Design and Installation of Large Diameter
Slipliner Pipe in Lakeland, Florida
Joseph E. Hickle, Jr., P.E.l
Kenneth L. Glasgow, ASCE affiliate2
The application of practical design criteria to determine the suitability of large diameter sewer pipe for rehabilitation by sliplining is presented in light of a real project installed in Lakeland, Florida during 1994. The project consisted of rehabilitating 6.71 km (22,000 ft) of gravity sanitary sewer mains ranging in size from 61 cm (24 in) to 122 cm (48 in) diameters. The determination of pipe rehabilitation techniques with consideration to flow monitoring and televised inspection is presented.
The appropriate pipe stiffness to resist long term hydrostatic buckling and short term grouting pressure is noted. The application of field controls to insure proper installation and acceptance testing is discussed. Practical cleaning processes, pit preparation, pushing techniques, and field fabrication techniques are reviewed. An innovative method of making closures is described. Methods of grouting a small annular space are described along with methods of building bulkheads, grout pressure control and grout delivery .
The City of Lakeland, Florida desired to perform a Sanitary Sewer Evaluation Study (SSES) of its Eastern Trunk Sewer in 1992. This program was intended to determine the conditions of the existing sewer and to recommend how the sewer system could be rehabilitated by currently available methods.
1 Project Engineer, Camp Dresser & McKee, 1715 Westshore Blvd, Suite 875, Tampa, FL 33607
2 Regional Manager, Lamson Vylon Pipe, 25701 Science Park Drive, Cleveland, OH 44122
The system comprised of approximately 83 manholes and 6.7 km (22,000 ft) of gravity sanitary sewers ranging from 61 cm (24 in) to 122 cm (48 in) in diameter. The majority of the sewers were reinforced concrete pipe (RCP) or vitrified clay pipe (VCP) which were installed approximately 50 years ago. Several system problems were recorded and the City chose to take a proactive approach to investigating the condition of its sewer system and planning for rehabilitation prior to system failures. The City retained Camp Dresser & McKee, Inc. as the engineer for the system evaluation and subsequent rehabilitation design project. Kimmins Contracting Corporation was the rehabilitation contractor for the project.
A program to observe rainfall induced flows in the sewer was conducted for 30 consecutive days. Rainfall was measured during this period to develop some correlation of the system response time to precipitation.
All manholes were entered and inspected for structural integrity and leaks. Each sewer run between manholes was televised and videotaped to record the actual condition of the sewer. Numerous problems were recorded that lead to design decisions of how certain sections of the sewer line should be considered for rehabilitation. Manholes were to be either lined with fiberglass inserts or coated with cementitious material by a spraying process.
Video inspection of the sewer revealed corrosion, offset or dropped joints, broken joints, and protruding laterals. Many of these pipe defects often call for point repairs. In addition to defect identification, service laterals were also noted for future reference. Proper consideration of existing pipe conditions and surface inspections enabled the contractor to locate the optimum position for insertion pit placement.
Hydraulic capacity analysis
An existing condition hydraulic analysis was performed to determine the flow capacity of the sewer. The methodology used was to determine flow characteristics according to the Manning Equation and to calculate the hydraulic grade line to investigate any sewer surcharging or partial flow for design flow rates planned by the City .Observed flows during a thirty day period were measured as well as precipitation to determine if any rainfall induced correlation could be established. Rainfall data only supported the belief that the sewers did receive infiltration and needed to be rehabilitated so that the City could reduce treatment volumes at its wastewater treatment facility.
Each run of existing sewer was noted for the type of pipe material, roughness, diameter, and slope between manholes. The Manning Equation was then calculated for each run of sewer to compute the hydraulic grade line for design flows planned in the system. This would determine if adequate flow capacity would exist after sewer rehabilitation.
A post rehabilitation capacity analysis was performed in the same manner as the existing condition analysis with the exception of modifications to pipe diameter and roughness. Generally, pipe diameter reduction with smoother slipliner materials retains adequate system capacity. Observed peak flows as well as planned design flows were induced on the rehabilitated sewer system and a resulting hydraulic grade line calculated. In areas where chemical grout sealing was proposed, the pipe diameter would remain the same, but the roughness factor would be similar to that of normal concrete with n=0.Ol3. Other segments proposed to be repaired by sliplining were estimated to be improved to a relatively smooth roughness factor between 0.009 to 0.011.
Rehabilitation methods consideration
The SSES considered the methods of chemical grouting leaking pipe joints, sewer line rehabilitation by a sliplining method, or complete pipe run replacement. Unit costs for sewer rehabilitation by one of the four conventional methods of cured-in-place, sliplining, spiral wound, or complete excavation and replacement were considered.
Excavation and replacement was rejected for further design consideration due to excessive costs and disruption to traffic and local residents. Cured-in-place technology presented the highest unit costs for sewer rehabilitation, but had to be accepted in areas of the sewer system where hydraulic capacities were marginal and the design team wanted to retain as much of the original pipe diameter as possible. In other areas of the sewer system, diameter reduction by sliplining rigid pipe into the host pipe was acceptable when hydraulic concerns were not as critical.
Project design and specification
Construction plan and profile drawings were prepared by the design engineer to control the installation of the slipliner process. The construction documents addressed where each type of sewer system rehab method should be employed to construct the project. The contractor chose the final locations of the insertion pits from suggested locations provided by the engineer. Occasionally, planned insertions pits would be abandoned due to conflicts observed underground once a pit had been started.
Various types of leak-free, corrosion resistant pipe materials were considered as suitable. slipliners, such as polyvinyl chloride (PVC), high density polyethylene (HDPE), fiberglass and cured-in-place liners. Sufficient pipe stiffness to withstand buckling pressures and acceptable flow capacities were considered prior to inclusion into the specifications.
Initial technical specifications for the project were assembled, in part, from similar slip lining projects, listing acceptable pipeline material types and construction requirements to accomplish a successful rehabilitation of the existing sewer system. A variety of sewer system rehabilitation methods were considered and included in the technical specifications for the project. Inclusion of segmented, closed-profile PVC
slipliner pipe and various manhole lining materials were late entries in the technical specifications. These materials were specified only after exhaustive investigations ensured that they were suitable for sanitary sewers and met long-term criteria established by the engineer .
Due to the critical nature of the existing sewer, the most stringent of these investigations involved the suitability of closed-profile PVC slipliner pipe. Prior to this project, the engineer was unaware of closed-profile PVC slipliner, although it had seen limited use in the southwestern United States. This PVC slipliner met, and in many areas, exceeded the required design criteria. The following are the criteria under which the slipliner pipe materials were reviewed and accepted.
Trenchless technology was chosen to rehabilitate the sewer system primarily due to cost factors and minimal impact to traffic, businesses and local residents. Sections of existing sewers planned to be sliplined were investigated for practical locations to construct insertion pits.
System rehabilitation would be accomplished with the use of cured-in-place pipe, pushing rigid slipliner pipe, and pulling flexible HDPE pipe where appropriate. Since cured-in-place pipe could conform to the walls of the host pipe, it would be used where a reduction in the diameter of the existing pipe needed to be minimized to preserve as much hydraulic capacity as possible.
In areas of the sewer system where hydraulic capacity was not critical, the next smaller diameter of slipliner pipe would be pushed into the host pipe from insertion pits. In some sewer runs where 7.62 cm (3 in) downsizing was desired, the existing pipe had settled and misaligned pipe joints would not allow the pushing of a rigid slipliner pipe. In these rare situations, the contractor would have to adapt his construction method to use the more flexible HDPE pipe that would be pulled from manhole to manhole rather than being pushed. This method would require that a bypass pumping system be established. Since using slipliner pipe would create an annular space, the annulus between the host and liner pipes needed to be grouted. Grouting pressures would be limited so that the slipliner pipe would not buckle during grouting of the annulus.
Contractor's material selection
The low bidder selected to use segmented, closed-profile PVC slipliner, as provided by Lamson Vylon Pipe, as the primary slipliner material for this project. Although the material cost of the PVC slipliner exceeded the material cost of the butt-fused HDPE, installed cost for the PVC slipliner was much less. Among the deciding factors involved in the contractor's cost analysis were:
Vylon Slipliner is a segmented, closed-profile PVC slipliner pipe meeting the requirements of ASTM F794. The joint consists of a gasketed fiberglass coupling which is designed to join the pipe by insertion into an internal channel machined into the ends of the pipe (see figure). The groove created by this routing process enables the joint to articulate 2° .The pipe's unique joint geometry creates a flush exterior and interior allowing for greater annular clearance than conventional joining methods.
The internal I-beam structure between the dual walls receives the axial loading transferred across the coupling and transmits the loading throughout the entire wall section. Epoxy fills the void between the first and second rib, thus, providing additional support and raising allowable jacking loads.
Prior to any construction activity, cleaning and video inspection were required. F or initial cleaning, a bucket machine (similar to a drag line) was employed to remove large build-ups of solids and debris. A final cleaning using a standard jetting nozzle and vacuum truck removed most residual solids.
Proper consideration of existing pipe conditions, necessary point repairs and surface geography enabled the contractor to locate the optimum position for insertion pit placement. Since the PVC slipliner could be pushed equally as well upstream or downstream, flow direction was not a consideration when selecting pit location.
Construction and preparation procedures remained similar throughout the entire project, regardless of pipe diameter. Due to shallow ground water conditions, a well pointing system was normally established around the proposed pit area. Typical length of an insertion pit was approximately 6 m (20 ft). The width of each pit varied by diameter, but was typically 2.4 m (8 ft). Excavation depth was ordinarily limited to the springline of the existing pipe (1/2 OD). A trench box and steel sheeting were placed and a concrete floor was poured at this point to stabilize the work area.
The crown of the existing pipe was removed by saw. cutting along the springline. Crown sections were stored intact to serve as a cover when construction procedures were interrupted. With the old sewer pipe interior exposed, a piece of PVC slipliner was lowered into a cradle formed by the invert of the host pipe. This test piece would be pulled through the existing pipe in both directions to ensure that the slip liner could fit through the entire section to be lined.
When the PVC slipliner was inserted, the gasketed coupling was oriented at the leading end. This enabled the bullnose of the rubber gasket to absorb any impact received on the leading edge during insertion. To distribute the jacking loads evenly throughout the slipliner, a push ring was supplied by the pipe manufacturer .
The sewers that were sliplined were typically flowing between 1/3 and 1/2 full, depending on the time of day and recent rainfall. Once the slip liner pipe was placed in the cradle and submerged, it was pushed into the host pipe. The slipliner pipe was pushed into place with the assistance of a backhoe (Caterpillar 225). The contractor found that placing the bucket on the push plate and crawling the backhoe forward or backward provided the most even and consistent jacking loads.
The trailing two feet of each slipliner joint was left extending into the pit to allow for joining the next piece of slipliner. To prevent the slipliner from moving while being joined, the contractor used air bags to anchor the slip liner pipe. These bags were placed in the annulus at the 10 o ' clock and 2 o' clock positions and inflated, thereby creating a wedge effect that anchored the slipliner during joining.
With this system, the contractor was able to achieve an installation rate of one 4.6 m (15 ft) joint every three minutes or 90 m/hour (300 ft/hr), even though the sewers were flowing as much as half-full during installation. Sliplining continued through manholes where the host pipe was properly aligned. The greatest jacking distance encountered on this project was approximately 610 m (2000 ft) and required only seven hours of actual installation time. A 45° elbow at a manhole prevented pushing any further .
When pushing slipliner in two directions from a single installation pit, a closure piece was needed to join the slipliner together. Lamson Vylon Pipe provided a field routing tool in order to create a factory-style joint on the jobsite. This portable router allowed the contractor to cut pipe to length in the field, create a new joint, and close the slipliner pipe at the pit without creating a hydraulic disturbance at the closure.
When a push was completed, bulkheads were built at each manhole interface. Bulkhead construction was completed first at the upstream end and continued downstream as the annulus drained. Since the annulus was never entirely dry due to groundwater infiltration, bulkheads were constructed of virgin oakum and chemical grout (3M #5600) below the spring line. This not only created an initial seal, but also aided in constructing a 20.3 cm (8 in) thick structural bulkhead with hydraulic cement. Two tubes were built into each bulkhead to allow for grout injection or venting. After constructing bulkheads, service laterals were excavated and reconnected. Prior to reconnection, waste from the service laterals had been allowed to flow into the annulus, thereby, not interrupting local sewer service.
A subcontractor performed annular grouting services. The slipliner pipe was grouted from manhole to manhole. In areas where a four inch annulus was present, the grout was pumped in multiple lifts in order to prevent floating the slipliner. In areas where a one inch annulus was present, a monolithic pour was performed. The annular grout consisted primarily of Type II Portland cement, pozzolanic fly ash, and water. To improve flowability, sand content was kept to a minimum. Grout pump pressures were normally limited to 34.5 kPa (5 psi); however, in areas where long grout runs or tight annular spaces were encountered, pump pressures in excess of68.9 kPa (10 psi) were necessary.
The grout was pumped into the annulus through the two tubes built into the bulkhead. As grout was pumped, annular sewage was monitored as it flowed out of the vent
tubes at the other end of the annulus. The annular sewage flowed through the vent tubes as it was being displaced by the heavier grout. As the volumes of grout approached the theoretical annular volume, pump speed was slowed, pressures were observed and the color of the displaced sewage was noted. When grout began venting from the annulus, pumping ceased. As a quality control measure, a post-grouting video was performed to ensure that all joints remained sealed and no buckling of the pipe wall had occurred during grouting.
The Eastern Trunk Sanitary Sewer of the City of Lakeland, Florida was successfully rehabilitated using cured-in-place pipe and PVC slipliner pipe as the primary trenchless technology methods. This rehabilitation has reduced infiltration and inflow to the wastewater treatment facility and has restored the structural integrity of the system for extended service life without major disruption to local businesses, traffic, schools, and local residents. This project is noteworthy since new sliplining materials were used with great success.