Construction of roads over swamps require special attention. Either the soil has to be removed or otherwise,it has to be strengthened by suitable means after ascertaining the properties of the soil.
Building Roads Over Swamps Building roads in challenging terrain and swamps or bogs requires special attention compared to construction in ordinary soil. Swampy or marshy lands are periodically or permanently inundated by water. Swampy soil This type of soil soil contains contains large quantities quantities of vegetable vegetable matter, clay clay and mud. mud. Swamps are are classied into bracish or fresh water depending on the salt content of the water. The depth of this particular layer may vary from shallow to deep. !or construction of roads, shallow deposits can be removed and deeper deposits strengthened by suitable means. Surveys and investigations There are are certain basic basic design controls controls and and criteria which which govern govern the geometric geometric features of of a highway. They are topography topography,, tra"c, speed, capacity, capacity, vehicle vehicle and control control access. access. #lso certain surveys surveys and investigations are to be conducted so that construction failures can be avoided or minimi$ed. Embanment subsidence, %ooding of road, land&slides, deposition of sand dunes, erosion are some of the problems.
Types of Surveys The di'erent di'erent surveys surveys to be conducted conducted in connection connection with the the construction construction of roads roads are( A. Transport planning ).
Tra"c surveys
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+ighway inventory
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-avement deterioration
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#ccident studies
B. Alignment and route location /.
0es study
1.
2econnaissance survey
3.
-reliminary survey
4.
!inal location survey
C. Drainage studies 5.
+ydraulic studies
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Subsurface drains
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Cross drainage
D. Soil survey )*.
0es study
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Site reconnaissance
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Soil e7ploration and sampling
E. Pavement design investigations )/.
Soil properties and strength
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8aterial survey
Soil nvestigations nvestigations
Soil investigations are very important to assess the properties of the e7isting soil and to decide upon the improvements to be made to strengthen it. Purpose of soil investigations ).
To determine the nature and physical properties of the soil to be used for embanment
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To facilitate the design of slopes of embanments and cuts
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To locate suitable borrow pits
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To determine the construction techniques
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To determine the surface and subsurface drainage requirements
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To determine the suitability of local materials
3.
To determine the need for sub grade treatment
!et"ods of determination of soil pro#le !or determination of soil prole, trial pits or borings are to be made. The tests to be conducted for determination of the suitability of the soil for embanment construction are( 9radation test, liquid limit, plastic limit, shrinage limit, optimum moisture content, ma7imum dry density, di'erential free swell :for highly plastic clays;.
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Design Approac"es for Swamps Compared with ordinary soil, for construction of roads in swamps, special treatment and techniques are to be adopted depending on the characteristics of the underlying soil. They are( ).
-artial or total removal of undesirable material
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Stage construction and surcharge ll
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=se of sand drains
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=se of light weight material
Partial or total removal of swampy soil The undesirable material is e7cavated and removed. Bog blasting is a method in which a layer of sand is deposited over the undesirable soil and the soil below is blasted. The blast disperses the clay sideways and the resultant cavity is occupied by the overburden sand. Stage construction and surc"arge #ll #s consolidation taes long time, system of stage construction is adopted so that embanment settlement and further construction proceed side by side. #lternatively, the embanment is constructed to an elevation higher than the nal requirement. The surcharge so placed accelerates the settlement.
-se of vertical sand drains >ertical sand drains consist of bore holes of 6. to 6.1 m diameter, dug through the swamps and lled with sand. The depth of the borehole can be )6m to *6m. The drains are spaced * m to 1 m centre to centre both transversely and longitudinally. # sand blanet layer of ) to )./ m is placed on top of the vertical sand drains. The sand drains facilitate quic removal of water from the soft strata as the embanment load squee$es the water from the pores. The drainage of water helps in the following ways( ?uic settlement of the ban taes place and shear strength of soil is increased. -se of lig"tweig"t materials Settlement can be minimi$ed by reducing the weight of the embanment. !or this, light materials such as cinder, %y ash and saw dust can be used in embanment. Soil sta&iliation The soil properties can be changed either by addition or by mechanical blending of di'erent soil types. The additives normally used are lime, cement, sodium silicate, calcium chloride and bituminous materials.
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