Dynamic analysis of a smooth penetrometer free-falling into uniform clay

Abstract

The finite-element analysis of free-falling objects penetrating soil deposits is one of the most sophisticated and challenging problems in geomechanics. A robust numerical method will be described here for dealing with such complex and difficult problems. The approach is based on the arbitrary Lagrangian-Eulerian (ALE) method of analysis, the main features and challenges of which are described briefly in the paper. The ALE method is then employed to perform a parametric study of a perfectly smooth penetrometer free-falling into a uniform layer of clay, which deforms under undrained conditions. The effect of the mechanical properties of the clay soil on the penetration characteristics is presented, and an approximate, closed-form expression is derived for the dynamic penetration factor, N dp : Comparisons are made between the deduced values of N dp and published values of the conventional cone factor, N c , and comparisons are made with experimental data to validate the approach.KEYWORDS: numerical modelling; offshore engineering; shear strength; site investigation; soil/structure interaction L'analyse aux éléments finis d'objets en chute libre péné-trant dans des dépôts de sol est un des problèmes les plus sophistiqués et difficiles de la géomécanique. La présente communication décrit une méthode numérique robuste pour résoudre des problèmes aussi complexes et difficiles. Cette approche est basée sur la méthode d'analyse Lagrangien-Eulérien arbitraire (ALE), dont les principaux problèmes et caractéristiques sont décrits brièvement. On utilise ensuite la méthode ALE pour effectuer une étude paramétrique d'un pénétromètre parfaitement lisse tombant dans une couche d'argile uniforme, qui se déforme dans des conditions non drainées. On présente l'effet des propriétés mécaniques du sol d'argile sur les caractéris-tiques de pénétration, et on dérive une expression à forme fermée pour le facteur de pénétration dynamique N dp : On effectue des comparaisons entre les valeurs déduites de N dp et des valeurs publiées du facteur de cone N c , pour procéder ensuite à des comparaisons avec des données expérimentales pour valider cette approche.

Validation of impact penetrometer data by cone penetration testing and shallow seismic data within the regional geology of the Southern North Sea

“…For cohesive sediments such as soft clays, either side friction along the rod needs to be taken into account, or the penetrometer needs to be designed (e.g., as full-flow penetrometer, or by the incorporation of friction reducers) to minimize friction at the interface between rod and soil (cf. Lunne et al 1997;Randolph 2004;Chung and Randolph 2004). At peak deceleration of 20 m/s 2 , the buoyancy of the rod immersed in the sediment amounts to approx.…”

“…Correction of cone resistance data Due to the mechanical design of the cone, the cone resistance measured by the instrumented tip of the impact penetrometer q dyn cone needs to be corrected for the unequal area effect (Lunne et al 1997) to obtain the corrected total cone resistance:…”

“…Typically, a reference velocity of 0.02 m/s is chosen to compare impact penetrometer data with static velocity CPT measurements. The soil viscosity coefficient has been applied for dynamic penetrations with penetrometers in clayey sediments, where it ranges between 0.04 and 1.50 (Dayal and Allen 1975;Dayal 1980;Randolph 2004;O'Loughlin et al 2004O'Loughlin et al , 2013Einav and Randolph 2005;Aubeny and Shi 2006;Low et al 2008;Zhou and Randolph 2009;Young et al 2011;Nazem et al 2012;Steiner et al 2012Steiner et al , 2014Airey 2013, 2014). Applications of the strain rate correction on silty sediments yield a soil viscosity coefficient between 0.13 and 0.36 (Perlow and Richards 1977;Biscontin and Pestana 2001).…”

“…Applications of the strain rate correction on silty sediments yield a soil viscosity coefficient between 0.13 and 0.36 (Perlow and Richards 1977;Biscontin and Pestana 2001). Several authors employing lightweight dynamic penetrometers penetrating sands at high rates have used values of 0.7-1.5 for the soil viscosity coefficient λ, although Dayal (1974), Dayal and Allen (1975), and Lunne et al (1997) concluded from laboratory observations that the strain rate correction is applicable only to cohesive sediments.…”

“…Measurements are taken in discrete intervals as a function of soil depth while maintaining a constant (static) penetration velocity. Correction and interpretation of these data are well assured by a broad theoretical (Baligh 1985;Teh 1987) and empirical background (see Lunne et al 1997 for review). Modern CPT instruments are equipped with strain gauges, and pore pressure is typically measured by means of pressure transducers.…”