November 2011, Vol. 66 No. 11
Features
Design Approach, Inspection For Manhole Rehabilitation Technologies
Editor’s Note: This is the final installment of three articles focusing on the current state of manholes, manhole assessment and manhole rehabilitation. Parts one and two were published in the September and October editions of Underground Construction.
The design of manhole coating and lining systems must take into consideration a number of conditions in the manhole, both as a whole and as individual components. When evaluating the nature of the coating or lining that will work best, a number of conditions should be defined.
If the manhole is structurally sound but showing signs of infiltration or inflow, a combination of grouting, patching and joint seals, applied either internally or externally, might be one answer.
If the manhole is brick and the mortar has deteriorated, the joints are leaking but display no real sign of corrosion, then a combination grout, patching, cementitious coating and joint seals might be one answer.
If the manhole is precast concrete, structurally sound but leaking at the riser joints, internal seals or grouting might be options.
If the manhole is precast concrete and the wall mortar has deteriorated and the steel cage is visible, then a wall build-back and a polymer top-coat or a cured-in-place liner might be the answer.
Many answers
There are a multitude of other scenarios that can be further illustrated but the point is, each individual manhole or group of manholes may require a specific selection of products, to accomplish a long-term rehabilitation of the structure.
Further, it must be determined whether or not the manhole is in jeopardy of structural failure. Unlike a pipe which may go through various stages of failure leading to total collapse, a manhole is either extensively deteriorated but structurally okay, or it has collapsed. Also unlike a pipeline, a manhole is a vertical structure where the hydrostatic pressure, earth loading and live loading from the surface is constant at any specific elevation. Loads on the manhole will, however, increase at the deeper elevations.
So what formula should be used? Should it be the formula contained in the ASTM F1216 Appendices that is typically used for pipe liner design? Or should the formula be specific to manholes and incorporate the compressive strength of the material to calculate the buckling load and thereby determine the required wall thickness of the new product? When installed, the new product should be equal to the full loading characteristic provided by the existing manhole. In either event, the formula should be standardized for the industry and require the same criteria for all coating and lining technologies.
To further explore this topic, Underground Construction sought input from a municipal manager, product manufacturer and a consulting engineer for their responses to several questions.
The participants included:
Edward Carpenetti, P.E., P.G., principal civil engineer, Infrastructure Systems Group, Sewer Rehab Unit, Washington State Suburban Sanitary Commission. The commission allows installation of product approved by its Material Evaluation Committee.
Chip Johnson, P.E. director, business development, Sprayroq Inc., manufacturer of structural and corrosion barrier products SprayShield Green 1 and SprayShield Green 2 (elastomeric corrosion barrier products).
Ed Kampbell, P.E. pipeline rehabilitation technologies specialist, Jason Consultants, a consulting engineering company that performs technical reviews and advises clients on alternatives that are appropriate for projects.
What is your definition of full structural rehabilitation, and what is the role of the existing manhole to continue functioning as the structural component after rehabilitation?
Carpenetti: “Full structural rehabilitation is a stand-alone product that does not depend upon the original structure for strength or function. The structural repair should be able to function even if the original structure crumbles.”
Johnson: “The existing manhole essentially acts as a ‘form’ for the installation with partially deteriorated structures handling the forces generated by the hydrostatic head and from the static water table and those fully deteriorated structures requiring that the installed technology will withstand all live and dead loads that are acting on the rehabilitated structure. Any reinforcement from the existing structure offers additional strength in the finished rehab installation, but is not dependent on that.”
Kampbell: “A fully structural rehabilitation means that the existing soil-structure interaction system has been renewed to its original structural functionality with a new service life; it’s water-tight and in a long-term stable condition. Any remaining competent material becomes a part of the new soil-structure interaction system once rehabilitation has taken place.”
What design approach should be used for manhole rehabilitation, and is ASTM F1216 applicable?
Carpenetti: “Manholes are installed by stacking individual components on top of one another. Therefore, no single design approach is applicable — each existing structure and surrounding area should be evaluated on a case by case basis to determine the best way to rehabilitate a manhole. In my opinion, ASTM F1216 is applicable for sewer mains, not manholes. The forces acting on a manhole are different than the forces acting on a sewer main since a manhole is orientated perpendicular to the ground surface as opposed to parallel.”
Johnson: “With our structural spray wall product, the design can be for partially deteriorated or fully deteriorated manholes per ASTM F1216-10. This standard has been the industry wide accepted protocol for buried circular geometries for conduit and manhole design for 20-plus years.”
Kampbell: “A manhole is a vertical structure and any rehabilitation alternative used must be designed accordingly. The design appendix in ASTM F1216 is based upon a classical elastic buckling theory for an ‘infinitely long tube’ (L > 4.9r (r/t)½) with essentially a uniform external loading acting upon it (the manhole’s loading is increasing with depth). Further, the close-fit liner can be subject to varying dynamic loadings in a vertical (end) orientation. To validly use the classical buckling theory in its F1216 embodiment, a 48-inch diameter manhole would have to be 68-feet deep for a 0.5-inch thick liner. So the design appendix of F1216 is not a viable design tool. In my engineering judgment, a close-fit liner system installed in a manhole must be designed to resist hydrostatic buckling only. For proper insight into the analysis of buried silos, I recommend looking at Reynold Watkins book, Structural Mechanics of Buried Pipes. In a reasonably circular shape, the performance limit is ring compression yield strength. Olivier Thépot in his 2001 paper, A New Design Method for Non-Circular Sewer Linings, presents a design method for evaluating the effect of external long-term hydrostatic pressure, acting on a thin lining. It takes into consideration the exact geometry of the lining (curve radii) and the beneficial interaction with the host structure that may be attenuated by an annular void space. Provided the host structure’s shape meets the criteria given therein it would be my choice for close-fit liner design analysis; otherwise, one must employ a non-linear finite element analysis.”
Should coatings, that bond, be designed the same as liners, that don’t typically bond?
Carpenetti: “No, coatings that bond need to withstand the stresses, such as freeze thaw cycles, acting on the existing manhole structure. Therefore they need to be flexible so that over time, they do not crack, fracture or break.”
Johnson: “No, dependence on bond strength can be a risky proposition in the sewer environment. Standards such as ASTM F1216 do not assume adhesion and while conservative, minimize the human installation application factor.”
Kampbell: “Taking credit for bonding requires that it can be quantified up front for the purposes of the analysis — this, I think, would put a challenging burden on a liner system provider to do the testing that can substantiate the value that should be used. Given the many variances in the host material’s competency and the level of attention to preparing the surface for lining that the contractor must be able to ensure would not be wise. Further, the design analysis method I recommended in the previous question demonstrates that the liner is going into a compressive state that ultimately results in a buckling failure. My judgment is that all lining systems should be designed as if there were no bonding taking place.”
We can therefore, assume that if a product bonds or does not bond to the host structure that the acceptance of the product should always be through verifiable quality assurance and testing.
There seems to be some differences of opinion but it would seem that all will agree that whichever approach to design is implemented the criteria used should be consistent and be verified through proper trained, certified inspection and standard testing procedures.
To confirm that a coating or lining has been properly installed several procedures are commonly followed.
First, the manufacturer’s specification sheet and installation requirements for the specific technology, followed by the applicator, should be a guideline to be studied and evaluated and used as inspection criteria for proper installation.
In the case of coatings and linings, that are promoted to bond to the substrate, a trained and certified inspector must enter the structure during the preparation stage as well as after the material is applied. The preparation inspection should include visual and detailed substrate examination. Running a gloved hand over the substrate surface or using a scraper type tool should not result in material becoming loose from the surface. If so, then further cleaning and removal of deteriorated material is required before the coating is applied.
Second, once the material is applied and cured, the trained inspector must enter the manhole to perform a detailed visual inspection of the finished surface and note any potential defects. Holidays and pinholes can best be detected upon entry into the manhole structure.
Third, the quality of the applied product must be assured through required testing, that is applicable to the material applied and as specified in the contract documents.
Should all manhole rehabilitation products require inspection and testing to verify proper installation?
Carpenetti: “Yes, inspection and testing is paramount to the successful completion of a manhole rehabilitation project. A lot of products get a bad rap due to improper installation. The best products in the world will fail if they are installed incorrectly.”
Johnson: “Absolutely. Testing such as holiday testing (on corrosion protection installations, typically 125-250 mils) and adhesion testing promote good installation practices. Also, verification is needed from a third-party, independent testing of physical properties performed by certified materials testing company, specifically those that use the same physical properties in their thickness designs. An example would be both short-term and long-term flexural modulus that is used in ASTM F1216-10.”
Kampbell: “Construction observation is a bargain for the owner when the inspectors are properly trained in how to identify critical steps in the lining’s installation that assure long-term performance. Yes, there should be construction observation of the installation process and quality assurance testing used to document the success of these installations.”
It seems that there is, at least, a consensus among the participants that good qualified and certified inspection is a must for all manhole rehabilitation activities. NASSCO is in the final stages of developing an Inspector Training and Certification Program (ITCP) for manholes that is designed to educate inspectors and engineers on different technologies and how to properly inspect and test the products during and after installation.
Even though a deteriorated manhole is typically still capable of sustaining external soil loads (unless, of course, it is actually collapsed) the design of a full structural manhole within a manhole is not typically required. What is required are qualified technology installers and inspectors that are not only trained, but understand products and failure mechanisms that can be prevented during coating or lining applications As the manhole rehabilitation industry continues to grow we may see additional advances and a consensus on the best approach for structural design when required.
Editor’s Note: Senior Editor Jeff Griffin contributed to this article.
FOR MORE INFORMATION:
NASSCO, (410) 486-3500, www.nassco.org
Comments