Sieving Error from Dry Separating Silt Soils

Silt SoilsThe dry-separation method is an alternative to the Silt Soils wet-preparation in the current European Standard for the determination of particle size distributions by the sieving of soils. Due Silt Soils to the risk of error, dry-separation is cautioned against in the standard; however, there is no additional guidance as to when it is unsuitable nor for the magnitude of error that it may introduce. This study investigates the dry-separation method as an alternative by comparing with the conventional method of Wet-preparation in terms of particle-size distributions of eight cohesionless sand-gravel soils with varying amounts of nonplastic fines. The findings indicate a gradually increasing sieving error for fractions at minus 0.5 mm with the amount of fines in the soil, and depending on the fines content of the soil, dry-separation introduced errors upwards of 45% in silt-sand-gravel soils. An empirical best-fit formula is proposed for the estimation of the error using the dry-preparation method on this type of soil. Furthermore, to avoid sieving errors, the results suggest that the dry-separation method should not be used for silt-sand-gravel soils exceeding 2% silt size fractions.

The process of obtaining the particle size distribution (i.e., the gradation) of a soil incorporates several sequential steps that are usually comprised of an initial weighing, an initial oven-drying, a second weighing, washing (removal of the fines), i.e., particles finer than 0.063 mm according to the current European Standard [1] or 0.075 mm according to the American equivalent [2], a second round of oven-drying, then a third stage of weighing, and finally the sieving of the remaining fractions of the soil. The sieving is usually performed by shaking the soil through a stack of sieves of different size opening. The sample results can thereafter be determined by the weight in terms of the size ranges. The final product, the particle-size distribution curve, is used in geotechnics for many purposes, i.e., analysis, design, prospecting, and for determining engineering properties [2], to name a few. The oven-drying stage is the most time-consuming step; at 110˚C ± 5˚C it typically requires 24 hours to complete.

When the process includes removing the fines by washing, it is called wet-preparation [1]. Dry-separation, on the other hand, is an alternative method to wet-preparation in the European Standard (but not in [2]) that allows one to bypass the washing stage and continue straight to the sieving stage. In the following discussion, these methods will be abbreviated as “wet-prep” and “dry-sep”, respectively. Naturally, a dry-sep method will save processing time; however, ref. [1] cautions against it by stating that “Wet preparation is preferred for soils with particles smaller than 0.063 mm, as use of dry-separation method may introduce significant errors”. However, no further guidance is given as to when the dry-sep method is unsuitable nor to the magnitude of the error that it may introduce if it is used inappropriately. Since it is less time-consuming, the dry-sep approach is advantageous when there are time- or economic constraints and in special cases, such as when the original soil must be preserved. However, sieving errors may arise for other reasons, i.e., sieve overloading (or under), errors due to particle properties and shape [3] or due to the formation of fine-particle aggregates that may lump together [4].

In this paper, particle-size distributions from the dry-sep method are compared to those of the conventional wet-prep method. Eight nonplastic silt-sand-gravel soils with varying amounts of fines are studied. It will be shown that the sieving error caused by using the dry-sep method increases with the amount of fines, generally resulting in errors in the minus 0.5-mm range, which may produce a notable underestimation of the finer fractions of the soil (e.g., the fines content).

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