Challenges in Large Diameter Water Pipelines

Proceedings CIGMAT-2005 Conference & Exhibition Challenges in Large Diameter Water Pipelines David H Marshall, P.E., Engineering Services Director Tarrant Regional Water District, Forthworth, Texas 76196 E-mail: DMarshall@trwd.com The Tarrant regional Water District is a raw water supplier in North Central Texas, serving communities in ten counties, including Fort Worth and surrounding communities The District has twenty-six municipal customers who supply treated water to about 1.5 million people Water use in Tarrant County is about 92% of the District’s total use The District’s supply, shown on the map below, consists of four supply reservoirs and two pipelines The two western reservoirs, Bridgeport and Eagle Mountain hold about twenty percent of the total supply, and release water down the Trinity River to Tarrant County Water from Cedar Creek is pumped using a 72-inch diameter, sixty-eight mile long pipeline and an 84inch diameter six-mile long pipeline to Tarrant County Water is pumped from Richland Chambers in a 90-inch diameter seventy-two mile long pipeline and a 108-inch diameter sixmile long pipeline to Tarrant County Since Cedar Creek and Richland Chambers Reservoirs are eighty percent of the water supply, the reliability of the pipelines is essential The Cedar Creek Pipeline was constructed in 1972 The pipeline has suffered nine failures, the first in 1981 Eight of the nine failures have been due to corrosion of the prestressing wires One failure has been due to hydrogen embrittlement damage All have been catastrophic failures, with a large hole in the pipe being formed and the loss of millions of gallons of water One area of the Cedar Creek Pipeline was under an impressed current cathodic protection system, with the on-current voltage set at 1.2 volts The hydrogen embrittlement segment that failed was connected to the rectifier The Richland Pipeline was constructed in 1988 The Richland pipeline has suffered thirteen failures: four thrust restraint failures, six corrosion failures, one due to hydrogen embrittlement, one from a spigot cracking, and one from operator error All but the thrust restrain and spigot cracking was catastrophic failures Richland Chambers had no cathodic protection, and the embrittled segment was probably due to the wire characteristics being influenced by light surface corrosion of the wire The District recognized in 1989 that the pipe integrity was lacking and began to study and mitigate the problems The entire system was cathodically protected using sacrificial zinc anodes to minimize further embrittlement damage Water hammer was identified as a possible cause of cracking the outer mortar and mitigated by using programmable logic I-1 Proceedings CIGMAT-2005 Conference & Exhibition controllers on the pump control valves to prevent premature closing The pipelines were also internally inspected and obvious damaged segments repaired or replaced Beginning in 1998, the District employed the Pressure Pipe Inspection Company (PPIC) to inspect the pipeline using the remote field eddy current/coupled transformer system About 20 miles have been inspected annually and in January 2005 the inspection will cover the last twenty miles of the system Results of the PPIC inspections revealed that of the 32232 segments inspected prior to January 2005, 800 segments have wire breaks The Cedar Creek Pipeline has about four percent of the segments damaged, a total of 718 segments The inspection of the Cedar Creek pipeline near the rectifier location revealed about nine percent of the pipe was damaged, likely due to embrittlement The Richland line has about 0.5 percent of the segments damaged, a total of 82 segments Of those 800 damaged segments, about 70 have been replaced or repaired Repairs were done the follow winter of the inspecting, generally choosing the pipe with 50 or more broken wires The first repairs of the embrittled area revealed that although the pipe may be damaged with 100 broken wires, the force required to remove the pipe showed the pipe had a lot of residual strength Embrittled pipe was then replaced when over 125 broken wires were detected in subsequent work During the initial investigation of the pipeline integrity problems, the District contacted other agencies that were facing similar issues A number of agencies jointly funded a study by Simpson Gumpertz & Heger (SGH) to develop a simplified finite element analysis to determine residual strength of damaged pipe The District employed SGH to examine the pipe designs specific to the District, develop and calibrate the model for embrittlement damage and determine the residual strength and repair priorities of the damaged pipe Yehuda Kleiner, of the National Research Council of Canada, through an AWWARF study, introduced a new approach to modeling the deterioration of buried pipes, using a fuzzy rulebased, non-homogeneous Markov process This deterioration model yields the possibility (as opposed to probability) of failure at every point along the life of the pipe Kleiner expanded this approach by expressing the possibility of failure, as a fuzzy number, and then coupled it with the failure consequence (also expressed as a fuzzy number) to obtain the failure risk as a function of pipe age The District participated in this study and will build on the model Based on the strength model and inspection result, there are several hundred segments of pipe that have enough damage to warrant concern The cost to repair or replace a single segment varies from about $35,000 to $75,000 Using $40,000 as n average cost, 300 pipe segments would cost $12,000,000 Through analyses to reduce the uncertainty in some parameters (inspection error, water hammer pressures, rate of deterioration as it becomes available) and using the fuzzy logic modeling, a logical prioritization and timing of replacement of the damaged pipe segments may be developed to keep risk to a minimum and efficiently replacing pipe when needed Weld after Backfill Sequence for Use with Steel Water Pipelines Dennis Dechant, P.E., Chief Corporate Engineer Northwest Pipe Company, Saginaw, Texas 76179 I-2 Proceedings CIGMAT-2005 Conference & Exhibition The Weld After Backfill sequence is a procedure developed and tested by the steel pipe industry to eliminate costly standby time for equipment on large diameter water pipelines The procedure allows pipe to be placed, coating applied, and backfilled so that construction can proceed Welding on the pipeline can then be done on the inside of the pipe as pipelaying continues thus saving the equipment and crews numerous hours of waiting for the weld to be completed This paper will present the testing done to develop this procedure and present numerous applications where this procedure has been utilized with great success The main obstacle to implementing the Weld after Backfill sequense was the potential damage to the pipe coating that might occur due to the high temperatures that develop during the welding process Deep Foundations for Transportation Facilities Mark McClelland, P.E., Geotechnical Branch Manager Texas Department of Transportation, Austin, TX 78701 E-mail: mmcclell@dot.state.tx.us I-3 Proceedings CIGMAT-2005 Conference & Exhibition The Texas Department of Transportation (TxDOT) is a major user of deep foundations to support transportation facilities, namely bridges In the past 12 months, approximately 850,000 linear feet (160 miles) of piling and drilled shaft have been let to contract TxDOT bridges are currently founded exclusively on deep foundations Spread footing foundations were used occasionally into the 1970’s to support bridges in competent soil or rock, but the increasing economy of drilled shafts resulted in an end to the use of spread footings With current emphasis on scour vulnerability of bridges, we are particularly fortunate to have the vast majority of our bridges over waterways on either piling or drilled shafts States with significant inventories of structures on shallow foundations are now spending considerable time and resources to manage scour at those bridges Driven piling constitute approximately 30% (250,000 lf) of the deep foundations let to contract in the past year Over 99% of the piling driven on TxDOT projects are prestressed concrete piling, with only a few hundred feet of steel H-piling driven each year Concrete piling are more effective than steel H-piling in our softer soils because they are displacement piling, which seem to develop more reliable skin friction than non-displacement piling Texas has an extremely well developed and competitive precast concrete industry, and prestressed concrete piling are very cost effective in soft soils, with in-place prices running $25 to $40 per linear foot Concrete piling are used extensively along the Gulf Coast and to a lesser extent in northeast Texas Common prestressed pile sizes are 16, 18 and 20 inch square For larger structures 24 and even 30 inch square piling have been utilized For small structures, piling are often driven in a trestle bent configuration In trestle bents the piling function as both foundation element and the bridge column, and are embedded directly into the cap supporting the bridge beams This provides rapid and cost effective construction, as the need to form and place a footing and separate column are eliminated This configuration also works well in standing water where placement of footings underwater requires construction of an expensive cofferdam Trestle pile bents are limited to structures with relatively low heights (2000 m) cables without appreciable degradation of the signal caused by variations in cable resistance which can arise from water penetration, temperature fluctuations, contact resistance, or leakage to ground This factor, coupled with the elegance and ruggedness of Geokon designs results in sensors which exhibit excellent long-term I-10 Proceedings CIGMAT-2005 Conference & Exhibition stability and which are ideally suited for long-term measurements in adverse environments Instruments manufactured by Geokon are used primarily for monitoring the safety and stability of civil and mining structures such as dams, tunnels, mine openings, foundations, piles, embankments, retaining walls, slopes, subway systems, underground powerhouses, bridges, culverts, pipelines, shafts, slurry wall excavations, braced excavations, tiebacks, nuclear waste repositories, ground water remediation schemes and the like Geokon manufactures a complete line of geotechnical instruments including extensometers, piezometers, strain gages, crackmeters, jointmeters, load cells, settlement sensors, pressure cells, inclinometers, dataloggers and many other custom items made to order Geokon is committed to providing its customers with out-standing products and services that meet or exceed quality expectations As a result, Geokon has been awarded ISO 9001:2000 registration from both ANSI-RAB, USA and UKAS of Great Britain Subsurface Utility Engineering Peter Borsack, P.E Cobb, Fendley & Associates, Inc., Houston, TX 77040 Subsurface Utility Engineering (SUE) is the practice of locating and mapping of underground utilities, surveying the information obtained and producing a drawing that is representational of what utilities exist underground Utilities that are buried underground can be the most critical element during the design and construction process If these utilities are bypassed when the design and construction planning are underway, cost estimates and schedules will often have to be adjusted well after the conceptual phases of project management have come and gone Because utilities are frequently added, relocated or removed it has proven very difficult for these owners to have an accurate record of them The benefit of having SUE information includes giving the excavator additional resources to keep the project successful, minimize cost by preventing damage to the utility and maintaining a good relationship with the public by minimizing utility service disruptions Projects which might benefit from SUE are I-11 Proceedings CIGMAT-2005 Conference & Exhibition ones that include utility maintenance, new utility installation, demolition of existing structures or construction of new structures, new roadway pavement, pavement improvements and traffic control device installation SUE can be used by a client, such as an engineering firm when the information sought may provide key details required to make correct and effective decisions during the design phase of the project and not at the point when the contractor has already mobilized and begun work The unseen information can help avoid utility conflicts, reduce change orders in the field and help the contractor maintain the critical path and stay within project budget Subsurface utility engineering as defined by the ASCE is a branch of engineering practice that involves managing certain risks associated with utility mapping at appropriate quality levels, utility coordination, utility relocation design and coordination, utility condition assessment, communication of utility data to concerned parties, utility relocation cost estimates, implementation of utility accommodation policies and utility design SUE incorporates traditional engineering practices such as utility records research, relocation cost estimates, utility relocation design and plotting utilities from records Two terms must draw distinction when discussing elements of SUE deliverables Designation is the science of searching for a utility via subsurface geophysical means to gain information as to the alignment and depth of the utility Location is the actual exposing of the utility for survey so that precise geophysical coordinates and elevations are acquired based on a known survey datum These distinctions are clearly defined in the different quality levels of utility identification that are delivered to the client Four Quality Level attributes are attached to plotted utilities indicating how the utility data is developed Utilities identified as Quality Level D are plotted based on available utility records, verbal testimony and indications made from field visits Utilities identified as Quality Level C have been identified based on surface appurtenances, such as valves, manholes, vent pipes and then the Level D information is correlated with these surveyed surface features Quality Levels C and D are still considered incomplete when the utility needs to be “located” Quality Level B includes the act of “designating” utilities based on geophysical instrumentation Finally, “locating” a utility by Quality Level A SUE involves non-destructive excavation by means of airvacuum or water This excavation creates a small test hole in which each utility can be identified in size, material, depth or cover and location Quality Level A SUE is the most accurate and also the most expensive method So the engineer or even the contractor with his conceptual estimate and schedule may begin to determine if there is enough money to mitigate the risk of unknown underground utilities Thus, a decision must be made as to which utilities should be designated and which ones should be located Level B SUE designation incorporates the science of transmitting a signal to a utility using conduction (metal to metal) or induction and receiving a return signal from that I-12 Proceedings CIGMAT-2005 Conference & Exhibition utility However, it is not just a matter of designating a utility by means of signal transmission Correct designating takes into account the physical characteristics of the utility system which impact the accuracy and knowing which field conditions can be overcome and which ones can cause impedance Successful designating also takes into account the metallic continuity of various types of utilities, that is, knowing what to hook on to and what not to hook on to Factors which affect designating are types of soil, moisture content of the soil, proximity of other utilities, depth of target utility and presence of other metallic objects, such as rails, guy wire anchors and even chained link fences The electromagnetic spectrum encompasses a wide range of frequencies (30 kHz to 300,000 MHz) SUE uses a small window in this spectrum to locate utilities but can span a range from as low as kHz to as much as 500 kHz However, the higher the frequency the higher the risk in finding the utility that is being sought The higher frequencies (considered radio frequencies) should be confined to situations where lower frequencies not work and where there is a low chance for energizing nearby conductors that will distort the signal and misinform the ‘locator’ Lower ranges of frequency, the audio frequencies, are the most successful frequencies in designating Characteristics of this frequency include the following: resistance to energizing stubs, dead-ends and poorly grounded utility laterals This range is useful in identifying utilities that are in congested areas because of the ability to limit the effects of in-ground induction For conductive designation, some utilities are installed in the ground with tracer wires that provide the most direct circuit The transmitter’s signal can not follow any other path other than the feeder cable itself, which is adjacent to the utility This minimizes conductance from other sources and is extremely helpful when the locator needs to record a depth Other utility material factors that effect designation are conductivity of material, size of material, continuity, grounding, splices, joints or transitions and insulation Inductive conduction is made transmitting a signal without metal to metal contact with the utility by placing the transmitter directly on the ground or by using inductive clamps An inductive signal can travel along a metallic conductor The transmitter must be directed properly over the utility to induce a signal onto the buried utility What the locator is trying to accomplish in either method is a complete circuit between the transmitter (source), the utility (the medium) and the grounding device (closed door) by introducing a current through the earth (another medium) The earth and the utility have enough capacitance to transfer the energy through the complete circuit Frequency selection is important in order that only the capacitance being targeted is from the utility being designated Designating requires field experience and resource utilization Quality Level A Utility Location is the most accurate means of mapping a utility but it requires the utilization of the other three levels in order to pick the right test hole I-13 Proceedings CIGMAT-2005 Conference & Exhibition location It can be used in all kinds of soil, and can even be used under concrete If Levels B, C and D are used correctly, one will have a utility to map once the earth or pavement is temporarily removed and the utility is exposed I-14 ... 144 inches in diameter have been used in special circumstances The most common configuration for drilled shafts is the use of single 30 or 36 inch diameter shafts directly supporting matching diameter. .. column height increases, drilled shafts can simply be increased in diameter and reinforcing to provide the necessary structural capacity Single shaft/column elements of up to 96 inches in diameter. .. structures, piling are driven in groups and capped by a structural footing A separate reinforced concrete column is extended up from the footing and into the cap Most pile footings contain 3, or piling,