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CLASSIFICATION OF LOESS TYPE SOILS OF CENTRAL MONGOLIA BY INDIRECT SIGNS AND
FOUNDATION DESIGNING ISSUES
Nyamdorj S.
Mongolian State University of Science and Technology
Ulaanbaatar. Mongolia DOI: 10.5281/zenodo.6532889
ABSTRACT
Quality assessment of Physical and mechanical properties of the loess type of sandy loam and loamy sand soil distributed in the Orkhon-Selenge region is growing to be more vital. In this region, the distribution of loess type soil in the top layer of soil is relatively low with a thickness of 4.0-10m and it shows subsiding effects due to techno genic saturation. With an intention to assess the saturated properties of soil, many linear regression equations are iterated in numerous laboratories and in the field. Furthermore, the cause of deformation of buildings constructed in the region was identified. The efficient use of structures of optimal foundation types in the region were recommended and conclusion has been made.
Keywords: construction site, lithological cutting, technogenic saturation, base soil subsiding, pile foundation, soil solidification, optimal use.
Introduction. Estimation of subsidence properties of deluvial-proluvial clayey soils in Central Mongolia is a top priority since these soils serve as the basis for most of the buildings and structures under construction. They are common in wide intermountain valleys and in the middle and lower parts of the gentle slopes of local uplands. It is these areas that are primarily subject to construction development. The Darkhan-Seleng and Erdenet-Orkhon regions have well-developed road, rail and other structures, industrial and agricultural sectors.
Engineering-geological research. Most researchers studying loess rocks see the characteristic features of their appearance in the increased content of silt particles (more than 50%), low natural humidity (up to 15%) and significant porosity (more than 42%-45%) [2:4:5]. With these parameters, regulatory documents [1] associate indirect indicators by which soils can be classified as subsiding. It should be noted that some sandy-loamy varieties common in the Central regions
of Mongolia, although they do not fit into the above criteria, often have subsidence, both under domestic and additional loads [2]. The predisposition to subsidence, along with other features of the composition and properties, make it possible, with a certain conventionality, to classify these soils as loess type deposits.
Studies of physical and mechanical properties based on the results of 782 laboratory experiments found that clay soils are characterized by subsidence properties when soaked under a pressure of 0.2-0.3 MPa [2]. These are the most common loads in the foundations of buildings under construction, so the assessment of subsidence in the range of given pressures is of paramount importance. For the territory of the above named regions, it was found that the dependence of subsidence with other physical properties of rocks (porosity, humidity, etc.) weak, which limits the prospects for using indirect methods in surveys and building design. This is explained by the fact that subsidence does not depend on one indicator, but is a function of many
arguments that were formed under the conditions of a specific physical and geographical situation during the period of sedimentation and subsequent times.
First of all, these include: natural humidity (W), degree of water saturation (Sr), bulk density (pd), type and strength of structural bonds, material composition, magnitude and nature of stresses acting on the soil. Most of these factors are interdependent and interrelated. Therefore, one-dimensional correlations between subsidence and individual soil indicators are weak.
One of such indirect criteria for assessing subsidence is the degree of moisture. The data of laboratory studies have shown that clay soils, which are naturally in a water-saturated state, are practically not subsiding, since they have already had a destruction of non-water-resistant structural bonds and degradation of subsidence signs has begun. For the Darkhan region, an indirect criterion for the loss of subsidence properties is the value of Sr=0.6; and for the Erdenet region Sr=0.7.
Based on the results of analyzes of laboratory studies, a comparison was made of the estimates of the possibility of manifestation of subsidence signs according to the established values Sr=0.6 for Darkhan and Sr=0.7 for Erdenet with direct determinations of the relative subsidence in compression devices. It turned out
that the number of cases when the clayey soils of Darkhan with a degree of moisture Sr=0.6 are characterized by subsidence properties is very small 2-3% of cases at pressures of 0.2-0.3 MPa.
For clayey soils of the Erdenet region with a degree of humidity Sr=0.7, the number of cases when the value of the relative subsidence coefficient (e) exceeds 0.01 is also small. At a pressure of 0.1 MPa, it is equal to 3%, at a pressure of 0.2 MPa it is 9%, and at a pressure of 0.3 MPa it increases to 15% of the total number of samples [4]. These data indicate that the indicator can be used to satisfactorily separate potentially subsidence from non-subsidence soils. Analysis of the influence of natural humidity on the degree of manifestation of subsidence signs did not reveal a clear relationship as an indicator of the degree of flatness.
Study of subsidence and compressibility. Settling deformations are the process of soil compaction due to the violation of structural strength when wetted under pressure. Since the main factor influencing the magnitude of subsidence deformations is "undercom-pact", which is reflected in the density index (porosity coefficient (e)). The systematization of the results of studies of the dependence of relative subsidence, initial soil density and vertical pressures is shown in Table I.
Table-I
The probability of manifestation of subsidence depending on the coefficient of porosity of the soil (e) and pressure (P)
Coefficient porosity, e Pressure, under conditions of sample soaking, MPa
0,1 0,2 0,3
Number of samples Probability, % Number of samples Probability, % Number of samples Probability, %
0,300-0,409 1 5 - 5 | 2 - 7 -
0,410-0,509 11 14 - 12 13 20 13 6 16 70 29
0,510-0,609 10 34 23 24 12 28 30 22 18 27 44 41
0,610-0,709 23 32 27 10 29 20 20 30 44 27 50 48
0,710,0,809 17 18 30 3T 34 10 30 10 44 15 52 50
0,810,0,909 15 10 40 33 26 T 81 100 35 18 74 75
>0,910 3 2 75 50 15 100 14 93 100
Note: in the numerator are the indicators of Darkhan, in the celebrity are the indicators of Erdenet.
Table-II
Generalized table of the basic physical and mechanical parameters of loess type soils of the Darkhan-Selengi region
№ об ело* rp означены унт» толщина СЛ01. Il грана состав, H 20 40 60 60 влажность. % 0.10 0.20 i i плотность сухого грунта, г/ем' 150 160 170 TRC.10 LiacmHOCTi 010 0.20 0.0 относительна* просадочностъ 0.02 0.03 0.04 0.05 0.06 внд грунтов
1 0.5-1.1 |S s суглинок средней плотности
2 1.1-4.2 I § itratft суглинок бежевого цвета, пористый н малоалажныЙ
3 4 2-7.5 * длыватый супевь желто-серого цвета, аорнстый, маловлажный
........
---
\
4 il I I I I Jll 7.5-126 ! M I/ суглинок красно-корнчнего цвета, с галнчннковымн включениями, влажный
5 \; \ \VV\ \ ^ 126-139 Ш крупнооблоыочный грунт с супесчанным заполнит слом, влажный
/. Ху/ /// / 139-151 / s
илолшиелом. ыааашй
p 7 \ Ш ш \ 151-17 6 i E кр>ттнооблпмочный грунт с песчаииш моолннгыом. влажный
Analysis of the results shows that as the porosity coefficient e increases from 0.300-0.409 to 0.910 at pressures of 0.1; 0.2; 0.3 MPa, the probability of occurrence of subsidence deformations increases [2].
Regression equation and probability diagram. Based on the results of numerical studies, using the R-Plus software package, linear regression equations were built and generalized tables of the main physical and mechanical indicators by region were compiled.
lgS1= -1.7445r +0,886e0-2,15; n=69;
SUl.q8i)=02U6: (1)
Prl.OKTC/CM2 P;=2.0KFCCM"
lgS2= -1.8065r +0,885e0-2,093; n=121;
SOCT(W=0,2115; (2) lgS3= -1.6415r +0,922e0-1,9972; n=147;
SOIT(W=0,4387; (3)
The equations make it possible at the stage of preliminary assessment to make a qualitative forecast for the tendency of loess type soils to subsidence, and according to the diagram (Fig. 1) one can estimate the probability of subsidence.
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Условные обозначения
Вероятность большее 50% меньшее 504 меньшее 15% меньшее 5%
Figl. Drawdown Probability Diagram
Causes of deformation of buildings and structures. Most public and civil facilities are equipped with water heating, water supply and sewerage, i.e. when designing, it is necessary to take into account the possibility of water leakage from networks and soaking of base soils and bedding under floors. It is also necessary to take into account: cases of soil soaking with surface water as a result of improperly executed territory planning or lack of necessary elements of water protection and improvement of hard surfaces, drainage trays, ditches; in cases of soaking with water that appears on
the sites as a result of accidents at water-containing structures or as a result of their improper operation [2]. In recent years, there have been more and more cases of a sharp increase in soil moisture, a significant rise in the level of groundwater as a result of technogenic factors. There are many cases of deformations and defects of buildings and structures on subsiding soils [2].
The reasons that led to the deformations, in general, were the following shortcomings and violations made by prospectors, designers, builders or operators:
1. Incomplete information about subsidence soils given in the reports of engineering and geological surveys, complete or partial failure to take into account the phenomenon of subsidence in projects.
2. Violation by the builders of the instructions of the norms and rules for the production of works and recommendations for designing the construction of bases and foundations, for compacting the bedding under the floors and backfilling the sinuses of pits.
3. Intensive soaking of soils as a result of water leakage from networks or surface water due to improper operation of networks, unfulfilled or incorrectly executed vertical planning of the building site.
For a complete characterization of subsiding soils, the designer needs to have a number of additional indicators of their properties, information about the ground-water regime, construction experience in the area, and recommendations for using soils as foundations.
Design methods. In construction practice, there are many methods for designing and constructing buildings and structures on subsidence soils [1:5]. If it is possible to soak foundation soils, the following methods are most often used:
A. Elimination of subsidence properties of soils;.
B. Cutting subsidence soils with pile foundations;
Q A set of measures, including the preparation of
the foundation, waterproofing and constructive measures.
Compaction with heavy rammers with soaking up to Wopt, in our opinion, is a fairly simple, highly mechanized and reliable method. It is well mastered by builders and widely used. However, almost everywhere builders do not fulfill one of the most important conditions of this method - control of soil density in the lower part of the compacted zone. As you know, the density should be at least 1.65 t/m3. It is known from literary sources and according to our research that when rammers weighing 5.0 tons are dropped from a height of 67 m with a base diameter of 1.5 m, soils are compacted to a depth of 2.0-3.0 m, subject to preliminary soaking [5].
The device of highly compacted soil pads with body reinforcement is the replacement of subsiding soil within the entire part of the deformable compaction zone with non-subsiding soil. This method is used in cases where it is impossible to use heavy rammers (for example, closer to the existing building), or there are no equipment and mechanisms. Coarse-grained sands or sand-gravel mixtures are used as a replacement soil. We recommend filling the soil cushion in layers with compaction by hand rammers or rollers to a density of 1.65 t/m3. This method is more time-consuming and more expensive than the first one, since it requires excavation and transportation of a layer of subsiding soil, delivery, laying with compaction and reinforcement of the cushion soil. But in some cases it is the only possible one for the reasons mentioned above.
Pile foundations for buildings during construction on subsidence soils, first of all, type 1 in terms of subsidence, have found wide application in Mongolia. There, driven prismatic and pyramidal piles, bored piles are used with great effect. Driven prismatic piles are less effective in loess type subsiding soils, but if it
is necessary to completely cut through subsiding soils, their efficiency increases. According to the results of field tests of driven pile foundations in the city of Darkhan, it was established that the bearing capacity of piles during soaking decreases by 1.75-2.45 times [2:3:4]. Therefore, it is necessary to design pile foundations taking into account the soaking of the soil until full water saturation. The use of pile foundations increases the degree of industrialization of construction, but entails the need to solve a number of complex technical and organizational problems, setting up the production of piles, creating an organization for pile work and pile testing.
Foundations in Rammed down tamped pits with and without broadening at the lower end. This method is widely used in foundation soils according to type I subsidence. High bearing capacity for vertical loads and with soil compaction around the foundation is its feature. Based on the results of field tests of foundations in rammed down tamped pits, it was established that the bearing capacity under the action of vertical and horizontal loads or settlement of these foundations varies greatly depending on the type and volume of materials used for widening [1:2:3].
A set of measures is recommended for use in the case of construction on soils with type II soil conditions in terms of subsidence, however, some of them are also used for type I. So, regardless of whether we prepare the base by compacting or cutting, we attach great importance to waterproof measures. When laying out the general plan, we strive to place all water-containing and conductive structures, hard-surfaced sites, silo trenches in places where leakage or runoff from them on neighboring buildings and structures will not adversely affect, if possible, lower in relief or on another natural water intake line._We provide blind areas around buildings with a width of up to 1.5 m._In necessary cases, we design upland Kanavos and drainage trays [1].
CONCLUSION:
1. Subsiding loess soils are spread over more than 30% of the total area of the territory of Mongolia; Particularly harsh continental climate (annual temperature range from -35°C to +35°C) creates special conditions for the formation of loess type soils. Significant seasonal freezing (from 2.8 m to 4.5 m) of the soil, due to small snow, vegetation cover and a long period of negative temperatures with severe frosts, will add to frost weathering, which is expressed in an increase in the silt fraction due to the crushing of sand particles and a decrease in structural strength of soils, which ultimately increases their subsidence.
2. Significant seasonal freezing (from 2.8 m to 4.5 m) of the soil, due to small snow, vegetation cover and a long period of negative temperatures with severe frosts, will add to frost weathering, which is expressed in an increase in the silt fraction due to the crushing of sand particles and a decrease in structural strength of soils, which ultimately increases their subsidence.
3. A distinctive feature of the building properties of the subsiding soils of Mongolia are relatively low humidity (W<0,06-0,08), density (pd= 1.42-1.58g/cm3), degree of moisture (Sr<0.50) and the minimum porosity is approximately equal to (n=40 %) and
more. The content of sandy and silty particles is more than 50% in sandy loams and loams. The mineralogical composition is relatively constant both in plan and in depth. By subsidence, mainly type I, very rarely type II. According to lithology, Quaternary loess type deposits occur from the daytime surface of the earth at a depth of 4.5 to 10.0 m, below which are hard clay with pebble inclusions and coarse-grained soils with various aggregates. The groundwater level in its natural state is lower by 15.0 m and deeper.
4. As a result of experimental and theoretical studies, I came to the conclusion that under the conditions of subsiding loess type soils of Mongolia, the most appropriate methods are driven and bored piles, foundations in rammed pits, compaction of soils with a heavy rammer under conditions of preliminary soaking of the base soil, and soil cushions with reinforcement and with different sole configurations.
5. As a result of field and laboratory tests of the compactibility of the subsiding loess type soil of the city of Darkhan, it was found that the ratio of the deformation moduli at natural humidity and water-saturated state is 3-5 times, after compaction this ratio decreases to 1.2-2 times. Consequently, the structure of compacted loess rocks is more resistant to external influences (pressure combined with soaking) than uncom-pacted ones. Due to this, the foundation of buildings
becomes more resistant to man-made soaking during operation.
References
1. Building codes and rule 2016. BNbD 2.02.0194. (Updated edition BNbD 50.01-16) Design of foundations and foundations of buildings and structures-UB. 1995 (2016).
2. S. Nyamdoij. 2003. Abstract for the degree of doctor (Ph). Study of the design features of foundations and foundations in loess-like subsidence soils of Mongolia. - Ulaanbaatar. 2003
3. Yu.A. Bagdasarov, S. Nyamdoij. 1991. Results of tests on foundations in rammed down foundation pits in Darkhan (MPR). Vol.28, No.2, March-April, 1991. Page 95-98. Consultants Bureau, New York.
4. S. Nyamdorj, A. Anand, M. Myagmarzhav. 1988. Study of the bearing capacity and development of technology for the production of driven pile foundations in subsidence soils. -Scientific report, -UB., 1988. 135 Pages.
5. S. Nyamdorj, A. Anand, M. Myagmarzhav. 1988. Study of the development of technology for the production of work on compaction of subsiding soils of the base. - Scientific report. -UB., 1989. 86C.
6. S. Nyamdorj, R. A. Mangushev. 2018. Russian-English-Chinese Dictionary of Construction Geotech-nics. Volume-2. 25,000 words and phrases. 723 pages. 2018. Ulaanbaatarr.
