SL 88 (Outer Loop), Lubbock, TX, TxDOT

IEA is the prime consultant responsible for providing plans, specifications, and estimates (PS&E) services for SL 88 Segment 3.

When complete, SL 88 Segment 3 will connect US 87 to US 82 located approximately 7 miles South of downtown Lubbock, Texas. The proposed SL 88 freeway will consist of 6 mainlanes (3 lanes in both the Eastbound and Westbound direction) along with 3-lane parallel frontage roads in each direction. There will be a multi-level interchange providing non-stop connections between US 87 and SL 88. The design and construction of the proposed project will be broken into approximately 2-mile long sections over a multi-year implementation process.

Roadway: IEA is responsible for the project design, which includes design of mainlanes, frontage roads, ramps, U-turns, local streets, and a 5-level interchange. There will be overpass bridges at each major thoroughfare. To improve traffic flow, U-turns will be provided under SL 88 mainlanes, along with left turn lanes and selected use of right turn lanes on the frontage roads and thoroughfares.

IEA, in collaboration with TxDOT, is designing several features with a focus on driver and pedestrian safety, including signalized intersections with cross walks and pedestrian push buttons, sidewalks, and a 10-foot wide Shared Use Path along US 87. This Shared Use Path will accommodate both pedestrians and cyclists.

Bridges: IEA is responsible for the PS&E of 2 direct connector bridges for level 3 and level 5 at the SL 88/US 87 Interchange and 2 mainlane bridges on US 87. Design challenges include complex interchange geometry, superelevation transitions, unbalanced 2-span continuous steel plate girder design, hammerhead and multi-column inverted-T bents with flared aesthetics, and multi-shaft footings.

Drainage: The flat terrain in this area presents many drainage design challenges. Unlike topography in other parts of the state, there are no major stream crossings available to drain storm sewers. Instead, runoff water is accommodated via Playa Lakes that are scattered randomly throughout the Lubbock area and most of the Texas High Plains. These naturally occurring shallow lakes are usually ephemeral. They receive most of their water during the spring and fall, and sometimes dry out completely during the winter and summer months. Some Playa Lakes retain water year-round and even serve as a water-feature for local communities.

These Playa Lakes require complex hydrologic and hydraulic analysis beyond what is typical for TxDOT projects. So, IEA is using a complex modeling program called Interconnected Channel and Pond Routing (ICPR) to perform hydrologic and hydraulic analysis. The software is an unsteady state model that can analyze the water as it flows from one body of water to another and through buried storm sewer pipes and culverts. The use of ICPR proved especially useful on one stretch of the project near the SL 88/US 87 interchange. At this location there are 2 nearby playa lakes that needed to be analyzed, including one that encroaches on the SL 88 Right of Way. Using ICPR, IEA was able to analyze the water flow and develop solutions to mitigate the impacts to the existing playa lake storage. IEA determined that 4 interconnected detention ponds within the TxDOT right of way would be required to mitigate the loss in the playa lake floodplain storage. In addition to providing flood storage, the detention ponds created outfall locations for the 14 storm sewer lines that drained the mainlanes, frontage roads, and 5-level interchange bridges.

Pictured: 3-D Exhibit of an On-Site Detention Pond