Characterisation of a high plasticity marine clay using a T-bar penetrometer

A series of laboratory-scale T-bar penetrometer tests have been conducted on a clay bed virgin consolidated from reconstituted high plasticity marine clay. This investigation was mainly concerned with the effects on the penetration resistance of rate of penetration and the presence of free water on the surface of the clay bed. The rate of penetration varied between 0.005mm/s and 50mm/s. The results showed that the nature of soil resistance was 'undrained' over the range of speeds studied, and the resistance showed a marked viscous rate effect. The virgin consolidated clay bed exhibited an increase in penetration resistance by up to 35% for a factor 10 increase in rate of penetration much larger than values previously reported for kaolin. The presence of water on the surface of clay bed had a profound impact on penetration resistance, particularly on the remoulded strength obtained by taking the T-bar through successive penetration and extraction cycles. This was true even when the remoulding cycles were conducted without the T-bar breaking through the clay surface.

A constitutive description of the inelastic response of ceramics

The objective of the article is to present a unified model for the dynamic mechanical response of ceramics under compressive stress states. The model incorporates three principal deformation mechanisms: (i) lattice plasticity due to dislocation glide or twinning; (ii) microcrack extension; and (iii) granular flow of densely packed comminuted particles. In addition to analytical descriptions of each mechanism, prescriptions are provided for their implementation into a finite element code as well as schemes for mechanism transitions. The utility of the code in addressing issues pertaining to deep penetration is demonstrated through a series of calculations of dynamic cavity expansion in an infinite medium. The results reveal two limiting behavioral regimes, dictated largely by the ratio of the cavity pressure p to the material yield strength σY. At low values of p/σY, cavity expansion occurs by lattice plasticity and hence its rate diminishes with increasing σY. In contrast, at high values, expansion occurs by microcracking followed by granular plasticity and is therefore independent of σY. In the intermediate regime, the cavity expansion rate is governed by the interplay between microcracking and lattice plasticity. That is, when lattice plasticity is activated ahead of the expanding cavity, the stress triaxiality decreases (toward more negative values) which, in turn, reduces the propensity for microcracking and the rate of granular flow. The implications for penetration resistance to high-velocity projectiles are discussed. Finally, the constitutive model is used to simulate the quasi-static and dynamic indentation response of a typical engineering ceramic (alumina) and the results compared to experimental measurements. Some of the pertinent observations are shown to be captured by the present model whereas others require alternative approaches (such as those based on fracture mechanics) for complete characterization. © 2011 The American Ceramic Society.

Probabilistic soil identification based on cone penetration tests

In geotechnical engineering, soil classification is an essential component in the design process. Field methods such as the cone penetration test (CPT) can be used as less expensive and faster alternatives to sample retrieval and testing. Unfortunately, current soil classification charts based on CPT data and laboratory measurements are too generic, and may not provide an accurate prediction of the soil type. A probabilistic approach is proposed here to update and modify soil identification charts based on site-specific CPT data. The probability that a soil is correctly classified is also estimated. The updated identification chart can be used for a more accurate prediction of the classification of the soil, and can account for prior information available before conducting the tests, site-specific data, and measurement errors. As an illustration, the proposed approach is implemented using CPT data from the Treporti Test Site (TTS) near Venice (Italy) and the National Geotechnical Experimentation Sites (NGES) at Texas A&M University. The applicability of the site-specific chart for other sites in Venice Lagoon is assessed using data from the Malamocco test site, approximately 20 km from TTS.

Uplift resistance of buried pipelines at low cover-diameter ratios

Reliable estimates for the maximum available uplift resistance from the backfill soil are essential to prevent upheaval buckling of buried pipelines. The current design code DNV RP F110 does not offer guidance on how to predict the uplift resistance when the cover:pipe diameter (H/D) ratio is less than 2. Hence the current industry practice is to discount the shear contribution from uplift resitance for design scenarios with H/D ratios less than 1. The necessity of this extra conservatism is assessed through a series of full-scale and centrifuge tests, 21 in total, at the Schofield Centre, University of Cambridge. Backfill types include saturated loose sand, saturated dense sand and dry gravel. Data revealed that the Vertical Slip Surface Model remains applicable for design scenarios in loose sand, dense sand and gravel with H/D ratios less than 1, and that there is no evidence that the contribution from shear should be ignored at these low H/D ratios. For uplift events in gravel, the shear component seems reliable if the cover is more than 1-2 times the average particle size (D50), and more research effort is currenty being carried out to verify this conclusion. Strain analysis from the Particle Image Velocimetry (PIV) technique proves that the Vertical Slip Surface Model is a good representation of the true uplift deformation mechanism in loose sand at H/D ratios between 0.5 and 3.5. At very low H/D ratios (H/D < 0.5), the deformation mechanism is more wedge-like, but the increased contribution from soil weight is likely to be compensated by the reduced shear contributions. Hence the design equation based on the Vertical Slip Surface Model still produces good estimates for the maximum available uplift resistance. The evolution of shear strain field from PIV analysis provides useful insight into how uplift resistance is mobilized as the uplift event progresses. Copyright 2010, Offshore Technology Conference.

Uplift resistance of buried pipelines in blocky clay backfill

This paper presents the results from 10 minidrum centrifuge tests conducted at the Schofield Centre, compiled with 4 additional test results from Thusyanthan et al., 2008. All these tests were designed to measure the uplift resistance of a pipeline installed into stiff clay by trenching and backfilling, then uplifted approximately 3 months after installation. All tests were conducted at 1:30 scale using soil obtained from offshore clay samples. Experimental results show that clay blocks remained intact after 3 prototype months of consolidation, and were lifted rather than sheared during pipe pullout. The uplift resistance therefore depends on the weight of the soil cover and the shearing resistance mobilised at the softening contact points between the intact blocks and within the interstitial slurry. Slow drained pullout led to lower resistance than fast pullout, indicating that the drained response is critical for design. The varying scatter shows that peak uplift resistance is very sensitive to the arrangement of the backfill blocks when the cover and pipe diameter are comparable to the block size. Copyright © 2009 by The International Society of Offshore and Polar Engineers (ISOPE).

On the use of densification as a liquefaction resistance measure

The use of densification to improve the performance of shallow foundations during the centrifuge modeling of earthquake-induced liquefaction on level sand deposits is discussed. The densification of liquefiable ground provided protection against or significantly reduces liquefaction-related damage. Propagation of accelerations in the deposit exhibited considerable distinct features according to the relative density of the sand in the model. It was found that during the first couple of cycles, the dense soil amplifies the fundamental frequency component of the earthquake and preserves the higher frequency components.

Mechanisms of projectile penetration in Dyneema® encapsulated aluminum structures

Polymer composites comprising ultra-high molecular weight polyethylene (UHWMPE) fibers in a compliant matrix are now widely used in ballistic applications with varying levels of success. This is primarily due to a poor understanding of the mechanics of penetration of these composites in ballistic protection systems. In this study, we report experimental observations of the penetration mechanisms in four model systems impacted by a 12.7 mm diameter spherical steel projectile. The four model targets designed to highlight different penetration mechanisms in Dyneema® UHWMPE composites were: (i) a bare aluminum plate; (ii) the same plate fully encased in a 5.9 mm thick casing of Dyneema®; (iii) the fully encased plate with a portion of the Dyneema® removed from the front face so that the projectile impacts directly the Al plate; and (iv) the fully encased plate with a portion of the Dyneema® removed from the rear face so that the projectile can exit the Al plate without again interacting with the Dyneema®. A combination of synchronized high speed photography with three cameras, together with post-test examination of the targets via X-ray tomography and optical microscopy was used to elucidate the deformation and perforation mechanisms. The measurements show that the ballistic resistance of these targets increases in the order: bare Al plate, rear face cutout target, fully encased target and front face cutout target. These findings are explained based on the following key findings: (a) the ballistic performance of Dyneema® plates supported on a foundation is inferior to Dyneema® plates supported along their edges; (b) the apparent ballistic resistance of Dyneema® plates increases if the plates are given an initial velocity prior to the impact by the projectile, thereby reducing the relative velocity between the Dyneema® plate and projectile; and (c) when the projectile is fragmented prior to impact, the spatially and temporally distributed loading enhances the ballistic resistance of the Dyneema®. The simple model targets designed here have elucidated mechanisms by which Dyneema® functions in multi-material structures. © 2014 Elsevier Ltd.