As storms and torrential rain hit Granbury, Texas back on 10 May in 2015, what residents were not expecting to find as the rain subsided the following morning was a large sinkhole in the lakeside parking lot outside Brookshire’s supermarket.
The 90 ft wide and 30 ft deep hole was just a short distance from Brookshire’s store.
Measuring over 90 ft wide and 30 ft deep, what was initially thought to be a sinkhole turned out to be collapsed earth under a section the parking lot, resulting from the ground becoming saturated. This was caused after an old 7ft diameter storm drain had collapsed following several inches of rain falling in a very short space of time. This 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, literally washing away the earth supporting the City’s No.4 wastewater/effluent lift station and creating the large crater. Early estimates put the potential repair cost to the City in the region of US $500,000.
Such was the damage to the underground wastewater pipelines and effluent lift station, coupled with the potential danger to the public, Granbury City Council declared it a local disaster. Their immediate action was to seal-off the area and bring in specialist engineering crews to stop water flowing from the broken pipes into the nearby Lake Granbury. The swift action in rerouting the water flow away from the area, using temporary pumps and pipelines, enabled engineers to start shoring up and stabilizing the damage on the Monday.
Having successfully eliminated the possibility of wastewater and effluent flowing into Lake Granbury, the pressing concern was to assess the damage and formulate a programme to replace the damaged lift station. Clearly a new one would not only have to stand up to future floods and storms, but 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 highly specialised area of environmental engineering, Granbury City Council called on the resources of Abilene-based Enprotec/Hibbs & Todd, Inc. (eHT), with whom it had worked on several projects over a number of years, and Pump Solutions, Dallas/Fort Worth. Scott D. Hay, eHT Vice-President takes up the story. “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,” relates Hay. “When our local office was called in and visited the site, it became clear to us that this was not really a sinkhole.”
Hay continues: “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 comprised an 84 inch 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 this revealed that 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/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 in to quick sand, 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 lines and bringing in a by-pass 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,” says Hay. “
Rebuilding the pump station
It was only when eHT commenced working on the site that more information about the original lift station was forthcoming. A major issue was that the installation was pumping into a 12 inch force main and that the pumps did not come even 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 related to the integrity of the force main, particularly in respect as to whether it would it be compromised in its ability to perform a full 12 inch flow. As a result a decision was made to address this issue by installing new pumps capable of delivering an increased flow rate at new head conditions.
Although the temporary pumping system was operating effectively, Granbury City Council recognised 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 deliver a fully operational lift station within a three-week time frame.
KSB Amarex KRT-F submersible pump employs a vortex impeller.
“I saw things happening that took place in time frames that I had never experienced before,” continues Hay. “A major part of this was Pumps Solutions’ ability to deliver two KSB KRT-F submersible pumps, along with the wet well made by US 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. US 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. Whilst fibreglass wet wells are popular because of price, they are not as robust as concrete wet wells. Pump Solutions delivered the wet well structure and pumps in a remarkably short space of time.”
Pump Solutions worked closely with KSB USA to identify the right pumps and types of impellers for the job. The preferred design of impeller 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. Being such a high profile project Pumps Solutions and eHT 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 no actual specification was developed. The sole objective was to get the pump station up and running within the City’s exacting time frame.
The replacement lift station in full working order.
Pump Solutions was the company that Scott Hay turned to as both companies 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,” explains Charles Norman, Pumps 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, pre-cast 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 1160 rpm, with a design point of 975 US gpm at 30 ft TDH for a velocity of 3 ft/sec. and fitted with 1058 inch (270 mm) diameter vortex impellers.
As a major distributor of KSB’s pumps, Pump Solutions has ready access to the pump manufacturer’s large stocks of pumps and components in the USA. Also Pump Solutions has the capability to meet most requirements from its own stock-holding resources in Texas. However, the job required two pumpsets, which were non-standard stock items. Coupled with the hydraulic requirements and the specification of vortex impellers, Pump Solutions approached KSB USA, but the best lead time that they 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,” explains Norman. “Having to source the vortex impeller pumps, wet well, piping, wet well access hatch and lift station control panels was a very tall order. Normally a lift station of this size and specification would take up to 12-14 weeks to complete. 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 of the wrong dimensions, so Charles Norman had to start calling other KSB distributors around the USA to see if any of them had impellers in stock. Unfortunately, he drew a blank, but another solution was proposed by KSB’s application engineers. This was to use one of their European castings suppliers to manufacturer two new impellers and have them air-freighted to KSB’s plant in Henrico VA for pump assembling and testing. The castings were made and air-freighted from Europe in two, weeks giving KSB one week to do the assembly and testing.
Having solved this challenge, there was also the matter of sourcing the wet well access hatch and the control panels. Access hatches must be dimensioned for the actual wet well, so have to be purpose made, a process that can take up to at least four weeks. Luckily for Pump Solutions, its regular hatch supplier, US Fabrications in Florida, had unit one of the right dimensions left over from another project. This was shipped to the pre-casting plant making the wet well and cast in place.
The final element was getting the control panels manufactured by Quality Controls & Integration in New Prague, Minnesota and supplied to site, a process that normally takes 6-8 weeks. “We wanted to give Granbury true state-of-the-art controls for the lift station,” says Norman.” Our supplier knew exactly what we wanted, designing a touch screen 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.”
So how was all this achieved by Pumps Solutions? “Through our branches in Dallas, Austin and Houston, we have established a supplier base that sees great benefits in collaboration,” says Norman. 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 and 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.”
Summary
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 further away from the area were the land collapsed and also away from the underground drain. New ground works had to be put in place, with some rerouting of the pipes 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.
