SETTLEMENT MODELLING OF EMBANKMENTS ON SOFT SOILS
Rapid industrialisation and population explosion has forced to develop new towns or satellite towns near the main urban centres. Lack and various constraints for good terrain to allow development of an efficient road network has led to venturing for space, for building infrastructure, on discarded areas. Engineering properties such as very high compressibility and low shear strength generally characterize marine clay deposits, which are otherwise too soft. Such deposits are generally encountered in the near coastal areas of almost the entire world, along seashores, in creeks, tidal flats and formerly submerged areas.
Marine clays in New Bombay area are pedologically regarded as having been formed mainly by deposition of fine materials under water. They are underlain by stiff, compact murrum with boulders, followed by basaltic rocks. However, in some locations the clay deposits are found at a relatively shallow depth (i.e. 0.3 to 1.0m.). This clay stratum is noticed to be relatively stiff, possibly due to desiccation effects, and is greyish blue in colour. At the same time, in some other locations these deposits are extending very deep, upto a maximum of 10.0m, also. In some places the clay underlies a layer of loose or cemented sand about 2.0 to 5.0m thick. The characterization of Bombay marine clay in the twin city area has been done by Katti et al. (1977).
Road networks are the backbone of a planned city, in particular a developing city. In the Twin City of New Bombay (being developed by CIDCO) there are many important roads which are required to trek the soft marine clay deposits. Due to high compressibility and low shear strength of the marine clays, road embankments placed over them need rigorous soil investigation and geo-technical expertise for its design.
Settlement and stability are the two main factors that govern the design of an embankment. Settlements occurring over a period of time can be estimated using one-dimensional consolidation theory (Terzaghi, 1943) in combination with the theories proposed by Boussinesq (1885), Jurgenson (1934) and Newmark (1942) for calculating the stresses in the soil mass. Even though, stability of an embankment is one of the most important criteria for its design, it is not being investigated here in so as not to deviate from the main theme of the study. As such, the present study only deals with the settlements of embankments built on soft soils. This can be achieved through a sophisticated instrumentation program to record; total pressure, vertical settlements and horizontal movements of the subsoil and pore water pressures generated.
The obtained data can be put to use by evolving a relationship between the soil properties and the settlement undergone. At the same time, a co-relation between the predicted and the measured settlements can also be developed, for ready use, by the practising engineers.
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