The methods are described in the book of D.Sc. A. Gerasimov “Foundations in Permafrost” (sections 5, 7, 8, 10 - 17):
https://sites.google.com/site/permafrostfoundations/english-version
The objective of the present research is to develop the rheological principles of deterministic and probabilistic methods for analysis of permafrost foundations and in such a manner to increase the loads allowable on these foundations.
Profit:
- An original technique to determine reliability of foundations.
- An original technique to determine the reliability coefficients for loads and soils temperature, as well as design loads upon foundations of the buildings being erected on permafrost.
- The possibility to apply the results obtained in practice is provided because all the solutions are presented in the form acceptable for designing and are followed by the numerical values of the coefficients applied and calculation examples.
- A decrease in the design values of temporary loads upon foundations by 10-100%.
- An increase in bearing capacity and decrease in settlements of frozen soils up to 300% (in comparison with other probabilistic models) due to the probabilistic variability of loads and soils temperature while constructing and maintaining a structure.
ApproachesThe working hypotheses approved experimentally and substantiated theoretically:
- values of the design loads upon structural elements are conditioned by necessity to ensure reliability of foundations and structures;
- the loads on foundations and temperature of permafrost soils are random processes, but at simplified analysis they can rated as random or deterministic magnitudes;
- the frozen soil deformation and strength are random functions of time; by the end of the design service life, their long-term random values can differ significantly from the «instantaneous» ones: the long-term strength can differ several times and in some cases - even tens of times, and deformations can differ tens, hundreds of times and even greater;
- the random character of variation of load, temperature and frozen soils properties while calculating their long-term bearing capacity and both steady and unsteady creep strains can be considered by means of linear summation of damage, theory of strengthening and summation of plastic and viscous creep deformations; at simplified analysis an evaluation of bearing capacity can curried out by simplified means basing on the long-term values of ground resistance;
- the determination of design loads acting upon foundations (snow, wind, crane, people, equipment, etc।) and the probabilistic analysis of the frozen foundations should be fulfilled by the offered method based on the cumulative probability model which is single for both groups of the limiting states, taking into account the rheological properties of frozen soils and ice and the random character of variation of external loads and actions in time; considering the external loads and actions as random processes and taking into account the dependence of strains and strength of frozen soils and ice upon time, one can significantly increase correctness of determination of the design values of bearing capacity and settlement of foundations.
The results of the experimental studies on strength and creep of frozen soils and ice at variable loads, published in the literature and specially fulfilled in order to substantiate the accepted design models, are thoroughly analyzed. The statistic variability of long-term strength and creep parameters of frozen soils and ice is studied relying on the statistic-correlation analysis of the test results.
Based on the literature data and theoretical models, the temperature variability of permafrost during a year and in the course of a building maintenance is analyzed in probabilistic terms, and the probabilistic parameters of the mechanical properties of soils and their temperature are found out for application in the probabilistic analysis.
According to the literature data and climate reference books, the probabilistic characteristics of the external loads acting upon foundations are analyzed, particularly the short-term ones having an increased variability (climatic loads, people, equipment, etc.). Also, the periodic variability of these characteristics is studied and the dependence upon time in the course of maintenance of a building is determined.
The reliability of foundations is investigated in terms of the theory of probability, including the theory of random processes. The probability of foundations safe working is determined through analysis by two groups of the limiting states, including necessary relationships between loads and mechanical properties of soils. A probabilistic cumulative model is formed, as well as a semiprobabilistic cumulative model allowing to calculate a foundation, using particular reliability coefficients (for a load, temperature, soil, etc.).
The estimation of the design load values is made by the semiprobabilistic method, in accordance with which the probabilistic variability of a single or several factors is considered, provided that the others are predetermined. The appearing errors are taken into account. The formulas to calculate bearing capacity and settlement of frozen soils and ice, taking into consideration the probabilistic variability of loads, are received along with the appropriate calculation examples. The problem of combining the loads is investigated and necessary recommendations are given.
The environmental loads (snow and wind), just like the bearing capacity of soils and foundations upon vertical and horizontal loads, vary periodically. The coordination of their design values during a year is considered and the possibility for refinement of the design loads upon foundations depending on the accepted calculating scheme is determined (analysis of the bearing capacity of frozen soils, frost heave, piles for horizontal wind load). The correctness of using the principle of coordination is analyzed on the basis of the proposed cumulative model and some appropriate calculation examples are given.
The main results of the present work are as follows:
- a method for determination of safety of the permafrost, while calculating its bearing capacity and settlement taking into account deformation and failure of frozen soils in time, statistic indeterminacy of their mechanical properties and alteration of load and temperature in the form of random processes and random magnitudes; the above method results in the methods for determination of particular reliability coefficients and design parameters which can be used when calculating foundations;
- some practical methods are offered to analyze foundations on the permafrost, taking into account the random variability of the climatic and other loads, along with the simplified methods for consideration of the statistic nature of loads of different types while calculating bearing capacity and settlement of frozen soils and ice;
- some lowered design values of short-time loads transferred onto foundations and structures are recommended.
The conducted investigations will allow not only to increase trustworthiness of determination of the soils' reological properties and formulate the reological principles for the probabilistic analysis of foundations, but also to refine the design values of settlement and bearing capacity of foundations and the external actions by determining the former with consideration for the random variability of loads, temperature and properties of soils.
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