Silt Fence may be the most common, and most maligned, temporary sediment control BMP in use today. Poorly installed silt fence is the hallmark of most SWPPP implementations.

There are lots of reasons for the failure of typical silt fence installations, But the primary factors are inferior materials and improper installation. Either of these factors can cause a failure of this BMP, but the fact is that most silt fence implementations today are burdened with both of these factors. It's a circumstance that guarantees failure.

The inferior materials issue comes up in installations incorporating a "Contractor Grade" filter fabric. These fabrics rarely meet DOT or local specifications, but they're cheap, and since the level of sophistication in readily identifying such inferior fabrics once they're installed is relatively low—they get installed in a "cosmetic" SWPPP installations fairly often. They reveal themselves over time, but by then the damage is done. Ironically it's these inferior installations that often utilize another of the sure fire failure generators—installation on inferior fence posts, placed too far apart. Wood posts (often "sticks" is a better descriptor) are used in these installations because they're cheap. The fact that they're difficult to drive to a depth that will provide the necessary support is not taken into consideration. As a consequence they're highly susceptible to failure under load., and unprofessional appearance in general.

Another favorite for inferior installations is the prefabricated silt fence. Commonly available at home handyman stores, these rolls of filter fabric (often a "Contractor Grade" fabric to further compound the problem) come with wooden sticks stapled in place at 8' or 10' intervals. Trenched-in (or more typically hammered in at surface level) with dirt piled on the loose flap of filter fabric, these implementations are almost completely useless and provide little beyond cosmetic compliance — and that's assuming the inspector is driving past at a high rate of speed!

The bad news is that even the best of typical silt fence installations—those that use high quality materials, suffer from the most common failure generator of them all — the trenched installation. Operating from specifications dating back as many as 25 years, the trenched installation is at the heart of most silt fence problems. While some specifications call for filling the trench with gravel, it's not highly practical and rarely done in the field. Typically, after installation of the filter fabric, the trench is simply  backfilled with the spoil from the original excavation, or more accurately, spoil from one side of the excavation. The spoil pile on the  other side is now behind the installed fabric. During a rain event the loosened soil absorbs water and becomes saturated rapidly — piping, undercutting and washouts are virtually guaranteed.   

Compaction of the spoil in trenched installations is usually called for in the specs, but it's rarely attempted in the field. It's difficult to accomplish in any case. Once the posts have been installed in the trench, it's nearly impossible to compact. The washout problem is compounded by the trencher's action, which more often than not creates a bowl-shaped trench rather than the neat, square bottomed trench found in specification details. Then there's all the air space created by the excavated grass and weeds that are turned back into the trench when it's filled with spoil. These air spaces aren't easily compacted, and they are prime targets for infiltrating water.

In recent years there has been an effort to compensate for the failure in the trench by reinforcing the trenched silt fence with wire mesh. Beyond the obvious, and environmentally painful, landfill disposal problem associated with all that wire that can't be readily reused — it's a costly solution in search of a problem. High quality filter fabric doesn't require reinforcing for a normal installation assuming it's properly installed.

Today's high quality silt fence fabric is designed to handle the loads that may be generated by water and sediment loads if support posts are properly installed and properly spaced.  Although wire mesh reinforcement may be valuable in providing additional strength for the severe loading that might be found in implementations where silt fence is probably not the best BMP solution in the first place (as in continuous flow channels or as the sole control at the toe of a steep hill, for instance)—it's just not required for strength in typical silt fence installations. Most importantly, the issues that drive the failure of the trench aren't even addressed by the wire.

High failure rates. High cost of maintenance. The trenched silt fence just doesn't get us where we need to be on almost any count. There has to be a better solution.

The Silt Fence Solution
We believe that the best solution to the silt fence problem involves discarding the trenching method all together, in favor of slice-inserting the silt fence fabric. This methodology, as exemplified by the patented Tommy Silt Fence Machine, slices the earth, disrupting the soil upward just enough to open a slit into which the filter fabric is fed to a depth of 8-12". The sliced earth closes around the fabric as the machine passes. The closed slice containing the fabric is then mechanically compacted by the weight of the tractor pulling the Silt Fence Machine as it makes several passes over the slice, after insertion is complete. Since the posts haven't yet been installed, they aren't there to inhibit compaction. The minimal soil disruption minimizes moisture loss which also contributes to better compaction. The result is that the soil is fully compacted around the fabric, back to near it's original state.

This mechanized approach to silt fence installation not only provides better functionality of the silt fence due to the nature of the insertion process, but it also minimizes the variables associated with the consistency and varying abilities of an installation crew. Windy conditions aren't a problem for slice-insertion installers either, as they can be for trenching crews who must fight the wind to keep the fabric taught longitudinally while dumping enough soil into the trench to hold the fabric in place.

Slice-inserted silt fence can be installed on steep slopes because the minimally displaced soil holds in place regardless of the incline, making for a good installation. In fact, one of the unique benefits of this technology is it's  usefulness in almost any conditions, from rocky soil to wetlands with standing water and fluid mud.

We use steel posts with the slice-insertion system because of their  ability to be driven more easily to proper depth (in most cases 24"). They are driven adjacent to the inserted filter fabric at intervals appropriate for the needs of the specific implementation (most often on 6' centers), with cable ties in the upper portion of the fabric.

An EPA study co-sponsored by the manufacturer concluded that the minimal soil disruption and compaction attributes of this technology were directly related to effectiveness. In those tests, slice-insertion had no failures, where trenching applications had many.

By utilizing the latest technology in silt fence installation, complying with ASTM D6462, rather than relying on outdated methods, we insure that our installations are more than cosmetic compliance. We install silt fence that works.

For more information on Silt Fence Problems, please contact us at 832.456.1000.