Burrowing in marine muds by crack propagation: kinematics and forces.

The polychaete Nereis virens burrows through muddy sediments by exerting dorsoventral forces against the walls of its tongue-depressor-shaped burrow to extend an oblate hemispheroidal crack. Stress is concentrated at the crack tip, which extends when the stress intensity factor (KI) exceeds the critical stress intensity factor (KIc). Relevant forces were measured in gelatin, an analog for elastic muds, by photoelastic stress analysis, and were 0.015+/-0.001 N (mean +/- s.d.; N=5). Measured elastic moduli (E) for gelatin and sediment were used in finite element models to convert the forces in gelatin to those required in muds to maintain the same body shapes observed in gelatin. The force increases directly with increasing sediment stiffness, and is 0.16 N for measured sediment stiffness of E=2.7 x 10(4) Pa. This measurement of forces exerted by burrowers is the first that explicitly considers the mechanical behavior of the sediment. Calculated stress intensity factors fall within the range of critical values for gelatin and exceed those for sediment, showing that crack propagation is a mechanically feasible mechanism of burrowing. The pharynx extends anteriorly as it everts, extending the crack tip only as far as the anterior of the worm, consistent with wedge-driven fracture and drawing obvious parallels between soft-bodied burrowers and more rigid, wedge-shaped burrowers (i.e. clams). Our results raise questions about the reputed high energetic cost of burrowing and emphasize the need for better understanding of sediment mechanics to quantify external energy expenditure during burrowing.

Seismic response of bridges to differential support ground motion

A methodology is presented to generate spatially varying seismic ground motion time histories at a number of prescribed locations on the ground surface, compatible with prescribed response spectra and durantion of strong ground motion, and reflecting the wave propagation and loss of coherence effects. The prescribed locations can correspond to different local soil conditions and therefore different response spectra can be assigned to each location.This methodology is then used in a case study of the SR14/15 Interchange that has partially collapsed during the 1994 Northridgeearthquake. This case study includes a set of different scenario earthquakes with different velocities of wave propagation, different angles of incidence of seismic waves with respect to the axis of the bridge, and combinations of vertical and horinzontal components of ground motion. It was concluded that the maximum stress at critical locations of the bridge can show increases up to 18

, compared to the case of identical support ground motion. (AU)