Road pavements in the coastal Niger Delta are under perennial flooding and ingress of undrained water into the pavement layers through cracks, joints, voids, and embankments. Studies have shown that water in road bed leads to increased unit weight under saturated conditions, reduced shear strength, excess pore water pressures, increased seepage pressure and reduced effective stress constituting the major culprit in road failures. This study involved field boring and sampling of 18 holes to a depth of 2m, monitoring of climate induced detrimental environmental variables and geotechnical laboratory testing including identification, classification and soil grading tests and results analyzed using empirical relationships. Hydraulic conductivity was determined using the Kozeny – Carman equation and Hazens method for plastic and non-plastic subgrades respectively. Subgrades were classified as A3, A-7-5 and A-7-6 AASHTO soil groups and poorly graded CI – CL (USCS), Kaolinitic clayey silts. The hydraulic conductivities vary from 1.66 x 10-4 – 9.604 x 101 cm/sec, degree of saturation from 36.1 – 121.1%, submerged unit weight from 5.77 – 14.49KN/m3, critical hydraulic gradient from 0.59 – 1.48 and void ratios from 0.52 – 1.28. The effective stress under dry and submerged road conditions range from 1.731 – 4.347KN/m2 and 1.554 – 3.903 KN/m2 respectively indicating a % reduction >10% in all the samples under a seepage pressure of 1.02 – 6.43 KN/m2. Subsurface drainage was considered based on piping ratio which range from 1.32 x 10-3 – 3.60 x 10-3 and permeability ratio which varies from 0.00947 – 0.90 indicating that subgrades are unsuitable filter materials in the road bed foundation. Recommended design options for pavement sustainability include increasing pavement elevation above the historical average flood level and raising the road bed thickness by filling with free draining materials such as graded sand, provision of subsurface drains and geosynthetic or geotextile materials to aid water egress.
Keywords: Eastern Niger Delta., Subgrade, effective stress, hydraulic characteristics, pavement, subsurface drainage
