TRI LAB UPDATES
LONG-TERM DESIGN FLOW - PART 2
(Vol. 1, No. 8)

Will the soil environment cause geocomposite drains
to lose flow capacity over time?

(Read Long-Term Design Flow - Part 1)

This Lab Update is the second in a two-part review of long-term design flow issues . and answers. Following is some background on the calculation of long-term design flow, discussion of one industry proposal, and an outline of TRI's proposed approach to testing the long-term flow capacity of geocomposite drains.

In-plane Drainage
In-plane drainage is described in terms of flow, q, or transmissivity, 1. A product's allowable transmissivity can be described as follows:

1_allow = ______________1_ult________________
                 RF_CR x RF_INTR x RF_CC x RF_BC

Where:
1_ult = transmissivity from QC testing
RF_CR = Creep reduction factor (RF)
RF_INTR = Intrusion reduction factor (RF)
RF_CC = Chemical precipitate Clogging RF
RF_BC = Biological growth Clogging RF

One Current Industry Proposal
Recent industry discussions have included a proposal that determination of 1_allow should require only creep and intrusion measurements as follows:

1_allow = 1 _{100 Hr (with site-specific sub and super stratum)}.
             _________________________________________
                        [(RF_CR) (RF_CC x RF_BCdefault]

Where:
1 _{100 Hour} = transmissivity measured using site-specific boundary conditions and 100 hour seat time

This proposed "simplification" is in recognition that the industry has yet to develop a reasonable protocol for quantifying the chemical and biological clogging potential associated with product-specific and site-specific conditions. In lieu of test data, the following "default" reduction factors have been suggested and may be added to the above equation.

Concern about Baseline Transmissivity
Of some concern is the reliance on a transmissivity test that requires a 100 hour (4 day) seat time. This promises to minimize the number of replicates, leading to designs that are based on very limited data. Conversely, using the transmissivity results from quality control tests (1_ult) provides a statistically significant basis for deriving design/specification values since these tests are run routinely under standard boundary conditions.

Notes on Geotextile Intrusion
TRI believes that intrusion reduction factors related to geotextiles can be determined via independent short-term testing and then applied to the measured flow accordingly. TRI's experience with transmissivity testing of geocomposite drains overlain by soils has indicated that intrusion primarily occurs as the normal load is being applied, causing immediate flow reduction. Conversely, time-dependent flow capacity is affected much more significantly by long-term creep of the core or geonet. This may not be true if the geotextile is not laminated.

TRI's Proposed Testing Protocol
Based on the discussion above, TRI proposes the following approach to product testing:

1 _allow = ________________1_ult_______________
            RF_CR x RF_INTR x (RF_CC x RF_BC)default

• 1_ult = Transmissivity from QC testing - This data will be provided and certified by the manufacturer.
  
• RF_CR = Creep reduction factor -
  
  • Step 1. Determine thickness vs. flow using various normal pressures.
    
  • Step 2. Determine thickness vs. time for various normal pressures.
    
  • Step 3. Determine flow vs. time for various normal pressures using results of Steps 1 and 2.
    
• RF_INTR = Intrusion reduction factor -Using two different soils (sand and clay), determine flow vs. normal pressure for increasing time, until flow stabilizes.
  
• RF_CC and RF_BC = Chemical precipitate and Biological growth Clogging reduction factor - Carefully select appropriate default values.

Details on Testing
Task 1: Establishment of Normal Compressive Load vs Thickness Relationship - Geocomposite is tested for 10,000+ hour creep resistance using a combination of conventional and SIM test procedures. Selected normal compressive loads should bracket anticipated field conditions. Conventional testing will be performed to confirm and augment the SIM data.

Task 2: Thickness Dependent Transmissivity Testing - Using the data generated in Task 1, TRI selects specific thicknesses under which to perform transmissivity testing. Each test will be performed at the selected thickness with the following hydraulic gradients: 0.1, 0.25, 0.33, 0.5, and 1.

Task 3: Intrusion - A matrix of short-term transmissivity tests is performed using two different soils and three different normal loads. One long-term transmissivity test (100 hr seat time) is performed with each soil at the highest normal load and a series of different gradients.