Earthwork Notes

Earthwork

Definition

  • Earthwork involves excavating, transporting, and compacting earth materials.
  • It utilizes a wide range of earthwork equipment varying in size, capacity, and efficiency.
  • Scales range from small works like ditches and trenches to large earthworks such as highways and dams.
  • Completion of earthworks on schedule is crucial for overall project success.

Importance of Earthworks

  • Stakeout works must be carried out prior to any action.
  • Provides access for machinery, trucks, and ramps.
  • Excavations are performed before earthworks begin; this normally starts with clearing the land of plants, weeds, and garbage. This is called CLEARING.
  • Excavation follows clearing to reach foundation levels, done manually (pick and shovel) or mechanically (excavators).

Earth Works Excavation Types

  • Clearance: Movement of lands above the starting plane of the building.
  • Emptying: Occurs when the starting plane is below ground level.
  • Terraced: Earthwork to bring ground below the starting plane up to the same level.

Success Factors

  • Adequate site investigation and practical designs.
  • Selection and efficient use of appropriate plant types and sizes.

General Considerations

Site Consideration
  • Nature of excavation
  • Availability of work area
  • Disposal of soil
  • Existing services and structures
Ground Conditions

Various Techniques of Excavation and Their Choices

  • Bulk excavation
  • Rock excavation
  • Trench excavation
  • Support of excavations
  • Embankments

Site Consideration Details

Nature of Excavation
  • For large cut and fill operations, a detailed plan for spoil and plant movement is required.
  • Consider the excavation’s situation (reduced levels, road works, trenches, basements, or pits), time available, and sequence of earthworks.
  • Fill areas require retaining walls or drainage before spoil deposition, especially when sequencing operations.
  • Extended work, like pipeline trenches, can be divided into sections and progressed simultaneously.
Availability of Work Area
  • Work area is the total space for plant manipulation and material storage; it doesn't include site administration or accommodation areas.
  • The total work area should be indicated on a site plan.
  • Sufficient work area may allow for battered excavation sides, free from obstructions.
  • A study of available space and the effect of progress on it is essential.
Disposal of Soil
  • Methods:
    • Storage in spoil heaps for later use or removal.
    • Immediate use for backfilling elsewhere on-site.
    • Immediate removal from site to other destinations or dumping areas.
  • Material used for backfilling must be suitable.
  • Separate cut material into categories: suitable for filling and other for removal.
  • Consider the position of spoil heaps to avoid interference or danger.
  • Consider stresses from spoil heaps on structures or services.
  • Control lorries for maximum utilization of earthmoving plant.
  • Prevent spillage of soil on public highways.
Existing Services and Structures
  • Establish the position of all pipes, cables, and underground services, and clearly mark them.
  • Disturbed services should be carefully unearthed and supported to prevent damage.
  • Adjacent existing structures will require support during excavations, which can include:
    • Cut Off Walling: Watertight wall of clay or concrete to reduce seepage.
    • Sheet Piles: Piles driven side by side to retain earth or prevent seepage.
    • Underpinning: Strengthening and stabilizing existing foundations.
    • Strutting and Shoring:

Ground Conditions

  • Required ground support depends on soil strength, excavation depth, and duration the excavation remains open.
  • Consider soil stability changes due to adverse weather.
  • Site investigation assists in determining:
    • Ground support requirements.
    • Method for keeping excavation free from water.
    • Plant to be used.

Various Techniques of Excavating and Their Choices

Bulk Excavation
Rock Excavation
Trench Excavation
Support Excavation
Embankments

Bulk Excavation

  • Removes large amounts of material, reducing the general level to near formation; uses large, efficient excavators.
  • May include:
    • Cuttings
    • Cut and fill areas
    • Basements and large pits
    • Hand excavations
Cuttings
  • Used for canals, roads, etc.
  • Plant use depends on:
    • Quantity of soil to be transported
    • Disposal unit distance
    • Ground conditions and water level
  • Examples of Plants:
    • Bulldozers
    • Scrappers
    • Draglines
    • Combination of plant track mounted excavation
    • Face shovel
Shallow cut and fill
  • Shallow cut and fill operations occur in road works and airfield construction.
  • In shallow excavation the plant may have to stop Work to prevent damage of the formation level, or temporary works may have to be provided over large areas, either way resulting in extra cost.
  • Areas of cut and fill can be adequately drained by temporary trenches, which into the final sub-grade drainage.
  • The formation level may be protected against water and the drying-out action of wind and sun, by some form of waterproof dressing.
Basement Construction

  • Synonymous with deep pit excavation (over 4.5m4.5m deep).

  • Methods of excavation:

    • Open cut excavation
    • Cut and cover techniques
    • Top-down techniques
    • Composite techniques

  • 'Deep pit' applies to excavations over 4.5m4.5m deep.

  • Shallow pits (1.5 m deep) present little problem.

  • Medium pits (1.5 to 4.5m4.5 m deep) require careful plant selection and ground support.

  • Supporting methods:

    • Unshored excavations
    • Shored excavations
    • Dumpling method
    • Cut-off walling method
      • “Cut-Off” means to either stop the flow, to “Dam” the underground stream, or to surround and seal-off an area, to prevent water inflow and water outflow from the encircled area
      • principal types of vertical cutoff walls are sheet pile walls, geomembrane walls, and slurry trench cutoff walls.

  • The method being chose have an effect on the choice of plant to be used.

  • Unshored excavations can be assumed there is an ample working space around the excavation to allow battering of the excavation.

    • This will allow more freedom of choice in excavating plant than the other methods.

  • With shored excavations, the shoring can be made watertight by sealing the joints of sheet piling, thereby eliminating the free flow of water.

Hand Excavation
  • Uses pneumatic tools like clay-spades and picks.
  • Occurs in heavily supported excavations with limited room for machines.
  • Necessary near services subject to machine damage or for removing obstacles.
  • Spoil is put into skips for removal.

Rock Excavation

  • Varies depending on:
    • Types of materials
    • Quantity involved
    • Condition of site
    • Equipment available
  • Methods:
    • Breaking by hand (hammer & wedges)
    • Pneumatic breaker
    • Drilling with pneumatic machine: freezing liquid
    • Drilling with pneumatic machine: blasting
      • the first three methods are suitable where any of the following conditions prevail:
        • The noise of blasting would cause annoyance
        • Adjacent buildings may be subject to damage.
        • Blasting may cause inconvenience or stoppage of traffic.
        • Landslides or rock falls might result.
        • Accurate cutting is necessary and excessive 'over break’ would be uneconomical.
      • Overbreak is defined as the unwanted removal of rock beyond a specified maximum excavation perimeter

Trench Excavation

  • Choice of Methods depends on:

    • Purpose for which the trench is being excavated
    • The nature of the ground
    • The time scale of the work
    • Number of obstructions
    • The location of the trench
    • Groundwater conditions

  • Methods:

    • Full depth, full length excavation
    • Full depth, successive stages of excavation
    • Stage depth, successive stages

  • Full depth, full length excavation

    • This method is suitable for long narrow trenches of shallow depth in which the machine completes the trench non-stop ahead of any other operation.
    • This method is suitable for pipelines and sewers

  • Full depth, successive stages of excavation

    • This method is suitable for deep trenches where several operations of work can proceed in sequence; this would prevent stretches of trench from being left open too long and thereby being subject to collapse.

  • Stage depth, successive stages of excavation

    • This method is suitable for very deep trenches in confined areas or adjacent to existing property.
    • It involves the support of the trench as the work proceeds and is most suited for operations such as deep foundations and underpinning.

Support of Excavations

  • Governed by:
    • Type of soil
    • Groundwater condition
    • Depth and width of excavation
  • Soil types:
    • Loose sand, gravel, and silts
      • Since such soils are likely to slump quickly, the support must be placed immediately after excavation takes place'.
      • This means that deep excavations will have to be dug in stages:
        • The first stage by machine
        • and, after supporting the first stage, any subsequent stages by hand or by grab.
      • Hand excavation is very expensive and therefore the use of driven sheet piles may be more suitable for deep trenching.
      • Support requires continuous support, such as:
        • Trench sheeting (lightweight pressed steel sheets)
        • Steel sheet piling
    • Compact sands and stiff clays
      • Support achieved using open timbering support/Pooling boards at intervals of approximately 1 meter.
      • If the soil is subjected to drying out and crumbling, the spacing of the pooling Boards can be reduced accordingly.
      • Dry clays which take up rain water and expand: this produces extra stress in the struts and allowance should be made for this at the design stage.
    • Rocks
      • Support to rock excavation depends to large extend on the type of rock and the slope of rock strata.
      • In the case of unstable rock faces, open timbering should be used to prevent any slump/collapse.
      • Where the depth of excavation is excessive, the rock face may be stabilized by rock bolting.
      • Rock bolting consists of solid steel rods which are fixed in deep drill holes by means of wedges, sleeves or grouting process; light steel sections or steel plates are used to support the rock face through which the rods are threaded.
      • Support of these materials may be expensive; It may be more economical to cut these back to a safe angle of repose if space allows such treatment.

Embankments

  • A wide wall of earth or stones built to stop water from flooding an area or to support a road or railway.
  • Construction depends on:
    • The purpose for which the embankment is constructed, e.g. the loads involved.
    • The stability of the ground on which the embankment is to be constructed.
    • The cost of obtaining suitable fill material.
    • The consolidation of the fill in the embankment under the proposed loads.
    • The extent to which the strength properties of the fill may be affected by the method of construction.
    • The difficulties in construction during adverse weather, when using clays and fine sands.
  • The method of constructing an embankment will depend upon the extent of the works, the type of fill material being used and the nature of the site.
  • The site must be stripped of all vegetable matter.
  • Fill material should be tipped and spread in layers of such a thickness that it can be compacted to, the required density – which will be established by laboratory testing
  • Where large volumes of fill are involved, the density factor obtained in the laboratory may not be achieved on site.
  • Difficulties in varying the moisture content of large volumes of earth between the two stages of excavating and filling
  • This discrepancy can be minimized by the correct selection of compaction plant: the plant most suited will depend on the soil type and its working moisture content.

Embankment slope

  • The safe angle of any embankment slope will depend on the nature of the fill material used and the height of the bank.
  • The safe angle will range from as much as 45′45' for rock waste fill down to as little as 20’ for some clays.
  • Slopes may also be stated as ratios and percentages. e.g: a slope of 1:3 or 3333%, a slope of 1:5 or 2020%