Civil engineering and building works

2.9 Interpretation of site investigations

Whereas a factual report of any site investigation should always be compiled for record purposes, there will be occasions when interpretations of the factual findings are required in addition. Such interpretive reports are usually published in a separate cover from the corresponding factual data. They may be compiled by one or more staff members from a contracting, con­ sulting or client organisation. Naturally the authors should be fully qualified and highly competent people who, from their specialist experience, are fully able to translate the facts into sound geotechnical advice. It is also of prime importance that the subject matter of the interpretive report is agreed by the parties concerned before the report is commissioned. Depending on the report content, the authors may have a ground engi­ neering, geotechnical or geological specialism — the all important issue is that the authors are selected for their eminence in the matters requiring interpretation.

The subject matter of interpretive reports varies con­ siderably, depending on the terms of reference for the site under consideration. But a typical report might give advice on the following matters:

•Safe bearing capacity of soils and rocks.

’• Settlement criteria relative to major structures — immediate settlement, long-term settlement, creep considerations and differential settlement.

•Dewatering requirements and solutions.

•Design slopes for cuttings and embankments.

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• Compaction and settlement of back-filling and embankments.

Chapter 3

concern the development and structure of the crust around the site and its state of stress and current dynamics, and extend from the surface to the greatest depths permitted by the evidence. The investigation may include the monitoring of local seismic events on a specially-commissioned microseismic network.

The data absorbed into the study are of numerous types and drawn from many sources, but resolve into two principal categories — the record of the earth­ quakes themselves (as recovered from historical and instrumental sources), and the evidence of crustal movement, past and current (as inferred from geo­ logical and geomorphological studies). One major task is to reconcile these two, very different, types of information.

Using established empirical correlations relating isoseismal areas with relevant instrumental determin­ ations of surface wave magnitudes and focal depths, these latter parameters are estimated for each earth­ quake.

The completeness of any catalogue of earthquakes is dependent on the production and preservation of con­ temporary accounts, and hence not only on the disposi­ tion of recording centres and the state of communica­ tions around them, but also on the present day survival and availability of their records. Thus some gaps in the historical earthquake records may simply be cultural.

a function of the severity and extent of the effects of the earthquakes that occurred. On the basis of his­ toriographical research, two thresholds are chosen to define through history those earthquakes which, had they occurred, must have been reported in surviving accounts (Set 1), and those which are likely, but not certain, to appear in the record (Set 2, which includes Set 1). Small earthquakes falling below the lower threshold (Set 3) are taken to have been recorded fortuitously and, although possibly significant in terms of their location, have little statistical importance.

3.1 Geology

Knowledge of the surface geology across the region of

of satellite imagery and local field studies supple­ mented by aerial photographs. The subsurface strati­ graphy is known from deep boreholes and, in combina­ tion with geophysical data, helps to map subsurface structure. An exhaustive review of the geological and geomorphological literature is supported by fieldwork in pursuit of evidence for recent crustal movement. Finally, at locations indicated by the field evidence, radiometric and palaeontological dating is carried out to constrain last dates of fault movement.

3.2 Earthquakes

The seismological database incorporates all earth­ quakes known from historical and instrumental sources. 'The historical record is critically re-evaluated from the primary sources using a data searching tech­ nique designed historiographically to minimise the chance of 'missing' any earthquakes or of failing to recover any reports of significant felt (niacroseismic) effects. Secondary sources are analysed to identify and explain shortcomings (particularly multiple entries or incorrect dates) in earlier catalogues. Descriptions of fell effects of earthquakes are assessed with regard to their contemporary cultural contexts and ‘intensities’ are attributed on the basis of comparison with standard scales of diagnostics such as the Modified Mercalli Scale, most of which extend a scale of 1 to XII.

Each macroseismic earthquake is treated by mapping the assessed intensities and drawing isoseismals, i.e., intensity contours enclosing common levels of severity. In many cases only the ‘felt area' of the earthquake, as enclosed by isoseismal III, can be represented, but for a few larger events isoseismals are drawn for higher intensities. From the mapped data, the ‘macrocentre’ (the centre or peak of the intensity field) is located and graded according to the inherent uncertainty of this location, which depends on both the quantity and the quality of the data available to constrain it. Four grades are used: better than 5 km uncertainty; better than 10 km, better than 20 km and worse than 20 km uncertainty.

3.3 Crustal dynamics

Because the historical record of seismicity offers only a brief and incomplete picture of contemporary crustal dynamics (and hence the controls on future seismicity) supplementary sources of information are researched.

Evidence of relative movements of the surface, determined over a variety of timescales, can indicate local and regional concentrations of crustal deforma­ tion. However, geological observations of crustal

movement are only of direct relevance to the contem­

Britain that the Current Tectonic Regime has not persisted for longer than about 8-6 million years, since

the past 2 million years has removed much potential evidence for recent tectonic movements so that, in general, only information for the past 10 000 years of the post-glacial Holocene remains. Observations from tide gauges and from repeated geodetic levelling sur­ veys should provide some insight into contemporary vertical movements, although the duration of accurate monitoring means that any tectonic signal can scarcely be resolved from the inevitable determination errors.

The implications, in terms of direct measurement of deformation patterns, from all three of these data sources is compared Xvith the strain rates inferred from the historical record of seismicity.

Measurements of the state of stress from in-situ borehole observations are not widely available but observations from southern and southwest England, as well as more widely across northwest Europe, confirm a regionally consistent northwest to southeast principal horizontal compressive stress.

The record of seismicity is examined for any correla­ tion with crustal provinces defined by the geological data. Also, in seismic hazard assessment a primary uncertainty is whether or not a specific fault is active; determination of this ‘active status’ is by criteria, utilising both geological and seismological evidence, which have been developed specifically for use in" Britain.

The seismotectonic constraints identified by these studies are incorporated in the construction of a model for the computation of the seismic hazards at the site.

3.4 Ground motion hazard

Earthquake ‘ground motion’ describes the pheno­ menon of shaking as the earthquake waves arrive at, or travel across, the surface of the earth. By virtue of geo­ metrical spreading and material damping, the severity of shaking reduces with distance from the .focus of the earthquake until eventually wavetrains are so reduced in energy that they can be observed only by extremely sensitive instruments. Although engineering signifi­ cance is attached only to the area within which so called ‘strong ground motion’ is experienced, this can still extend over considerable lateral distances depending on the size of the earthquake and its depth. Ground motion is conventionally described in terms of peak free field horizontal acceleration expressed as a percen­ tage of gravity.

A numerical formulation has been developed for treating the inherent random uncertainty in frequency and location of earthquakes within sources and in the