Engineeringgeological survey in the permafrost regions

Engineering-geological and geocryological investigations carried out to help in the design of various structures are termed engineering-geological surveys. The first practical work in the world on the problem of geocryological investigations for construction purposes was that by V.N. Yanovskiy, Methods of investigation of permafrost for engineering construction purposes, published in the 1950s in which experience in the development of the European North and Siberia in the 1930s and 1940s was outlined. At the present time engineering-geological surveys are standardized. The basic document on the problems of surveys in the region of unfrozen as well as perennially frozen ground is the Building Code and Regulations (SNiP) 1.02.07-87 'Engineering surveys for construction purposes'.

According to the Building Code 1.02.07-87 the main problem for engineering-geological surveys is the integrated study of the engineering-geological conditions within the locality where construction is planned. This includes the topographical, geomorphological, seismic and hydrogeological conditions, geological structures, composition, state and properties of the soil, geological processes and phenomena and changes in conditions following construction, in order to get necessary and sufficient information to justify structural designs taking into consideration the harmonious exploitation and protection of the geological environment, as well as to make forecasts concerning changes in the engineering-geological conditions in the course of construction and operation of enterprises, buildings and structures. The engineering-geological survey is closely associated with and precedes the design of the particular structures. This dictates the specific study of the engineering-geological conditions of the locality selected for construction purposes. Clearly the engineering-geological support for the design of hydroelectric stations, underground structures, motor and rail roads, industrial and civil structures requires specific integrated geo-

cryological investigations. Therefore the main points and the details of an engineering-geological survey must depend on the particular type of construction.

The requirements for the individual kinds of engineering-geological work, are determined by the number of design work stages, the complexity of engineering-geological conditions, and the character and degree of importance of the buildings and structures being planned.

Normative documents used today provide for a survey in three main stages. By a stage is meant the finished portion of the particular engineering-geological survey distinguished by the degree of detail and the procedure for the work depending on the number of structural design stages. In line with the established design procedure the engineering survey is performed to develop: 1) predesign documentation - technical and economic substantiation (TES) and technical-economic estimation (TEE) for construction, for enlargement, modernization and reequipment of buildings, buildings and structures in service, of overall plans for cities, settlements and populated rural areas; of projects of cities planning industrial zones (regions), of projects of planning in detail; of structural models and overall plans for industrial centres; 2) designs (contractor's designs) for plant, buildings and structures; 3) service forms and records of undertakings, buildings and structures. The name of the survey stages accord with the design work stages. Therein lies the succession and interrelation of engineering survey and design work as a single process.

The engineering-geological (engineering-geocryological) survey is performed under a program developed by surveyors (engineering geologists, geocryologists, hydrogeologists) on the basis of the technical assignment given by the designing organization. The successful survey has a number of requirements imposed by the technical assignment in accordance with particular features of the structures under design and their characteristics. These details are different at various design work stages. Thus at a predesign stage the technical assignment must contain the following data: the character of construction (new construction, modernization, enlargement, reequipment) and of the enterprises under design, the importance of the buildings and structures and the effect of the structures under design on the environment. When industrial and civil construction is carried out it is necessary to have a schematic of the overall plan for the site representing the most economical arrangement of the structures under design taking into account technological and other particular features of their mutual arrangement.

In the course of survey at the design stage the technical assignment must contain data on the justification of the principle of using ground as founda tion base, on the proposed types of foundations, on the laying depths of subsurface portions of buildings and structures and on the interaction of the structures under design with the geological environment.

The technical engineering survey assignment for developing working documentation must include data on which principle is accepted for using ground as a foundation base, on the technological function and heat regime of the structure under operation, on the way of laying service lines and on the measures providing the accepted principle of construction and control of the cryogenic processes.

Study and assessment of the terrain (for alternative sites for route sections) is to justify selection of the most favourable site for the particular structure to be located, constructed and to operate, taking into consideration protection of the structure against dangerous geological processes and protection of the geological environment. At this stage the principle of using perennially frozen ground as a foundation base (by keeping it frozen or by thawing it) and the estimated cost of construction are determined, and the area under survey is two times or more greater than that of the particular construction site. Sometimes not one site but a set of competing sites, which can be widely separated, is considered.

The survey at this stage includes the basic work in the field, laboratory and office. The work includes collection, analysis and classification of materials from the preceding survey, materials from the geological survey, and engineering-geological and hydrogeological work and other data on natural conditions within the region under design. So, the landscape is classified with preliminary compilation of a landscape map and the engin-eering-geocryological conditions are noted on this base, using results of interpretation of air photos.

The field work includes reconnaissance, geocryological study of the area to improve and extend the accumulated data, to give a comparative assessment of the complexity of geocryological conditions within the area under investigation, to establish the distribution and development of dangerous geological processes, to reveal changes in geocryological conditions within the developed terrain, as well as analysis of construction experience. In the course of the reconnaissance, field observations, geophysical investigations, drilling with soil sampling, groundwater sampling and laboratory investigations of the samples are carried out.

In poorly known regions where geological investigations have not been performed earlier and for which construction experience is absent, it is necessary to carry out the survey of sites at the 1:25 000 -1:10 000 scale and of linear structures at the 1:50 000 - 1:25 000 scale.

During the office work period the report on the survey is prepared, in which natural and engineering-geocryological conditions within the territory are described. An essential requirement for the report is a forecast of changes in the geocryological situation under natural conditions and in the course of development of the site. The report is supplemented with the map of landscape types, a geocryological map with the terrain classified by construction conditions and with engineering-geocryological sections, graphs and tables of the permafrost and ground ice properties. When incorporated in the technical assignment the report is supplemented with maps of types of seasonal ground freezing and thawing, ground ice content, permafrost thickness and cryogenic processes and phenomena.

Comparative assessment of the construction sites and of the routes of linear structures and alternatives, with justification of the selection of the best and determination of the construction principle to be followed, are the result of work at the predesign stages of the engineering geological survey.

The engineering-geological survey within the selected site (design stage) must provide the materials necessary and sufficient to justify the particular site for the buildings and structures, to select the approach to their construction, to determine measures for engineering protection of the ground, for geological environmental protection and for developing the designs for construction and organization of the work. For these purposes an integrated study of the engineering-geological conditions within the site or route under construction is carried out. This investigation includes field, laboratory and office work. The engineering geological survey and experimental field studies of ground composition and properties are the main methods for field investigations. The survey is carried out at a larger scale compared with that being used at the predesign stage, 1:5000 - 1:2000 for industrial and civil engineering structures as a rule and 1:10000 - 1:5000 for the routes of linear structures.

Beginning from the stage of the survey, permanent observations including those of permafrost temperature dynamics, of cryogenic processes, of seasonal freezing and thawing dynamics, and of changes of properties of thawing and freezing ground are considered in the design according to the normative documents.

In the course of office analysis of survey materials, the geocryological assessment of the site with forecasts of the changes in permafrost conditions in connection with construction are made, recommendations are developed on the arrangement of the constructions under design, on their base and foundation design, and measures for controlling the cryogenic process are considered. Compilation of a report is the result of the survey in the same manner as at the first stage. The report must contain data on geocryological conditions within the site for construction, results of field and permanent investigations of ground properties and improved recommendations on the selection of the particular principle for use of the ground as a base.

The engineering-geological survey task for particular building sites (the stage of working plans) consists of the determination of geocryological conditions and of the state and properties of sites for particular buildings and structures, necessary to design foundations according to the technical approach accepted. Engineering-geocryological mapping is not carried out at this stage. Surveys include engineering-geological (engineering-geo-cryological) prospecting consisting of drilling, geophysical investigations, pilot field investigations (testing of frozen ground by a heated probe, of ground pillars for shearing, of ground by static loads on piles, etc.) within the limits of buildings and structures. Fixed observations initiated at the second stage are continued.

In addition to overall geocryological characterization of the site the data necessary for foundation design are included in the report on the survey results. When the permafrost is used as a foundation base on the first principle, the design mean annual ground temperatures at depth in the geological section, the design values of pressure on the permafrost, of permafrost shearing strength, results of field testing, data on frost susceptibility of the seasonally thawing layer and results of permanent observations are presented. When the permafrost is used as a base on the second principle, the physical and thermophysical characteristics for heat engineering calculations for thawed and thawing soils at various depths within the thawing zone under the structure are presented, as well as the design strengths of the soils after being thawed, data on frost susceptibility of the seasonally frozen layer and the results of field and permanent observations.

The concepts presented are common to all kinds of on-land industrial and civil engineering. Developments such as hydrotechnical construction, underground construction and agrobiological utilization of the permafrost areas require additional survey. The content and volume of this work are regulated by normative documents of departmental ministries. In some cases a specially designed survey is carried out for an individual design, according to the particular technical requirements.

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