Flood control project a unique solution for city
Overland Park, Kan., is a typical 1950s-’60s midwestern suburb from its cul-de-sacs and 50-foot-tall oaks right down to its open drainages that fail to meet city stormwater standards. Many of its neighborhoods were constructed before city incorporation in 1962, when stormwater drainage was often unregulated. Natural drainages left to carry stormwater through the city were inadequate from the start, and flooding of streets and homes worsened as development continued. For the city and homeowners, unsightly and eroding banks and accidental drownings were further concerns.
In 1993, the Overland Park Public Works Department tackled the open drainage problem in one particularly flood-prone neighborhood just south of Kansas City. There, an unimproved channel wound through established backyards between double-cell, concrete box culverts under busy streets at both ends. Construction on complete drainage improvements in the area was expected to carry flood flows without event.
The project, however, was anything but straightforward. The 20-plus homeowners along the alignment quickly organized over concerns about construction noise, neighborhood safety, property damage and general inconvenience. And though they generally supported the project — having signed an Improvement District petition and agreeing to help with funding — skeptics remained.
Additionally, the technical complexities of the project, including a winding storm sewer alignment (with three vertical grade changes and 11 horizontal curves and requiring excavation within five feet of some homes), a narrow and rock-bottomed drainage channel, little or no staging area, limited site access and the need to maintain channel capacity during construction, posed numerous problems. For culvert placement, the four-foot-deep natural channel required excavation to 10 feet through solid limestone, and space constraints allowed channel widening to just 20 feet to accommodate the planned 18-foot-wide double boxes.
“Needless to say, nobody on the city engineering staff was excited about designing or constructing this project,” says Dan Miller, the city’s special projects engineer. “It had been on our books for 10 years, and we were still wary of the whole thing.” Indeed, a number of people deemed placing 1,134 feet of snaking culvert in 192 sections with little room to maneuver, all in three-plus months to get the sod cover on before winter, an insurmountable task.
The project did get underway, however, and immediately the weather began to add to the problems, with the area experiencing the wettest summer on record — 51 days of significant rainfall during the 136 days of construction. Yet, despite everything, the project succeeded — the only completed major city-administered project that summer.
Taking no chances, the city specified precast concrete at bidding, eliminating flood complications associated with cast-in-place work. Heavy rain, Miller reasoned, might wash out formwork or uncured concrete, blocking the channel and necessitating considerable re-work. Avoiding liability related to increased flooding as a result of city work was paramount, and there was no travel space for concrete trucks and no on-site storage for formwork lumber.
Precast construction presented its own problems but offered additional advantages beyond flood control. A solely precast design for example, eliminated the long process of cast-in-place detailing for the city, shifting the detailed design burden to the precast supplier.
Additionally, in a further break from custom, the city specified construction methods, requiring use of a crane to set in the concrete boxes. Miller wanted all bidders for the project to be prepared and up to the task.
AROUND AND AROUND
Following preliminary design and alignment (and before bid letting), the city involved Kansas City Concrete Pipe, Shawnee, Kan. Preparing for contractor bids, the pipe company refined the city’s design to accommodate its own production methods and equipment. Specifially, the company eliminated eight of 11 sharp directional changes made through hand-fabricated elbows, replacing them with gradual curves stretched over four or five prefabricated, beveled joints. Prefabrication of joints reduced costs, and the gradual curves improved the culvert’s flow characteristics. In the end, 23 right-hand and 33 left-hand bevels (at 2.2[degrees]) were manufactured, along with three 10.7[degrees] hand-fabricated elbows. Strict tolerances were maintained for both curved and straight sections.
Extreme care in manufacturing, however, was not enough. Even a small error in culvert placement around a curve could pull the line off significantly along straight sections. The proximity of yards, houses and trees precluded any such deviation.
So the city took prevention one step further. “We made it bulletproof,” Miller says. On the city’s shop drawings, the exact centerline and bottom elevation of each joint and section were overlaid. Later, the surveyor staked from the CADD drawings, setting up the contractor for continual alignment checks.
“There was no way we wanted to be two feet off at the end of a 200-foot straightaway,” Miller says. “There’d be no way to fix that at the next curve.”
The project’s excavator, Miles Excavating, questioned the common wisdom about dropping culverts into place from downstream, carrying them in by crane atop boxes already in place.
The fear, with which Miller agreed, was that in-place boxes would shift out of line under the back-and-forth weight of the crane. And crane rental was expensive.
The problem was solved using equipment the excavator already had. With a Caterpillar 330 track hoe, the alignment was accessed first from a midpoint cul-de-sac (phase one), then from the upstream street bounding the project (phase two), always carrying boxes downstream and working exclusively within the channel. In transport, the boxes hung from special hooks designed and fitted by the Caterpillar dealer. Before the start of each phase, the entire alignment for that phase was excavated to the cul-de-sac, using the track hoe to rip the channel bottom. Excavated soil and rock were hauled away.
Grade was checked at each spot with a rotating laser. Set atop the in-place line after rough grading by track hoe, the laser scanned the entire placement area for each box. Workers then hand-graded to get within one-quarter-inch of the grade specified.
With less equipment at the site, there were no trucks backing up alongside to haul away excavated dirt and rock. Also, excavation disturbances of noise, dust and vibration were confined timewise. Once culvert placement was underway, the excavator moved in and out of each yard in just a few days, undelayed by dig-as-you-go excavation.
In-place culvert sections were backfilled every few days then final-graded. Elimination of crane travel atop the culvert allowed inlets and weep boxes to be cast into the culverts, not field-cut or field-built. And the culverts — no longer expected to carry a loaded crane — were redesigned at a significantly lower weight.
At summer’s end, the culvert lay exactly on grade and just 20 inches beyond design length. Sod went down in October, closing the project on time and on budget with no change orders. And, although not all homeowners wanted the work done, all were pleased with both the process and results.
Additionally, nominated by the city for its part in the project, the excavation firm won 1993 Contractor of the Year honors from the American Public Works Association’s Kansas City Chapter. Second place followed at the national APWA competition for projects under $2 million.