November 2022 Vol. 77 No. 11
Features
ASCE MOP 145 Update on the Design of Flexible Liners for Gravity Pipes
Norman E. “Ed” Kampbell, P.E. | Principal, Rehabilitation Resource Solutions
(UC) — Engineering is always advancing its methods of investigation and analysis, at a slow but pragmatic pace, as new information comes to light and is properly vetted.
In November 2021, the American Society of Civil Engineers (ASCE) issued a new Manual of Practice (MOP) for the Design of Close-Fit Liners for the Rehabilitation of Gravity Pipes. The purpose of the MOP was to provide good guidance to engineers tasked with making wall thickness designs of liners, such as cured in place pipe (CIPP).
The new MOP covers more than just the equations involved in this analytical approach. It covers soil-structure interaction, condition assessment, and information on the liner material/system alternatives that are appropriate to use. It applies to essentially all the geometrical shapes in which gravity pipes are constructed.
Prior to its issuance, engineers relied primarily on the non-mandatory Design Appendix X1 of the ASTM Standard F1216. Written in 1989, it applied only to circular shapes with a stated limitation of no more than 10-percent ovality at the time of lining.
Background
At that time, there was no thought as to how to design liners for egg-, elliptical- or arch-shaped pipe geometries. The egg-shaped and ovoid shaped sewers in the UK were later covered under the WRc Sewer Rehabilitation Manual (SRM).
As one would expect from a first-generation proposed design methodology, Design Appendix X1 was intended to provide a conservative approach to CIPP designs for pipes in the 8-to-12-inch range, which essentially made up most of the rehabilitation work at that time. It was not envisioned that it would later be routinely applied to large-diameter piping, as great as 96-inches.
ASTM F1216 Design Appendix X1 was based on two design conditions: partially deteriorated and fully deteriorated. The partially deteriorated design equations (X1.1 and X1.2) were created from Timoshenko’s buckling equation for an un-encased thin tube submerged in water. For the equation to more accurately predict real-world buckling inside of an existing pipe, a multiplier, K, was added.
Research by Aggarwal and Cooper in 1984 found that on a 10-inch diameter pipe, K varied between 3.6 and 21.4, with the value being skewed toward the higher values. The mean value from 49 tests was 12.4.
The researchers cautioned that the distribution of their recorded K values did not approximate a “normal distribution;” 46 of the 49 tests gave a value greater than 7. Thus, in Appendix X1, a minimum value of K equal to 7 was recommended, where full support of the existing pipe was found.
No information on the large variability of K was cited and no notice was made of the effect that the magnitude of the gap had on this value. The smallest K value of 3.6 was found when the annular gap was 5.3 percent.
In addition to a K value, Equation X1.1 had a C value added to reflect the impact of the host pipe’s ovality on the liner’s buckling resistance. Equation X1.2 (an Aggarwal analytical derivation with help from Roark) was to be used “if the original pipe is oval,” but it is my understanding that many do not believe in the validity of X1.2 and have long since discontinued use of the equation.
For the fully deteriorated design condition equations, X1.3 and X1.4 were given. Equation X1.3 was taken from the direct bury design of that time for fiberglass pipes, such as centrifugally cast glass fiber reinforced pipe (CCFRP). I say that because, over the years, this equation has been modified significantly by the industry it was borrowed from, while the F1216 Design Appendix has not kept up with those changes.
Equation X1.4 defined the minimum thickness the minimum handling stiffness required to survive the rigors of storing and transporting a manufactured pipe to the job site and burying it in a trench. However, it’s not a real-world requirement for a liner that is inflated inside an existing host pipe structure. Hence the need for a new design methodology reflecting a liner installed inside of an existing pipe.
Practical, timely updates
The new MOP 145 is based on 20-plus years of engineering discussion and research. At the heart of the new design method is the Glock buckling model, as modified by Olivier Thepot for handling non-circular geometries. Glock’s work focused on the buckling of a tube that is encased inside an existing tube or pipe, which mirrors the real world of a CIPP or other flexible pipe liner.
Ironically, it was available at the time the Timoshenko equation was adopted. However, it existed in an obscure German writing that was not familiar to the writers of the design appendix. This is a game-changer for the engineering community, as it now has a buckling equation based on the conditions the CIPP (or other flexible liner) would see in the ground going forward.
The new MOP allows the engineer to arrive at a minimum wall thickness required for essentially all of the geometries that a CIPP or other flexible liner could be installed: circular, elliptical, egg-shaped, arch and even box. Further, the way the design methodology evaluates the liner, it is not limited in size like the equations in the ASTM F1216 Design Appendix X1. It performs a true structural analysis looking at the axial and bending stresses of the proposed liner as a whole structure.
One of the many benefits that comes with the implementation of the new MOP 145 is engineers are now able to better assess the finished quality of the CIPP installation, based on the gap created during processing of the liner. In the past, it has been common to merely “seal the ends” of the CIPP with resin or hydraulic cement mortar. But the buckling failure resistance of the CIPP is a function of the annular gap, as discussed earlier.
In the new design process, the engineer chooses an appropriate annular gap, based on past experiences for the size and shape of the host pipe or using the minimum 0.04 inches recommended in the MOP, to determine the thickness that will withstand the anticipated external hydrostatic head pressure from the groundwater.
If that gap is later exceeded by the contractor’s installation process, the engineer can assess the resulting gap’s impact on the liner’s long-term survivability and, if there’s an issue with the as-constructed gap, resolve it before accepting that liner. Not addressing it can lead to disastrous results.
As an example of an installation with an excessive annular gap, I was called on to review installation of a failed liner in a horizontal elliptical concrete pipe. The dimensions of the pipe were 48 inches tall by 76 inches wide. The resultant annular gap, after curing the liner, was approximately 1.5-inches.
At the time of installation, the contractor simply sealed the ends of the pipe, per normal practice, hiding the large annular gap, because the focus was to ensure the groundwater would not leak into the ditch at the open pipe ends. The pipe had a relatively shallow bury, with an 8.2-foot maximum external hydrostatic head assumed by the engineer during the design phase. The installed liner, however, failed in buckling with less than four feet of hydrostatic head.
Equations in the new MOP predicted this would happen. One can only imagine the legal battles that followed this failure. In hindsight, the engineer could have required the liner be grouted to produce the correct annular gap and the failure averted.
Another benefit of the new MOP 145 design methodology is the fit and finish that can be achieved in non-circular sewers, such as egg- and arch-shaped. An overly conservative thickness can make it difficult to get a liner to form properly in the tight bottom radius of an egg-shaped pipe or the corner radii of an arch-shaped pipe.
This lack of proper fit has led to much friction between the engineer, owner and contractor, over the years. Having an efficient thickness, without the extra thickness from over conservatism, yields a liner that is much more compliant (i.e., tightly fitting) with the shape of the host pipe.
Comprehensive design capabilities
This new design methodology also allows the wall thickness to take into account the variation in the curvature around the perimeter of a liner in a large-diameter pipe. Failure in buckling occurs at the flat spots in these pipes and being able to define the magnitude and shape of a particular flat spot is imperative in producing a structurally efficient liner.
Calling out that the “ovality” of a 96-inch diameter sewer is 3.5 percent doesn’t convey the information necessary to design a large-diameter pipe liner. One needs to define the changes in the radii around the periphery of the pipe and the arc lengths involved, in order to properly assess this local defect in geometry. The MOP has many modifiers (reduction factors) to address not only annular gap, but also four-hinge-type ovality, elliptical ovality, flat sections, intrusions (with flat sections), and combinations of these imperfections, as normal occurs in the real world.
The biggest benefit of the new MOP is the analytical methodology it brings to all sizes of engineering firms – from one-person engineering to large, multi-discipline firms. It doesn’t require an ongoing investment in a Finite Element Analysis (FEA) program and the frequent model building required to ensure the analysis performed is correct for a particular project site.
Further, written in a Load Resistance Factor Design (LRFD) format means the engineer can assign load factors and resistance factors to match the reality of the relative unknowns used in pipeline rehabilitation designs for a particular project site. Live loads and groundwater pressures are multiplied by 1.6. Dead loads are multiplied by 1.2.
Resistance factors are the material resistance factors that account for lower values of the material properties in the field, versus the manufacturer’s published values obtained in laboratory testing. Examples of these for flexural strength are 0.85 for liners impregnated and cured on-site, versus 0.9 for liners resin impregnated off-site and cured on-site, and 1.0 for factory manufactured thermoplastics (PVC, PE) installed via fold and form.
The long-term flexural modulus for CIPP is multiplied by a factor of 0.8. This will eliminate a lot of heated discussions between contactors and engineers, when the field samples test below the manufacturers’ published values, since the engineer would have accounted for that possibility during the design phase of the project.
At first blush, the design equations in chapter 5 of the MOP can appear quite daunting. The reality is that they can be put into an Excel spreadsheet, automating the design for the engineer. In fact, as of this date, several firms have already written their own design tool or are in the process of doing so.
To give the engineering community its first comprehensive design manual for flexible liners, the ASCE task group had to cover all the variables that might come into play in the wall thickness design. Engineering firms can simplify their own tool to reflect the types of defects they typically see in the sewers they propose to rehabilitate.
Payoffs of a more efficient wall thickness, a better fitting (and thus finished) liner to the irregularities of the host pipe, and the ability to confidently assess the finished quality of the installation before acceptance, alone, are worth the effort of migrating to the new MOP sooner rather than later.
ED KAMPBELL is a technical consultant to the trenchless pipeline rehabilitation industry specializing in CIPP, deformed/reformed HDPE, fold and form PVC, spray in place liners (both cementitious and polymeric), and large-diameter segmental slip-lining. Kampbell’s work expertise includes the engineering design of these renewal pipeline systems, development of the proper installation processes to suit a wide variety of applications and environments, and raw materials design, selection and handling of these trenchless systems.
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