Earth pressures on twin positive projecting and induced trench box culverts under high embankments

Although twin culverts are often preferred for watercourse crossing, little is known about the earth pressures they experience under high embankments. Centrifuge test results have been used in this research to verify a numerical model used to evaluate culvert spacing and compressible zone geometry for twin positive projecting and induced trench box culverts. Lower pressures were calculated for twin positive projecting culverts than for a single culvert. For the induced trench case, one compressible zone spanning both culverts was the preferred geometry for culverts spaced at 0.5Bc and 1.0Bc (where Bc is the culvert width), while two zones 1.2Bc wide were found to be optimal for 1.5Bc spacing. The twin configurations generally resulted in slightly higher vertical and lower lateral earth pressures than a single culvert configuration. The base contact pressures were 25%–76% greater than the top pressure plus dead load because of shear stresses mobilized along the sidewalls; however, they were 41%–47% lower than the pressures for positive projecting configurations. The maximum bending moments calculated for the induced trench culverts were 54%–59% lower than for the positive projecting ones. Induced trench construction therefore appears to be viable for twin box culverts, provided that frictional forces along the sidewalls are taken into account.