One of the most important regulatory requirements on landfill bottom lining system drainage layer is that the maximum head of leachate over the liner should not exceed 1 ft. When this requirement was developed, the consensus was that the drainage layer consisted of granular materials. Later, when geonets and geocomposites entered the market, the unwritten consensus among solid waste engineers and regulators was that the maximum head of leachate at the base should not exceed the thickness of the geonet or geocomposite drainage layer.
With that in mind, the reduction in hydraulic transmissivity of geocomposite laid over steeper slopes can adversely affect the maximum leachate head over the liner. When hydraulic transmissivity value reduces due to steeper slope at the base, the hydraulic conductivity reduces in turn as well. Reducing hydraulic conductivity results in an increase in the maximum head of leachate passing through the geocomposite.
Read Dr. Ali Khatami’s design advice for cell base slopes under these circumstances to maximize hydraulic transmissivity; recently published in the winter edition of Talking Trash.
Sometimes geosynthetic material specifications for a specific project, i.e., lining system or final cover system, is a performance-based specification which does not specify the type of product for use in construction. What does the engineer need to do when the selected contractor submits a product for approval in accordance with a performance-based specification? What should the engineer do when the owner purchases the material and identifies a product for use based on the performance-based specification?
Specifications that SCS has prepared are performance-based and include a qualifying procedure whether the product is introduced by a contractor or owner. This qualifying procedure is specifically left to the engineer to carry out by laboratory testing of typical samples of the specific product for use in construction. Typical reported values by the manufacturer or test results submitted by the contractor or owner are not acceptable under these procedures. Since the engineer is taking the liability of accepting a specific type of product for his or her project, the engineer should have the right to perform laboratory testing before the product is approved for use in the project, that only makes sense in the world of taking liabilities!
The testing performed by the engineer for qualifying a product do not count toward conformance testing of materials delivered to the site. The qualifying procedures are solely for accepting a certain type of product to be used in the project, but the specific rolls of pre-qualified product manufactured for use in engineer’s project must go through the required conformance testing specified in the specifications before use in the project.
The process of qualifying a product, ordering the qualified product, and performing conformance testing on the pre-qualified materials takes time. Engineers need to consider the amount of time necessary for the involved stages of approval into the construction schedule. If using material purchased by the owner, the owner needs to keep the timeline in mind to allow the engineer to carry out all necessary testing for the approvals to be in place before construction begins.
Repeating the qualifying procedure for a product from one project to the next depends on how the performance-based specification is written. Sometimes, the engineer accepts a product that was qualified for use in a prior project as long as the product has not changed since last used in accordance with statements by the manufacturer. If the performance-based specification includes such options, SCS highly recommends identifying the period between a prior project and the next project in the specification. In some cases, this means the product must go through a qualifying process even if it has not changed for many years but the previous set of qualifying data is older than a certain number of years. The period is based on the engineer’s judgment, but most professionals normally use five years in their specifications. During a five-year period, if the product changes or there are indications that the product might have changed due to recorded changes in certain reported values by the manufacturer, the qualifying process must be followed irrespective of the number of years passed since a recent past project to maintain quality and minimize risk.
Questions? Contact the author, Ali Khatami.