To run a gas pipeline, contractors cut a right-of-way typically 75 to 150 ft. wide through highly varied terrain. The concealed pipe can be buried on a slope between 6 to 20 feet deep, depending on steepness and transitioning from the previous and to the next. This particular slope, approx. 21,000 sq. ft., had developed “slumps” in the right-of-way due to a likely combination of poor compaction and subsequent saturation – resulting in shallow plane sliding failures which may slide 4 to 10 feet deep into the slope, causing ‘scarps’ – areas of exposed dirt and shelves along the slope.
The solution called for putting the slope back to its original topography and elevations, compacting as they go up, fixing the slumped areas. The contractor enlisted the services of a geotechnical engineering firm to determine the depth of the potential failure plane. The engineer used the information to determine the most economical and safe solution for long term protection of the buried pipe. In this case, it turned out soil properties rather than erosion was the issue. After determining that there were no other obstacles such as buried tree roots, logs and rocks, the slope was re-graded and compacted properly and covered with EnkaMat R45.
THE ENKA SOLUTION
It was determined that the failure plane was approx. 10 ft. deep. A system was designed which would use percussive anchors driven through the TRM. EnkaMat R45 was selected, in part due to its high modulus, high tensile strength biaxial geogrid (3,000 lbs/ft ) which is integrated into the EnkaMat during production, to spread the point load created by the Percussive Driven Earth Anchors (PDEA)
supplied by Platipus USA. Without that load spread, the tension would sink into the soil and stabilization would eventually be lost. The PDEA was driven 12 to 15 feet deep to intercept the failure plane, also anchoring into the stable soil which is beyond it. Also recommended were two rows of vertical wick drains driven into the plane horizontally to relieve any pore water pressure within the slope. The passive
drain system is driven in at a slight angle to allow perched water to freely flow to the slope face. Platipus reports that this simple technique of removing the water from the slope greatly reduces the risk of future slope failure.
INSTALLATION BENEFITS & RESULTS
EnkaMat R45’s unique physical and mechanical properties allows for hydraulic mulch filling of its 3D matrix with the strength required to spread the load of the PDEA.
Other woven products don’t provide the openness to allow the mulch to touch the soil. The 95% open matrix allows the mulch through to help bind all of the elements together. Once completed and the grass is established this slope will now be able to withstand heavy maintenance equipment such as tractors pulling ‘bush hog mowers’ and the occasional ATVs and dirt bikes operated by the property owner or pipeline personnel.
In summary, the contractor was able to use a soft armor solution using existing soil, eliminating the need for mass excavation and importing new, more suitable soils; the properties of the slope were modified by putting in the anchors, consolidating the soil and intercepting the failure plane. The final solution was much less expensive than cabled concrete mat or rock riprap. The contractor was confident enough to provide a one year warranty against additional failure. The land owner uses the property as before. The gas company is happy with the repair project and is planning to use the same engineered solution on adjacent slopes.