The plastic limit of clays

The plastic limit of soils was first described by Atterberg in 1911. The thread-rolling test was standardised at the US Public Roads Bureau in the 1920s and 1930s, and has subsequently become one of the standard tests of soil mechanics. This paper reviews the original definitions of plastic limit as proposed by Atterberg, and proposes that the brittle failure observed in the plastic limit test is caused by either air entry or cavitation in the clay. Critical state soil mechanics is used to show that the observed range of undrained shear strengths of soils at plastic limit is consistent with this hypothesis. The fallacy that strength at plastic limit is a constant is highlighted, and the implications for geotechnical practice are discussed. </jats:p

Microbially Induced Calcite Precipitation (MICP) to mitigate contact erosion in earth embankment dams and levees

Internal erosion of water retaining structures (such as earth embankment dams, levees and dykes) is a major geotechnical problem. Contact erosion is a specific type of internal erosion that occurs at the interface between fine and coarse soils, for instance along the downstream edge of the core-filter interface, and can lead to earth dam failure due to internal erosion. Although in new dams this may be avoided by fulfilling the filter criteria or with the construction of barriers, retrofitting older structures often entails significant design and construction costs due to the uncertainties surrounding their materials and behaviour. In this context, microbially induced calcite precipitation (MICP), a bacteria-induced bio-mineralisation process capable of binding soil particles in situ, provides a cost-effective alternative for contact erosion control. However, it is necessary to establish a solid understanding of how biogenic cementation occurs at the interface between fine and coarse sands and its influence on the erosion and hydro-mechanical characteristics.The author would like to acknowledge Mr Chris Knight for the production of the EFA device and EPSRC for funding this studentship as part of the Centre for Doctoral Training in Future Infrastructure and Built Environment (EP/L016095/1)

Liquefaction Induced Settlement of Structures

Soil liquefaction following earthquakes leads to excessive damage to a wide variety of structures. Settlement and rotation of structures following liquefaction have been witnessed in many of the recent earthquakes. Investigation of the mechanisms of failure of structure when the foundation soil suffers either partial or full liquefaction is therefore very important. Dynamic centrifuge tests were conducted at Cambridge and elsewhere on different boundary value problems in which liquefaction of soil models was investigated. Excess pore pressure data and the settlement data for the particular structure that is being investigated are recorded during the centrifuge tests. In this paper the centrifuge test results from a range of structures will be considered. The co-seismic and post seismic settlement of structures will be considered separately along with the excess pore pressure recorded generated during the cyclic loading. It will be argued that the co-seismic component of the settlement is much larger than the post-seismic settlement in many of the structures considered. Accordingly a hypothesis that the hydraulic conductivity k of the liquefied soil during the earthquake shaking is much higher than the normal hydraulic conductivity is proposed. A discussion on the micro-mechanical reasons for this increased hydraulic conductivity is presented

Flume study on the effects of microbial induced calcium carbonate precipitation (MICP) on the erosional behaviour of fine sand

Tangential flow-induced interface erosion poses a major threat to a wide variety of engineering structures, such as earth-filled embankment dams, and oil- and gas-producing wells. This study explores the applicability of microbial induced calcium carbonate (CaCO3) precipitation (MICP) by way of the ureolytic soil bacterium Sporosarcina pasteurii as a method for enhancing the surface erosion resistance of fine sand. Specimens were treated with cementation solution concentrations between 0·02 and 0·1 M, and the erosional behaviour examined in a flume under surface-parallel flow and increasing shear stress. Photographs, cumulative height eroded-time series and erosion rates were obtained as a function of specimen height, MICP treatment formulation and calcium carbonate content. Results showed that while untreated specimens eroded primarily in particulate and mass form, MICP-treated specimens were characterised by a block erosion mechanism. Further, erodibility was found to depend on the calcium carbonate content and the cementation solution concentration. To understand this, a systematic study of the calcium carbonate crystal sizes and distributions was undertaken through X-ray computed tomography. Fundamentally, the effectiveness of MICP for erosion control was found to be dominated both by the precipitated calcium carbonate content and microstructural features, with higher contents and larger crystals yielding lower erodibility values. Additionally, crystal growth mechanisms varied depending on the cementation solution concentration.</p

Centrifuge Modelling of Earthquake Effects in Uniform Deposits of Saturated Sand

Centrifuge models representing level uniform saturated deposits of relatively loose and dense sand were tested at Cambridge University\u27s Schofield Centre to clarify the behaviour of these deposits under earthquake loading. The excess pore pressure, vertical propagation of the accelerations and ground surface settlements resulting from a model earthquake are presented and discussed. The results show that, for similar dynamic loading, the models undergo large shear stiffness degradation resulting from significant pore pressure build up, this taking place at a slower rate in the dense sand. As a result of the cyclic loading, the models suffer settlements, occurring mostly during the event, that are noticeably smaller in the dense model. The upwards propagation of the accelerations through the model depends on the relative density of the sand and changes during the seismic event, following degradation of sand mechanical properties. Large short-duration acceleration spikes are observed near the surface of the dense model, corresponding to large amplification of input acceleration. The results presented and discussed contribute to the understanding of the basic mechanisms of earthquake-induced liquefaction and the use of densification as a measure to mitigate its effects

Characterisation of CaCO3 phases during strain-specific ureolytic precipitation

Numerous microbial species can selectively precipitate mineral carbonates with enhanced mechanical properties, however, understanding exactly how they achieve this control represents a major challenge in the field of biomineralisation. We have studied microbial induced calcium carbonate (CaCO3) precipitation (MICP) in three ureolytic bacterial strains from the Sporosarcina family, including S. newyorkensis, a newly isolated microbe from the deep sea. We find that the interplay between structural water and strain-specific amino acid groups is fundamental to the stabilisation of vaterite and that, under the same conditions, different isolates yield distinctly different polymorphs. The latter is found to be associated with different urease activities and, consequently, precipitation kinetics, which change depending on pressure-temperature conditions. Further, CaCO3 polymorph selection also depends on the coupled effect of chemical treatment and initial bacterial concentrations. Our findings provide new insights into strain-specific CaCO3 polymorphic selection and stabilisation, and open up promising avenues for designing bio-reinforced geo-materials that capitalise on the different particle bond mechanical properties offered by different polymorphs

Assessment of bridge natural frequency as an indicator of scour using centrifuge modelling

Funder: Gates Cambridge Trust (GB)Abstract: One of the most prevalent causes of bridge failure around the world is “scour”—the gradual erosion of soil around a bridge foundation due to fast-flowing water. A reliable technique for monitoring scour would help bridge engineers take timely countermeasures to safeguard against failure. Although vibration-based techniques for monitoring structural damage have had limited success, primarily due to insufficient sensitivity, these have tended to focus on the detection of local damage. High natural frequency sensitivity has recently been reported for scour damage. Previous experiments to investigate this have been limited as a result of the cost of full-scale testing and the fact that scaled-down soil-structure models tested outside a centrifuge do not adequately simulate full-scale behaviour. This paper describes the development of what is believed to be the first-ever centrifuge-testing programme to establish the sensitivity of bridge natural frequency to scour. A 1/60 scale model of a two-span integral bridge with 15 m spans was tested at varying levels of scour. For the fundamental mode of vibration, these tests found up to a 40% variation in natural frequency for 30% loss of embedment. Models of three other types of foundation, which represent a shallow pad foundation, a deep pile bent and a deep monopile, were also tested in the centrifuge at different scour levels. The shallow foundation model showed lower frequency sensitivity to scour than the deep foundation models. Another important finding is that the frequency sensitivity to “global scour” is slightly higher than the sensitivity to “local scour”, for all foundation types. The level of frequency sensitivity (3.1–44% per scour depth equivalent to 30% of embedment of scour) detected in this experiment demonstrates the potential for using natural frequency as an indicator of both local and global scour of bridges, particularly those with deep foundations