These photographs show what happened during the first rainy season after the explosive eruption of Mount St. Helens in 1980. The eruption generated a large pyrocalstic surge and tephra fall that deposited loose gravel-sized and sand-sized rock debris to a thickness of about 1 m (3.2 ft) and leveled nearly all vegetation in this area, about 8 km (5 mi) northeast of the volcano.
Deposits of volcanic ash, including quantities removed from roadways, etc., continue to be troublesome because the ash is blown by the wind and is tossed up by passing vehicles. A vareity of materials can be used to suppress blowing dust through formation of a crust. This is generally a highly successful approach, at least until the crust is broken. The U.S. Army Corps of Engineers has used some of these and made certain recommendations, thoting that manufacturer's instructions should be followed and that the Corps does not guarantee or accept responsibility for any specific product. Specifics on costs and decisions as to which materials to use vary according to local circumstances.
Agricultural LimeChemical analyses of various samples of the ash material show that while its composition does vary, its content of silica, alumina, and iron oxides does meet the minimum ASTM chemical requirements for a cement binder (known technically as Type N or F pozzolan), and compares favorably with such materials used in the concrete for many Corps of Engineers construction projects in the Pacific Northwest. In the binding reaction, the oxides in the volcanic ash combine chemically with hydrated lime to form cementing compounds similar to those formed in the hydration of Portland Cement. At least 5% hydrated, agricultural type lime (by weight) would be required for this reaction to occur. The cementation process proceeds considerably slower than for Portland Cement, but in time, cement mortar strengths may be attained. Dust control would be effected by the formation of a crust on a stockpile or roadside accumulation which would be resistent to wind erosion if left undisturbed. The lime should be applied to the surface of the ash in the form of a slurry suspension or a solution, by means of a spray bar. If applied in a lime-water solution, several applications will likely be required to supply a sufficient concentration of lime to the ash surface because of the lime's low solubility (approximately 1.1 lbs/100 gallons of cold water). For existing gravel surfaced roads, some practical use could also be made of the ash material itself by mixing it with either in-place or borrowed sands and gravels, a minimum of 5% (by weight) of lime, and sufficient water for compaction placement, to form a low quality type concrete similar to soil cement or cement treated base. This procedure could serve to increase the quality of existing gravel surfaces or could be used to produce a stabilized base course.
Lignin SulfonateAn ammonia base wood liquor by-product of the paper pulp industry that can be used for dust control, it is moderately hygroscopic (retains moisture) and the wood sugars act as a binding agent. It has been found not to work well on materials such as decomposed granite which remain coarse upon weathering. It is used as an annual dust control measure in some Eastern Washington counties with applications effective for about four months. It is also sprayed on dust to facilitate blading into windrows for pickup. It is produced in the form of a 50 percent solids liquid, and is generally diluted with two to four parts water. For availability, price information and application rates, contact any major paper manufacturer.
Lion PrimeA penetrating asphalt compoent (similar to items known in the trade as MC 30 or MC 70 asphalt cutback with kerosone) was developed for soil stabilization and dust control for helicopter landing areas in Vietnam. It is used by some states for stabilization of road shoulders and is also used in spray application on coppermine tailings to control very fine powdery dust. It will effectively penetrate two to three inches and act as a binding agent, but will decompose after several years.
CoherexA dust palliative also developed for helicopter landing areas in Vietnam, is an asphaltic wax emulsion (similar to items known in the trade as CRS-1 and CRS-2) used by lumber companies and the Forest Service as a dust palliative on forest roads. It makes dust fines heavier so that passing trafffic will not cause them to rise higher than about three feet. In Redmond, Oregon, a 4:1 dilution was sprayed on volcanic ash stockpiles to form a wind-resistant crust on the surface. For traffic use, a standard application is ½ gal/sq yard of the 3:1 or 4:1 solution. However, it can be used in up to 10:1 dilutions for forming light membranes. The Washington State Highway Department has made trial applications along I-5 at dilutions of up to 8:1.
Emuslified Asphalt SS-1-1-1This is a hard base emulsified asphalt which can be diluted with water up to 10:1. It should not be used in areas of pedestrian traffic because it will stick to shoes. It is available from most major asphalt companies, who can also advise on costs and application rates.
Slow Cure OilsThis term is applied to soft asphalt whcih will also control dust but does create an oily surface. Costs and applications rates are also available from most major petroleum companies.