Summary A classification of mudrocks for engineering purposes was considered necessary in order to improve the common qualitative use of a variety of geological terms. There has been a lack of agreement between authors on the precise definition of these terms. Mudrock is defined as a fine- to very finegrained, siliciclastic sediment or sedimentary rock. Quantitative boundaries for this group of rocks are proposed using meaningful soil and rock index parameters, at changes in mechanical or material behaviour. The basic division is between fine-grained engineering soil, non-durable rock and durable rock, at compressive strengths of 0.6 and 3.6 MN/m 2 . The mudrock soils are classified according to the British Standard Classification of Soils (BSCS) with the addition of the term silty clay. Fissile or anisotropic rocks are termed shales, including clayshales where non-durable. Isotropic ones are classed as siltstones, mudstones and claystones according to their quartz contents. Tectonically induced strength and fabric anisotropy can be important and may result in slaty shales where the rock microfabric has been well aligned. An upper compressive strength limit of 100 MN/m 2 is proposed to distinguish diagenetic mudrocks from their metamorphic equivalents.

Soil gas surveys have been carried out at research sites in Great Britain and Italy to test soil gas geochemistry as a site investigation technique for the detection of faults and discontinuities. At a site on Oxford Clay in Gloucestershire, soil gas anomalies of high He, Rn and CO 2 and low 02 were shown to correspond to the outcrop of a fault, identified by drilling and geophysics. Other apparently random anomalies remained unexplained and lateral migration of gas through superficial horizons complicated interpretation. Using three parallel sample lines at the same distance apart as the sample spacing it has proved not only possible statistically to remove spurious anomalies, but also to enhance and concentrate clusters of high values resulting from gas migration.This method was used to investigate a fault in Neogene clay at Narni in Italy where a gas pathway permeable to Rn and CO 2 was identified, corresponding to a geoelectrical anomaly indicating displaced strata. Further work was carried out over a fault revealed by trenching in the Caithness Flags in Scotland. For each three-line traverse an integrated gas anomaly map was produced to aid interpretation by allowing both sharp and diffuse anomalies to be more easily identified.

Summary The Dorset coast is an area with a long history of landslide activity and coastal erosion. Geologically the area is composed of the clays, mudstones and limestones of several divisions of the Lower Jurassic (Lower Lias) which are overlain unconformably by Cretaceous clays and sands (Gault and Upper Greensand.) Areas of serious instability appear to occur in close association with naturally occurring reservoirs of ground water, of which Black Ven at Char-mouth is a good example. Problems connected with a small coastal landslip near a private housing estate at Char-mouth were studied and shown to be related to a Pleistocene mudflow. The main purpose of the investigation was to suggest remedial works but the subsidiary aim, forming the theme of this paper, was to assess the usefulness of shallow depth geophysical and geotechnical methods not normally used in instability studies.

Variations in seismic velocity with depth have been measured in the Middle Chalk at the proposed 300 GeV proton accelerator site at Mundford, Norfolk, where they could be compared with the results obtained from a geological and geotechnical survey. It is shown that the seismic velocity increases with depth in clearly defined steps, and that these steps broadly correspond with some of the grades in the engineering classification adopted by the geotechnical survey. Correlation has also been obtained between the seismic results and the thickness of the overlying sandy drift, and with tectonic structures in the chalk. On a mesuré les variations de la vitesse de propagation des ondes sismiques en fonction de la profondeur dans le Calcaire Moyen du site qui a été proposé à Mundford (Norfolk), pour l'accélérateur de protons de 300 GeV où on pouvait les comparer aux résultats obtenus à partir des études géotechniques et géologiques. On montre que la vitesse de propagation augmente en fonction de la profondeur suivant une série de valeurs bien définies, et que ces valeurs correspondent approximativement à certaines catégories résultant de la classification visuelle de l'étude géotechnique. On a également obtenu une corrélation entre les résultats des vitesses de propagation et l'épaisseur de la couche détritique sableuse de surface, ainsi qu'avec les structures tectoniques du calcaire.

Well–to‐well seismic measurements are adapted to civil engineering problems by use of a sparker as a source of seismic signals and an 80 kHz hydrophone as a receiver. For display a Tektronix 549 oscilloscope is used. Field application shows that the delineation of interfaces between homogeneous strata and the detection and delineation of localized and irregular features is possible from inter‐well travel times. In‐situ measurement of the compressional wave velocity in a medium is often complicated by refraction and wave guide effects. The degree of fracturing cannot be estimated from travel time measurements alone in a tightly jointed, saturated, rock mass, but it may be possible to correlate variations in pulse shape and length with this parameter.