Change of properties of soils in regions of natural disasters

CHANGE OF PROPERTIES OF SOILS IN REGIONS OF NATURAL DISASTERS

Shestakova, A.N., GRIGORIEVA, I.Yu.
Moscow State University, Geological faculty, Department of engineering and ecological geology, 119132, Leninskie gory, 1, Moscow, Russia
e-mail [email protected], [email protected]

1. Introduction
The consequences of natural disasters have composite, complex nature. The problem is complicated by that in terrains of the natural disasters, subject to effect, the different constructions and objects place, the emergencies on which one are capable many times over to increase damage from natural disasters. To a similar kind of constructions can relate the objects of a petroleum industry. The contamination of terrain by oil and petroleum is one of consequences of natural disasters. The estimation of a rate of fouling and its influencing on properties of soils is essential and actual for today by a problem of engineering-geological researches in regions of natural disasters.
The properties of the soils, contaminated by oil, differ from properties of initial soils. We conducted special experimental researches on analysis of nature of change strength and water-physical characteristics of dispersed soils at oil contamination.
The estimation of strength contaminated by oil and petroleum of soils is important problem of engineering-geology in the regions of natural disasters. This process influence on engineering-geological and ecological conditions: the contamination by petroleum promotes not only loss of prolific grounds, but also as a result of change of properties of soils - activating of a number of geological processes, i.e. essential change engineering-geodynamical conditions of terrains.
In a case if the similar changes will not be taken into account, it can result in composite emergencies, scale contaminations and other unfavorable consequences. The count of changes, descending in soils, will allow to prevent secondary effects of natural disasters, to improve stability of protective buildings.
At the forecast and calculation of a design of protective constructions it is necessary to allow for probable changes of properties of foundation soils of these constructions.
As object of researches air-dry samples of loess of a natural structure were used. Samples ??????? have been selected from Privoljskaya heights (the sample 1), having a grainy microstructure, Minusinskiy intermountain deflection (Greate Salba - the sample 2), possessing by grainy-aggregative microstructure, and the South-Tadjik depression (Adyrniy - the sample 3) with an aggregative microstructure. Frequently in areas of distribution of loess there is a problem of oil pollution and, as consequence, a problem of occurrence of acts of nature.

2. Methods
We conducted special experimental researches on analysis of nature of change strength and water-physical characteristics of dispersed soils at oil contamination. Samples were tested on axial compression on installation P-12 (figure 1). The parity of phases in petropolluted soils is presented in figure 2. The estimation of change of strength of loess was in parallel spent at water-saturation.
In the literature many attempts of the analysis of influence of various factors on a condition and properties of disperse breeds are known. In V.I. Osipov's opinion [4], primary factors of strength of soils are the type of contacts and size of contact interaction.

Figure 1. The scheme of electromechanical lever press P-12

Figure 2. The parity of phases in petropolluted soils

In given paper strength of single contact has been received by means of dependence according to which strength of disperse structure (??) at simultaneous destruction of contacts is proportional to average force of contact interaction between particles (?1) and to number of the destroyed contacts in unit of the area of a surface of destruction ?:
Pc=P1 ? ? (1)
The given parity establishes communication between classical representations about durability of disperse systems and its microstructural features. Having defined sizes P and ? , it is possible to calculate average durability of individual contact ?1 and by that to estimate size of structural communications. At calculations of quantity of contacts on cm2 of the area have been used globular and bidispersial models.
Most simple is the globular model offered by Rebinder P.A., Schukin E.D., Margolis L.J. [5] for structures, combined particles of the spherical form and having porosity more than 48 %. Later this model has been widespread to structures with porosity in an interval from 48 % up to 26 %. For construction globular model are used the rectilinear chains consisting of spheres concerning with each other of identical diameter (figure 3). Chains are located in three mutually perpendicular directions and, being crossed, form units of structure. The way of packing is characterized by structural parameter N - an average of particles from unit up to unit. If N=1 the structure with simple cubic packing turns out, at fractional value N the system is characterized by wrong alternation of units. In such model porosity n is unequivocally connected with parameter N. This dependence is given usually graphically in the form of function (figure 4):
f(n) =1/N2 (2)

Dependence between number of contacts on unit of a surface of destruction, parameter N and average radius of a structural element can be written down as follows [5]:
?= l/(4?r2 ?N2) (3)

Figure 3. The scheme of globular model of disperse porous structure: N - an average of particles from unit up to the unit, defined by porosity of structure; r - average radius of particles [5]

Sokolov ?.N. [6] offered bidispersial model by means of which it is possible to estimate quantity of contacts in the system combined large (with radius R) and fine (with radius) particles (figure 5). According to this model, total of contacts to equally quantity of contacts between large particles (?R), increased by number of contacts between fine particles (?r), being within the limits of a contact platform between large particles.
Sizes ?R and ?r are accordingly from following expressions:
?R =3?z?(l-n)/(8???R2) (4)
?r =(?R??r ?R2)/(2??r??R?r2), (5)
where ?R , ?r , ?r , ?R , R, r - accordingly density, the maintenance and average equivalent radius of large and fine particles; Z - coordination number, n - porosity.


Figure 4. Dependence of parameter 1/N2 from porosity of structure n [5]

The coordination number (Z) does not depend on the size of structural elements in globular model and is defined exclusively by its porosity. For finding Z there is a number of the simple expressions received by different authors. V.Field [1] has offered following dependence:
Z= 12/(1 ?) (6),
where e - coefficient of porosity.
V.Gray [2] has received a number of other dependences:
Z=3,1/n ? Z=2?exp(2,4n) (7)
where n - porosity.
The total of contacts in bidispersial model is as follows:
? = ?R ? ?r =[3?z?(1-n)? ?R ? ?r]/[16???r2 ? ?r??R ] (8).


Figure 5. The scheme of bidispersial model of disperse porous structure: R and r - average equivalent radius of large and fine particles [6]

3. Discussion of results
The questions considered in given paper, concerning changes of structural strength of loess at oil pollution, allow to give general conclusions.
1. Characteristics of strength at interaction of loess with oil in a greater degree depend on features of structure and structure of a loess, rather than at interaction with water (with other things being equal) [3]. Dependence of strength of loess from type of structure is levelled at water-saturation and more brightly expressed at interaction of loess with oil (figure 6 and 7).

Figure 6. Relationship between degree of water saturation (Sr) and strength of soils (R, MPa)

Figure 7. Relationship between degree of oil saturation (Soil, g/g) and strength of soils (R, MPa)

2. Relations between type of a microstructure of loess and quantity of contacts in the best degree is reflected at calculation on bidispersial model, that is a lot of contacts on 1 cm2 of the area is characteristic for aggregative type of a microstructure, smaller - for grainy type of a microstructure. Calculation on globular model does not allow to reveal similar dependence as morphological features of structure of loess a various class of a microstructure are not considered (figure 8).

Figure 8. Parity of quantity of the contacts received on globular and bidispersial models, for different types of microstructures

3. Comparing the received values of strength of single contact on globular model, it is possible to observe communication with a microstructure of loess: the greatest value of strength of single contact are characteristic for loess with an aggregative microstructure, the least - with a grainy microstructure (figure 9), and the given dependence is not observed by the received results on bidispersial model
(figure 10).

Figure 9. Parity of strength of single contact in initial, petrosated and water-sated on globular model for different types of microstructures

Figure 10. Parity of strength of single contact in initial, petrosated and water-sated on bidispersial model for different types of microstructures

4. At calculations on used models the tendency of reduction of strength of single contact for the same loess in sequence is observed: an initial loess - the petrosated loess - the water-sated loess (figure 9 and 10).
5. At saturation of loess there is a gradual transition from mixed (solid and adsorbed water) type of contacts to capillary water owing to what strength of soils falls. And at interaction of a loess with water this transition is expressed more intensively, than at interaction with oil.

4. Conclusion
In a case if the similar changes will not be taken into account, it can result in composite emergencies, scale contaminations and other unfavorable consequences. The count of changes, descending in soils, will allow to prevent secondary effects of natural disasters, to improve stability of protective buildings.
At the forecast and calculation of a design of protective constructions it is necessary to allow for probable changes of properties of foundation soils of these constructions.

References

1. Field, W.G., 1963,To wards the statistical definition of a granylar mass, Proc.4-th Australia-New Zealand conf. On solid mechanics, pp. 143-148.
2. Gray, W.A., 1968, The packing of solid particles. Chapmen and Yall Ltd., 238 p.
3. Grigorieva I. Yu., Shestakova A.N., 2005, Change of proporties of soil contaminated by oil (in Russian), Scool of ecological geologie, conf., pp. 308-310.
4. Osipov, V.I., Sokolov, V.N., 1995, Factors and mechanism of loess collapsibility, Kluwer Academic Pubplishers, Netherlands, pp. 49-63.

5. Rebinder, P.A., Schukin E.D., Margolis, L.Ya., 1964, About mechanical strength of porous bodies (in Russian), DAN, Vol.154, ?3, pp. 695-698.
6. Sokolov, V.N., 1990, Engineering-geological classification of clay microstructures, Proceedings of the Sixth International Congress of the International Association of Engineering Geologists, Balkema, Rotterdam, Brookfield, pp. 753-760.