Other Considerations
Constructability and Sequencing
The difficult terrain encountered on side-hill retaining wall projects not only make them difficult to design, but construction isn’t any picnic either. One of the major considerations is access to the wall line. If the terrain is too steep, a bench or pioneer road will have to be cut at the wall line to permit construction. In some cases, special equipment such as limited access rigs for micropiles or crane-mounted pile drivers will be needed, which of course raises the cost of the wall.
Another significant issue is the availability of space to lay back slopes during construction. In a roadway widening example, existing traffic usually has to be maintained. So there may not be enough room to open up the slope for a MSE wall. Or in other cases, temporary shoring may be required so that the permanent wall can be constructed. This makes sequencing of construction of particular importance.
Sequencing of construction is also important for other wall systems. For example, with anchored wall systems to ensure that the cantilevered portion of the wall does not deflect too much before the anchors are installed. Another example would be with storm drain installation to avoid disturbing compacted wall backfill or worse, reinforced backfill for MSE walls.
Finally, in the case of walls with drilled or driven elements (piles or shafts for an SPL wall, micropiles, sheet piles), the material type can cause significant headaches during construction. An engineer may think that piles can be driven (or shafts drilled) in a particular soil or weathered bedrock material, but a contractor may disagree (or at least make a claim). If you are in hard rock, make sure you report the highest unconfined compressive strength you determined in the lab and somehow work it into the plans or specifications that the contractor’s equipment should be capable of drilling through rock of that strength.
Detailing
Once you go through all of the investigation steps (outlined below) and design procedures, someone must communicate these designs on the plans. For the more complicated wall systems, this often requires detailing various structural and non-structural elements. Examples of this would be cap beam reinforcement in micropile walls, anchor plate and trumpet designs for tieback walls, storm drain and catch basins located behind the walls, bearing pad thickness in between MSE walls or between precast concrete lagging panels, geotextile fabric behind MSE wall joints and pipe or other utility penetrations through the wall.
Construction QA/QC
This is not a problem specific to side-hill retaining walls, but because of the terrain and the consequences of failure, proper construction QA/QC is a particularly critical element of side-hill wall projects. Potential problem areas in this department could include improper compaction around deadman anchors leading to excessive deflection, improper compaction of backfill leading to settlement problems, improper treatment of anchor/tieback connections leading to potential corrosion issues over the long term and shortening of service life, poor quality backfill being used for MSE walls, bent or broken connections of MSE wall reinforcement causing reduction in service life, wall internal drainage systems being compromised because of poor construction…and an infinite number of additional possible problems.
