ABSTRACT: The fundamental soil parameter used in Eurocode 7 is the characteristic value Xk, from which the design value Xd is derived. These soil parameters are defined and compared to todays practice. A simple approach is proposed for the determination of the characteristic value. If offers an existing to incorporate experience or judgmental knowledge as well as test values to produce characteristic values Xk. The resulting characteristic values are equally valid using EC7 as well as for today’s design methods. The values used in design calculations are termed design values and these are obtained from tests results via derived values and characteristic values.
Eurocode 7, the new harmonised European code for geotechnical design, is due to be introduced in Lithuania and for geotechnical engineering since natural soils are extremely variable in their properties and the rational selection of suitable design parameters is generally one of the most difficult part of a design.
For geotechnical design tasks some kind of a conservatively chosen mean value, often based on local experience or subjective information, needs to be selected. If not entirely based on experience only, in practice the information on the mean properties is sometimes supplemented by relative few test values.
In this paper, practical definitions of the fundamental soil parameters as used in EC7, such as the characteristic value Xk and the design value Xd, are presented.
2. SAFETY CONCEPTS IN GEOTECHNICAL ENGINEERING
The traditional or current design method is based on the global safety concept. All variables such as loads, soil parameters, and calculation procedure are used at a level of a cautiously selected mean for the design computations.
Eurocode 7 uses the limit state format with a set of fixed partial factors or safety, which is at present considered the most rational design
procedure for geotechnical routine problems. Within this framework the characteristic values of soil properties have to be selected in a well-defined way if a specified safety level shall be obtained. Variability or uncertainty in soil layers is only taken into account in selecting the characteristic soil values accordingly. The characteristic value Xk is therefore the fundamental soil parameter in EC7.
3. CHARACTERISTIC SOIL PROPERTIES
In principle it would be possible to measure the soil properties everywhere in the affected volume and determine the statistical mean values of that zone. In this case we would know the true mean value of the selected layer and could use it for further analysis.
In practice, however this procedure is quite impossible. In reality, therefore the true mean properties of a layer do remain unknown and estimates of the true mean have to be basis of experience or only small numbers of tests.
The true mean properties of a layer do remain unknown and estimate of the true mean have to be made on the basis of experience or only small numbers of tests.
The different meanings of the above mentioned values as well as soil parameters referred to in Eurocode 7, will be defined and also explained in practical terms:
- mean value Xm
- characteristic value Xk
- design value Xd
The mean value itself is not used in Eurocode 7. Instead the characteristic value, begin the best possible estimate of this mean value of the selected layer, is used in place of the mean.
The characteristic value is the best estimate of the “unknown” statistical mean Xm of a soil layer. According to EC7: The characteristic value shall be selected with the aim that the probability of a more adverse (mean) value governing the behavior of the soil and rock in the ground is not greater than 5 %.
In EC7 the degree of conservatism or caution is further defined as follows:
- The probability of a more adverse real mean value governing the behavior of the soil layer should not be higher than 5 %.
- We want to be approximately 95 % confident that the real statistical mean of the layer is superior to the selected Xk.
The design values Xd for soil parameters are generally derived from the characteristics values Xk.
Based on statistical sampling and estimation theories, the characteristic value Xk can be described by following general expression:
where kn is a statistical coefficient and VX is the coefficient of variation of the parameter X defined as:
where sX is the standard deviation of the n sample test.
4. Geotechnical section layers statistics
When working with several CPT files in a single project, we are targeting towards to the identification of soil layers that reappear in the various CPT profiles. Some of these layers most probably will be assigned with an identical code name, for example Clay_1 layer that appears in CPT-01, 06, 08, 09 and 10. Each Clay_1 layer carries information like average values of various parameters but what we probably need is to present single values when talking about this clay layer. CPeT-IT, adopted the methodology proposed in Eurocode 7, to calculate Minimum Characteristic Values. The analysis is performed mainly for qt but the software will present the characteristic values calculated for all estimations. By examining characteristic values we may adjust and finally propose single values in your final report.
The characteristic values Xk used in today’s approach (global factor of safety) as well as in EC7 are for all practical purposes equivalent and should be of the same magnitude.
In today’s practice Xk is selected primarily based on experience and personal judgment. This is a well-proven practice and can, or even should, still be employed for EC7. Difficulties however can arise as soon as test values are available in additional to local experience. EC7 however does go one step further and presents a definition of Xk in qualitative and even quantitative terms. This puts the proposed safety system on a more rational basis. Xk in EC7 constitutes an essential part of the safety
chain governing the entire design process.
The statistical mean value Xm itself is not used in EC7, but will always be replaced by corresponding characteristic value Xk.
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