ASTM D 6459 State-of-Practice

1. ASTM D 6459 State-of-Practice Presented to D18.25.02 June 2011 By Joel Sprague, P.E. TRI/Environmental

2. D 6459 Overview ASTM D 6459 is an international standard that simulates full-scale conditions “typically found on construction sites” as outlined in the standard’s significance and use statement. Test results have been correlated with “real world” performance and reported in, Slope Erosion Testing – Identifying “Critical” Parameters, (2008 IECA). The paper presented an analysis of large-scale slope erosion testing protocols used by three different labs and demonstrated that at least one protocol (ASTM D 6459) produces actual performance results that correlate with the theoretical results predicted by the Revised Universal Soil Loss Equation (RUSLE). Tilting bed protocols did not correlate well. This correlation validates that the test provides relevant input to the Revised Universal Soil Loss Equation. Numerous, comprehensive test reports of products independently tested in accordance with the standard are publically available at www.NTPEP.org.

3. D 6459 Overview ASTM D 6459 sets the following specific test parameters: • 8 ft wide x 40 ft long bed; • 3:1 slope; • 12 inches of compacted soil veneer (over underlying natural soil layer); • Soil veneer is to be compacted to 90 ±3% Std Proctor density when first placed; • The soil surface is to be tilled to a depth of 4-inches, raked level, and lightly compacted with a turf roller (TRI uses an empty 24-in diameter x 48-in long turf roller that weighs 160 lbs.); • The same soil preparation applies to slopes that receive an erosion control treatment (RECP) and to unprotected (control) slopes; • The erosion control treatment is to be installed as directed by the client and reported; • Rainfall drop size distribution range is dictated (and based on natural rainfall) and must be calibrated/documented and reported; • Rainfall uniformity over the test bed is dictated and must be calibrated/documented and reported; • Rainfall intensity is dictated and must be verified/reported during the test; • Wind must be not be over 5mph when testing. The wind speed must be measured and reported. (TRI’s test slopes are fully enclosed and not subject to wind.) • All runoff is collected (water and sediment), quantified, and reported; • 1 control slope is to be tested for every 3 protected slope; • Test results are to be reported as the ratio of soil loss from the protected slopes to soil loss from the control slopes.

7. Insufficient Differentiation? One historical objection has been that the test method does not sufficiently differentiate between product types. Some believe that this is a deficiency in the test protocol. However, others believe it more likely that when very different products perform similarly, it is because they have been installed using installation details uniquely designed (by the manufacturer) for the test conditions, i.e. 3:1 slopes, sandy-loam soil, high intensity rainfall. Most product literature provides guidance in selecting installation details for the specific conditions. Such variations as anchor density, size, pattern, and type (RECPs) and coverage rate and curing time (HECPs) can dramatically influence product performance. Thus, the installation details are reported along with the resulting test results. The data presented above reasonably supports the test’s ability to differentiate products appropriately for the conditions tested.

8. Seriously Flawed? Another objection that has been heard is that the test method is “seriously flawed” and the ASTM balloting process has been too slow to make changes. As noted above, like all test methods, this method should be continuously improved based on experience. Yet, results of this test have been shown to correlate well with “real world” performance, so any proposed changes are being thoroughly investigated by the ASTM task group governing the standard before being implemented. Work is ongoing in this regard.

9. D 6459 vs. Tilting Beds All other currently used slope testing protocols use tilting beds that allow for the soil layer to drain from below. This prevents the soil layer from becoming saturated under heavy rainfall simulations. This also creates a soil condition that cannot exist in the real world. The ballot author references Lal, 1994, as recommending the use of bed drainage when testing where “runoff and soil loss are the primary indicator of differences in the treatments”. While Lal was referring only to small plots, he goes on to clarify that these “small [drained] plots do not give complete information about the erosion process”. Clearly, there are times when the primary indicator of differences in the treatments is whether mass wasting occurs or not. Thus, it would not be appropriate to have drained beds when testing these products.

10. D 6459 vs. Tilting Beds The tilting bed slope test configuration has been used to isolate surface dynamics from full-depth slope stability issues, and thus has been shown in this limited context to segregate between surface-treatment technologies. However, global erosion phenomenon, including infiltration and associated hydraulic loading, warrant the use of large – real world slope tests, such as ASTM D 6459, for field performance investigations. For example, it has been documented that erosion control products that resist surface erosion dynamics by preferentially encouraging infiltration may also increase the risk of slope instability via mass wasting.

11. D 6459 vs. Tilting Beds Other commonly used protocols use large, uniform sized raindrops rather than a range of drop sizes as found in nature. The rainfall is then reported in terms of a hypothetical amount of kinetic energy that has been applied. The argument for using large drop sizes has centered on the need to maximize the aggressiveness of the rain storm event in order to delineate between competing erosion control technologies, especially those that rely on a chemical agent to provide localized fiber-to-fiber bonding. While calculating the kinetic energy of single-sized spheres is quite straight forward, it is not at all clear how, or if, this can be accurately done for an actual rainfall distribution until the measurement of drop sizes and their proportional makeup of the rainfall can be more definitively measured.

12. D 6459 vs. Tilting Beds Finally, all other testing protocols use test slopes that are shorter (< 40 ft) and narrower (< 8 ft) limiting the extent to which natural erosion mechanisms can develop. The ballot author references Lal, 1994, as noting that “experience has shown that 5 m is about the minimum slope length that will adequately represent a rill system in an up-and-down-hill plot.” In the very next sentence Lal goes on to say, “A better length is at least 10 m.” It seems clear that longer is better (i.e. less flawed).

13. Improving D 6459 • Since the standard test protocol requires construction of the test slopes in-situ, and rainfall drop size and distribution representative of actual rainfall, definition and control of these variables can and should be refined with continuing development of control mechanisms. • Slope construction, especially final surface compaction, is currently only qualitatively defined in the standard. This could be better quantified based on experience to-date. • Rainfall drop size distribution requires balancing flow, pressure, and spray head selection. The proper balance is then judged by a periodic drop size measurement technique using pans of flour and the sifting of dried “beads”. An improved drop size measurement technique, such as real-time measurements, would make continuous controls possible.