B4.1 Significance
Characteristic valuesof geotechnical parameters are fundamental to all calculations carried out in accordance with the code. Their definition, in geotechnical terms, has been the most controversial topic in the whole process of drafting Eurocode 7. Some of the more difficult issues will be addressed here. More straightforward matters will be left to Part C. The most important text is in EC7, 2.4.3.
Two factors underlie the importance and controversy of characteristic values.
a Calculations are to be carried out by applying partial safety factors to characteristic values in order to obtain design valuesof parameters. The partial factors are specified by the code, so the selection of characteristic values is the main point in calculations at which engineers are to apply their skills and judgment, with the possibility of dangerous mistakes.
b Engineers have always had the responsibility for selecting values of material parameters for calculations. This process has sometimes been referred to as a ‘black art’, and it is difficult to find helpful advice on the thought processes necessary to derive appropriate values from site investigation and other information. In particular, the degree of conservatism necessary in choosing values for design purposes is rarely discussed.
Eurocode 7’s definition of characteristic values is intended to make full use of the skills and judgment of experienced engineers, whilst helping less
experienced engineers to choose values which are both reasonable and safe.
This was, and remains, a major challenge.
B4.2 Characteristic values in Eurocode 1 and in structural design Characteristic values, as used in Eurocode 7, are intended to comply with Eurocode 1 as far as possible, whilst remaining true to principles of sound geotechnical engineering. Although it arguably remains within the spirit of Eurocode 1, the definition adopted for geotechnical purposes differs from that of Eurocode 1 in some important respects. To understand this, it is necessary first to consider what Eurocode 1 says about characteristic values, Xk.
Eurocode 1, Subclause 9.3.3 states:
The design value Xdof a material or product property is generally defined as:
Xd= ηXk/ γMor Xk/ γM
where:
γMis the partial factor for the material or product property, given in ENVs 1992 to 1999, which covers:
● unfavourable deviations from the characteristic values;
● inaccuracies in the conversion factors; and
● uncertainties in the geometric properties and the resistance model.
ηis the conversion factor taking into account the effect of the duration of the load, volume and scale effects, effects of moisture and temperature and so on.
Characteristic values are introduced in Eurocode 1 Section 5 thus:
(1)P Properties of materials (including soil and rock) or products are represented by characteristic values which correspond to the value of the property having a prescribed probability of not being attained in a hypothetical unlimited test series.
They generally correspond for a particular property to a specified fractile of the assumed statistical distribution of the property of the material in the structure.
(2) Unless otherwise stated in ENVs 1992 to 1999, the characteristic values should be defined as the 5% fractile for strength parameters and as the mean value for stiffness parameters.
Note: For operational rules, see annex D, for fatigue, information is given in annex B.
(3)P Material property values shall normally be determined for standardized tests performed under specified conditions. A conversion factor shall be applied where it is necessary to convert the test results into values which can be assumed to represent the behaviour of the material in the structure or the ground (see also ENVs 1992 to 1999).
This text specifies the following features:
a Characteristic values take account of the statistical distribution of the property. That is, the range of uncertainty of the property is relevant to their selection.
b They can normally be derived by a statistical process applied to a series of tests on specimens of the material. However, in principle they relate to a hypothetical, unlimited test series, so some correction may be required when test series are limited.
c For strength properties, they are to correspond to the 5% low fractile of the test results; this is the strength below which 5% of test results fall.
d Nevertheless, the characteristic values are said to represent the behaviour of the material in the structure or the ground, and corrections to test results may be needed in order to achieve this.
e For stiffness, mean values are to be used. This is considered further in B4.12.
These definitions of characteristic value are clearly intended to be general.
Eurocode 1 does not at this point mention the mode of failure or type of limit state being discussed, or the severity of its consequences.
In structural design, characteristic values are generally defined using statistical procedures applied to the results of tests on material specimens.
The specimen is generally not obtained from the structure and its relationship to material in the structure depends more on control of workmanship than on the designer’s observation or judgement. In this respect, the definition of characteristic value for ground materials given in Eurocode 7 is distinctly different.
B4.3 Characteristic values used in geotechnical design
In Eurocode 7, the characteristic values of geotechnical material parameters are based on an assessment of the material actually in the ground and the way that material will affect the performance of the ground and structure in relation to a particular limit state (EC7, 2.4.3(2,3,4)). Field and laboratory tests are to be used, but they are only one means of assessing what is in the ground;
characteristic values are not derived directly or solely from the test results.
Statistical manipulation of test results will generally have only a minor role in this process, if any. The resulting value is inevitably subjective to some extent, being influenced by the knowledge and experience of the designer. However, this is considered preferable to an alternative, mechanical approach which has arithmetic objectivity but jettisons established engineering knowledge.
In many situations, the known geology of a stratum, and existing
experience of it give a fairly good indication of its parameter values. Soil tests are used as a check. It is good practice to base the selection of characteristic values on a combination of well established experience and the test results (EC7, 2.4.3(2,4)). If unusually good test results are obtained, engineers will normally spot this and treat them with greater caution, unless further investigation is possible to establish that they are relevant. Unusually bad results may lead to further investigation, or may otherwise be taken at face value unless the evidence of other experience is overwhelming.
Construction activities may affect the properties of the ground, adversely or beneficially (EC7, 2.4.3(4)). Common examples occur during boring or driving of piles, or excavating to a level on which concrete will be cast. In many cases this will occur after any investigation and testing are complete.
Nevertheless, the characteristic value is to account for these construction PART B IMPORTANT FEATURES OF EUROCODE 7 PART 1
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effects. Information from previous experiences and publications will contribute to the selection of characteristic values in these circumstances.
Having reviewed these items, EC7 says that the characteristic value of a soil or rock parameter shall be selected as a cautious estimate of the value affecting the occurrence of the limit state (EC7, 2.4.3(5)). This is standard engineering practice. The relationship of the ‘cautious value’ to mean values will be considered in B4.9 to B4.11 below.
B4.4 Characteristic values dependent on failure mode
The characteristic value of one parameter in one stratum is not necessarily the same for two different failure modes. It may depend on the extent to which a particular mode averages out the variable properties of the stratum (EC7, 2.4.3(4, 6)).
Figure B4.1 shows a small industrial building, founded on pad footings near a long slope. The underlying materials are estuarine beds, mainly of sands with some impersistent lenses of clay occurring at random. In this type of situation, the design of the footings would probably assume that they might be founded on clay, the more adverse condition for foundation design.
(An alternative could be, in some cases, to require an inspection and probe at each footing, so avoiding this adverse condition.) On the other hand, when the possibility of a large slip surface is considered, it is inconceivable that this will lie entirely, or even mainly in clay. In this type of situation, the
characteristic values for strength parameters of the beds would be different for the footing design and for the slip, though their safety is controlled by the same stratum in both cases.
Figure B4.2 shows results of a CPT test in a mixed, estuarine deposit which has been overconsolidated, variably, by desiccation. A piled foundation is to be constructed in this material. If the piles are of fixed length (perhaps limited by construction equipment), the characteristic values of soil strength for the base and shaft may be quite different. The shaft averages the properties of a large amount of material, from many periods of deposition, whilst the base could be formed in one of the weaker layers. In this case the characteristic values of soil strength for the shaft would be higher than that for the base, in the same deposit. On the other hand, if the construction process allows the base to be tested, by pile driving for example, the characteristic value for the base could be higher that the averaged value used for the shaft. This discussion must also be modified to take account of any systematic variation of strength with depth.
Estuarine beds Sands with Clay lenses
Figure B4.1 Small building on estuarine beds near slope
0
5
10
Depth (m)
0 5 10 15 20
Cone Resistance (MPa)
Figure B4.2 CPT results in variable deposit
B4.5 Which value – peak, critical state, residual, mobilised ...?
The question has been asked: Whichvalue is the characteristic value? It is sometimes necessary to chose from one of the following, depending on circumstances:
a peak, critical state or residual shear strength;
b ultimate strength or a ‘mobilised’ value;
c strength of intact material or strength on joints;
d strength at first loading or after repeated loading;
e stiffness of intact rock or of the jointed material;
f stiffness on first loading, or on unload-reload.
In all cases, the answer of Eurocode 7 is: the one that is relevant to the prevention of the limit state under consideration. EC7 does not differ in this respect from normal practice. For some particular situations, the code is able to specify which of these values is relevant. For example, where concrete is to be cast against ground, which might therefore be disturbed, the critical state value for the angle of shearing resistance is required (EC7, 8.5.1(4)). In considering rocks, a study of the joint patterns will determine whether intact or joint strength is relevant (EC7, 3.3.9).
This answer to the question is not the same as: the one which would become relevant if the limit state was not prevented. For example, in most plastic clays, if a slip occurred, the angle of shearing resistance would eventually fall to the residual value. Nevertheless, it is not necessary to design for residual strength in clays which have not previously slipped. Similarly, it may be unnecessary to design for critical state values, though brittleness and ductility must be considered, as noted in EC7, 2.1(9) and C2.1.
Generally the strength to be used in Eurocode 7 is the maximum available to prevent collapse, not a mobilised value.
B4.6 Relationship to other texts and practices
With regard to characteristic values, the intention of the drafters of EC7 was to clarify existing practice, rather than to introduce something new. The main problem was the difficulty of defining existing practice. Nevertheless, some texts give helpful indications of the way in which parameter values are to be chosen and it is relevant to compare these with characteristic values.
CIRIA Report 104 suggests that design may be based on moderately conservative values of parameters. ‘Moderately conservative’ is defined (p 40) as meaning conservative best estimate. It could be objected that the latter term is contradictory, since a value cannot be both conservative and a best estimate simultaneously. CIRIA 104 states that this approach is used most often in practice by experienced engineers. The authors consider that the conservative best estimate values of CIRIA 104 are essentially the same as the characteristic values of EC7.
In BS 8002, design values of soil strength (ie values entered into calculations) are derived by factoring representativevalues. For effective stress parameters, there is a further requirement that the design value must not exceed the representative critical state value. A representative value is defined (1.3.17) to be a conservative estimate of the mass strength of the soil.
‘Conservative values’ are further defined (1.3.2) as values of soil parameters which are more adverse than the most likely values. They may be less (or greater) than the most likely values. They tend towards the limit of the credible range of values. The authors suggest that this definition makes representative values essentially the same as moderately conservative values in CIRIA 104 and characteristic values in EC7.
The Dutch standard NEN 6740 (in Dutch) provides a more statistical approach to derivation of characteristic values. German recommendations for waterfront structures (EAU (1980, p38)) discuss the statistical background to characteristic values, and also provide some more pragmatic suggestions:
When a large number of shear parameters have been determined, the characteristic value can also be estimated as being that value which occurs immediately below the PART B IMPORTANT FEATURES OF EUROCODE 7 PART 1
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mean of all tests made ... With only three determined values, which have been obtained from three samples of the investigated layer taken at well separated
locations, the lowest value may also be used as the characteristic value if the values do not differ too much from one another.
B4.7 Why are structural and geotechnical characteristic values different?
The designer of a structure is concerned with the properties of materials which generally do not exist at the time of design, but which can be specified with fair precision. The range of uncertainty of their properties is fairly well known, and, in many cases, may be better understood by the drafters of codes than by designers in practice. Hence, it is appropriate that codes give specific rules about the measurement of characteristic values and that the possible range of uncertainty is entirely accommodated in factors prescribed by the code writers.
In geotechnical design, however, the designer is in possession of information not available to the code drafters. He knows where the site is located, what is its geology, and he has test results, relevant publications, observations of nearby constructions, and so on. The designer is therefore in a much better position than the code drafter to make allowance for the range of uncertainty of the parameter values. It is this extra information which
Eurocode 7 requires the designer to incorporate in his selection of characteristic values.
B4.8 Relationship to mean values
EC7 says that the characteristic value of a soil or rock parameter shall be selected as a cautiousestimate of the value affecting the occurrence of the limit state (EC7, 2.4.3(5)). The probability that the characteristic value will, in fact, prevail in such a way as to govern the occurrence of a limit state is fairly remote, nominally 5%.
It has been suggested that the characteristic value should be defined to be a
meanvalue. Unfortunately, there is some confusion about different meanings of the word ‘mean’. For the purpose of this discussion, three mean values will be defined: statistical, spacial and probabilistic.
a A statisticalmean will be taken to be the simple average of established data.
These could typically be test results, adjusted where necessary to allow for differences between the test and field situation.
b A spacialmean is the average of a parameter over some space. This could be the volume which is compressed under a load or the surface over which a slip might occur. Many limit modes are governed by the average
performance of such a volume or surface, and for these a spacial mean of the parameter value is appropriate. The decision to use a spacial mean does not dictate the degree of pessimism which may be attached to the chosen value.
c A probabilisticmean is a value, taken from a range of uncertainty, such that the value which will actually be found to govern the limit mode has a 50%
chance of being worse than the probabilistic mean. Most often, this probability must be assessed by the engineer in advance of the actual events. One advantage of using a probabilistic mean is that it is equal to the statistical mean value of a set of relevant test results, provided they have been adjusted for any difference between the behaviour of the soil in test and in situ.
In many situations, the characteristic value required by EC7 should be a cautious assessment of a spacial mean. If there is to be, in fact, a 5% chance that a worse value will govern field behaviour, then the cautious spacial mean will be much less pessimistic than the 5% fractile of relevant, adjusted test results. This reflects the fact that many limit modes average out the variabilities of a lot of ground.
Figure B4.3 shows the results of a series of test results from which a Young’s modulus is to be obtained for calculation of settlement beneath a foundation. There is a clear increase of stiffness with depth, but the designer has checked that there is no systematic variation of the test results with position on the site, so the variations shown can be treated as random. The foundation will load a large volume of ground and it is unreasonable to assume that its settlement could be determined by material from the lower end of the range of variation, such as the lowest 5% fractile. A suggested profile representing a cautious mean is shown on the figure. This could be used as the characteristic stiffness, varying with depth.
Figure B4.4 shows the same data and characteristic profile as Figure B4.3, with the statistical mean, obtained by linear
regression, added. It can be seen that these two are fairly similar in this case. In general, where the range of possible parameter values is narrow, it will be acceptable to adopt a statistical mean as the characteristic value, the cautious spacial mean. However, where the range of values which could govern the limit mode is large, the cautious spacial mean should be more pessimistic than the statistical mean.
Characteristic values for stiffness parameters are considered further in B4.12.
There are some situations in which spacial means are not relevant, or, at least, must be chosen so specifically that they are not easily recognised as means. For example, if stability of a rock cutting is being considered, the mean strength of the rock may be of no relevance: what is needed is the strength along joints which are inclined towards the cutting. It could be argued, of course, that it is the (spacial) mean strength of the joints which is needed.
Similarly, for a small foundation or the base of a pile, the mean strength of the stratum may be irrelevant if there is a possibility that the small zone of soil affecting the foundation is less strong. In
Similarly, for a small foundation or the base of a pile, the mean strength of the stratum may be irrelevant if there is a possibility that the small zone of soil affecting the foundation is less strong. In