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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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