Processes of coastal bluff erosion in weakly lithified sands, Pacifica, California, USA

River bank erosion occurs primarily through a combination of three mechanisms: mass failure, fluvial entrainment, and subaerial weakening and weathering. Subaerial processes are often viewed as ‘preparatory’ processes, weakening the bank face prior to fluvial erosion. Within a river basin downstream process ‘domains’ occur, with subaerial processes dominating the upper reaches, fluvial erosion the middle, and mass failure the lower reaches of a river. The aim of this paper is to demonstrate that (a) subaerial processes may be underestimated as an erosive agent, and (b) process dominance has a temporal, as well as spatial, aspect.  Bank erosion on the River Arrow, Warwickshire, UK, was monitored for 16 months (December 1996 to March 1998) using erosion pins. Variations in the rate and aerial extent of erosion are considered with reference to meteorological data. Throughout the first 15 months all erosion recorded was subaerial, resulting in up to 181 mm a−1 of bank retreat, compared with 13 to 27 mm a−1 reported by previous researchers. While the role of subaerial processes as ‘preparatory’ is not contended, it is suggested that such processes can also be erosive.  The three bank erosion mechanisms operate at different levels of magnitude and frequency, and the River Arrow data demonstrate this. Thus the concept of process dominance has a temporal, as well as spatial aspect, particularly over the short time-periods often used for studying processes in the field. Perception of the relative efficacy of each erosive mechanism will therefore be influenced by the temporal scale at which the bank is considered. With the advent of global climate change, both these magnitude–frequency characteristics and the consequent interaction of bank erosion mechanisms may alter. It is therefore likely that recognition of this temporal aspect of process dominance will become increasingly important to studies of bank erosion processes. Copyright © 2001 John Wiley & Sons, Ltd.

Subaerial river bank erosion processes and their interaction with other bank erosion mechanisms on the River Arrow, Warwickshire, UK

In impaction grafting of contained bone defects after revision joint arthroplasty the graft behaves as a friable aggregate and its resistance to complex forces depends on grading, normal load and compaction. Bone mills in current use produce a distribution of particle sizes more uniform than is desirable for maximising resistance to shear stresses. We have performed experiments in vitro using morsellised allograft bone from the femoral head which have shown that its mechanical properties improve with increasing normal load and with increasing shear strains (strain hardening). The mechanical strength also increases with increasing compaction energy, and with the addition of bioglass particles to make good the deficiency in small and very small fragments. Donor femoral heads may be milled while frozen without affecting the profile of the particle size. Osteoporotic femoral heads provide a similar grading of sizes, although fewer particles are obtained from each specimen. Our findings have implications for current practice and for the future development of materials and techniques.

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Mechanical considerations in impaction bone grafting.

In this study, kaolinite and bentonite were investigated for the effect of thermal treatment on particle size, as well as water content, specific gravity, plasticity, activity index, swelling and compression index and strength properties. At first, kaolinite and bentonite were characterized using XRD and TG standard methods. All sieved samples were subsequently heated to various temperatures (100, 200, 400, 500 and 600°C respectively), and kept for 24 h at the maximum temperature. Odometer cells were used to describe the compressibility properties (loading and unloading) of the compacted-treated samples. Triaxial compression tests were carried out to determine thermal treatment effects on the strength parameters of all samples. It was found that kaolinite becomes non-plastic at 400°C and bentonite at 500°C. Grain size growth with increasing thermal treatment plays indeed a major role. Thermal treated kaolinite and bentonite up to 400°C drastically reduces their swelling behavior. Strong correlations between the angle of shearing resistance and thermal treatment of both materials and correlation coefficients have been demonstrated. The results showed that kaolinite and bentonite can be used as good stabilization materials if they are thermal treated; kaolinite up to 400°C and bentonite up to 500°C.

	 

	Key words: Kaolinite, bentonite, thermal treatment, index properties, scanning electron microphotograph.

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The effects of temperature on the characteristics of kaolinite and bentonite

From earlier research it has been suggested that chironomid larvae may significantly affect the physical properties of cohesive sediments, leading to chemical alteration in lake ecosystems. The present study was aimed to evaluate the effect of Tanytarsini larvae on the physical properties of cohesive sediments, both in controlled environments and in natural environments. For this the sediment surface was examined by low temperature scanning electron microscopy (LTSEM) and direct measurement of the shear strength. The larvae are able to modify the surface sediment significantly, e.g., by secretion of silk, which forms a matrix at the sediment surface in the vicinity of the larval tubes. The shear strength increased with greater sediment depth in all the experimental treatments, except where the densities were high. Considerably higher shear strength was observed in the natural sediment than in the experimental set up. A very significant density-dependent effect was observed within the top 1-cm by tube building Tanytarsini larvae. The relationship between larval densities and shear strength was not significant for 2 deeper sections (1–3 cm). The tube length was significantly greater in experimental containers with the low density (4960 ind. m−2) than in containers with high larval densities (19840 and 39680 ind. m−2).

Alteration of biogenic structure and physical properties by tube-building chironomid larvae in cohesive sediments

This book, which is intended to serve the needs of the undergraduate civil engineering student, provides information on the following aspects of soil mechanics: basic characterisitcs of soils; seepage; effective stress; shear strength; stresses and displacements; lateral earth pressure; consolidation theory; bearing capacity; stability of slopes; and ground investigation. Worked examples are included throughout the book.

R F Craig

Papers

Soil mechanics. 5th edition