Bridge Approach and 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. A sleeper slab is a reinforced concrete block that supports the end of the concrete approach slab at the approach roadway end. Sleeper slabs are commonly used when preformed elastomeric compression joint seals are not used between the concrete pavement sections. To stabilize the sleeper slab, injection tubes shall be inserted to a minimum depth of 5 feet (minimum of 2 feet below the bottom of the sleeper slab) and then typically at a second elevation approximately 10 feet 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 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.