is also known as the back hoe. It is an excavating equipment comprising of a digging bucket fitted to the end of a two-part articulated arm. The back hoe, as the name suggests, is normally mounted at the back of a tractor.

Compaction

Soil compaction entails mechanically increasing the density of soil. It is a very significant part of the building process. It is done to prevent settlement of soils, which normally results in cracking and collapse of structures. Compaction increases the load bearing capacity of soil. In its natural state, soil is not very dense and cannot carry heavy loads. Soil compaction also provides stability. The earth materials will not expand and contract differentially after the compaction. Compaction also reduces water seepage into the soil. Compaction can be applied through 4 types of efforts: vibration, impact, kneading, and pressure. Compaction force can be either static or vibratory. Static force refers to the dead weight of the machine that applies a downward force on the soil surface and compresses the particles. Vibratory force refers to a rapid sequence of blows on the soil surface, usually generated by a rotating eccentric weight or the combination of a piston and a spring. Proper compaction requires the soil to have the optimum amount of moisture. Too little moisture will not result to the soil being compacted to its maximum density. Too much moisture leaves the soil with water voids, and this reduces the bearing capacity of the soil. The proctor test is usually used to determine the maximum possible dry density of the soil and the optimum moisture content that will lead to this.

Compaction Equipment

Smooth wheeled roller

Smooth-wheeled rollers have a circular wheel that crushes the soil and flattens it. They are mostly used in road construction. They use both their dead weight and vibratory action to compact the soil. The smooth wheel roller can have either a single drum or double-drum. The single drum roller is normally fitted with pneumatic tires at the back.

Sheepsfoot Roller

Sheepsfoot rollers are used for compacting soils with fine particles such as heavy clays and silty clays. Mostly, such soils are encountered in the construction of banks and embankments, and subgrade layers in road construction.

Pneumatic Roller

Pneumatic rollers typically have rubber tyres. They are used for compaction of soils which contain a mixture of coarse and fine grains. They are also very common in tarmacking of roads.

Vibrating Roller

The vibratory roller is used for small scale compaction of soil and bituminous materials. Their compaction effort is not as large as that of the bigger wheel rollers.

Rammers

Rammers are used to compact small areas by providing impact load to the surface. The smaller rammers can be handheld, while the bigger rammers are machine operated. The compacting plate can be anywhere between 15 cm to 40 cm. They can even be dropped from a height to compact rock fragments.

Vibratory Plate Compactor

Vibratory plate compactors are used for compaction of small areas where the bigger compaction equipment cannot fit. The area of the compaction plates can vary between 0.16 and 1.6 m2. The weight varies between 100 kg and 2 tonnes.

Conclusion

Earthworks is an Engineering process that requires a lot of planning for it to be successful. The design on paper must first be analyzed and interpreted correctly on the ground. The correct equipment must be transported to the site, and the right teams mobilized for the work. Care must be taken not to destroy utility lines that may be hidden underground.

References

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