Bridge Approach/Departure Slabs

Problem:

Bridge approach and departure slabs often settle several inches or more causing potential unsafe driving conditions. The settled approach slab and highway pavement are impacted by cracking and stresses that either reduce the pavement life or require additional maintenance costs. Additionally the bridge itself is subjected to increased impact loading which is damaging to the structure. The settlement is most often caused by loose or poorly compacted foundation soils. The settlement can also be caused by water penetration under the slab and out under the abutment wall or wing walls, carrying soils out from under the slab.

Solution:

Perform Dynamic Cone Penetrometer (DCP) tests to determine the depth(s) of the weak soils. Bridge approach slabs that have sleeper support slabs shall have all drill holes fully sleeved by tubes into the base soils to prevent any injection of material between the sleeper slab and the pavement. To stabilize the sleeper slab, injection tubes shall be inserted to a minimum depth of 5′ (minimum of 2′ below the bottom of the sleeper slab) and then typically at a second elevation approximately 10′ below the pavement surface. Material shall be injected in each tube until the soils are stabilized as evident when movement of the pavement is detected. After the soil is stabilized beneath the sleeper slab, injection may be continued to lift the sleeper slab and pavement to original grade. Based upon the DCP tests, additional levels of injections may be required to provide adequate stabilization. Stabilization of the soils will provide proper support for the heavy sleeper slab to mitigate any future movement. If there is any concern over loss of soil beneath the abutment wall, a row of injection tubes shall be inserted to the proper depth(s) and material injected to stabilize the soils and to close off any pathways for water to travel that were carrying soils out from under the abutment wall.

Expected Results:

  • Foundation soils are sufficiently stiffened to support the load and mitigate future settlement.
  • Sleeper slab, approach slab and pavement raised to grade.
  • Impact loading from launching trucks onto the bridge eliminated.

Benefits:

  • Zero daytime lane closures.
  • Sleeper slab is properly supported.
  • Approach slab and roadway are not lifted off the sleeper slab.
  • Pathways for water to travel through the system are cut off so soils remain within the pavement system
  • Injection can be accomplished in wet soil conditions as the URETEK 486 STAR hydro-insensitive polymer will form a dimensionally stabile polymer even when injecting into saturated soils. The expansion process will also drive the water out of the soil system.

Alternates:

  • Utilization of concrete slurry directly under slab has many weaknesses:
    • Difficult to control the lift.
    • Material setup time is long and material can travel far beyond the point of installation – often times requiring significant additional material and/or causing environmental concern.
    • Vibration from traffic in adjacent lanes can cause water separation and thus a poor end product.
    • Does not have the tensile strength to hold up under the vibration of traffic.
    • Difficult to control – if there is a blow out – there is a significant delay while the material hardens before injection can continue.
    • While cement slurry is often used to fill large voids, it is ineffective where there are small pockets of void or weak soils as it is difficult to identify exactly where those voids or weak soils are and should be injected.
  • Polyurethane injection directly beneath the concrete to fill voids and lift has many weaknesses:
    • Material can easily travel from under the sleeper slab and lift the approach or pavement off the sleeper slab – leaving the sleeper slab down. This places polyurethane material between the concrete roadway and the concrete sleeper slab which will eventually crush, causing re-settlement.
    • Foundation soils that cause the settlement are not repaired.
    • Channels for water to travel through the foundation soils are not eliminated.
    • Polyurethane material has a very high R value (no impact when mixed with soils). If several inches of polyurethane material is between the concrete and the foundations soils, the bridge approach panel is now susceptible to icing faster than it is not isolated from mother earth. When the bridge approach re-settles, that icing situation will present an additional safety problem.

Reference Documents:

  1. ADR Forensic Photos – the entire premise of this research project for the Navy was to stabilize the un-compacted fill in 20’ Diameter bomb craters. The photos demonstrate how the injection process will lock up and compact the loose soils beneath a bridge approach and provide sufficient support for the heavy sleeper slab. View Photos.
  2. NAVFAC Press Release – Confirms the success of the URETEK Deep Injection Process to stabilize un-compacted fill to support pavement. View Press Release.
  3. URETEK R-Value Testing – Provides information on the insulating characteristics of the polyurethane by itself and when injected into soils. View Report.