Permafrost soils are common for extensive areas of our country and when building engineering structures in such conditions, it is necessary to take into account the factor supporting subbase on the permafrost soil, which may initially have high stress-related characteristics. But the permafrost degradation phenomenon with frost retreat no less common. It can be happening both for natural reasons due to global warming, and for man-made reasons, when interference during construction and further the structure itself initiates subsoil frost retreat. For the most part, permafrost soils lose their strength and elastic properties during the frost retreat, turning into an almost liquid consistency, which entails the failure of the supports in the bearing capability and position stability. With the road network development and the bridge’s construction in the northern regions at the end of the 60s — early 70s of the XX century, the first principle of the design was widely applied. It was for use and maintenance of the subsoil in a frozen state, which means not require a deep foundation required. However, after 20 — 30 years of operation, cases of unexpected and significant bridge piers sagging began to be noted, as it turned out, was the result of permafrost soils frost retreat to the entire depth of the foundation. It was also noted that in some cases the bridge piers sagging is preceded by its frost boil phenomena, which appear shortly before piers sinking and almost complete failure. In this article case of an unexpected bridge piers failure examined on a specific bridge. After 30 years of proper functioning, pierces received such significant sagging, so in just three years that got to the point when it’s not only traffic safety but the cohesiveness of the entire structure. And before that, a frost boil was noted at some of the bridge pierce. The purpose of this study is to analyze the frost boil mechanism and sagging of the bridge pierce’ foundations due to the progressive permafrost degradation and to suggest restoring and maintaining ways for pierces’ operation capacity. Overall, the reasons and principles of permafrost subbase degradation were analyzed in this paper. through calculations using a specific example, significant sagging and bridge pierce bearing capability loss observations were justified and substantiated the effectiveness of the proposed in this article structural reinforcement measures.

Temporary bridges are necessary to ensure movement during the construction or repair (reconstruction) of capital bridge structures, operational communication of interrupted tracks in various emergency situations, for one-time or seasonal transport communications. For this purpose, reusable mobile prefabricated structures are advisable. Inventory sets of collapsible bridges were designed and manufactured primarily in the interests of the military department, but are currently widely in demand and are used in the civilian sector of bridge building because of their economy, mobility, and accessibility in transportation. Among others, including modern collapsible bridge designs, the middle automobile collapsible bridge (CAPM), developed in 1968 and modernized in 1982 for the needs of the USSR Ministry of Defense, occupies a special place. In the process of withdrawing the CAPM sets stored in storage to the civilian construction sector, a significant demand for these structures was found to be due to their following advantages: complete staffing with all elements of the bridge, including supports; the possibility of overlapping spans of 18.6, 25.6, 32.6 m with the dimensions of the driving bed 4.2 m for single-track and 7.2 m for double-track passage Passport carrying capacity is designated as 40 tons for single-track passage and 60 tons for double-track passage. Since, according to a number of geometric and technical parameters, CAPM designs do not fully comply with the requirements of modern standards for capital bridges, their application is mainly oriented as temporary. It should be noted that with minor refinement – the installation of modern fences and a double-track transverse arrangement of sections for single-lane traffic, it is possible to achieve compliance with the required geometric parameters of the driving bed and the total lifting capacity for bridges on public roads of IV and V of technical category. The article considers the design feature of the pin mounting joints of sections of a collapsible span as a factor determining the carrying capacity, the nature of general deformations and, as a result, affecting the transport and operational characteristics of the bridge structure. The purpose of this study is to analyze the operation of the pin mounting joints of the sections of the spans of the CAPM with the assessment of the stress state of the elements of the connection node. We consider the novelty in the consideration of the issue to be an assessment of the strength of the elements of the pin joints and its effect on the general deformations – the deflections of the main beams.