Retaining Walls: Failure and Repairs

The retaining walls (spandrels and approaches) in a stone arch bridge are prone to failure thanks to their relatively thin width. However, there are several things that can be done to repair and prevent these failures.

The retaining walls in a stone bridge consist of the spandrels and any approaches. These walls are often a weak point in the structure, due to their relatively thin width. In this post we investigate these common failures and how they can be remedied and prevented.

Typical Failures and Their Causes

Spandrel walls and approaches are a rather common failure point in stone bridges. Usually freeze-thaw cycles eventually succeed in upsetting the equilibrium of these gravity walls, while in some instances saturation of the fill caused by flooding overloads and bursts the walls. One factor to keep in mind is that the walls tend to weaken over time if the mortar is not maintained. This weakening reduces the friction so critical for the strength of a masonry wall, and may be the cause of failure in some walls, as loss of friction means sliding is much easier. Other failures on newer stone bridges can be from inadequate wall thickness (a.k.a. poor initial design) and impacts from debris or (commonly) vehicles. Below are some examples of failed walls.

Damage to Clements Bridge — Undermining may have played a role in the failure of this approach on a stone bridge (note the island of masonry marking the original end of the bridge) but another possibility is waterlogged fill, as the fill all seems to have been loose soil.

Front Circle Crack — This stone bridge had loose fill and was built with extremely thin walls. This appears to have allowed the force of the fill to burst out the bottom of the spandrel wall, taking part of the arch along with it.

Glencoe Township Culvert — These walls have been failing over the years, primarily from loss of mortar and impacts it would seem. The small stones used makes the loss of mortar more serious than would have been the case if large blocks were used. Note how the damage has started at the top of the walls. Once the top stones are gone, the next layer becomes vulnerable and so on until the wall is reduced to rubble.

20th Street Access Culvert — Another example of deterioration caused by loss of mortar and impacts. On this example the spandrel walls are almost completely gone, leaving only the arch. This is a striking example of how the arch is far stronger than the walls.

NE160th Street May Branch Whitewater River Bridge — The replaced section of wall (repaired with concrete blocks) likely failed due to freeze-thaw cycles perhaps coupled with waterlogging.

Strengthening the Walls

The best means of strengthening the walls is (where applicable) keeping the mortar in good condition. Regular maintenance can go a long way towards preserving a structure. Another method used to strengthen a wall is adding an additional wall, usually of concrete, behind the outside stone wall in order to effectively thicken it.

Private Stone Bridge — Since some of the top stones on this 100-year-old private stone bridge were becoming dislodged, the landowner poured concrete behind the top layer (just visible through the fill on the near side of the bridge) to help protect them.

Tie rods can and have been used to strengthen the wall, typically by tying the two spandrel walls together thereby providing tensile strength. However, this method detracts from the appearance of the bridge, and if not done properly merely results in a small section of stable wall around which the rest of the structure is breaking up. For large-scale problems, the best solution is to arguably tackle the problem at the fill itself.

Strengthening the Fill

Strengthening the fill is usually easier to do and far more successful than strengthening the fill-retaining walls. One of the most common solutions is the removal of the fill altogether and the replacement of it with lightweight concrete. Concrete, being solid, should exert no force against the walls once it sets up (use caution while pouring to avoid collapsing the retaining walls). Be aware, however, that the outside skin walls can still collapse under the right conditions, unless they are bonded to the concrete.

Other solutions involve geotextiles and reinforced earth methods, all with the idea of reducing or eliminating the fill’s tendency to push over walls. All of these methods can and have been used with success.