When storms and torrential rains hit Granbury, Texas, on May 10, 2015, residents were not expecting to find, as the rain subsided the following morning, a large sinkhole in the lakeside parking lot outside Brookshire’s supermarket.
Measuring more than 90 ft wide and 30 ft deep, what initially was thought to be a sinkhole turned out to be collapsed earth under a section of the parking lot, resulting from the ground becoming saturated. This happened after an old 7-ft-diameter storm drain collapsed following several inches of rain falling in a short amount of time, and was accompanied by a landslip as the ground was washed away down the adjacent hillside.
The collapse of the buried storm culvert was due to water building up and then leaking, washing away the earth supporting the city’s No. 4 wastewater and effluent lift station and creating the large crater. Early estimates put the potential repair cost to the city at approximately $500,000.
Such was the damage to the underground wastewater pipeline and effluent lift station, coupled with the potential danger to the public, that Granbury City Council declared a local disaster. Its immediate action was to seal off the area and bring in specialist engineering crews to stop water flowing from the broken pipe into the nearby Lake Granbury. The swift action in rerouting the water flow away from the area, using temporary pumps and pipeline, enabled engineers to start shoring up and stabilizing the damage the next day.
Having successfully eliminated the possibility of wastewater and effluent flowing into Lake Granbury, the pressing concern was to assess the damage and formulate a program to replace the damaged lift station. Clearly, a new one would not only have to stand up to future floods and storms, but also employ pumping technology that would provide the best solution for handling wastewater and effluent in a safe and environmentally acceptable manner.
With this being a specialized area of environmental engineering, Granbury City Council called on the resources of Abilene, Texas-based Enprotec/Hibbs & Todd Inc. (eHT), a firm it had worked with on several projects over a number of years, and Pump Solutions of Dallas/Fort Worth.
“This particular area within the city of Granbury is not prone to flooding and this disaster only came about because the old, deep underground drain failed,” said Scott D. Hay, vice president of eHT. “When our local office was called in and visited the site, it became clear to us that this was not really a sinkhole.
“Looking at the deed records for the area when it was developed in the 1960s, the natural surface water flow was into the nearby Lake Granbury. The drainage system constructed at the time [included] an 84-in. corrugated metal drainpipe buried around 50 ft deep, and over the years this had been added to and modified. It served its purpose very well, but when we undertook a close inspection, there was corrosion in certain areas and many joints had opened up. Thus, when the storm surge hit and water built up, the water escaped through these weak spots. The disaster was compounded by the collapse of the No. 4 wastewater and effluent lift station. This had been built, unknowingly, over the top of the drain some 20 ft or more below, so when the water leaked out it turned the ground fill into quicksand, resulting in the lift station collapsing.”
The original lift station was a duplex unit containing pumps sized to handle flows prevailing at the time of its construction in the 1970s. “The city took the lead in stopping up all the water and sewage line and bringing in a bypass pump at the start of the emergency. It was a rapid and successful response on behalf of the city. Our Granbury city office was then called to the site to get a handle on the environmental issues and stabilize the situation for the long term,” Hay said.
Rebuilding the Pump Station
When eHT commenced work on the site, more information about the original lift station emerged. The installation was pumping into a 12-in. force main and the pumps did not come close to providing adequate velocity to maintain a flow that would scour and clean the main to prevent it from being plugged by waste and solids. The prime concern was related to the integrity of the force main, particularly in respect to whether it would it be compromised in its ability to perform a full 12-in. flow. As a result, a decision was made to install new pumps capable of delivering an increased flow rate at new head conditions.
Although the temporary pumping system was operating effectively, Granbury City Council recognized the urgency in getting a permanent lift pump station up and running. Having settled on a course of action, eHT was given a brief that would require the company to deliver a fully operational lift station within three weeks
“I saw things happening that took place in time frames that I had never experienced before,” Hay said. “A major part of this was Pump Solutions’ ability to deliver two KSB KRT-F submersible pumps, along with the wet well made by U.S. Composite Pipe, in a very short space of time. Having previous experience with both companies, it was an excellent choice to work with them on this project. U.S. Composite Pipe offers a steel reinforced, polymer aggregate mix wet well that gives the robustness of concrete without it having to be coated for additional corrosion resistance. While fiberglass wet wells are popular because of price, they are not as robust as concrete wet wells.”
Pump Solutions worked closely with KSB U.S.A. to identify the right pumps and impellers for the job. The preferred impeller design was a vortex impeller specified by Hay. In his opinion, the vortex was the best type for the application, even though there had never been a problem with grit, solids or ragging. This being such a high profile project, the team wanted to put in the best possible option for current and future demands on the system. In fact, such was the speed of the project that no actual specification was developed. The sole objective was to get the pump station up and running within the city’s exacting time frame.
Pump Solutions and eHT had a well-established working relationship on wastewater and water supply projects. “When we got the call from eHT, we visited the site to see what was required and asked to come up with a plan,” said Charles Norman of Pump Solutions. “Because of the time frame, we had a lot of hoops to jump through. Our solution was based on the existing conditions and the technical brief given in respect to flow, hydraulic duty points and head, and it was our job to deliver a lift station complete with pumps, precast wet well with access cover and the necessary control panels for the station.” The pumps that were finally selected were two 24-hp KSB KRT-F models running at 1,160 rpm, with a design point of 975 gal per minute at 30 ft total dynamic head for a velocity of 3 ft per second and fitted with 10.58-in.-diameter vortex impellers.
Sourcing Supplies
As a distributor of KSB pumps, Pump Solutions has ready access to the pump manufacturer’s stock of pumps and components, as well as the capability to meet most requirements from its own stock-holding resources in Texas. However, the job required two pump sets, which were non-standard stock items. Pump Solutions approached the manufacturer, but the best lead time it could offer for new pumps to meet these specifications was 12 weeks.
“The city needed the entire pump station solution delivered in just three weeks,” Norman said. “Having to source the vortex impeller pumps, wet well, piping, wet well access hatch and lift station control panels was a very tall order. The advantage of working with KSB is that they have wet well pumps where the impellers can be swapped out. Their inventory revealed two K-type multi-vane impeller pumps in their warehouse, which we could swap to F-type vortex impellers. What’s more, the hydraulics and motors were a good fit for the job, so we had a potential solution.”
There was, however, another challenge. The vortex impellers in stock were the wrong dimensions, so Norman called other distributors around the U.S. to see if they had impellers in stock. Unfortunately, he drew a blank, but KSB’s application engineers suggested using one of the company’s European casting suppliers to manufacture two new impellers and have them air-freighted to the KSB plant in Henrico, Va., for pump assembly and testing. The castings were made and air-freighted from Europe in two weeks, giving KSB one week to do the assembly and testing.
There also was the matter of sourcing the wet well access hatch and control panels. Access hatches must be dimensioned for the actual wet well, so they must be purpose-made, a process that can take four weeks or longer. Pump Solutions’ regular hatch supplier, U.S. Fabrications in Florida, had a unit of the right dimensions left over from another project. This was shipped to the precasting plant that was making the wet well and cast in place.
The final element was getting the control panels manufactured by Quality Controls & Integration of New Prague, Minn., and supplied to the site, a process that normally takes six to eight weeks. “We wanted to give Granbury true state-of-the-art controls for the lift station,” Norman said. “Our supplier knew exactly what we wanted, designing a touchscreen panel that enables easy and rapid data acquisition. Linked to a submersible transducer in the wet well, this monitors the water level in the wet well, raises alarms to indicate any changes taking place, monitors pump performance and delivers flow trends flow charts so that city engineers can see what is going on in real time and historically.”
Teamwork was the key factor in Pump Solutions’ ability to complete the project in the city’s short timeframe. “Through our branches in Dallas, Austin and Houston, [Texas,] we have established a supplier base that sees great benefits in collaboration,” Norman said. “A collective effort by all parties is the only way we achieved success. As a distributor we can only satisfy customers’ needs with the help of others around us. It starts with the system being designed by quality, experienced engineers. And then it’s up to our manufacturer to produce a product within the required time frame. Without all parties coming together as a team, we fail.”
The stability of the new lift station had to be given major consideration, and for this reason it was not built on the original site. A decision was made to move the lift station farther away from the area where the land collapsed and also away from the underground drain. New ground works had to be put in place, with some rerouting of the pipe and associated infrastructure.
This disaster has had a positive outcome for the city of Granbury in that it now has a robust and efficient lift station that has increased pumping capacity to accommodate future growth, and ensures the safe transfer of wastewater and effluent away from the Brookshire’s parking lot and Lake Granbury.
Download: Here
