In today’s construction world, the general contractor has to determine a schedule that will maximize efficiency and minimize downtime. Much consideration must be given to what and when activities must occur to avoid major, costly issues down the schedule. A common contractor question is about the timing of under slab utility installation when the building is supported by ground improvement. Should utilities be installed before or after the ground improvement? The answer is: it depends on factors including the pipe elevations and locations.
For utilities that run about two feet or less below the floor slab, such as shallow water and sewer, plumbing or electrical lines, contractors are typically in the clear. These utilities do not impact ground improvement design or foundation support. It is important to place compacted backfill over the pipes, though, as backfill supports the floor slab support.
It is the deeper utility pipes that create the challenges. These are pipes that are at least two feet below the floor slab and directly impact the timing, integrity and location of ground improvement elements. Here are a few best practices to coordinate the utility planning with ground improvement.
1. SCHEDULE EARLY AND COMMUNICATE OFTEN
It may be more efficient to install deep utility pipes before the ground improvement contractor comes to the site. Be sure to communicate with the ground improvement designer on the backfill that is planned and work with the geotechnical engineer of record for backfill requirements.
After backfilling, the pipe alignments will need to be clearly marked to avoid damage during the ground reinforcement installation. Contractors should also discuss with the designer the distances that the pipes need to be away from the ground improvement, as to not cause damage to the newly installed utilities.
2. IDENTIFY PROXIMITY AND ALIGNMENT
If deep utilities are installed after the ground improvement elements, then contractors have other challenges and considerations to work through with the ground improvement team.
For the utilities that run alongside the footings, the pipes must be installed outside of the “no-dig zone of influence” (seen in illustration), which helps to maintain the integrity of the reinforced ground support.
If the utilities cross beneath footings, a common occurrence for service lines entering the building, the ground improvement elements can be situated to bridge or straddle the utility. This bridging design will support ease of installation during the ground improvement phase. In the field, utilities may also need to be adjusted to be centered between installed ground improvement elements. It is always best to plan these utility locations prior to the ground improvement installation.
3. COLLABORATE ON DESIGN CHALLENGES
Of course, every project is different, and sometimes the timing or location of utilities may not be known. Contractors may also deal with an older utility that must be removed or left in the ground. Design accommodations can still be made for complex utility placements through a detailed evaluation by the geotechnical design team. Sometimes the solution calls for a bridge method, as described earlier, or installation of additional reinforcing elements.
Even the most carefully coordinated utility installation plans can result in unavoidable conflicts with the no-dig zone. When it is not possible to avoid the no-dig zone next to the installed reinforcing elements, solutions can still be found through careful selection of backfill materials and sufficient compaction to repair or sustain foundation integrity.
Utility questions are common, and this article just scratches the surface. Consult the geotechnical engineer of record and the ground improvement designer to handle utility construction situations. The geotechnical teams work to avoid those issues that will cause future challenges for the schedule and, ultimately, provide the owner with a well-constructed product.