Field Performance of Prefabricated Vertical Drain to improve soft clay beneath the high railway embankment in Bangladesh-Paper

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The study evaluates prefabricated vertical drains (PVDs) used with staged loading to improve soft clay beneath the Khulna–Mongla railway embankment in Bangladesh. The embankment is generally 5–13 m high, while soft clay layers are approximately 9–25 m thick. PVDs were installed in a triangular pattern at 2.5 m spacing and to a depth of 7.5 m.<br /><br />Settlement was monitored weekly using settlement plates; installed piezometers and inclinometers did not function. At eight locations, measured settlements ranged from 251 to 1,783 mm, differing substantially from design predictions of 652–759 mm. The results indicate that some sections settled far more than predicted, while others settled less. The authors attribute discrepancies to limited or inadequate subsurface data, assumptions that soil conditions were uniform, possible soil disturbance during drain installation, and smear zones around the drains that may restrict water flow. Monitoring also continued longer than anticipated.<br /><br />Post-construction investigations and stability analyses using Low’s method found factors of safety above the cited minimum of 1.25. No cracking, heaving, or other signs of embankment instability were observed. The findings suggest that the embankment remains stable, but highlight the high compressibility and variability of the soft clay and the need for more representative ground investigations and improved settlement predictions in design.

Keywords

prefabricated vertical drains

soft clay

staged loading

embankment settlement

Khulna-Mongla railway

Bangladesh

settlement monitoring

smear zone

stability analysis

ground investigation