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Trenchless Technology

Introduction

In modern U.S. cities, piping services are complex and marvelous. Butaverage city dwellers don't know about buried pipes, could care less,and simply take them for granted. They cannot contemplate the con-sequences if these services were to be disrupted. Cities can improveonly to the extent that city service systems are improved.Improvement is slow because buried pipes are out of sight and, there-fore, out of mind to planners and to sources of funding. Without main-tenance of the piping systems, cities can only deteriorate. Withoutwell-maintained cities, the quality of human life deteriorates.

Pipeline engineers and city managers are sobered by the present-day reality of deteriorating pipe systems. Leaks in buried (out of sight)sewer pipes are either overloading treatment plants or are chargingthe soil and groundwater with contamination. The first thought is toreplace the pipes. But many sewers have served so long that they areovergrown with streets and buildings. Excavation and replacementbecome an unattractive remedy

Among the alternatives to replacement by excavation are trenchlesstechnologies. Small-diameter gas lines are being jetted into place.Large-diameter traffic tubes and tunnels are being bored into placeand lined. Moles and directional drilling are evolving with remarkablesuccess.

Might something be done to rehabilitate existing pipelines? In fact,many sewer lines could handle increased sewage loads (1) if ground-water infiltration were eliminated by stopping leaks and (2) if flowrates could be increased by smooth-lining the pipes. Plastic pipeinserts are successful and attractive. They can be inverted, folded, orswaged; then inserted, inflated, and heated to thermoset the plastic.Leakage is stopped. Plastic inserts provide resistance to corrosion andto abrasion of sediment flushed along the pipe. Plastic inserts evencontribute significantly to the structural integrity of the conduit.

But plastic has lower strength and lower stiffness than do most ofthe older, traditional materials. So how do flexible plastic pipes holdup under external water pressure? If leaks are stopped by inserting aplastic liner into a deteriorated sewer pipe (casing), groundwater nolonger drains into the sewer pipe and the water table rises. Still thecasing leaks, so external water pressure must be resisted by the liner.The conditions exist for buckling of the liner if external pressure isincreased. A typical scenario for failure is the following.

The empty liner floats up, leaving a gap on the bottom where theexternal pressure (head h) is greatest. The liner is flattened a bit on

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the bottom because the perimeter shrinks under pressure.Consequently, the radius of curvature is increased. Both the increasedradius of curvature and the loss of support, at the point where pres-sure is greatest, are the conditions for buckling of the liner. If pressureis increased, the liner will buckle. Because of plastic creep over thelong term, the perimeter shrinks even more over time and the condi-tions for buckling worsen. What is the time to failure? What is thedecrease in failure pressure in 50 years—or 100 years?

Tests at Utah State University (USU) have given some answers tothese questions. Failure was defined as the maximum pressure whenthe liner is just on the verge of buckling. Buckling is the reversal ofcurvature. It is the result of instability and might be initiated by aslight glitch (holiday) in the material of the liner, by a slight deviationof the shape, or over a period of time.

Data from the report "Long Term External Hydrostatic PressureTesting of Encased Insitupipes" show that long-term failure pressure isabout one-half the short-term (quick-load) failure pressure. The ratio V2of long-term to short-term failure pressures applied to all Insitupipestested with approximately the same D/t ratio. With ample safety factor,long-term design can be based on the half-ratio rule of thumb.

Except for an allowance for long-term plastic creep, the structuralperformance and performance limits of plastic pipes are based on thesame generic properties required of all flexible pipes, including metals,composites, etc. Of course, pipe performance must not exceed perfor-mance limits. We refer to performance limits rather than failurebecause failure implies rupture or complete collapse. Performance lim-its usually fall short of failure. Performance limit is usually defined asexcessive deformation of the pipe. Deformation includes rupture, buck-ling, ring deflection, puncturing, denting, etc.

Design of pipe liners

For a pipe liner in a casing, internal pressure is usually of no concern.Even if the liner inflates, it is confined by the casing as an innertubein a tire. External pressure on the liner causes ring compression stressof a = P(OD)/(2£), where P is the external pressure, OD is the outsidediameter, and t is the wall thickness. The ring compression stressmust be less than the yield strength of the pipe wall. If steel has ayield strength 8 times as great as that of PVC, then the PVC pipe lin-er wall must be 8 times as thick as the equivalent steel pipe liner wall.This can be demonstrated by a section of pipe placed in shaped blocksand loaded to crushing of the pipe wall.

Buckling of the pipe liner wall is more complicated. It depends uponboth the yield strength of the pipe wall and the pipe stiffness. But pipe

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stiffness depends upon wall thickness, modulus of elasticity E, shape,and degree of confinement by the casing. Because there are so manyinteractions, wall buckling of the liner is best found by experience,either from tests or from performances and failures in service.

Bases for evaluation of liners

Structural evaluation of liners must include wall strength to resistring compression and pipe stiffness to resist buckling. Both rigid lin-ers (such as mortar liners) and flexible liners (such as plastics) mustmeet the same requirements. Both must be designed for long-term ser-vice. Long-term service includes deterioration, corrosion, abrasion,and creep in the case of plastic liners. Long-term service for creepmeans long-term, persistent pressure. Design by ring compression isbased on long-term strength. If pressure is only instantaneous, ringcompression design is based on short-term strength. Short-term exter-nal pressure may be caused by a sudden vacuum inside the liner. Itshould be emphasized that testing is important in order to evaluatethe performance and performance limits of liners.

Liners in broken casings

The question arises, Do liners reestablish any of the original strengthof broken casings? Often the soil backfill retains the casing which con-tinues to perform as a conduit. Of course, if horizontal soil supportwere lost, the pipe would collapse. Collapse could occur if sidefill soilwere fine enough to be washed into the pipe through the cracks, leav-ing voids on the sides of the casing. But what if the ring deflection ofthe casing were to increase, say, due to increased surface loads or dueto partial loss of horizontal side support? The results of tests per-formed at Utah State University show that for a typical Insitupipeinstallation in prebroken pipes, the vertical soil load at any given pipedeflection is roughly 1.5 times greater for the casing with theInsituform lining than for the casing with no lining. This is a signifi-cant increase in strength in the event that ring deflection increases. Ifthere is no increase in ring deflection, at least the margin of safety isincreased by roughly one-half.

Design specifications for plastic inserts should be based on provenperformance—a track record. In general, design specifications areeither procedural or performance. Procedural specifications spell outthe details of manufacture and installation. Performance specifica-tions describe the required performance. "Turnkey" projects are typi-cal of performance specifications. Details on how to do it are left to theengineer and manufacturer. After the project is completed, the owneronly has to turn the key and operate it with assurance of adequate per-

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formance for the design life of the project. Many products, such ashome appliances, are sold on the basis of performance specifications—with a guarantee that performance will be satisfactory over the life ofthe product.

In buried flexible pipe design, procedural specifications have beenthe traditional basis for design. Pipe materials, shape, strength, mod-ulus, seams, etc., are all spelled out. Soil type, placement, compaction,and zones of backfill soil are all carefully specified. Even the installa-tion procedure is described in detail.

In reinforced-concrete pipe design, from experience, pipe design is socomplex and specialized that pipeline engineers favor performancespecifications, leaving the burden of pipe manufacture to the special-ists. Besides the complexities of forming and casting the pipes, designdetails include a multitude of variables such as reinforcing steel—size,strength, smooth or deformed, spacing, directions, bonding, shear-steel, cages, longitudinal steel, etc. Likewise, the concrete is a functionof many variables such as strength, aggregate size and distribution,water/cement ratio, type of cement, admixtures, and length of pipe sec-tions. Consequently, engineers who specify reinforced concrete pipe,write performance specifications based on the D load strength of thepipe. The D load strength is essentially a parallel-plate load to failure.A section of pipe is compressed between the two heads of a testingmachine. The D load is the load per unit length of pipe at failure.Failure is defined either as the load at the opening of a 0.01-in crackin the wall of the pipe, or the maximum load that the pipe section cantake. The pipe engineer must then relate D load strength to anticipat-ed loads: internal pressure, external pressure (soil, water table, andpressure due to live loads), and soil bedding conditions. The pipe isspecified by performance, i.e., the minimum D load. The D load isensured by testing a statistically representative number of the pipesections.

The design of plastic inserts for rehabilitation of deteriorated pipes,like that of reinforced concrete pipes, is specialized and complex.Specialists are emerging with technology based on testing and onexperience with in-service performance. They are identifying the mostimportant performance limits, such as resistance to persistent exter-nal hydrostatic pressure for a period of 50 years. Long-term testing isessential because plastics creep. Long-term performance cannot sim-ply be related to strength regression test data. As the plastic insertcreeps, it changes shape with consequent increase in stress. Stressdoes not remain constant as reported by strength regression data.Long-term performance tests are essential.

Whenever performance specifications are in conflict with proceduralspecifications, performance usually prevails over procedure. Courts

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usually construe for (i.e., weigh more heavily) performance specifica-tions and construe against procedural specifications.

Legal liability for performance

As products and installation methods become more complex and morespecialized, legal obligations of the manufacturer are tightened. The tra-ditional caveat emptor (let the buyer beware) approach is yielding to doc-trines such as strict liability. According to caveat emptor, once the productis sold, the contract is executed, ownership is transferred, and the previ-ous owner has no further liability. This doctrine began to change whenguarantees and warrantees became part of the sales contract. Statutesof limitation (time limit for filing a lawsuit) were adjusted in commonlaw to accommodate warrantees. Further changes are occurring as a doc-trine of strict liability takes shape. Strict liability holds that the statuteof limitations for filing a lawsuit against any previous owner, or anyoneinvolved in manufacture, handling, marketing, dealership, repair, modi-fication, installation, etc., of a product, begins at the time of injury—notthe time of sale. The manufacturer has continuing liability and legalexposure. The legal exposure may be mitigated by modification or abuseof the product, or by reasonable anticipated wear or deterioration.

Testing of Insituform Pipes

Introduction

Insitupipes in broken buried rigid pipes stop leaks. But to what extentdoes the liner contribute to the structural strength and shape of the bro-ken pipes? The cracked rigid pipe will take some additional load with-out collapse. The Insitupipe liner itself has structural strength and hassignificant pipe stiffness. What is the strength of the composite ring, i.e.,of the cross section, of buried, broken, lined pipe? Because theoreticalanalyses are extremely complex and because of the many assumptionsneeded for solution, full-scale physical tests were undertaken. Two full-scale tests were performed in the large soil cell at Utah State University.

Procedure

The experiment comprised two tests, each with two parallel test sec-tions, in the USU large soil cell shown in Fig. 8.1. In each test, the twoparallel test sections were 30-in pipes placed in the soil cell separatedby a spacing of 7.5 ft center to center. The test sections were 20 to 25 ftlong. The height of soil cover over the tops of the test sections was 3 ft.The bedding was firm and uniformly compacted soil. The pipe-zone

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Figure 8.1 The testing of Insituform pipe in the USU large test cell.

backfill soil was silty sand placed in layers and compacted to a uniform density. A vertical soil load was applied by 50 hydraulic cylinders attached to 10 beams, as shown in the photograph. Vertical diameters of the test sections were measured after each increment of load.

Each of the two parallel pipe sections in the first test was made up of 4-ft lengths of unreinforced concrete pipes, 30-in inside diameter. These were class 3 pipes with a minimum specified three-edge bearing strength of 3000 1bsAin ft. The joints were tongue-and-groove. No gas- kets or sealants were used at the joints. Each test section comprised five of these 4-ft-long pipe sections for a laid length of 20 ft. Both of the test sections in the first test were broken. An unbroken pipe 4 ft long was placed on each end of each of the parallel test sections to serve as a transition. Access pipes were placed in tandem with each of the tran- sition pipes to provide for entrance of personnel. After backfill was placed, one of the test sections was lined with Insituform pipe. See Fig. 8.2. The 10 broken sections of concrete pipe were cracked in a three- edge-bearing device. The average ultimate load was 3806.4 lb/lin ft of pipe. The standard deviation was 398.04 1bAin ft of pipe. Before load- ing in the three-edge-bearing device, each pipe section was banded with steel bands and stuffed with three 14-in-diameter paper sono- tubes to serve as mandrels for holding the circular pipe cross section during transportation and installation in the soil cell. Figure 8.3 is a photograph of broken rigid pipes.

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The two test pipes in the second test were Insitupipes that had been inverted and cured in paper sonotubes of 30-in ID. One of the Insitupipes was made from a standard resin with modulus of elastici- ty of 300 to 400 kips/in2. Its thickness was about 21 mm. The other was a formulation of resin with a modulus of elasticity of about 500 to 600 kips/in2. Its thickness was about 16.5 mm.

For each test, two parallel test pipe sections were placed in the cell on a level soil bedding. A 12-in uncompacted lift of backfill soil was located on each side of both test sections and was hand-shoveled into place under the haunches. Shoveling or shovel-slicing of soil under the haunches is a typical procedure on the job. Backfill soil was then brought up in l-ft lifts to 3 ft above the tops of the test pipe sections. The surface was leveled and covered with steel plates onto which the hydraulic cylinders would bear for loading the cell.

For the first test, each of the soil lifts was dropped into place from a conveyor and leveled, but was not mechanically compacted. Moisture content was kept on the dry side of optimum so that the soil density was as uniform as possible under the weight of the soil itself. The aver- age soil density was 75.7 percent AASHTO.

For the second test, each of the l-ft lifts of backfill soil was leveled and then compacted by one pass of a vibroplate compactor. The aver- age soil density was 83.4 percent AASHTO T-99.

Figure 8.4 Equipment for inverting the Insitutube.

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Figure 8.5 Process of inverting the Insitutube.

Test 1

This test comprised two parallel 20-ft-long test sections of 30-in-ID rigid pipes that had been previously cracked by a vertical line load in a three-edge-bearing test device. The cracks occurred approxi- mately at 3, 6, 9, and 12 o’clock. The pipes were so oriented in the soil cell that the top cracks in all five pipes in each test section were at the top and were in line. One of the two test sections was lined with a 21-mm-thick Insitupipe. The objective of the first test was to provide a direct comparison between the structural performance of two buried broken rigid pipes under increasing vertical soil pres- sures, one test section lined and the other unlined (see Figs. 8.4 and 8.5 for process). Structural support is tantamount to an increase in safety factor or a margin of safety against further deformation or collapse. Collapse of broken rigid pipes can occur if cracks in the pipes allow leaks large enough for in-migration of soil particles from around the pipe, thus leaving an empty vault in the soil at the sides and over the pipe. A soil vault is the prime condition for col- lapse of a broken rigid pipe. With no side support, the broken pipe collapses when the soil vault collapses. The test also provided data for comparing the load-deflection diagram with the load-deflection relationship predicted by theory.

With the two test sections positioned in the soil cell, the sonotube mandrels were removed. Access pipes were then located in line with

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the test sections for entrance of personnel. The first backfill soil liftwas placed on the bedding, shoveled under the haunches, and leveled.A second lift was then placed and leveled. With two lifts of soil backfillto support the broken rigid pipes, the steel bands holding the pipestogether were cut and removed. The backfill was placed in 1-ft lifts,but was not compacted. The soil lifts were continued on up to 3 ft abovethe tops of the test sections. Soil embankments were then shaped upat the ends of the cell. Loading beams were lowered and pinned intoplace. A preliminary vertical soil pressure of 1450 lb/ft2 was appliedwith a corresponding pipe deflection less than one percent. This con-figuration was established as the configuration of the broken rigidpipes for Insituforming. Pipe deflections during loading were based onthis initial pipe deflection as zero and on this vertical diameter of thebroken rigid pipes. Insides of pipes were cleaned, and an Insitutubewas placed and inverted in one test pipe. For the Insitupipe with wallthickness of t = 21 mm, the dimension ratio DR was in the range of 36to 38. The Insitutube was inverted at the recommended pressure headusing a polyester resin and standard cure.

Vertical loads were applied in increments equivalent to about 6 ft ofsoil cover at a unit weight of 120 lb/ft3. After each increment of load,the vertical ring deflections were measured at various locations. Allother pertinent observations were recorded. This procedure continueduntil a soil load of 8700 lb/ft2 was reached, which is equivalent to 72.5ft of soil cover at unit weight 120 lb/ft3. Measurements and observa-tions were recorded, and the test was terminated.

Results of test 1

Pertinent observations from test 1 follow:

1. The Insitupipe contributes significant strength to the pipe-soilsystem. The strength contribution is the result of two phenomena, thereinforcement phenomenon and the stiffener phenomenon, asexplained in the next paragraphs.

2. As the soil load increased above P = 2200 lb/ft2, the unlined testsection began to deflect. The lined test section did not begin to deflectuntil the soil load was 2900 lb/ft2. This increase in strength is the rein-forcement phenomenon. The Insitupipe serves as reinforcement. Ascracks inside the rigid pipe widen at 6 and 12 o'clock, the Insitupipeholds the cracks together.

3. Above a soil pressure of 2900 lb/ft2, the load-deflection curveswere approximately linear up to about 10 percent deflection, but theslope of the curve for the lined pipe is 1.5 times as steep as that of theunlined curve. This is the stiffener phenomenon. This increase instrength is the contribution of pipe stiffness by the Insitupipe.

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4. The safety factor due to the reinforcement phenomenon is 1.4. The safety factor due to the stiffener phenomenon is 1.5. The two are not cumulative, but clearly the safety factor is not less than 1.5, even if bond is not achieved. In this test, the soil load was increased. Therefore, the pipe deflection increased. In practice, the pipe deflec- tion does not increase-it has already occurred, probably at the time the rigid pipe broke. Therefore, the increased strength contributed by the Insitupipe is available as a margin of safety of at least 1.5. 5. It is noteworthy that neither of the two test sections collapsed

completely, even though both were deformed beyond what most engi- neers would accept as performance limits.

6. As the pipe deflection increases in the broken rigid pipe sections, the cracks widen and the potential for leakage increases. If the leak- age allows for in-migration of soil particles into the pipe, in time an empty soil vault will develop around and over the pipe. The pipe loses its sidefill soil support. When the soil vault becomes large, it collapses and soil falling on the broken pipe collapses the pipe. The photographs of Figs. 8.6 and 8.8 show the potential for leakage and in-migration of soil particles into the unlined section. No such leakage potential occurred in the lined section. See Figs. 8.7 and 8.9.

7. At the highest vertical soil pressure of P = 8700 lb/ft2, a discon- tinuous, longitudinal hair crack was observed inside the pipe in the

Figure 8.6 The inside of the broken rigid pipe test section after loading to a vertical soil pressure of 5040 lb/ft2 (equiva- lent burial depth of 42 ft).

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Figure 8.7 The inside of the lined broken rigid pipe test sec- tion. The vertical soil pressure is 5040 lb/ft2 (equivalent bur- ial depth of 42 ft).

Figure 8.8 The inside of the broken rigid pipe test section after loading to a vertical soil pressure of 8700 lb/ft2.

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Figure 8.9 The inside of the lined broken rigid pipe test section. The vertical soil pres- sure is 8700 lb/ft2.

crown of the Insitupipe. This was not a leak, but is indicative of the high tensile stress in the Insitupipe due to increasing deflection of the rigid pipe. The probability of such a crack in practice is low because the broken rigid pipe does not continue to deflect.

Test 2

The purpose of the second test was to provide a comparison between two 30-in-OD buried Insitupipes-one of standard resin formula- tion, with a 21-mm wall thickness, DR = 36 to 38; and the other of a new resin formulation with a 16.5-mm wall thickness, DR = 46 to 48. The two test sections were located in parallel in the soil cell, and so, for comparison, were subjected to the same backfill soil condi- tions and the same vertical soil pressures. This test provided a quantitative comparison of the load-carrying capacity of each Insitupipe and provided load-deflection diagrams of each for com- parison.

The Insitupipes for this test were formed inside paper sonotubes. Soil was placed to at least 1 ft over the two sonotubes into which Insitupipes were formed. This soil cover provided a uniform insulation

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and heat-transfer medium during curing and cooling of the two 30-ftsections of Insitupipe.

The two parallel test sections of the Insitupipe were placed in the testcell. Access pipes were located in tandem at the other end of each testsection. Backfill was placed in 1-ft lifts. Each soil lift was compacted byone pass of a vibroplate compactor. Compaction was continued in liftson up to 3 ft above the tops of the test sections. The average soil densi-ty was 83.4 percent AASHTO T-99. Vertical soil pressure was appliedin increments of 50 lb/ft2 in the hydraulic cylinders. Measurements andobservations were the same as those in the first test.

Results of test 2

Data for standard Insitupipes and Insitupipes with additive A are asfollows:

Data Standard Insitupipe Type 2 InsitupipeOD = outside diameter, in 30 30t = wall thickness, mm 21 16.5DR = dimension ratio 37 47E = modulus of elasticity, kips/in2 350 550

1. The ratio of pipe stiffnesses for the type 2 Insitupipe and the stan-dard Insitupipe is R = 0.75. Despite the greater modulus of elastic-ity E for the type 2 Insitupipe, its lesser wall thickness prevails andthe pipe stiffness is only three-fourths as great as that of the stan-dard Insitupipe.

2. The standard pipe deflected slightly less than the type 2 pipe.

3. No distress was observed in either of the test pipe sections. Even atvertical soil pressure of 7300 lb/ft2, both pipes would perform ade-quately in service.

Trenchless Technology Methods

Trenchless technology methods include all methods of installing orrenewing underground utility systems with minimum disruption of thesurface or subsurface. The demand for installing new underground util-ity systems in congested areas with existing utility lines has increasedthe necessity for innovative systems to go underneath in-place facili-ties. Environmental concerns, social (indirect) costs, new safety regula-tions, difficult underground conditions (existence of natural or artificialobstructions, high water table, etc.), and new developments in equip-ment have increased the demand for trenchless technology.

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