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La formula di Kozeny-Carman (K-C), proposta da Josef Kozeny nel 1927 e modificata da Philip C. Carman nel 1938, può essere utilizzata per stimare la conducibilità idraulica delle terre sature quando sono noti il grado di addensamento e la superficie specifica dei grani. Il principale limite all’utilizzo della formula risiede nella stima della superficie specifica, grandezza che di rado viene misurata direttamente, ma che è possibile stimare in modo indiretto. Nella presente memoria è stato proposto un procedimento per stimare la superficie specifica delle terre – combinando il contributo della frazione incoerente con quello della frazione argillosa – e l’applicabilità della formula di K-C è stata verificata mediante il confronto con i risultati di circa 130 prove di permeabilità condotte prevalentemente in cella triassiale. Il procedimento sviluppato per la stima della superficie specifica e la formula di K-C hanno fornito una buona stima della conducibilità idraulica nell’intervallo tra 1x10-11 e 5x10-6 m/s.
The saturated hydraulic conductivity of a soil can be estimated by using the Kozeny-Carman equation (K-C), developed by Josef Kozeny in 1927 and modified by Philip C. Carman (1938), provided enough information is given regarding soil packing and the specific surface area. The main limitation to the use and popularity of the K-C equation lays in the determination of the specific surface area of the soil, which is not frequently determined, but which can be fairly easily estimated with analytical methods or empirical correlations. For sands, the specific surface area can be simply calculated from the particle size distribution [FAIR and HATCH, 1933; CARRIER, 2003; CHAPUIS and LEGARÉ, 1992; CHAPUIS and AUBERTIN, 2003]. For plastic clayey soils the specific surface area can be related to the value of the liquid limit [MUHUNTHAN, 1991; MBONIMPA et al., 2002]. However, in the case of graded soils with equally significant amounts of clay, silt and sand, the evaluation of the specific surface area with one of the existing methods can overestimate (analytical method for sands) or underestimate (empirical correlation with liquid limit for clays) the soil hydraulic conductivity. This paper presents a method to evaluate the soil specific surface area which combines the contribution of the plastic clayey fraction and that of the coarse sandy fraction. The resulting value of the specific surface can be used to predict the hydraulic conductivity by means of the K-C equation. The applicability of the proposed method and its effect on the performance of the K-C equation has been verified by comparing the K-C estimated hydraulic conductivity with the result of approximately 130 conductivity tests on saturated soils in the triaxial apparatus (ASTM D 5084-00, Method A). The results presented here show that the Kozeny-Carman equation together with the proposed method for estimating the specific surface area predict the saturated hydraulic conductivity of a fairly wide range of clayey to silty-sandy soils, with measured hydraulic conductivity in the range 1×10 -11÷5×10 -6 m/s.
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