An innovative Protect-in-Place design concept for a pipeline on a Texas interstate highway expansion allowed the project to keep moving, while meeting the needs of all stakeholders.
One early morning in 2017, a contractor kicked off construction by drilling a bridge column foundation, a critical path schedule item for a major 4-mile urban interstate highway expansion project. They were completely unaware of what they were about to encounter. Just below the Earth’s surface, they hit steel. This article discusses the investigation into the major pipeline that they encountered, what was done to resolve the conflict, and how the project got back on track.
Hitting an unknown utility pipeline is a critical safety issue, and can adversely affect project costs and schedules. Today’s constrained public Right-of-Ways (ROWs) coupled with increasingly complex and expensive utility facilities have created the need for engineering-level utility data collection, conflict identification, analysis, and resolution. HDR’s Utility Engineering Solutions program provides consulting services on civil infrastructure projects for clients nationwide to solve these utility challenges.
In the case of the the column mentioned above, construction of its foundation was a key element for the critical path schedule of the project. The column would support the bridge bent for expansion that was needed for the primary live-traffic shift. Hitting unknown steel and stopping construction is a major challenge because every day that an interstate highway project is paused, valuable time is lost by the department of transportation (DOT), and the traveling public is impacted.
We approached this particular situation by conducting Subsurface Utility Investigations (SUE) immediately after the impact to better assess the situation. Using SUE Quality Level-A Test Holes, we determined that the steel plate was actually a major 39-inch diameter high-pressure water transmission pipeline, encapsulated within a five foot diameter protective steel casing. Previous phases of the project’s record findings had inaccurately mapped the location of the portion of the pipeline crossing the highway. An additional more extensive SUE Quality Level-D search of old project archives uncovered a key change order document. During a highway project 10 years prior, a ROW acquisition issue led to a last minute re-design change order that resulted in the water pipeline crossing being installed on the opposite side of the intersection. Unfortunately the utility owner and latest project designers were unaware of this change. To reduce the risk of similar future incidences from occurring, the ASCE Utility Engineering and Surveying Institute (UESI), along with the new Texas Chapter of UESI, is in the process of developing utility asset management and as-built standards.
We worked with the utility owner to determine that relocating the pipeline would potentially take a minimum of six months to design and permit, and an additional six months to construct. Furthermore, the water flow could only be shut off and switched over to the new pipeline during a low water demand period during the following winter. The pipeline was a major water transmission route that needed to remain in service, feeding a rapidly growing 100,000 person community. The pipeline would cost $1.5M to relocate, and the initial crucial traffic shift would need to be put on hold for a total of 18 months, stopping the entire first phase of the project. Such a delay was estimated to cost $6,000 per working day in contractual legal claims liabilities. The potential total financial impact of the conflict to the project was estimated to be $4.3M in losses.
Structural engineers were able to redesign the bridge column foundations to ‘straddle’ the pipeline and continue column foundation construction, but other potential problems remained that prevented continuation of the project. The bridge expansion also included a proposed 30-foot high retaining wall of very heavy compacted soil backfill, directly over the pipeline. Due to the weak, shifting clay soils along the Gulf Coast south of Houston, numerous stone columns were initially designed to be installed around the base of the proposed bridge expansion walls to stabilize the soil below. The construction method of these underground stone columns includes drilling, then pouring in the stones using heavy vibration and compaction. Although the pipeline was encased in steel, the utility owner’s engineer determined that the stone column vibrations and weight of the backfill could cause a burst or leak in the pipe at any point, now or in the future. Given that pipe would be under the new DOT bridge embankment structure, we could not risk a pipe burst or leak on a 39-inch high pressure pipeline because public safety, pipeline maintenance, and bridge infrastructure access could be negatively affected. The pipeline was also determined to be in conflict with designs for storm drainage culverts and the retaining wall base foundation.
Evaluating Resolution Options
These complex problems and time constraints required a complex solution in a timely manner. To explore and develop our resolution options, we initially organized group teleconferences and individual meetings with the stakeholders including the Texas DOT (TxDOT), the Federal Highway Administration (FHWA), the pipeline owner, the local city public works directors, the project construction inspection group, the roadway contractor, the highway users, and, course, the citizens who would be impacted by any significant interruption of their source of drinking water. As the feasibility diminished for other options, such as pipeline re-routing, traffic phasing adjustments and other alternatives, we advocated to the stakeholders for protecting the pipeline in-place. We gained a consensus for the pipeline to be left in-place and to remain in a stable condition. This required HDR to design a protective ‘shield’ around the pipeline, and for the construction group to plan other conflict mitigations. We drafted a Letter of Concurrence specifically for the protect-in-place solution, and it was signed by the utility owner and executed by TxDOT.
To initiate design, we reviewed previously built protect-in-place concrete pad designs, and determined that those were primarily designed only for impact protection and not suitable to satisfy the current structural requirements. The circumstances required a design that would completely resolve the risk that the heavy weight of the soil backfill may burst the pipe. We worked diligently with our structural engineers, geotechnical engineers, and drainage designers to propose various protection design solutions and alternatives.
The design team determined a custom cast-in-place steel reinforced concrete slab fully supported by reinforced concrete drilled shafts was the most suitable option to protect the pipeline. This design was chosen because it supported the entire weight of the future 30-ft high bridge backfill and retaining wall, and reduced the risk of the proximal construction activity affecting the pipeline. The structural engineers designed a 1.5-ft thick concrete slab reinforced with U1 and #6 steel rebar, and structurally determined the need for 17 drilled shafts inserted up to 70-ft deep into the clay soil (or until hitting bedrock) to support the concrete slab in place.
Ultimately, we designed a bridge under a bridge.
During final design, our geotechnical engineer applied the Morh-Coulomb theory to re-analyze the weak clay soils, and determined the bridge and backfill weight would slowly shift the clays diagonally, putting additional pressures on the pipeline. These soil pressures would shift the pipeline horizontally over time. The pipeline engineer determined this was an unacceptable risk, because any shifting would create the possibility of a major leakage under the bridge due to the unknown installation methods of the angled bends of the pipeline. To resolve this concern, we increased the drilled shaft diameters from 24 inches to 36 inches, essentially creating a protective wall between the bridge backfill weight and the pipeline to greatly reduce the risk of the pipeline shifting horizontally.
Regarding the other related conflicts, the stone columns were no longer needed in this area because the new underground protection bridge more than satisfied the soil stabilization requirements for the retaining wall. We coordinated design mitigations to adjust drainage culverts around the protection structure and the pipeline. Additionally, we re-designed the retaining wall to include vertical ‘slip-joints’ in the wall to resolve the issue of potential vertical wall panel displacements due to the substantial risk of variable soil settling around the proposed slab. After all conflict risks were resolved, the final protection design was QC’ed, signed and sealed, and delivered into the highway planset as a joint bid construction change order for an estimated $150,000 in construction costs.
Once the change order was executed, the contractor immediately restarted construction, carefully drilling the 17 36-inch diameter shafts supported by rebar cages. Then they built the slab formwork with the rebar, poured and cured the concrete, and continued the expansion of the bridge for the critical path traffic shift.
Overall the protect-in-place solution saved the project an estimated $4.1M dollars in potential costs by avoiding the immediate pipeline relocation cost and the pending delay liabilities. The protect-in-place structure benefited all of the stakeholders by allowing construction to continue and resolving safety concerns related to the pipeline stability and traveling public.
Although protect-in-place alternatives such as these have been utilized in the past, it is important to share them with the civil engineering community to inspire future conceptual designs. It is also important to display innovative designs using captive imagery to effectively illustrate alternative underground concepts so that they can be improved upon and utilized on other civil projects. On a subsequent highway project, we implemented a similar protect-in-place concept utilizing structurally supported box beams to resolve conflicts involving three high pressure gas lines, saving $6M in direct costs and an additional two years of schedule liabilities for easement acquisitions. This box beam strategy is an improved concept because it is more easily constructible and does not require custom formwork.
These innovative solutions serve to reduce costs to all stakeholders and save taxpayer money, while keeping projects progressing to completion. In the end, civil engineers work to not only to meet our clients’ budgets and deadlines, but for all of society’s benefit.