49 Large-scale field trials
Large-scale field trials should be carried out in such a manner that the ground is tested on a scale and under conditions comparable with those prevailing in the project under investigation. Such trials, however, are likely to be costly in terms of instrumentation, technical support for the co-ordination of the results and requirements of purpose-made equipment. Trials should include an appropriate range of investigation methods, tests and instruments as described in Sections 4, 5 and 7. Large-scale field trials are not standard tests, and should be designed to suit the individual requirements of the proposed works and the particular ground on which or within which these are to be performed.
NOTE On large projects, field trials can provide the necessary design parameters and valuable construction data on excavation, handling and placing, resulting in considerable savings and enhanced safety. Such methods and trials can usefully be extended to the construction stage and to monitoring the interrelated response of the ground and structure after completion under the working conditions.
49.2 Methods of instrumentation
COMMENTARY ON 49.2
Numerous techniques are available to be used in ground investigations to monitor movements and strains, total stresses and pore water pressures associated with known or suspected ground behaviour. Such movements can result from construction processes, potential stability failures, tunnelling, subsidence and ground response in large scale field trials. The types, advantages, limitations and appropriateness of the various techniques are discussed in detail elsewhere.
Field instrumentation is technology-based, which advances rapidly. Accordingly, users of instrumentation are encouraged to follow the developments by browsing manufacturers' websites and using other online sources of information (e.g. www.geotechnicalnews.com).
Ground movements are normally associated with stress redistribution and pore pressure changes which are characteristic of the particular ground and should be measured in terms of the displacement of points, which can be positioned on the surface of the ground or within the ground mass. The displacement of a point should be referred to a stable reference position, and sufficient measurements taken to define movement in three dimensions if this is required. The relative movement between adjacent points can be used to obtain strain.
Surface movements should be measured using one of the following:
- precise levelling;
- total stations (automatic or manually operated)
- photogrammetric methods; or
- global positioning systems (GPS).
NOTE 1 An accuracy of ±0,5 mm can be achieved with precise levelling and ±3 ppm for distance measurements over 2 000 m can be achieved using EDM (electronic distance measurement) instruments. Manually operated total stations can typically deliver an accuracy of ±2,5 mm in X, Y and Z coordinates of a monitoring point if the sighting distance is less than 75 m. Automatic (robotic) totals stations mounted on fixed platforms that don't move can deliver accuracies as good as ±1,0 mm if they are properly installed and maintained.
Care should be taken to position reference points away from the effects of movements due to load and water changes. Due account should be made for atmospheric distubances and care taken to avoid positioning instruments in locations that are adversely affected by wind, vibrations and construction activities.
Internal movements or displacements and stresses should be measured in boreholes or by direct placement of instruments within fill using the following techniques.
- a) Extensometers and settlement gauges:
- plate; and
- b) Lateral or horizontal movements measured by:
- tilt sensors
- inverted pendulums; and
- magnet plate gauges.
- c) Total stress monitored using:
- hydraulic cells;
- vibrating-wire cells;
- push-in type cells; and
- interface pressure measurements.
The techniques for the measurement of pore pressure response are covered in Clause 52.
NOTE 2 Many types of automatic data-logging are available. It is essential that great care is taken when maintaining and assessing the performance of the instruments and the quality of the data recorded.
NOTE 3 For the design of an instrumentation programme, see Section 8.
49.3 Trial embankments and excavations
COMMENTARY ON 49.3
The construction of trial embankments can serve a threefold purpose: the quality and compaction characteristics of available borrow material can be determined on the field scale and compared with laboratory test results; the characteristics and performance of placing and compacting equipment can be investigated; and the strength and settlement characteristics of the ground on which the embankment is placed can be examined. A trial embankment can be constructed in such a way that, where failure is of no consequence, it can be induced deliberately, either in the embankment alone or in the embankment and the foundations. Such failures sometimes occur in an unexpected manner, and precautions ought to be taken by the engineer to ensure that no injury to persons or unexpected damage is caused; even so, some installed instrumentation might be destroyed. The value of such a failure is that back analysis (see 49.5) can be used to check strength parameters.
Compaction trials can include experiments using differing borrow pit materials, layer thickness, amounts of watering and amounts of work performed in compaction. Measurements should be taken of in-situ density and water content and comparisons made both with laboratory compaction tests to obtain a specification standard, and with in-situ borrow pit densities, so that the degree of bulking or volume reduction can be estimated for given quantities (see BS 6031). Trials of equipment can also be undertaken. Care should be taken not to vary too many factors at the same time, otherwise the effects of variation cannot be estimated.
NOTE Trial excavations yield information on the material excavated and the performance of excavating equipment, and they also permit more detailed examination of the ground than is possible from borehole samples. Excavations can sometimes be used to test the short-term stability of excavated slopes. Trial excavations can be constructed deliberately to fail. However, failure in excavation, especially if deep, is correspondingly more dangerous than failure of fills, and increased vigilance is needed. Trial excavations also enable the response of the ground and groundwater to excavation to be measured.
Adequate instrumentation to trial embankments or excavations should be used, together with continuous observation (see 33.2), if the maximum information is to be gained. The scale of trial embankments or excavations should be carefully evaluated. The more closely the size of the trial approaches that of the actual works, the more directly applicable are the results obtained from the trial.
49.4 Construction trials
COMMENTARY ON 49.4
In many projects, considerable value can be derived from trials carried out before the commencement of the permanent works. Such trials permit the evaluation of the procedures to be adopted and the effectiveness of the various expedients. As with ail large-scale testing, a prior knowledge of the characteristics of the ground is essential. The results of the trial give an assessment of the properties of the ground and often enable the results to be correlated with those obtained from routine ground investigation methods.
A wide range of expedients should be tested in trials. Examples include: construction methods, such as pile tests; ground anchor tests; compaction tests for earthworks, experimental shafts and adits for tunnels; and construction methods such as grouting trials, trial blasts for explosives and dewatering trials.
49.5 Back analysis of full-scale performance
COMMENTARY ON 49.5
Natural or man-made conditions on a site sometimes produce phenomena that can be used to assess parameters which are otherwise difficult to assess, or that can be used to check the validity of parameters measured in the laboratory. Examples of such phenomena are slope failure and settlement of a structure. It might be possible, starting from the observed phenomena, to perform a back analysis and, in the case of a slope failure, to arrive at shear strength parameters that fit the observed facts.
Back analysis of settlements is also possible, but care should be taken in assessing actual loadings and the times when they have taken place. For a back analysis to be effective, it should be accompanied by a full investigation to determine the ground and groundwater conditions. The development of finite element techniques has greatly improved the ability to back analyse more complicated geotechnical structures.