Role of accuracy and quantity of field tests in engineering-geotechnical researches for construction

Abstract

The aim of this work is to summarize previously conducted studies on the optimization of the unequal geotechnical testing program and on the selection of the desired calculation indicator based on the results of such tests. The approximate, but quick and cheap tests (“express methods”) are recommended to be performed on a large scale and considered as a means of assessing the geotechnical structure of the site as a whole. It is proposed to carry out expensive “accurate” tests in a reduced volume and to use them as a means of correcting approximate tests. In the article, these issues are considered by the example of determining the bearing capacity of piles according to the data of static sounding (cone penetration testing – CPT), dynamic and static tests of full-scale piles. We propose the mathematical model for evaluating the informative content of the test complex, based on the concepts of information theory. The site is mentally divided into several sections, each of which is characterized by one of the possible values of the ultimate resistance of piles of a certain length. All variants of “placement in the plan” of possible values of pile resistances (“site images”) are considered. Initially, when nothing is known about the true value of the pile resistances in each section, all possible values of the pile resistances are assumed to be equally probable, i.e. the uncertainty of the situation is maximum. In the theory of information, such uncertainty is quantified by the value called entropy. When any test is performed at the site, the uncertainty decreases, and the more accurate the test the more significant is the decrease. The difference in entropy before and after the test represents the amount of information (in bits) that these tests carry. The calculations using this model showed that the information content of a large number of approximate tests can (due to heterogeneity of the soil) exceed the information content of small exact tests. Only one approximate test method can lead to the systematic error (overestimation or underestimation of the average value of the desired indicator). It is necessary to carry out control “exact” tests and approximate tests to eliminate such a danger. A technique is proposed for adjusting approximate estimates based on data from “accurate” tests, which ensures optimal “safety margins” in decisions being made.