A case study of a 3-D utility engineering project on a gas main installation allowed for decreased production costs and a shorter construction period
By Philip J. Meis
Utilities are universally a leading cause for construction delays and project cost increases. Urban projects are notorious for involving a congestion of active and abandoned infrastructure systems that are poorly mapped and intertwined with vital facilities that support traffic control, lighting, water, sanitary sewage, storm drainage, natural gas, power, and telecommunications for many critical commercial, government, and public operations. Ill-defined utilities wreak havoc on planned construction sequencing and activities, increase risk for damage, injury and claims, disrupt services and traffic flow, and heighten public discomfort.
Utility engineering (UE) allows project owners and their design team to address utility conflicts in a proactive manner and achieves even greater benefit with the introduction of 3-D design methods. Any of the nation’s leading construction firms will say that reliably qualified 3-D data on existing and proposed utility infrastructure is required to enable: 1) safety, risk reduction and damage prevention practices; 2) accurate planning, scheduling, and cost estimating; 3) expedited construction; and 4) cost savings and mitigated public disruption derived through compressed construction schedules.
“Utility engineering achieves even greater benefit with the introduction of 3-D design methods.”
A recent UE effort involving a 3-D method to survey, model, design, and construct a new Puget Sound Energy (PSE) 8-inch high pressure (HP) natural gas main installation along a 1-mile busy urban corridor, SR-510, in Lacey, Washington reaped tremendous value. The initial focus of our effort for PSE was to help identify a route for a new high pressure gas main which would minimize construction impacts to the local community and businesses. Our work included a utility investigation in accordance with American Society of Civil Engineers standard (38-02), and full development of a 3-D model of the existing utility infrastructure to assist designers in routing the new natural gas installation horizontally and vertically.
An iterative approach was used, in which an initial 2-D base map was created which depicted the horizontal position of the various utilities along the corridor. From this map, potential horizontal routes were identified and corresponding areas were isolated where additional 3-D utility data was required. 3-D data came from a variety of sources including vacuum excavations, GPR, and SPAR 300 data. The 3-D model presented the existing utility configuration in a highly useful, manipulable, virtual reality rendering.
The combination of 3-D QL B geophysical methods with strategically placed QL A test holes effectively reduced overall requirements for wholesale vacuum excavated QL A test holes by roughly 50%, which in turn mitigated project related costs, time and traffic disruption.
With the 3-D model, a final alignment for the proposed gas main was derived and one hundred seventy (170) potential hard (e.g., clash) and soft (e.g., issues related to constructability, safety, schedule impacting) conflicts, including 23 unknown and possibly abandoned features, were visually identified and managed within the UIM.
The 3-D model clearly demonstrated the optimal alignment, which had to encompass minimal traffic flow impact while balancing: 1) safety aspects appropriate or mandated for HP gas main installations, and 2) reasonable construction costs and duration. 3-D visualization made it straight forward for WSDOT to approve variances for: 1) an open cut in the road; and 2) those areas where a 5-foot cover requirement could not be maintained due to existing utility installations or other obstacles. This reduced risk, improved public safety, and averted significant project delays and increases to corresponding construction costs.
The contractor that was awarded the project had scheduled 10 weeks of field work for 2 crews to complete the installation; however, they soon found the 3-D utility information was so reliable that they were able to lay pipe to plan at their maximum rate of 300 feet per night. In the end the work was completed in 7 weeks and with only 1 crew, and without any change orders or issues arising from damage, delays, or unidentified utilities.
By developing a 3-D model of the existing utility installations, PSE was able to identify potential utility conflicts and constructability issues during the design process, well in advance to construction. This resulted in a reduced construction timeline, lower construction bids, and reduced impact to the traveling public.
Philip J. Meis, P.E. co-founded Utility Mapping Services, Inc. in 2002. He may be reached at email@example.com.