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ScienceDirect Procedia Engineering 189 (2017) 710 – 715
Transportation Geotechnics and Geoecology, TGG 2017, 17-19 May 2017, Saint Petersburg, Russia
Enhancement of subgrade’s bearing capacity in low water permeable (clay) soils Ludmila S. Blazhkoa, Valerii I. Shtykova, Evgenii V. Chernyaeva,* Emperor Alexander I St. Petersburg State Transport University, Saint-Petersburg, 190031, Russia
Abstract Today’s level of vibrodynamic loads performed by rolling stock should be compensated for new technical solutions in railway track construction, which ensure the minimum increase of permanent deformation during exploitation. The article presents the theoretically grounded solution for reduction of intensity of permanent deformation accumulation in low water permeable (clay) soils the bearing capacity of which is significantly changed during their excessive moisturising. The effective dewatering of such soils may be feasible when using “a coarse-grained drainage”. The authors demonstrate constructions of “coarse-grained drainages” and requirements for their calculations. © Authors. Published by Elsevier Ltd. This ©2017 2017The The Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the International conference on Transportation Geotechnics and Peer-review under responsibility of the scientific committee of the International conference on Transportation Geotechnics and Geoecology Geoecology. Keywords: Low water permeable (clay) soils, “coarse-grained drainage”, tensions.
1. Introduction The main requirement which is set for organisation of transportation process is ensuring safe railway traffic. Strength is one of the basic requirements set for a subgrade in general and for soils, constituting it, in particular. The subgrade should take the present and future permanent loads without failures and with a minimum intensity of permanent deformation accumulation.
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1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the International conference on Transportation Geotechnics and Geoecology
doi:10.1016/j.proeng.2017.05.112
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The deformation of a subgrade is the main problem occurring during organisation of train traffic. Additional conditions are the speed and quality of passengers’ carriage and goods transportation services. In many countries, due to a high competition of different means of transport, (road, air, rail transports) speed and weight of cargo trains enhancement are becoming the key objectives. One of the common problems occurring during achievement of the set tasks is an unsatisfactory state of a subgrade and/or soil of a foundation. In some regions where a subgrade has been exploited since XIX century its bearing capacity is now almost exhausted. The further increase is speed, axle loads or intensity of train traffic may be achieved only when a certain range of works is completed, which are to be aimed at creating the necessary bearing capacity of subgrade soils as a foundation taking a steady impact from the permanent way (weight of rails, rail fastening systems, rail seat) and the temporary one from the rolling stock, taking into a count constantly changing climate conditions. Performance of a complex of subgrade works (partial replacement of soils, forming right outline, rehabilitation and repair of drainage, protection and reinforcement structures) is extremely difficult due to certain reasons. The first and the most sound one is a lack of technical possibility of goods transportation at other directions (there are no spare routes), the second is a high cost of the works themselves. Besides, it should be noted absence of necessity in construction of extra railway tracks, because the existing ones, if reconstructed, could provide the necessary amount of goods transportation. 2. Experience The problem of reducing the intensity of accumulation of deformations in subgrade as a result of mismatch of strength properties has never lost its topicality. The soil of an exploited subgrade is not homogeneous and is able to change its properties with time, being influenced by exploitation and climate factors. In Russia, European countries and the USA several ways of improving the bearing capacity of subgrade soils are used: x alteration of a soil structure (by adding cementing agents into the soil); x adding anchor construction into the subgrade’s body; x laying of geosynthetic materials into the subgrade (geogrid, geotextile and expanded polystyrene); x organisation of a surface discharge; x regulation of subsurface waters’ impact; x installation of protection structures and other activities; There are other methods aimed at prevention and reducing of subgrade deformation. Among them there are: alteration in construction of the permanent way, e.g. by using ballastless track, bonding ballast with geocomposites or polyurethane sealant, construction of an elevated track. 3. Tasks of the Research The complex climate of Northwest Russia, length of lines built in XIX-XX centuries, changed exploitation characteristics of railway tracks, untimely or late repair works have led to formation of ballast pockets and softsinking spots. Naturally, the deformed subgrade cannot ensure the necessary location of a track panel, resulting in emergence of pumpings, sinkings and track twists along the line. This, in its turn, causes deterioration of carrying capacity of deformed sections due to limited speed of trains. Thus, maintenance labour costs in such sections are 1.5-3 times higher than in sections with a stable subgrade. [1] The most common reasons for deformation of subgrade in Northwest Russia are oversaturation after excessive rain and snowfall, unsatisfactory state and maintenance of the drainage systems. Subgrade in Northwest Russia is mainly made up from loam, whose bearing capacity (strength) and deformability most depend upon their moisture content. The task of the research was to develop a method to enhance the bearing capacity of subgrade soils by reducing its moisture content, including seasonal freezing. The specialists of the Department of Railway Track and the Department of Water Supply, Sewerage and Hydraulics of Emperor Alexander I St. Petersburg State Transport University have elaborated and substantiated the possibility of application of “coarse-grained drainage”, their construction and technical requirements, which ensure 100% drainage of excessive water from the exploitation area of the subgrade and, consequently, ensure its bearing
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capacity and prevention from deformation caused by frost boils. [2,3,4] 4. Solution The authors of the article suggest application of “coarse-grained drainage” (the main peculiarity – lack of drain pipe which is replaced with coarse-grained material with excellent filtering properties) in the subballast area of a railway track, which favourably differs from a classic system of drainage, namely: x simplicity of construction, actual application of one type of inert material, lack of joints and sophisticated junctions; x possibility of creating flat drain sections without digging into the base; x possibility of application of common construction machinery while performing earthworks and laying drainage material, which leads to significant increase of mechanisation of drainage laying works; x simplicity of quality control while performing “coarse-grained drainage” laying works; x possibility of giving up performance control of “coarse-grained drainage” under plates of the base, in turn points and differentials. In order to fulfill the set task, that is enhancement of bearing capacity of the subgrade made up from clay soils, there were two issues settled for the proposed technical solution. The first issue consisted in developing a method of hydraulic calculation of the suggested construction of “coarse-grained drainage” considering the climate and nature features in Northwest Russia. Fig. 1, right (B) illustrates on the solutions aimed at reinforcing the main platform of the subgrade by means of geogrids, where the drainage process is performed by the standard design of the drainage. On the left (B), there is a suggested decision, whereby the application of rectangular cross-section drainage trenches, calculated parameters ensure dewatering of the entire working area of the subgrade soils.
Fig. 1 – Construction scheme of railway track with various dewatering systems of subgrade working area: A) the suggested by the authors system of “coarse-grained drainage”, B) Standard design 1 “coarse-grained drainage”, 2 – waterproofing layer, 3 drainage filler, 4 drainage pipe, 5 geogrid.
The results of the theoretical works conducted by American scientists [5, 6] showed that in double-layer soils, underlaid by low water permeable (clay) layer containing a drain, nearly 18% per cent of water subject drainage flow into the latter one. For dewatering of loam soils the distance between drains must not exceed 2 metres, the requirement which is not met in traditional systems of subgrade drainage. Ineffective performance of a covered drainage in low water permeable (clay) soils is confirmed by studies conducted in SevNIIGiM (Northern Research Institute if Hydrology and Land Development) [7]. Thus, on the basis of the studies conducted by scientists in the fields of drainage of low water permeable (clay) soils, it can be stated that in these soils with a distance of 2 m. between drains it is impossible to ensure the needed bearing capacity using traditional dewatering of the subgrade. It should be noted that the depth at which the drainage laid in low water permeable (clay) soils does not fundamentally change its draining properties. According to SevNIIGiM, during 14 years of observation of test agricultural land the difference in flows out of drainages with laying depths of 0.7 m, 0.9 m, and 1.2 m has not been determined [7]. During six years, H. U. Tomson (Estonia) while observing work of drainages in “Pandera” section, characterised by subsurface hydraulic conductivity of 0.01 metres per day, has determined that with a distance of 10
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m between drains, the flow out drains, which are laid 1.1 m deep, is 1.3 times bigger than that from those which are laid 1.5 m deep. These data are confirmed by other foreign researchers. As a result, in a number of European countries the certain depth of laying drainage in low water permeable (clay) soils has been accepted: Germany – 0.6 – 0.75 m, France, The Czech Republic, Slovakia – 0.7 – 0.8 m. Application of materials with good filtering properties as a filling for covered gravity-fed drainages enhances the drainage flow up to two times by increasing the so-called catchment capacity of drains. In accordance with the above, drains should be placed directly in the subgrade, whereby “the coarse-grained drainage” should be applied which showed good performance in the field of hydraulic engineering. [8,9,10] As a result of the application of the design of drainage of load bearing area of subgrade soils (Figure 1 A) elaborated by the authors, it becomes apparent that by ensuring the needed drainage of clay soils their bearing capacity increases, thus guaranteeing the decrease in intensity of permanent deformation accumulation. Moreover, the necessity of application of geomaterials, e.g. geogrid, geonet and expanded polystyrene falls away. In order to determine parameters of “the coarse-grained drainage” design of subgrade soils, a set of methods was developed which includes: x estimation of water inflow into drainage; x estimation of parameters and selection of backfill materials; x hydraulic calculations of collectors; x estimation of subgrade soils freezing influence upon the drainage performance; Hydraulic calculation of “the coarse-grained drainage” consists in determining the maximum height of water rise in drains during wet years by the given filtering properties of infilling materials and distance between collectors, the drains run into. Moreover, there is a presented method of calculation of erosion possibility of subgrade soils in the mouth of “the coarse-grained drainage”. This calculation has already been elaborated and is successfully used both in land development and hydraulic engineering. The second issue is to determine the possibility of enhancing the weight of a train, including by enhancing the axle load while digging drainage ditches (of “the coarse-grained drainage”) in the subrail area. The issue was settled on the basis of Russia’s Railways actual Methods of accessing the impact of rolling stock on the track upon conditions of reliability process [11]. The Fig. 2 depicts scheme of transferring loads from wheels to the ballast and the subgrade influenced by a twoaxle bogie being the most widespread freight rolling stock operated on Russia’s railways.
Fig. 2 Scheme of transferring loads from wheels to the ballast and the subgrade influenced by a two-axle bogie
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Tables 1 and 2 show the results of calculation enabling to make the following conclusion: Table 1. Level of stress in the standard track construction Velocity, Level of stress in subgrade, MPa km/h summer winter direct 10 0,0404 0,0467 40 0,0444 0,0525 70 0,0491 0,0592 100 0,0547 0,0670
Table 2. Level of stress in the alternative track construction Velocity, Level of stress in subgrade, MPa km/h summer winter direct 10 0,0226 0,0248 40 0,0248 0,0279 70 0,0274 0,0315 100 0,0306 0,0357
The level of stress in the subgrade when using a rectangular cross-section drainage ditch for summer and winter conditions, taking into account the alteration in stiffness of subrail base is twice as little as compared to the standard construction. The Fig. 3 shows the correlation between the levels of stress in subgrade and the depth of drainage ditch. Apparently, that changing of the drainage ditch depth leads to reduce in stress the subgrade even if the train speed increases.
Fig. 3 The correlation between the levels of stress in subgrade and the depth of drainage ditch
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5. Conclusions: 1. The suggested construction of the rectangular cross-section “coarse-grained drainage” (characteristics are defined by estimation of the climate and nature features) in the subrail area, having a significantly higher water catchment capacity than that of a standard water drainage constructions, will surely reduce the moisture content of subgrade soils and prevent them from being excessively moisturised during the period of heavy rain/snowfall, consequently eliminating the risk of deformation caused by frost boils. 2. When using “the coarse-grained drainage”, the level of stress on the subgrade reduces nearly twofold, resulting in a significant decrease in intensity of permanent deformation accumulation and creates the possibility of enhancing the train weight. 3. “The coarse-grained drainage” enables water drainage even during the intensive snowmelt giving way to ground frost, due to a high water yield coefficient of a backfill material. The application of “the coarse-grained drainage” (Fig. 1 B) (the construction is confirmed by calculation) will help to solve the main problems of deforming sections of the subgrade made up from clay soils. The extensive moisturising of the subgrade due to the maximum level of rain/snowfall is also excluded.
References [1] I.V. Serebryanikov, Analysis of the state and basic problems of reinforcing of engineering constructions on the roads. Proceedings of the 2 d International scientific and technical conference. “Today’s problems of design, construction and exploitation of the subgrade and artificial structures” (November 22d November 29th, 2005). Moscow. P. 10-14. [2] L.S. Blazko, V.I. Shtykov, E.V. Chernyaev, Application of “a coarse-grained drainage” in railway construction. Transport infrastructure of Siberia. 2015. V.1. P. 589-593. [3] L.S. Blazko, V.I. Shtykov, E.V. Chernyaev Peculiarities of a sub-surface drainage performance in Spring time during its laying into a seasonal freezing zone. Proceedings: II International Workshop on subgrades in a cold region. Edited by A.L. Isakov and Ts. K. Liu. 2015. P. 91-94. [4] Blazko L.S., Shtykov V.I., Chernyaev E.V. Coarse-grained drainage: experience and perspectives. Railway transport. 2013. №11. P. 47-49. [5] James N. Luthin, R.E. Gaskell, Numerical solutions for tile drainage of layered soils. Trans. Americ. Geophys. Union, 1950, Vol 31, p. 595602. [6] D. Kirkham, Infiltration and permeability in soil overlying an impermeable layer. Americ. Geophys. Union, 1951, Vol 32, p. 422-442. [7] A.I. Klimko, Y.A. Kantsiber, L.M. Ermolina, Calculations of optimal parameters of agricultural drainage. Moscow, “Kolos”, 1979. P. 142. [8] V.I. Shtykov, A.V. Kozlova, Hydraulic calculation of the coarse-grained flat drainage in groundwater feeding. “B.E. Vedeneev VNIIG”, 2007. V. 247, P. 84-90. [9] Shtykov V.I., Bulganin E.V. Hydraulic calculation of a straight coarse-grained drainage of trapezoid cross-section// “B.E. Vedeneev VNIIG”. 2008. V. 252. P. 35-40 [10] Shtykov V.I., Gordienko S.G. Coarse-grained drainage: Hydraulic substantiation, calculation and efficiency. Saint-Petersburg. 1997. P. 224. [11] Methods of accessing the impact of rolling stock on the track upon conditions of reliability process 52/14 (Russian Railways Standard “ЦПТ 52/14”)
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