201 the design of earthworks

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The Design of Earthwork

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1.0 Introduction Earthwork: the processes whereby the surface of the earth is

excavated and transported to and compacted at anotherlocation

the introduction of the internal combustion engine, electricpower and hydraulic power have led to the development of awide range of earthwork plant (size, capacity and efficiency).

scale: ranges from small works (the excavation of ditches andtrenches for drainage and pits and trenches for foundations) to

the large earthworks (highways and dams).

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carried out at an early stage in a construction project,completion of the earthworks within the scheduled time is

often the key to the completion on time of the whole project

success often depends on:

an adequate site investigation and preparing practical andsatisfactory designs of the earthworks

the choice and efficient use of the correct types and size ofplant to meet the particular requirements of the site.

BS 6031 gives recommendations on the design and constructionof earthworks in general civil engineering schemes

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2.0 The Design of Earthworks2.1 Site or route selection selection factors:

the availability and cost of the land

planning, design, construction, environmental and economicconsiderations (choice of dam sites and the routes forhighways and railways)

the geological conditions and other geotechnicalconsiderations

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2.2 Environmental and economic considerations

the design & construction and the cost of earthworks aregenerally dependant on:

the environment in and around the site

the ground conditions within the site

the availability of materials for earthworks in the area.

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the landscape of the area should be taken into consideration,the earthworks should not disfigure but blend into the

environment:

suitable profiles of earthworks

amenity embankments

tree planting etc

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balance the amount of fill arising from cuttings with the amount offill required to construct the embankments - reduce the cost of

earthworks:

minimise the quantities of imported materials

minimise material to be disposed of off-site

consider natural and waste resources in the area, such as areproduced from the local mines, pits, quarries, power stations,etc. as fill required to construct the embankments

to incorporate lower quality local materials than to importhigher quality materials from some distance away - reduce

cost transportation and minimise disruption of the localenvironment (especially on larger schemes).

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Terangkan ciri-ciri sebuah projek kerja tanah yangdirekabentuk dengan baik.

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3.0 Design of cuttings and embankments3.1 Excavations carried out either as

part of the permanent works (e.g. cuttings) or

a temporary expedient in the construction of the works (e.g.for foundations and drainage)

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the sides of the excavations are required to remain stableduring their design life, can be achieved by

excavating the material to a stable slope angle

by retaining or supporting the material.

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3.1.1Slope

the stability of slope (natural and cut) is controlled by:

the nature, distribution, density and strength of the materials thatform the slopes

the groundwater conditions or porewater pressures

external or surcharge loadings,

the strength and disposition of any discontinuities.

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the effects of cutting

reduces the total stresses in the slope which leads to a reductionof the pore pressures - increases the stability of the slope in theshort term.

pore pressures will tend to rise to a new equilibrium value and thematerials in the slope may weaken, leading to a reduction in the

stability of the slope

occur rapidly in granular soils and well jointed rocks and there willbe little difference in the long and short term stability - analyses ofthe stability of slopes carried out by effective stress methods

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occur slowly in cohesive soils and rocks due to low permeability(weeks or even decades) - total stress methods of analysis are

used for the short term stability and effective stress methods forthe intermediate and long term.

weathering and erosion of slopes should be allowed for in thedesign

factors of safety applied to the design of slopes depend on:

the extent and reliability of the knowledge of the groundconditions

the consequences of failure

a good level of investigation, a factor of safety of between1.3 and 1.4 would be appropriate

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Typical batters of excavated slopes (as a guide and should notbe taken for final design purposes)

Material Slope batters* (vertical:horizontal)

Permanent Temporary

Massive rock 1.5:1 to vertical 1.5:1 to vertical

Well jointed/bedded rock 1:2 to 2:1 1:2 to 2:1

Gravel 1:2 to 1:1 1:2 to 1:1Sand l:25 to 1:1.5 1:25 to 1:1

Clay 1:6 to 1:2 1:2 to 2:1

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3.1.2Retaining Structures

Retaining and supporting structures (temporary works orpermanent works)

able to withstand the pressures generated by the in situ andbackfill materials, surcharge loadings and ambient

groundwater conditions.

a factor of safety of at least 2 is usually adopted.

ground anchors may be used as part of the retaining worksin both temporary and permanent excavations in soils and

weak rocks.

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rock anchors and bolts may be used with or withoutadditional structural support to increase the stability of rock

slopes.

3.1.3Groundwater problems

reduce the stability of slopes and retaining structures

make the excavated material unsuitable or troublesome foruse as fill within the works.

temporary or permanent dewatering and drainage measuresmay be needed.

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4.0 Embankments constructed from a wide variety of materials and for numerous

purposes:

load-bearing fills - roads and dams, require a higher factor ofsafety

non-load bearing fills - stockpiles or amenity embankments

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major factors to be considered in the design of an embankmentare

the stability

the bearing capacity

the settlement

the trafficability

the permeability.

The stability of an embankment vary with time and obviouswhen it is constructed of or on cohesive soils becauseconstruction of embankment will lead to:

increase of porewater pressures in the fill which may increase

or decrease with time (affect the stability of the slopes)

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porewater pressures in the foundation soils which decreasewith time (settlement of foundation)

4.1.1Stability

analyses of the stability of embankments (and theirfoundations) must be carried out to determine the lowestfactors of safety for the ranges of height, materials of

construction and foundation strata, with the porewaterpressures that may be generated during and after construction.

actions can be made to increase the stability when the factor ofsafety is inadequate:

reducing the angle of the side slopes

constructing a berm at the toe of the slopes

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using fill of higher strength

improving the strength of the fill by drying out or lime orcement stabilisation

emplacing drainage layers within the fill to reduce porewaterpressures

emplacing geotextiles within the fill to increase its overallstrength

partially or totally retaining the fill.

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Typical slope angles used for embankments (a guide and shouldnot be taken for final design)

Material Typical slope batters* (vertical:horizontal)

Hard rock fill 1:1.5 to 1:1

Weak rock fill 1:2 to 1:1.25

Gravel 1:2 to 1:1.25

Sand 1:25 to 1:1.5

Clay 1:4 to 1:1.5

4.1.2Bearing Capacity

the bearing capacity of an embankment will only be importantwhere the surface is to be loaded (e.g. a road, railway orfoundations)

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for roads, railways and runways, the California bearing ratio(CBR) of subgrade reaction is used to determine the bearing

capacity

plate bearing or triaxial test data are used for foundations.

bearing capacity of an embankment can be improved by:

using better quality fill such as gravel or rockfiil in the upperlayers

using material stabilised with lime or cement

improved with techniques such as vibroflotation or dynamic

consolidation.

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4.1.3Settlement of an embankment

arises from:

the settlement of the fill and/or the foundation strata

permeable material: settlement will occur during constructionand can be compensated for by the placement of additional

fill settlements continue for weeks or years after construction in

low permeability soil, thus measures must be taken toaccelerate consolidation

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Soil Stabilization action can be taken when the magnitude or duration of the

foundation settlements are excessive and/or the foundationsoils are unacceptably weak

reducing the angle of the side slopes

constructing a berm at the toe of the slopes emplacing geotextiles towards the base of the embankment

to reduce its deformability

the removal or displacement of soft or weak materials and

their replacement with suitable fill

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the construction of a wide and deep trench filled withgranular material

improvement of the ground by vibroflotation, dynamiccompaction or preloading

controlling the rate of construction to allow time for thefoundations to consolidate and increase in strength

decreasing the load applied to the ground by usinglightweight fill, such as pfa

the emplacement of vertical drains and/or a horizontaldrainage blanket beneath the embankment to accelerate the

dissipation of porewater pressures and settlement and toincrease the strength of the ground.

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5.0 Ground Improvement Techniques Partially sand replacement

1 m to 1.5 m deep

to provide a firm platform and to function as drainage

layer

Prefabricated vertical drains (pvd)

installed at 1.2 m square grids and down to firm strata (25m to 30 m deep)

0.5 m to 1.0 m surcharge to accelerate the consolidation(6 months)

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Geotextile

high stength polyester woven geotextile

to achieve the necessary FOS against slip failure

help to reduce differential settlement

Stone Column

300 mm diameter at 1.2 m square grids

to transmit loads to deeper stratum

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201 the design of earthworks

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