Railway Engineering 6th semester

Track Forces & Ballast (Lectures 4)

Subject : Railway Engineering

Department of Transportation Engineering and Management, UET Lahore.

Track ForcesThe forces on the track are:

Vertical Lateral (parallel to the ties) Longitudinal (parallel to the rails)

Vertical • Repetitive downward action of wheel load• Corresponding lift-up force on the ties away from the wheel load points. • Nominal vertical wheel force = gross weight of the railway car divided by the number of wheels • The vertical impact dynamic load has two components, a short-duration larger force and a

longer duration smaller force. The first is expected to be more harmful to the rails and ties, while the second does more damage to the ballast and track geometry.

• The major factors affecting the magnitude of the dynamic vertical forces are: 1. Nominal wheel load 2. Train speed 3. Wheel diameter 4. Smoothness of the rail and wheel surfaces 5. Track geometry 6. Track modulus or vertical track stiffness

Lateral• Wheel force transmitted through friction between the wheel and top of the rail and by the

wheel flange acting against the inside face of the rail head, particularly on curves • The design lateral wheel force depends upon a number of factors, including: 1. Vehicle speed 2. Track geometry 3. Elevation difference between the two rails at the same cross section

LongitudnalSources of longitudinal rail forces are:

• Speed • Locomotive traction • Locomotive and car braking

Railway Engineering 6th semester

• Expansion and contraction of the rails from temperature change • Track grade • The ratio of lateral to vertical force (L/V) is also important because it can cause loss of

alignment and even track buckling.

TRACK SYSTEM CHARACTERISTICSTrack system performance is a function of the composite response of the track components under the action of the train loads. Two response characteristics are important to consider in track design:

• Vertical track stiffness • Lateral track stability.

Stiffness• The vertical track stiffness k is the vertical load on one rail divided by the vertical deflection at

the loaded point. The subgrade has the greatest influence on the track stiffness k. • The track modulus u is the composite vertical support stiffness of the rails consisting of the

fasteners, ties, ballast, subballast, and subgrade

Vertical Response Model

Lateral Stability• Track buckling is a result of increasing longitudinal rail force from increasing temperature.

Buckling occurs in the lateral direction because this is the least stable direction. • The lateral resistance is greatest directly under the wheel loads because the weight of the

train increases the lateral restraint provided by the ties • Increasing resistance to buckling can be achieved by such means as increasing the ballast

shoulder width

Railway Engineering 6th semester

BallastThe second track component from the bottom. It has following sub-divisions:

Crib—material between the ties

2. Shoulder—material beyond the tie ends down to the bottom of the ballast layer

3. Top ballast—upper portion of supporting ballast layer that is disturbed by tamping

4. Bottom ballast—lower portion of supporting ballast layer, which is not disturbed by tamping and generally is the more fouled

Purpose of Ballast Section• Uniform Distribution of Traffic and track load over subgrade. Without Ballast, sleepers would

sink unevenly in subgrade • Track is anchored against both lateral and longitudinal movement • Immediate drainage & Reduced frost heaving. Some elasticity absorbs heaving in subgrade.

Railway Engineering 6th semester

• Maintenance operations are facilitated. Ballast is more readily tamped under sleepers and corrections made in line & surface when the track is up and away from the subgrade

• Resilience which absorbs some of the shock from dynamic loadings is found in ballast. Spring like action.

What must be the characteristics of a good ballast?1. Strength Must resist static + dynamic loads, brittle material is undesirable 2. Durability Resistance to abrasion + weathering, abrasion produces dust & fouls the material.

Weathering occurs due to freezing & thawing 3. Stability Rough, irregular particles in higher percentage to properly anchor the track 4. Drainability Voids b/w particles must be large enough for draining off the ground and surface

water 5. Cleanability Freedom from silt, clay, vegetation. Must be able to be cleaned easily: manually or

by machines 6. Workability Easily shoveled and tamped. Size of particles permitting flexibility in height of

surfacing lifts (layers) 7. Availability Material abundant near the site is preferable than the one which is to be hauled. 8. Cheapness Full economic cost of material. A ballast of high first cost may be cheaper in end due

to longer life, better quality & reduced maintenance requirements.

Kinds of BallastSee the next Page.

Testing of Ballast

Size and Gradation TestsUse is made of A.S.T.M. test "Method of Test for Sieve Analysis for Fine and Coarse Aggregates."

Tests for Deleterious Particles1. Material finer than a No. 200 screen shall not exceed 1 percent in prepared (stone, slag, washed

gravel) ballasts. A.S.T.M. “Test for Material Finer Than a No. 200 Sieve" is used. Particles decanted from the ballast-sample are screened, and the percentages of "fines less than No. 200" are obtained by dividing the weight of the sample before de canting by the difference in weights before and after decanting and sieving.

2. Soft and friable pieces must not exceed 5 percent in prepared ballasts. Pebbles crushing under specified static loads are reported as a portion of the original sample by weight in accordance with the "Method of Test for Quality of Soft Pebbles in Gravel," A.A.S.H.O.

3. Clay lumps must not exceed 0.5 percent as determined by the method of “Test for Clay Lumps in Aggregates”, A.S.T.M. With the sample spread in a shallow container, all clay lumps are crushed between the fingers. The clay-lump residue is then removed by screening, and the percentage of clay lumps determined by comparing the weights of the sample before and after screening.

Railway Engineering 6th semester

Abrasion TestsResistance to abrasion is determined in accordance with the current method of "Test for Abrasion of Coarse Aggregates by Use of the Los Angeles Machine," . A 5000-gram sample is subject to impact and abrasion in a hollow steel cylinder charged with cast-iron balls and rotated 500 times at 30 to 33 rpm. The material is then removed, sieved on a No. 12 screen, washed, dried and weighed. The difference in weights before and after rotating and screening, expressed as a percentage of initial weight, gives the percentage of wear. This should not exceed 40 percent except when experience and local availability may warrant acceptance up to 50 percent.

Soundness TestMade in accordance with "Test for Soundness of Aggregates by Use of Sodium Sulphate or Mag-nesium Sulphate," A.S.T.M. Quantities of sample retained on each sieve after screening are weighed and immersed 16 to 18 hours in a sodium sulphate solution at a constant temperature. Samples are then dried to constant weight at a specified temperature, cooled to room temperature, and reimmersed in a sodium sulphate solution. After the fifth cycle, the samples are washed, dried to constant weight, and sieved over the same screens on which originally retained. The weight of particles now passed by these screens expressed as a percentage of the original weight retained gives the percentage of loss.

ExperienceBallast which has in the past given a satisfactory performance may, because of its cheapness or availability, be continued in use even though it does not meet all specifications. The real economy of the material should, however, be determined by noting how its use effects other costs over a long period of time.

Selection of Ballast

Traffic RequirementsWheel loadings, volume and speed of traffic. Ultimate carrying capacity of ballast depends upon size of particle, smoothness of surface, and degree of irregularity, there is an advantage in the load-carrying capacity of coarser, rougher kinds of ballast. Crushed-stone ballast makes the best foundation for high-speed, mainline service.

Railway Engineering 6th semester

Unstable LocationsUnstable locations often require frequent additions of ballast as the roadbed and old ballast settle. It would hardly pay to make frequent renewals with expensive, high-grade ballast. The use of a cheaper material—cinders perhaps—is an alternative until stability has been achieved.

Size of ParticlesLarge sizes—up to 3 in.—possess more resistance and crushing and aresaid to hold line and surface better. This size is frequently recommended for heavily loaded freight lines. However, the flatter edges and angles on large particles may not grip the ties as well as the smaller sizes. Heavy materials being difficult to apply and work are therefore costly. Large sizes do not readily permit the light l-to-2 in. lifts desirable when surfacing track. Preferred size: ¾” to 1 ½” due to good workability but it could have drainage problems.

EconomicsThe cheapest ballast is that which over a period of years gives the lowest overall cost, including the items of first cost, frequency of renewal, track and equipment maintenance, and costs of train operation.

Transmission of Pressures through Ballast• By the principles of elastic deformation, one can forecast that pressures transmitted by a

loaded tie will be spread over a greater area with diminished unit pressure as the depth of section increases.

• Effects of adjacent tie loadings will be superimposed on each other, leading to a relatively uniform pressure's being transmitted through the lower depths of the ballast section and to the roadbed.

Design of Ballast Section1. Depth of Ballast below base of Tie

• If ballast pressure is not uniformly distributed to the subgrade, some of the ballast will be forced into the subgrade, causing inferior surface on the track and depressions in the subgrade surface which will form pockets and lead to instability

• Strength of supporting roadbed must be taken into account as well as the distribution of pressure at any given depth of ballast

• A rule given by Schubert is that depth of ballast should equal a + 8 in., where a is the space between the edges of ties in inches.

• On curves, these depths are measured under the low or inside rail, a greater depth being necessary under the outside rail to provide the necessary superelevation.

• Ultimate carrying capacity of the ballast is considerably increased by raising the level of ballast surface to the top of tie

Railway Engineering 6th semester

Transmission of Pressures through Ballast

Width of ballast section

• It is a function of the depth, tie length, shoulder width, and slope. • Width at the top must provide space for the tie • Outside the end of tie is a shoulder varying from 6 in. to 12 in. to give stability against the

lateral movement • Experience with heavy traffic indicates 12 in. to be a desirable width for gravel, 8 in. for

stone

3. Slope of ballast section

Slopes of 2:1 to 2 1/2: 1 are accepted

Functions of Subballast• Maintain separation between the ballast and subgrade particles. • Prevent abrasion of the hard subgrade surface by the ballast. • Reduce pressure from the ballast to values that can be sustained by the subgrade . • Intercept water from the ballast and direct it to the track drainage system. • Provide drainage of water flowing upward from the subgrade. • Provide some insulation to the subgrade to prevent freezing.

Railway Engineering 6th semester

• Provide some resiliency to the track.