Scary: Things that go bump…at the end of the bridge

Bump at the end of a bridge approach slab.

Full citation

Abu-Hejleh, N.M., Hanneman, D., Wang, T., Ksouri, I. (2008). Evaluation and Recommendations for Flowfill and Mechanically Stabilized Earth Bridge Approaches, Transportation Research Record: Journal of the Transportation Research Board, Volume 2045, No. 1, pp. 51-61.

Bullet List Summary

What follows is a bullet-list style summary of what I found were the interesting points. Its a good paper to read if you have time. You should be able to download the PDF from the link above, but it didn’t work for me. Free registration may be required.

CDOT bridges primarily use integral abutments. So typically the problems that occur are near the sleeper slab end of the approach slab. Since 1992 CDOT has had 3 alternatives for bridge abutment backfill:

  • Flow fill which was a standard from 1993 to 2001 (approx $76/CuYd in 2005$)
  • MSE Wall with Class 1 granular backfill, typically geofabric reinforcement, some geogrid (approx $37/CuYd in 2005$)
  • MSE Wall with Class B free-draining filter material (approx $57/CuYd in 2005$)

The goals of the study described by the authors were:

  • To document current CDOT practice
  • To evlauate performance and unit maintenance and lifetime costs of the approaches
  • To determine the causes of significant approach settlements

The complete details of the study were in CDOT Research Report 2006-2.

Some of the causes of approach settlement problems that the authors identified:

  • Drainage problems (including at expansion joints and cracks)
  • Built-in bump at the end of construction caused by not matching as-bult grades of sleeper slab with as-built grade of approach and bridge
  • Placement of high fills
  • Construction problems (typically in first year after construction)
    • Poor compaction of fill, including side slopes of abutments
    • Cold season construction (frozen ground that thaws later)
    • Placing compacted soil on dry side of optimum which can then compact when their moisture content increases
    • Settlement of foundation
Abu-Hejleh et. al. Figure 5a - Recommended supporting systems and drainage details for sleeper slab: placement of MSE wall under sleeper slab.

Abu-Hejleh Figure 5a – Recommended supporting systems and drainage details for sleeper slab: placement of MSE wall under sleeper slab.

Some of the resulting recommendations for mitigating the problem (my summary anyway, refer to the full paper to read in the Author’s own words):

  • Perform borings at sleeper slab locations
  • Apply seasonal correction factor to SPT values
  • Estimate post-construction settlement
  • Continue with the flowfill and MSE approaches but extend material under the sleeper slab
  • Continue to document the performance and costs of these systems
  • Compact fill wet of optiumum or moisten slightly after compaction
  • Preload approach fills if possible with temporary fill
  • Provide better support under the sleeper slab (use the MSE Class 1 backfil)
  • Improve drainage measures on bridge deck
  • Use drainage systems such as impervious membranes and collecter pipes under approach slab or under joints only.
  • Consider horizontal drainage systems above cohesive layers
  • Tune length of approach slab to limit angular distortion
  • Use smoothness criteria or some equivalent around bridge expansion joints
  • Install  sleeper slab higher to account for post construction settlement
  • Design approach slab to have asphalt overlay to allow correction for settlement
  • Require 18 mo. warranty on approach slab from contractor
  • Improve CDOT detail for bridge expansion joint to prevent cracking of approach slab allowing water to seep under sleeper slab