How are Roads Built Over Swamps?
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 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 contains large quantities of vegetable matter, clay and mud. Swamps are classified into brackish or fresh water depending on the salt content of the water. The depth of this particular layer may vary from shallow to deep. For construction of roads, shallow deposits can be removed and deeper deposits strengthened by suitable means.
Surveys and investigations
There are certain basic design controls and criteria which govern the geometric features of a highway. They are topography, traffic, speed, capacity, vehicle and control access. Also certain surveys and investigations are to be conducted so that construction failures can be avoided or minimized. Embankment subsidence, flooding of road, land-slides, deposition of sand dunes, erosion are some of the problems.
The different surveys to be conducted in connection with the construction of roads are:
A. Transport planning
○ Traffic surveys
○ Highway inventory
○ Pavement deterioration
○ Accident studies
B. Alignment and route location
○ Desk study
○ Reconnaissance survey
○ Preliminary survey
○ Final location survey
C. Drainage studies
○ Hydraulic studies
○ Subsurface drains
○ Cross drainage
D. Soil survey
○ Desk study
○ Site reconnaissance
○ Soil exploration and sampling
E. Pavement design investigations
○ Soil properties and strength
○ Material survey
Soil investigations are very important to assess the properties of the existing 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 embankment
○ To facilitate the design of slopes of embankments and cuts
○ To locate suitable borrow pits
○ To determine the construction techniques
○ To determine the surface and subsurface drainage requirements
○ To determine the suitability of local materials
○ To determine the need for sub grade treatment
Methods of determination of soil profile
For determination of soil profile, trial pits or borings are to be made. The tests to be conducted for determination of the suitability of the soil for embankment construction are:
Gradation test, liquid limit, plastic limit, shrinkage limit, optimum moisture content, maximum dry density, differential free swell (for highly plastic clays).
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:
○ Partial or total removal of undesirable material
○ Stage construction and surcharge fill
○ Use of sand drains
○ Use of light weight material
Partial or total removal of swampy soil
The undesirable material is excavated 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 surcharge fill
As consolidation takes long time, system of stage construction is adopted so that embankment settlement and further construction proceed side by side. Alternatively, the embankment is constructed to an elevation higher than the final requirement. The surcharge so placed accelerates the settlement.
Use of vertical sand drains
Vertical sand drains consist of bore holes of 0.4 to 0.6 m diameter, dug through the swamps and filled with sand. The depth of the borehole can be 10m to 20m.
The drains are spaced 2 m to 6 m centre to centre both transversely and longitudinally. A sand blanket layer of 1 to 1.5 m is placed on top of the vertical sand drains. The sand drains facilitate quick removal of water from the soft strata as the embankment load squeezes the water from the pores. The drainage of water helps in the following ways: Quick settlement of the bank takes place and shear strength of soil is increased.
Use of lightweight materials
Settlement can be minimized by reducing the weight of the embankment. For this, light materials such as cinder, fly ash and saw dust can be used in embankment.
Soil stabilization
The soil properties can be changed either by addition or by mechanical blending of different soil types. The additives normally used are lime, cement, sodium silicate, calcium chloride and bituminous materials. In mechanical stabilization, the stability of the soil is improved by blending the available soil with imported soil or aggregate.
Use of geo textiles
Geo textiles are synthetic fabric made out of nylon, polyester, and poly propylene. They are non- decomposable. They are used for the following purposes: ensureing physical separation of layers, acting as a filtering medium, working as a reinforcing layer permitting heavy loads on weak soils.
Use of geo grids
Geo grids are high strength plastic grids which are interposed either at the bottom of a layer or in a layer itself to increase the strength. When used at the bottom of a granular layer and above the sub grade, the grid increases the load bearing capacity of the soil.
Building a road over swamp is definitely a problem and as such should be avoided as far as possible. But when there is no other alternative, special techniques as described above should be taken to strengthen the swampy soil and then construct the road.