Criteria for Improving the Railway Embankment for High Speed Train
Project Submitted in
Partial Fulfillment of the Requirements for the Degree of
Bachelor of Technology
In
Civil Engineering
By
Mabud Sarkar
(CEN124020)
Under the supervision
Of
Prof. Md. Raghib Adil
Dept. of Civil Engineering
ALIAH UNIVERSITY
2016
CERTIFICATE
It is certified that the work contained in this project ‘Criteria for Improving the Railway Embankment for High Speed Train’ submitted by Mabud Sarkar for the award of B.Tech. is absolutely based on their own work carried out under my supervision.
I Wish his all success in their future endeavours.
Date: (Md Raghib Adil)
Professor
Dept. of Civil Engineering
Aliah University
ACKNOWLEDGEMENT
I take this opportunity to express my deep heartfelt sense of gratitude and thank my project supervisor Prof. Md. Raghib Adil, Department of Civil Engineering, Aliah University, for his invaluable assistance. I would like to express my special appreciation and thanks to my advisor Professor Dibyendu Sengupta you have been a tremendous mentor for me. Their innovative suggestion and encouragement has prevented me from ceasing faith and having patience in myself until the completion of the project.
I am extremely thankful to the university authority to provide me with all the amenities required for various experiments in my project. I am extremely grateful to all faculties of Dept. of Civil Engineering for their constant and steady support.
Last but not least I thank my parents, friends and others who helped us throughout the project.
ABSTRACT
A high-speed railway project for trains of speeds up to 160 km/h is recently introduced from Delhi to Agra (195 km long) in India. The ground improvement methods adopted in the project are vibro-replacement with stone columns, dry deep soil mixing (cement columns), geogrid-reinforced piled embankments with individual pile caps and removal/replacement works. This paper provides a detailed insight into the design and implementation of vibro-replacement and the deep soil mixing treatment methods used in the project. The use of plate bearing tests and field instrumentation to monitor the performance of the stone columns and soil mixing ground treatment methods is also discussed. This paper also provides a brief overview of other treatment methods implemented in this high-speed railway project such as a pile embankment with geogrids and removal/replacement works.
Railway tracks (rails and sleepers) are normally laid on a sub-structure that consists of two or more layers of different materials. The top layer (below the sleepers) is a layer of railway ballast. Below the ballast there might be layers of sub-ballast, a formation layer and/or the subground (the formation). Historically, the ballast layer performs the function of supporting the sleepers against vertical and lateral forces.
A railway track exposed to train traffic will degenerate. Track alignment and track level will deteriorate. Settlements of the track (loss of track level and alignment) require maintenance of the track; the track is aligned and lifted, and new ballast material is injected under the sleepers.
Explanations why track settlements occur are very scarce. Often, some parts of a track are more prone to settlements than other parts of the same track. So far, mainly the influence of such factors as axle load and train speed have been investigated. Having in mind that tracks subjected to the same load show different settlement behaviour, explanations of track settlements must be sought for in the track itself; not only in the loading of the track.
This review deals with railway ballast and railway track settlements. It also presents some mathematical and numerical methods dealing with the static and dynamic loading of the track due to interaction of train, track, and sub-structure.
KEYWORDS: Bearing capacity; Earthwork; Embankments; Geotextiles; High speed ground transportation; Piles (Supports); Railroad tracks; Soil stabilization; Speed
INTRODUCTION
Railways are one of the oldest mode of transportation systems started some 150 years ago under different traffic conditions as far as speed, axle loads and traffic intensity are concerned. Increasingly, there are greater demands from modern railway organisations to increase the axle loads and train speeds both for economic and environmental reasons. In addition to increased axle loads and train speeds, railway lines often have to cross over existing loose or soft cohesive deposits as a part of the alignment giving rise to the need for ground improvement. In order to achieve a high level of performance of the rail system, attention should be focused
on post construction settlements of the subsoil and factor of safety of the structure against slip failure. Different countries follow different sets of specifications for settlement and stability criteria for railway systems.
Apart from the settlement and stability criteria, another important criterion is to mitigate vibrations induced by high-speed trains in order to achieve acceptable dynamic performance of the rail system. By improving the subsoil characteristics, it is possible to mitigate the vibrations to the surrounding structures.
LITERATURE REVIEW
Relatively little of the existing literature addresses modal shift directly. Most studies that invoke the concept (either explicitly or implicitly) use it in the course of addressing the demand for HSR more generally. Mode shift is one of the major constituents of overall demand for HSR: Many passengers are expected to be persons who would otherwise have flown, driven, or used some other mode of transportation. As a rule, researchers are addressing modal shift when they explore “competition” between HSR and other modes. Most of the literature explored in this analysis explicitly uses the concept of competition to explore modal shift, although competition in the sense of economic battle is not the ultimate objective for HSR systems. HSR systems are intended to advance a number of policy aims, including environmental objectives, more rational allocation of public infrastructure, and other goals. However, to achieve these goals, significant modal shift to HSR is paramount, hence the literature’s emphasis on competition. In these studies, competition assumes a wide variety of forms, but it tends to focus on point-to-point travel times, costs, and the quality of the travel experience.
Before addressing the existing research on intermodal competition, it bears mentioning that scholars note two other potentially significant sources of demand for HSR services: (1) complementary demand and (2) induced demand. Complementary demand is created when passengers choose to use HSR service in concert with the use of another mode, such as when a person travels via HSR to connect to an airline flight. In this case, HSR does not subtract ridership from the air mode but helps enable the use and, perhaps growth in the use, of both modes.
OBJECTIVE
Modern railway infrastructure demands a high level of performance in terms of settlements and stability of the railway track. In areas where loose or soft cohesive deposits are found, ground improvement is often required to ensure the required level of performance.
In addition to increased axle loads and train speeds, railway lines often have to cross over existing loose or soft cohesive deposits as a part of the alignment giving rise to the need for ground improvement.
GROUND IMPROVEMENT TECHNIQUES
The presence of loose or soft soils along the alignment of railway tracks inevitably leads to problems in terms of post construction settlements, inadequate factor of safety against slip failure and problems associated with ground vibrations caused by high-speed trains. In order to overcome these problems, several ground improvement techniques are available and some of them are presented below.
VIBRO TECHNIQUES
The process of improving loose granular soils with depth vibrators started in the 1930’s and 25 years later with continuous development and modification of the equipment, additional refinements to the technique have been implemented in order to use the technology for treating soft cohesive soils as well. The depth vibrator as a ground improvement tool is used to solve a wide range of static, dynamic and seismic foundation problems by densifying loose granular soils (Vibro Compaction) and partially replacing soft cohesive soils with granular material (Vibro Replacement or stone columns). Where lateral support from in-situ soil is inadequate, the stone column may be grouted (Grouted Stone Columns) or concrete may be used (Vibro Concrete Columns).
Vibro Compaction
The basic principle behind the method is that particles of non-cohesive soil such as sand and gravel can be rearranged by means of vibration. The vibratory action of the depth vibrator is used to temporarily reduce the inter particular friction between the particles and rearrange them in a denser state. The vibrator penetrates the soil by means of water jets and once at full depth, it is gradually withdrawn leaving behind a column of well compacted soil. A schematic showing the process of vibro compaction is depicted in Figure 1. To achieve a mass densification, the entire area is compacted by column points in a triangle or square pattern.
This technique is well suited for the densification of relatively clean (fines content up to about 10 to 15%) granular soils such as sands and gravels. A major benefit of this method is that no additional materials are necessary which makes it a very economical technique.
Vibro Replacement (Stone Columns)
Vibro replacement is a technique used to improve sandy soils with high fines contents (>15%) and cohesive soils such as silts and clays. In this method columns made up of stones are installed in the soft ground using the depth vibrator. The vibrator is used to first create a hole in the ground which is then filled with stone during withdrawal of the vibrator. The stone is then laterally displaced into the soil following repenetration of the vibrator. In this manner a column made up of well compacted stone fill with diameters typically ranging between 700mm and 1,100mm is installed in the ground. Two methods of installation namely the ‘wet’ and ‘dry’ methods are available for the installation of the columns. In the wet method water jets are used to create the hole and assist in penetration. In the dry method the hole is created by the vibratory energy and a pull down force. Typical installation process in the case of dry method is schematically shown in Figure 2. This technique of soil improvement can be used for nearly all types of soils. Testing of the soil improvement, after installation of the stone columns in coarse-grained soils is usually performed with either static or dynamic penetrometer tests (CPT or DPT). However for stone columns constructed in fine-grained soils it is common practice to carry out load tests directly on the columns.
The Vibro Replacement technique provides an economic and flexible solution, which easily adapts to varying ground conditions. Using Vibro Replacement, the following geotechnical improvements are achievable:
a. Compaction of the subsoil and increase in density
b. Improvement in the stiffness of the subsoil to decrease excessive settlements
c. Improvement in the shear strength of the subsoil to decrease the risk of failure
d. Increase in the mass of the subsoil to mitigate ground vibrations
e. Ability to carry very high loads since columns are highly ductile
f. Rapid consolidation of the subsoil
Grouted Stone Columns (GSC)
A stone column depends on the lateral support offered by the in-situ soil for its stability and load carrying capacity. In organic soils such as peat, this lateral support may not be adequate or may diminish with time following decomposition. In such cases, columns can be constructed by binding the gravel/stones with a cement grout suspension. This can be achieved by the addition of cement suspension during the installation process, which combines with column material to form a grouted body. Typical installation process of grouted stone columns is schematically shown in Figure 3. The design of the external bearing capacity of the grouted columns is carried out as per normal pile design codes. The maximum vertical load per column generally ranges between 400 kN and 600 kN and is mainly influenced by the shape of the compacted toe (column base).
Vibro Concrete Columns (VCC)
This is a variation of the grouted stone column technique which forms a rigid pile like foundation element. In this technique concrete is pumped directly to the tip of bottom feed depth vibrator to form the column. Typical installation process of vibro concrete columns is schematically shown in Figure 4. Due to the formation of the base and its penetration into the compacted bearing strata, the columns are generally considered as end-bearing columns and can support high service loads. The internal load bearing capacity is dependent upon the grade of concrete, and is determined in accordance with standard design codes as in other in-situ pile foundation systems.
Vibro Concrete Columns are ideal for weak alluvial soils such as peats and soft clays overlying competent founding stratum such as sands and gravels, soft rocks etc. Working loads up to 750 kN can be achieved in appropriate soils. Where the VCCs are required to support structures, such as heavily loaded floor slabs, rafts, roads and embankment, the columns can be constructed with an enlarged heads as shown in Figure 5. The enlarged head serves to reduce punching shear and can either be used to give direct contact support to the slab or to provide a uniform bearing pressure through a geogrid reinforced granular mattress as shown in the Figure.
DEEP DRY SOIL MIXING
Dry deep soil mixing (DSM) technology is a development of the lime-cement column method, which was invented by Kjeld Paus almost 30 years ago. It is a form of soil improvement involving the introduction and mechanical mixing of in-situ soft and weak soils with a cementitious compound such as lime, cement or a combination of both in different proportions. The mixture is often referred to as the binder. The binder is injected into the soil in a dry form. The moisture in the soil is utilised for the binding process, resulting in an improved soil with higher shear strength and lower compressibility. The removal of the moisture from the soil also results in an improvement in the soft soil surrounding the mixed soil. Typical
execution process of dry deep soil mixing is schematically shown.
CASE HISTORIES
CASE HISTORIES FROM GERMANY AND AUSTRIA
Following the reunification of Germany in 1990 and the introduction of the high-speed railway system in the country, the existing railway network leading to Berlin required up-gradation. This called for extensive ground improvement works. Deep Vibro techniques were employed at several locations. The map in Figure 7 shows the locations/sites where ground improvement works were carried out. Figure 8 shows photographs from site showing 3 Vibrocats carrying out ground improvement works for the German ICE train system and a schematic of the high speed ICE Train with operating speeds of over 250 kmph founded on stone column.
Hamburg – Berlin High-Speed Line: Wittenberge Section (Vibro Replacement)
The subsoil underneath a 6-km stretch in the extension works to the Hamburg – Berlin route near Wittenberge was improved using Vibro replacement (stone columns). The improvement was necessary for the upgrading of a normal existing line to a high-speed line on rigid pavement (refer Sondermann 1996).
Following soil investigation works, compaction works were carried out using special equipment and utilising the redundant ballast. A 4-row layout of stone columns were installed with a horizontal spacing of 2m c/c and vertical spacing of 1.25m c/c on a triangular grid pattern. The columns were installed to depths between 3m and 7m. The diameter of the columns varied between 0.6m and 0.8m. A schematic showing the cross section and plan view of treatment
scheme is shown.
During the soil improvement works, measurements were taken by geophones to record the vibrations at depths between 2m and 3m. Evaluations of the results showed that the anticipated soil deformation caused by the vibrations induced by high-speed trains have already occurred during installation of the stone columns. The oscillation speed of the railway system is much slower than that of the soil improvement works. Even during the installation next to a service railway line (with speeds of up to 120 kmph), the vertical displacements of the rails were less than 3mm and the horizontal displacements were negligible.
Hannover – Berlin High-Speed Line: Schonhausen Embankment Section (Vibro Replacement)
A section of the Hannover – Berlin high-speed line between Hamerten and Stendal near to the Elbe bridge is constructed on a rigid pavement system. Due to the double tracking of the highspeed line, the Schonhausen embankment at the east side of the Elbe Bridge has been widened. The old embankment was inter-connected to the new extension backfill by stone columns on a rectangle grid spacing of 1.85m x 2.15m c/c as shown.
Hamburg – Berlin High-Speed Line: Vietznitz-Friesack Section
(Grouted Stone Columns)
The subsoil underneath the Hamburg – Berlin high-speed line at Vietznitz-Friesack section in the area of Havellander Luchs, was found to be made of peat sandwiched between sandy layers. Soil investigation at the site indicated a 2m thick sandy layer at the surface followed by a 2m thick organic layer of peat followed by dense sandy layers. In order to bridge organic layers of peat, partially grouted columns were used to transfer traffic loads to deeper strata. The soil profile along with treatment scheme is shown in Figure.
Austrian Railway line near Hollenegg (Vibro Replacement under operational railway line)
The railway line Graz - Wies - Eibiswald is one of the oldest railway lines in Austria. It was built in the second half of the 19th century. The subgrade material used at that time was locally available and in some parts poor quality fill. This lead to problems of serviceability and repeated maintenance during the operation of the track. Geotechnical investigations revealed that the upper meters directly below the tracks were made up of loose soils which required densification.
The Keller Vibro Replacement technique was chosen as an economical solution which allowed continued operation of the railway line during ground improvement works. There was no necessity for removal of the rails and the ballast. The works were carried out during the line block periods at night. For this purpose, a specially modified Keller Vibrocat including all necessary auxillary equipment was mounted on top of a railway wagon.
LIST OF GROUND IMPROVEMENT AND FOUNDATION TECHNIQUES APPLIED
Technique
Purpose
Vibro Replacement
Densification of loose silty sands
Mitigation of liquefaction potential
Deep Soil Mixing
Stabilisation of in-situ soils
Lime/Flyash Injection
Subgrade stabilisation
Compaction Grouting
Filling of voids in the shale
Avoidance of sinkhole formations
Densification of soil mass
Jet Grouting
To replace the existing support
To increase the slope stability
To stabilise the soil mass
Chemical Grouting
Stabilisation of granular soils
Solidification of sandy soils
Mini Piles
To form load bearing elements
To form a deep foundation system
GENERAL CONSIDERATION FOR HIGH-SPEED TRACK DESIGN
Presently all over the world non-ballasted track concepts are being applied, although still at a moderate volume. The great advantages of such structures can be summarized as follows:
• Reduction of structure height;
• Lower maintenance requirements and hence higher availability;
• Increased service life;
• High lateral track resistance which allows future speed increases in combination with tilting technology;
• No problems with churning of ballast particles at high-speed.
If the low-maintenance characteristics of slab track on open line are to be retained, great care
must be taken to ensure that the subgrade layers are homogenous and capable of bearing the loads imposed. The slabs may be prefabricated or poured on site. The high level of investment required has prevented widespread use of slab track on open line so far. However, on the basis of life cycle costs a different picture is obtained.
MODELLING RAIL TRACK, ELASTIC BEARING, AND SLEEPER SECTION
In order to model the rail track, its resistance to bending was simulated in the most accurate way possible, which is why the inertia of the modelled rail must be equal to that of the real rail.
The model also sought to make the vertical stiffness equal for all of the elastic bearings (see Fig 9). The vertical dimension and the modulus of elasticity were fixed so that the vertical stiffness of the element coincided with the stiffness of the elastic bearing provided by the manufacturer. For the high-speed line, the elastic bearings have a stiffness of nearly 500kN/mm.
Because the sleeper section is not constant along its entire length, the dimensions of its most representative section were used for the sleeper model elements. For each element (See Fig 10), the modelled flexural stiffness must be equal to the real flexural stiffness, as follows:
To obtain homogeneity in the calculations, the elements of the model had to have a constant width, which is not the case in the real sleeper. Thus, the model width must be considered to be an average width. This average width must be such that the load bearing surface in the model is equal to that in reality.
SLEEPER–BALLAST CONTACT
The sleeper–ballast contact zones contain a high concentration of strains. This local phenomenon requires refining of the mesh used to model these zones. However, applying this procedure is sometimes impossible because of the computational resources and model complexity needed. The most common alternative to modeling the contact zones is to use bounded degrees of freedom.
The use of bounded degrees of freedom requires the introduction of different nodes for each material at the contact surface. These nodes must move equivalently in the direction perpendicular to the contact plane (see Fig. 6). However, these nodes can move at different values in the directions parallel to the contact plane. This solution is effective because it solves the tensional discontinuities that appear at the interface between two materials that differ significantly in their stiffness. In this model, bounded degrees of freedom were used at the sleeper–ballast contacts.
BOUNDARY CONDITIONS
The model in this study differs from many existing models of railway track construction in which all vertical planes are constrained in all directions. In the planes that shape the slopes of embankments are left completely free, with no restrictions. In particular, the boundary conditions used here are as follows (see Fig. 12):
Fig. 12. Boundary conditions
a) In the vertical plans limits of the model, z=0 and z=7.20m, the boundary condition adopted is to impose the nullity of movement in the perpendicular direction to these plans (uz=0).
b) In the vertical plans limits of the model, x=0 and x=18.45m, the boundary condition is, like the previous one, to impose the nullity of movement in the perpendicular direction to these plans (ux=0).
c) In the horizontal inferior plan of the model y=-3m, the condition to be imposed is the null vertical displacement (uy=0).
MATERIAL CONSTITUTIVE MODEL
An elastic, isotropic, and linear model was used to develop a mechanic model of the rail tracks, elastic bearings, sleepers, and the granular material processed with cement. Finite strain was used to simulate the kinematics of
the continuous medium. Granular material treated with cement is considered to behave elastically, at least until it reaches a substantial percentage of its stress limit; one can assume its modulus of elasticity to remain essentially constant under normal stress.
To model the embankment material on which the track is laid, a perfect elasto-plastic behaviour was assumed. This assumption implies that reloading occurs in the same way as downloading, and that the material experiences no hardening (hardening parameter H=0).
HYPOTHESIS ADOPTED
The load application must be carried out in several stages. In the first stage, only the material’s own weight is considered until reaching the stress balance, while at later stages the loads due to the train are also taken into account. The stresses and displacements of interest are the ones that correspond to the application of the train loads; therefore, they can be calculated from the difference between the totals obtained after applying the train loads to the first stage.
Here, it was convenient to apply four load states due to the train passage, matching each state to the application of the static load per wheel in the four central sleepers of the model: T5, T6, T7, and T8 (See Fig 13).
To simulate the constructive process of an embankment and to ensure convergence of the solution, the first load state (only the material weight) was divided into 250 substeps of the gravitational load application and 50 balance iterations for each one. For railway loads, 15 sub-steps proved to be sufficient to achieve convergence. The program used is ANSYS Structural, which enables nonlinear analysis.
CONCLUSIONS
Increased railway speeds enhance the deterioration problems in the embankment–structure transitions, which has important implications for operating and maintenance costs, as well as for passenger safety and comfort.
From the analysis carried out in this study, the following conclusions are drawn:
a) The embankment must not be built on excessively compressible material on original ground. Such material must be replaced with another material or treated to obtain a modulus of elasticity corresponding to that of a material.
b) Deep vibro techniques and deep soil mixing methods have found extensive application worldwide and have proven to be flexible in the ability to treat a wide range of soils and site constraints/conditions and efficient in terms of time required to complete the treatment works and for consolidation.
c) The fact that they have been widely used is a confirmation that the techniques are technically sound and at the same time economical.
REFERENCES
Brill, G.T. and Hussin, J.D. (1992), “The Use of Compaction Grouting to Remediate a Railroad Embankment in a Karst Environment”, Proceedings of the Twenty-Third Ohio River Valley Soils Seminar, Louisville, Kentucky, October, 1992.
BROMS, B.B. (1999) “Design of lime, lime/cement and cement columns”, Proceedings of the International Conference on Dry Mix Methods for Deep Soil Stabilisation, Stockholm, Sweden, pp. 125-153.
Holm, G. (1999) “Applications of Dry Mix Methods for deep soil stabilisation”, Proceedings of the International Conference on Dry Mix Methods for Deep Soil Stabilisation, Stockholm, Sweden, pp. 3–13.
Holm, G. et al (2002), "Mitigation of Track and Ground Vibrations Induced by High Speed Trains at Ledsgard, Sweden", Swedish Geotechnical Institute, SD Report 10, Sweden, pp 1-44.
Moseley, M.P. and Priebe, H.J. (1993) “Vibro techniques”, Ground Improvement, Edited by M.P. Moseley, Blackie Academic & Profession, pp. 1 – 19.
Pengelly, A.D. (2000) “Ground Modification Techniques for Railroad Subgrade Improvement”. Hayward Baker Report, Maryland, USA.
Sondermann, W. (1996) “Soil Improvement by Vibro Replacement for Rigid Pavement Construction to the High Speed Railway System”, 3rd Geotechnique-Colloquium, Darmstadt, Germany, Technical paper 10-53 E.
Swedish Geotechnical Society (1997) “Lime and Lime Cement Columns”, SGF Report 4:95E, Linkoping.
Project Submitted in
Partial Fulfillment of the Requirements for the Degree of
Bachelor of Technology
In
Civil Engineering
By
Mabud Sarkar
(CEN124020)
Under the supervision
Of
Prof. Md. Raghib Adil
Dept. of Civil Engineering
ALIAH UNIVERSITY
2016
CERTIFICATE
It is certified that the work contained in this project ‘Criteria for Improving the Railway Embankment for High Speed Train’ submitted by Mabud Sarkar for the award of B.Tech. is absolutely based on their own work carried out under my supervision.
I Wish his all success in their future endeavours.
Date: (Md Raghib Adil)
Professor
Dept. of Civil Engineering
Aliah University
ACKNOWLEDGEMENT
I take this opportunity to express my deep heartfelt sense of gratitude and thank my project supervisor Prof. Md. Raghib Adil, Department of Civil Engineering, Aliah University, for his invaluable assistance. I would like to express my special appreciation and thanks to my advisor Professor Dibyendu Sengupta you have been a tremendous mentor for me. Their innovative suggestion and encouragement has prevented me from ceasing faith and having patience in myself until the completion of the project.
I am extremely thankful to the university authority to provide me with all the amenities required for various experiments in my project. I am extremely grateful to all faculties of Dept. of Civil Engineering for their constant and steady support.
Last but not least I thank my parents, friends and others who helped us throughout the project.
ABSTRACT
A high-speed railway project for trains of speeds up to 160 km/h is recently introduced from Delhi to Agra (195 km long) in India. The ground improvement methods adopted in the project are vibro-replacement with stone columns, dry deep soil mixing (cement columns), geogrid-reinforced piled embankments with individual pile caps and removal/replacement works. This paper provides a detailed insight into the design and implementation of vibro-replacement and the deep soil mixing treatment methods used in the project. The use of plate bearing tests and field instrumentation to monitor the performance of the stone columns and soil mixing ground treatment methods is also discussed. This paper also provides a brief overview of other treatment methods implemented in this high-speed railway project such as a pile embankment with geogrids and removal/replacement works.
Railway tracks (rails and sleepers) are normally laid on a sub-structure that consists of two or more layers of different materials. The top layer (below the sleepers) is a layer of railway ballast. Below the ballast there might be layers of sub-ballast, a formation layer and/or the subground (the formation). Historically, the ballast layer performs the function of supporting the sleepers against vertical and lateral forces.
A railway track exposed to train traffic will degenerate. Track alignment and track level will deteriorate. Settlements of the track (loss of track level and alignment) require maintenance of the track; the track is aligned and lifted, and new ballast material is injected under the sleepers.
Explanations why track settlements occur are very scarce. Often, some parts of a track are more prone to settlements than other parts of the same track. So far, mainly the influence of such factors as axle load and train speed have been investigated. Having in mind that tracks subjected to the same load show different settlement behaviour, explanations of track settlements must be sought for in the track itself; not only in the loading of the track.
This review deals with railway ballast and railway track settlements. It also presents some mathematical and numerical methods dealing with the static and dynamic loading of the track due to interaction of train, track, and sub-structure.
KEYWORDS: Bearing capacity; Earthwork; Embankments; Geotextiles; High speed ground transportation; Piles (Supports); Railroad tracks; Soil stabilization; Speed
INTRODUCTION
Railways are one of the oldest mode of transportation systems started some 150 years ago under different traffic conditions as far as speed, axle loads and traffic intensity are concerned. Increasingly, there are greater demands from modern railway organisations to increase the axle loads and train speeds both for economic and environmental reasons. In addition to increased axle loads and train speeds, railway lines often have to cross over existing loose or soft cohesive deposits as a part of the alignment giving rise to the need for ground improvement. In order to achieve a high level of performance of the rail system, attention should be focused
on post construction settlements of the subsoil and factor of safety of the structure against slip failure. Different countries follow different sets of specifications for settlement and stability criteria for railway systems.
Apart from the settlement and stability criteria, another important criterion is to mitigate vibrations induced by high-speed trains in order to achieve acceptable dynamic performance of the rail system. By improving the subsoil characteristics, it is possible to mitigate the vibrations to the surrounding structures.
LITERATURE REVIEW
Relatively little of the existing literature addresses modal shift directly. Most studies that invoke the concept (either explicitly or implicitly) use it in the course of addressing the demand for HSR more generally. Mode shift is one of the major constituents of overall demand for HSR: Many passengers are expected to be persons who would otherwise have flown, driven, or used some other mode of transportation. As a rule, researchers are addressing modal shift when they explore “competition” between HSR and other modes. Most of the literature explored in this analysis explicitly uses the concept of competition to explore modal shift, although competition in the sense of economic battle is not the ultimate objective for HSR systems. HSR systems are intended to advance a number of policy aims, including environmental objectives, more rational allocation of public infrastructure, and other goals. However, to achieve these goals, significant modal shift to HSR is paramount, hence the literature’s emphasis on competition. In these studies, competition assumes a wide variety of forms, but it tends to focus on point-to-point travel times, costs, and the quality of the travel experience.
Before addressing the existing research on intermodal competition, it bears mentioning that scholars note two other potentially significant sources of demand for HSR services: (1) complementary demand and (2) induced demand. Complementary demand is created when passengers choose to use HSR service in concert with the use of another mode, such as when a person travels via HSR to connect to an airline flight. In this case, HSR does not subtract ridership from the air mode but helps enable the use and, perhaps growth in the use, of both modes.
OBJECTIVE
Modern railway infrastructure demands a high level of performance in terms of settlements and stability of the railway track. In areas where loose or soft cohesive deposits are found, ground improvement is often required to ensure the required level of performance.
In addition to increased axle loads and train speeds, railway lines often have to cross over existing loose or soft cohesive deposits as a part of the alignment giving rise to the need for ground improvement.
GROUND IMPROVEMENT TECHNIQUES
The presence of loose or soft soils along the alignment of railway tracks inevitably leads to problems in terms of post construction settlements, inadequate factor of safety against slip failure and problems associated with ground vibrations caused by high-speed trains. In order to overcome these problems, several ground improvement techniques are available and some of them are presented below.
VIBRO TECHNIQUES
The process of improving loose granular soils with depth vibrators started in the 1930’s and 25 years later with continuous development and modification of the equipment, additional refinements to the technique have been implemented in order to use the technology for treating soft cohesive soils as well. The depth vibrator as a ground improvement tool is used to solve a wide range of static, dynamic and seismic foundation problems by densifying loose granular soils (Vibro Compaction) and partially replacing soft cohesive soils with granular material (Vibro Replacement or stone columns). Where lateral support from in-situ soil is inadequate, the stone column may be grouted (Grouted Stone Columns) or concrete may be used (Vibro Concrete Columns).
Vibro Compaction
The basic principle behind the method is that particles of non-cohesive soil such as sand and gravel can be rearranged by means of vibration. The vibratory action of the depth vibrator is used to temporarily reduce the inter particular friction between the particles and rearrange them in a denser state. The vibrator penetrates the soil by means of water jets and once at full depth, it is gradually withdrawn leaving behind a column of well compacted soil. A schematic showing the process of vibro compaction is depicted in Figure 1. To achieve a mass densification, the entire area is compacted by column points in a triangle or square pattern.
This technique is well suited for the densification of relatively clean (fines content up to about 10 to 15%) granular soils such as sands and gravels. A major benefit of this method is that no additional materials are necessary which makes it a very economical technique.
Vibro Replacement (Stone Columns)
Vibro replacement is a technique used to improve sandy soils with high fines contents (>15%) and cohesive soils such as silts and clays. In this method columns made up of stones are installed in the soft ground using the depth vibrator. The vibrator is used to first create a hole in the ground which is then filled with stone during withdrawal of the vibrator. The stone is then laterally displaced into the soil following repenetration of the vibrator. In this manner a column made up of well compacted stone fill with diameters typically ranging between 700mm and 1,100mm is installed in the ground. Two methods of installation namely the ‘wet’ and ‘dry’ methods are available for the installation of the columns. In the wet method water jets are used to create the hole and assist in penetration. In the dry method the hole is created by the vibratory energy and a pull down force. Typical installation process in the case of dry method is schematically shown in Figure 2. This technique of soil improvement can be used for nearly all types of soils. Testing of the soil improvement, after installation of the stone columns in coarse-grained soils is usually performed with either static or dynamic penetrometer tests (CPT or DPT). However for stone columns constructed in fine-grained soils it is common practice to carry out load tests directly on the columns.
The Vibro Replacement technique provides an economic and flexible solution, which easily adapts to varying ground conditions. Using Vibro Replacement, the following geotechnical improvements are achievable:
a. Compaction of the subsoil and increase in density
b. Improvement in the stiffness of the subsoil to decrease excessive settlements
c. Improvement in the shear strength of the subsoil to decrease the risk of failure
d. Increase in the mass of the subsoil to mitigate ground vibrations
e. Ability to carry very high loads since columns are highly ductile
f. Rapid consolidation of the subsoil
Grouted Stone Columns (GSC)
A stone column depends on the lateral support offered by the in-situ soil for its stability and load carrying capacity. In organic soils such as peat, this lateral support may not be adequate or may diminish with time following decomposition. In such cases, columns can be constructed by binding the gravel/stones with a cement grout suspension. This can be achieved by the addition of cement suspension during the installation process, which combines with column material to form a grouted body. Typical installation process of grouted stone columns is schematically shown in Figure 3. The design of the external bearing capacity of the grouted columns is carried out as per normal pile design codes. The maximum vertical load per column generally ranges between 400 kN and 600 kN and is mainly influenced by the shape of the compacted toe (column base).
Vibro Concrete Columns (VCC)
This is a variation of the grouted stone column technique which forms a rigid pile like foundation element. In this technique concrete is pumped directly to the tip of bottom feed depth vibrator to form the column. Typical installation process of vibro concrete columns is schematically shown in Figure 4. Due to the formation of the base and its penetration into the compacted bearing strata, the columns are generally considered as end-bearing columns and can support high service loads. The internal load bearing capacity is dependent upon the grade of concrete, and is determined in accordance with standard design codes as in other in-situ pile foundation systems.
Vibro Concrete Columns are ideal for weak alluvial soils such as peats and soft clays overlying competent founding stratum such as sands and gravels, soft rocks etc. Working loads up to 750 kN can be achieved in appropriate soils. Where the VCCs are required to support structures, such as heavily loaded floor slabs, rafts, roads and embankment, the columns can be constructed with an enlarged heads as shown in Figure 5. The enlarged head serves to reduce punching shear and can either be used to give direct contact support to the slab or to provide a uniform bearing pressure through a geogrid reinforced granular mattress as shown in the Figure.
DEEP DRY SOIL MIXING
Dry deep soil mixing (DSM) technology is a development of the lime-cement column method, which was invented by Kjeld Paus almost 30 years ago. It is a form of soil improvement involving the introduction and mechanical mixing of in-situ soft and weak soils with a cementitious compound such as lime, cement or a combination of both in different proportions. The mixture is often referred to as the binder. The binder is injected into the soil in a dry form. The moisture in the soil is utilised for the binding process, resulting in an improved soil with higher shear strength and lower compressibility. The removal of the moisture from the soil also results in an improvement in the soft soil surrounding the mixed soil. Typical
execution process of dry deep soil mixing is schematically shown.
CASE HISTORIES
CASE HISTORIES FROM GERMANY AND AUSTRIA
Following the reunification of Germany in 1990 and the introduction of the high-speed railway system in the country, the existing railway network leading to Berlin required up-gradation. This called for extensive ground improvement works. Deep Vibro techniques were employed at several locations. The map in Figure 7 shows the locations/sites where ground improvement works were carried out. Figure 8 shows photographs from site showing 3 Vibrocats carrying out ground improvement works for the German ICE train system and a schematic of the high speed ICE Train with operating speeds of over 250 kmph founded on stone column.
Hamburg – Berlin High-Speed Line: Wittenberge Section (Vibro Replacement)
The subsoil underneath a 6-km stretch in the extension works to the Hamburg – Berlin route near Wittenberge was improved using Vibro replacement (stone columns). The improvement was necessary for the upgrading of a normal existing line to a high-speed line on rigid pavement (refer Sondermann 1996).
Following soil investigation works, compaction works were carried out using special equipment and utilising the redundant ballast. A 4-row layout of stone columns were installed with a horizontal spacing of 2m c/c and vertical spacing of 1.25m c/c on a triangular grid pattern. The columns were installed to depths between 3m and 7m. The diameter of the columns varied between 0.6m and 0.8m. A schematic showing the cross section and plan view of treatment
scheme is shown.
During the soil improvement works, measurements were taken by geophones to record the vibrations at depths between 2m and 3m. Evaluations of the results showed that the anticipated soil deformation caused by the vibrations induced by high-speed trains have already occurred during installation of the stone columns. The oscillation speed of the railway system is much slower than that of the soil improvement works. Even during the installation next to a service railway line (with speeds of up to 120 kmph), the vertical displacements of the rails were less than 3mm and the horizontal displacements were negligible.
Hannover – Berlin High-Speed Line: Schonhausen Embankment Section (Vibro Replacement)
A section of the Hannover – Berlin high-speed line between Hamerten and Stendal near to the Elbe bridge is constructed on a rigid pavement system. Due to the double tracking of the highspeed line, the Schonhausen embankment at the east side of the Elbe Bridge has been widened. The old embankment was inter-connected to the new extension backfill by stone columns on a rectangle grid spacing of 1.85m x 2.15m c/c as shown.
Hamburg – Berlin High-Speed Line: Vietznitz-Friesack Section
(Grouted Stone Columns)
The subsoil underneath the Hamburg – Berlin high-speed line at Vietznitz-Friesack section in the area of Havellander Luchs, was found to be made of peat sandwiched between sandy layers. Soil investigation at the site indicated a 2m thick sandy layer at the surface followed by a 2m thick organic layer of peat followed by dense sandy layers. In order to bridge organic layers of peat, partially grouted columns were used to transfer traffic loads to deeper strata. The soil profile along with treatment scheme is shown in Figure.
Austrian Railway line near Hollenegg (Vibro Replacement under operational railway line)
The railway line Graz - Wies - Eibiswald is one of the oldest railway lines in Austria. It was built in the second half of the 19th century. The subgrade material used at that time was locally available and in some parts poor quality fill. This lead to problems of serviceability and repeated maintenance during the operation of the track. Geotechnical investigations revealed that the upper meters directly below the tracks were made up of loose soils which required densification.
The Keller Vibro Replacement technique was chosen as an economical solution which allowed continued operation of the railway line during ground improvement works. There was no necessity for removal of the rails and the ballast. The works were carried out during the line block periods at night. For this purpose, a specially modified Keller Vibrocat including all necessary auxillary equipment was mounted on top of a railway wagon.
LIST OF GROUND IMPROVEMENT AND FOUNDATION TECHNIQUES APPLIED
Technique
Purpose
Vibro Replacement
Densification of loose silty sands
Mitigation of liquefaction potential
Deep Soil Mixing
Stabilisation of in-situ soils
Lime/Flyash Injection
Subgrade stabilisation
Compaction Grouting
Filling of voids in the shale
Avoidance of sinkhole formations
Densification of soil mass
Jet Grouting
To replace the existing support
To increase the slope stability
To stabilise the soil mass
Chemical Grouting
Stabilisation of granular soils
Solidification of sandy soils
Mini Piles
To form load bearing elements
To form a deep foundation system
GENERAL CONSIDERATION FOR HIGH-SPEED TRACK DESIGN
Presently all over the world non-ballasted track concepts are being applied, although still at a moderate volume. The great advantages of such structures can be summarized as follows:
• Reduction of structure height;
• Lower maintenance requirements and hence higher availability;
• Increased service life;
• High lateral track resistance which allows future speed increases in combination with tilting technology;
• No problems with churning of ballast particles at high-speed.
If the low-maintenance characteristics of slab track on open line are to be retained, great care
must be taken to ensure that the subgrade layers are homogenous and capable of bearing the loads imposed. The slabs may be prefabricated or poured on site. The high level of investment required has prevented widespread use of slab track on open line so far. However, on the basis of life cycle costs a different picture is obtained.
MODELLING RAIL TRACK, ELASTIC BEARING, AND SLEEPER SECTION
In order to model the rail track, its resistance to bending was simulated in the most accurate way possible, which is why the inertia of the modelled rail must be equal to that of the real rail.
The model also sought to make the vertical stiffness equal for all of the elastic bearings (see Fig 9). The vertical dimension and the modulus of elasticity were fixed so that the vertical stiffness of the element coincided with the stiffness of the elastic bearing provided by the manufacturer. For the high-speed line, the elastic bearings have a stiffness of nearly 500kN/mm.
Because the sleeper section is not constant along its entire length, the dimensions of its most representative section were used for the sleeper model elements. For each element (See Fig 10), the modelled flexural stiffness must be equal to the real flexural stiffness, as follows:
To obtain homogeneity in the calculations, the elements of the model had to have a constant width, which is not the case in the real sleeper. Thus, the model width must be considered to be an average width. This average width must be such that the load bearing surface in the model is equal to that in reality.
SLEEPER–BALLAST CONTACT
The sleeper–ballast contact zones contain a high concentration of strains. This local phenomenon requires refining of the mesh used to model these zones. However, applying this procedure is sometimes impossible because of the computational resources and model complexity needed. The most common alternative to modeling the contact zones is to use bounded degrees of freedom.
The use of bounded degrees of freedom requires the introduction of different nodes for each material at the contact surface. These nodes must move equivalently in the direction perpendicular to the contact plane (see Fig. 6). However, these nodes can move at different values in the directions parallel to the contact plane. This solution is effective because it solves the tensional discontinuities that appear at the interface between two materials that differ significantly in their stiffness. In this model, bounded degrees of freedom were used at the sleeper–ballast contacts.
BOUNDARY CONDITIONS
The model in this study differs from many existing models of railway track construction in which all vertical planes are constrained in all directions. In the planes that shape the slopes of embankments are left completely free, with no restrictions. In particular, the boundary conditions used here are as follows (see Fig. 12):
Fig. 12. Boundary conditions
a) In the vertical plans limits of the model, z=0 and z=7.20m, the boundary condition adopted is to impose the nullity of movement in the perpendicular direction to these plans (uz=0).
b) In the vertical plans limits of the model, x=0 and x=18.45m, the boundary condition is, like the previous one, to impose the nullity of movement in the perpendicular direction to these plans (ux=0).
c) In the horizontal inferior plan of the model y=-3m, the condition to be imposed is the null vertical displacement (uy=0).
MATERIAL CONSTITUTIVE MODEL
An elastic, isotropic, and linear model was used to develop a mechanic model of the rail tracks, elastic bearings, sleepers, and the granular material processed with cement. Finite strain was used to simulate the kinematics of
the continuous medium. Granular material treated with cement is considered to behave elastically, at least until it reaches a substantial percentage of its stress limit; one can assume its modulus of elasticity to remain essentially constant under normal stress.
To model the embankment material on which the track is laid, a perfect elasto-plastic behaviour was assumed. This assumption implies that reloading occurs in the same way as downloading, and that the material experiences no hardening (hardening parameter H=0).
HYPOTHESIS ADOPTED
The load application must be carried out in several stages. In the first stage, only the material’s own weight is considered until reaching the stress balance, while at later stages the loads due to the train are also taken into account. The stresses and displacements of interest are the ones that correspond to the application of the train loads; therefore, they can be calculated from the difference between the totals obtained after applying the train loads to the first stage.
Here, it was convenient to apply four load states due to the train passage, matching each state to the application of the static load per wheel in the four central sleepers of the model: T5, T6, T7, and T8 (See Fig 13).
To simulate the constructive process of an embankment and to ensure convergence of the solution, the first load state (only the material weight) was divided into 250 substeps of the gravitational load application and 50 balance iterations for each one. For railway loads, 15 sub-steps proved to be sufficient to achieve convergence. The program used is ANSYS Structural, which enables nonlinear analysis.
CONCLUSIONS
Increased railway speeds enhance the deterioration problems in the embankment–structure transitions, which has important implications for operating and maintenance costs, as well as for passenger safety and comfort.
From the analysis carried out in this study, the following conclusions are drawn:
a) The embankment must not be built on excessively compressible material on original ground. Such material must be replaced with another material or treated to obtain a modulus of elasticity corresponding to that of a material.
b) Deep vibro techniques and deep soil mixing methods have found extensive application worldwide and have proven to be flexible in the ability to treat a wide range of soils and site constraints/conditions and efficient in terms of time required to complete the treatment works and for consolidation.
c) The fact that they have been widely used is a confirmation that the techniques are technically sound and at the same time economical.
REFERENCES
Brill, G.T. and Hussin, J.D. (1992), “The Use of Compaction Grouting to Remediate a Railroad Embankment in a Karst Environment”, Proceedings of the Twenty-Third Ohio River Valley Soils Seminar, Louisville, Kentucky, October, 1992.
BROMS, B.B. (1999) “Design of lime, lime/cement and cement columns”, Proceedings of the International Conference on Dry Mix Methods for Deep Soil Stabilisation, Stockholm, Sweden, pp. 125-153.
Holm, G. (1999) “Applications of Dry Mix Methods for deep soil stabilisation”, Proceedings of the International Conference on Dry Mix Methods for Deep Soil Stabilisation, Stockholm, Sweden, pp. 3–13.
Holm, G. et al (2002), "Mitigation of Track and Ground Vibrations Induced by High Speed Trains at Ledsgard, Sweden", Swedish Geotechnical Institute, SD Report 10, Sweden, pp 1-44.
Moseley, M.P. and Priebe, H.J. (1993) “Vibro techniques”, Ground Improvement, Edited by M.P. Moseley, Blackie Academic & Profession, pp. 1 – 19.
Pengelly, A.D. (2000) “Ground Modification Techniques for Railroad Subgrade Improvement”. Hayward Baker Report, Maryland, USA.
Sondermann, W. (1996) “Soil Improvement by Vibro Replacement for Rigid Pavement Construction to the High Speed Railway System”, 3rd Geotechnique-Colloquium, Darmstadt, Germany, Technical paper 10-53 E.
Swedish Geotechnical Society (1997) “Lime and Lime Cement Columns”, SGF Report 4:95E, Linkoping.
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