EXCAVATION CALCULATION BASIC INFORMATION AND TUTORIALS

Excavation is measured by the cubic yard for the quantity takeoff (27 cf # 1 cy). Before excavation, when the soil is in an undisturbed condition, it weighs about 100 pounds per cf; rock weighs about 150 pounds per cf.

The site plan is the key drawing for determining earthwork requirements and is typically scaled in feet and decimals of a foot. There is usually no reason to change to units of feet and inches; however, at times they must be changed to decimals. Remember that when estimating quantities, the computations need not be worked out to an exact answer.

Swell and Compaction.

Material in its natural state is referred to as bank materials and is measured in bank cubic yards (bcy). When bank materials are excavated, the earth and rocks are disturbed and begin to swell.

This expansion causes the soil to assume a larger volume; this expansion represents the amount of swell and is generally expressed as a percentage gained above the original volume.

Uncompacted excavated materials are referred to as loose materials and are measured in loose cubic yards (lcy).When loose materials are placed and compacted (as fill) on a project, it will be compressed into a smaller volume than when it was loose, and with the exception of solid rock it will occupy less volume than in its bank condition.

This reduction in volume is referred to as shrinkage. Shrinkage is expressed as a percentage of the undisturbed original or bank volume.

Materials that have been placed and compacted are referred to as compacted materials and are measured in compacted cubic yards (ccy). Bank, loose, and compacted cubic yards are used to designate which volume we are talking about.

Figure 9.1 is a table of common swell and shrinkage factors for various types of soils. When possible tests should be performed to determine the actual swell and shrinkage for the material.

FIGURE 9.1. Swell and Shrinkage Factors. (Solid rock when compacted is less dense than its bank condition.)

FOUNDATION EXCAVATION BASIC AND TUTORIALS

There are many different types of excavations performed during the construction of a project. For example, soil may be excavated from the cut or borrow area and then used as fill.

Another example is the excavation of a shear key or buttress that will be used to stabilize a slope or landslide. Other examples of excavations are as follows:

1. Footing Excavations. This type of service involves measuring the dimension of geotechnical elements (such as the depth and width of footings) to make sure that they conform to the requirements of the construction plans. This service is often performed at the same time as the field observation to confirm bearing conditions.

2. Excavation of Piers. As with the excavation of footings, the geotechnical engineer may be required to confirm embedment depths and bearing conditions for piers. Figure 1 presents typical steps in the construction of a drilled pier.

FIGURE 6.44 Typical steps in the construction of a drilled pier: (a) dry augering through self-supporting cohesive soil; (b) augering through water bearing cohesionless soil with aid of slurry; (c) setting the casing.


3. Open Excavations. An open excavation is defined as an excavation that has stable and unsupported side slopes.

4. Braced Excavations. A braced excavation is defined as an excavation where the sides are supported by retaining structures. Figure 6.45 shows common types of retaining systems and braced excavations.

Common types of retaining systems and braced excavations. (From NAVFAC DM-7.2, 1982.)

CIVIL CONSTRUCTION TRENCHING FOR PIPELINES BASICS AND TUTORIALS

TRENCHING FOR PIPELINES CIVIL ENGINEERING PROJECT BASIC INFORMATION
How To Make Trenches For Pipeline In Civil Engineering Project?

The hydraulic hoe or backacter is the machine most widely used for trench excavation for pipelines. In hard ground, rock or roads, the trenching machine might be used. Depths for water and gas pipelines are usually the pipe diameter plus 1 m.

For sewers, greater depths are often required to maintain falls. When flexible plastic pipes are used, especially in the smaller diameters, pipe joints can be made above ground, the pipe being snaked in. Bottoming of the trench can be achieved by using a straightedged bucket without teeth, and the backhoe can also place soft material or concrete into a trench on which to bed pipes or fully surround them.

Provided no men are allowed in the trench, timbering can thus be avoided. When large diameter steel pipes with welded joints have to be laid, a string of several pipes may be welded up alongside the trench, and dozers equipped with side lifting booms can lower the string of pipes into the prepared trench.

This reduces the amount of timbering and excavation of joint holes necessary which need only be arranged where successive strings have to be jointed together. The principal defects occurring on pipelines come from defective joints and pipe fracture due to settlement of a pipe on a hard band, large stone or lump of rock in the base of the trench.

The use of the hydraulic hoe makes the preparation of an even bed for the pipe easier to achieve, especially on suitable selected soft granular fill. However, the base of the trench and the bedding along each length of pipe must be carefully boned in before the pipe is lowered to ensure each pipe is fully supported along its body.


For non-flexible pipes of ductile iron, asbestos cement, steel or concrete it will be necessary to joint them after laying. Sufficient access is then required for the jointer to make the joint properly, and support to the trench sides will be essential in every case where there is not absolute certainty there can be no slip of material into the trench.

Falls of material into trenches are a major hazard in civil engineering, and adoption of a consistent, rigorously applied safety approach is the only way to prevent accidents. The damaging weight of even a small fall of earth must be borne in mind.

While it will be obvious that gravity sewers must be laid to a fall, it is sometimes not appreciated that pressurized trunk water mains should be laid to a minimum rise or fall. The preferred minimum gradients are 1:500 on a rising grade in the direction of flow; and 1:300 on a falling grade.

The former would be to an air release valve, the latter from the air valve to a washout or hydrant. Thus the levels of ground ahead of the pipelaying must be prospected to locate suitable high and low points, and intermediate points where an increase or decrease of grade is necessary.

The pipeline between such pre-determined points should follow an even grade. In flat ground it may not be possible to comply with the foregoing grades, but it is still advisable to give uniform rises to air valves and falls to washout positions.

In built-up areas pipelines can generally follow the requisite cover below ground surface because branches and connections will release air, and hydrants will be used as washouts. Backfill to pipes should always be of selected soft or fine granular material to 150 mm above the crown of the pipe.

Few contractors in UK would fail to do this, but on some contracts overseas the resident engineer may need to stop the contractor from dozing the excavated hard material straight back into the trench irrespective of the rocks it contains which would at the least damage the sheathing to pipes.

HAULAGE OF EXCAVATED MATERIALS IN CIVIL ENGINEERING PROJECTS BASIC AND TUTORIALS

HAULAGE OF EXCAVATED MATERIALS IN CIVIL ENGINEERING PROJECTS
What To Do With Excavated Materials In Civil Engineering Projects?


For large open excavations, such as when road cuttings have to be made and the material tipped to form embankments, or for building an earth dam from open borrow pit areas, the motorscraper is the most economical machine for excavating, transporting and placing clays and clay-sand mixes.

But the gradients traversed need to be gentle and the motorscraper cannot pick up hard bands of material or rock, unless ripping beforehand can break up the material sufficiently.

If hard or rocky material has to be excavated, the face shovel loading to dump trucks has to be used, the trucks commonly having a capacity of 50–60 t, sometimes larger. However, neither scrapers nor dump trucks can traverse public roads.

If the excavated material has to be routed off site via public roads to some dumping area, the excavated material has to be carted away by tipping lorries licensed for use on the public highway. Tipping lorries have a lesser capacity than dump trucks, usually in the range 10–30 t.

A factor often having considerable influence when needing to transport material along public roads, is the reaction of the local road and public authorities who may object to the extra construction traffic and mud on the roads. If the local authority has also to give planning permission for dumping spoil on some given land, such permission may only be granted subject to restriction on the size of lorries used and their frequency of passage.

This situation cannot be left for tenderers to find out; the employer has to obtain the necessary permissions and the contract must reproduce exactly the conditions laid down by the planning or other authority concerned and require the contractor to conform to them.

If the restrictions limit the size and frequency of tipping lorries, the contractor may be forced to temporarily stockpile excavated material on site and double handle it in order to conform to his intended programme for construction and the haulage conditions laid
down. This will raise his costs for excavation.

Assuming there are no planning restrictions, the contractor needs to choose that combination of excavating plant and haulage vehicles which achieves the required excavation rate at lowest cost. The face shovel or backhoe output must match the timing of empty vehicles back from the dumping ground and their loading capacity.

This means that the excavator bucket size and loading cycle time must be such that one haulage truck is loaded and moving away by the time the next vehicle arrives. Hence, the cycle loading time for the excavator must be known.

Thus if 10m3 haulage vehicles return at 5 min intervals, and the cycle loading time is 1.5 min, only three cycles of loading are possible so an excavator bucket size of 3.3m3 is required.

Alternatively if the cycle loading time could be 1.25 min a 2.5m3 bucket would suffice. Allowance has to be made for the bulking factor and unit weight of the material to be excavated.

The bulking of granular or soft material may range 1.1–1.3, through 1.4 for hard clays, to 1.6–1.7 for broken rock. Clays, clay–sand mixtures, gravels and sands may weigh 1.6–1.9 t/m3 in situ while rock and hard materials may vary 1.9–2.6 t/m3 in situ.

The excavator bucket size has to allow for the bulking factor: for example, a 2m3 bucket may only lift and load 1.4m3 loose material at 1.4 bulking factor, so it will need seven loading cycles to fill a 10-m3 tipper wagon. If this is too long a loading time for the required rate of output, an excavator with a larger capacity bucket is required.

Correct assessment of the bulking factor is financially important to the contractor, particularly in relation to the use of tipping lorries for offsite deposition of material. Whereas dump trucks used on site can be heaped, tipping lorries have a limited cubic capacity and payload, neither of which can be exceeded.

Thus if a bulking factor of 1.2 applies, a 10m3 lorry will take away the equivalent of 8.3m3 net excavation; but if the bulking factor is 1.35 the 10m3 lorry will take away only 7.4m3 net excavation. If the contractor has based his price on the former but experiences the latter, he would find his price for disposal of material off site 12 per cent too low.

This could mean no profit on the operation or a large financial loss, since there may be many thousands of cubic metres of material involved. In practice a contractor’s past experience will guide him as to what plant to use, taking into account many other practical matters which apply, such as reliability of different types of plant, need for standby, margins for hold-ups, length and nature of haulage road, cost of transporting plant to and from the job, and hire rates for different sizes of excavator and haulage vehicles.

EXCAVATING AND EARTH PLACING MACHINERY BASICS AND TUTORIALS

EXCAVATION AND EARTH MOVING EQUIPMENT AND MACHINES
What Are The Different Excavation and Machine Moving Equipment?


Bulldozers (‘dozers’) are used for cutting and grading work, for pushing scrapers to assist in their loading, stripping borrowpits, and for spreading and compacting fill. The larger sizes are powerful but are costly to run and maintain, so it is not economic for the contractor to keep one on site for the occasional job.

Its principal full-time use is for cutting, or for spreading fill for earthworks in the specified layer thickness and compacting and bonding it to the previously compacted layer. It is the weight and vibration of the dozer that achieves compaction, so that a Caterpillar ‘D8’ 115 h.p. weighing about 15 t, or its equivalent, is the machine required; not a ‘D6’ weighing 7.5 t which is not half as effective in compaction. The dozer cannot shift material very far, it can only spread it locally.

A dozer with gripped tracks can climb a 1 in 2 slope, and may also climb a slope as steep as 1 in 1.5 provided the material of the slope gives adequate grip and is not composed of loose rounded cobbles. On such slopes of 1 in 1.5 or 1 in 2 the dozer must not turn, but must go straight up or down the slope, turning on flatter ground at the top and bottom. It is dangerous to work a dozer (and any kind of tractor) on sidelong ground, particularly if the ground is soft.

Dozers cannot traverse metalled roads because of the damage this would cause, and they should not be permitted on finished formation surfaces. Sometimes a flat tracked dozer (i.e. with no grips to the tracks) can be used on a formation if the ground is suitable.

Motorized scrapers are the principal bulk excavation and earth-placing machines, used extensively on road construction or earth dam construction. Their movement needs to be planned so that they pick up material on a downgrade, their weight assisting in loading; if this cannot be managed or the ground is tough, they may need a dozer acting as a pusher when loading.

This not only avoids the need for a more expensive higher powered scraper, but reduces the wear on its large balloon tyres which are expensive. The motorized scraper gives the lowest cost of excavation per cubic metre of any machine, but it needs a wide area to excavate or fill and only gentle gradients on its haul road. It cannot excavate hard bands or rock, or cut near-vertical sided excavations.

The face shovel, or ‘digger’ can give high outputs in most types of materials, including broken rock. It comes in all sizes from small to ‘giant’; but for typical major excavation jobs (such as quarrying for fill) it would have a relatively large bucket of 2–5m3 capacity. The size adopted depends on what rate of excavation must be achieved, the capacity of dump trucks it feeds to cart away material, and the haul distance to tip or earthworks to be constructed.