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Earthworks and drainage 2513

25.1 Earthworks and drainage

25.1.2.2 Rock cuttings

British Railways has indicated safe slopes for rock cuttings and

angles of repose for rock embankments in a chart reproduced as

Table 3 in BS 6031. If steep cuttings in rock are essential, then it

is necessary to apply engineering geology concepts to assess

joint sets in relation to the direction of slope. Rock anchors,

rock bolts and sprayed concrete may be used to assure stability

before considering the use of mass-retaining walls. Chalk and

certain other soft rocks are susceptible to weathering and frost

action and may be protected by vegetation cover.

The contours of the territory to be crossed by a railway are

obviouslv decisiveas to its average gradient but they are also the

background to fixing the maximum permissible gradient within

the limits of tractive and braking adhesion. The lower the

difference between average gradient and maximum gradient the

greater is the practicable train load and the lesser are the

deviations from constant-speed running. The minimum curva-

ture to be used also determines the differences between the line

speed limit and local speed restrictions. Long curves of small

radius on heavy gradients may involve derailment hazards for

very long and heavy trains, arising from braking or tractive

effort surges along the train. Both maximum gradient and

minimum curvature have a large effect on the earthworks cost of

constructing a railway and, because of this, the ideal of con-

stant-speed running is often subject to heavy qualification. This

is particularly the case in mountainous country.

Railway alignment needs to be planned to give a volume

balance between excavation in cuttings and tipping in embank-

ments, subject to the material excavated being suitable for

tipping to form embankments and subject to minimizing the

haul of the excavated soil. Recourse to borrow pits for embank-

ments and spoil hauls for cuttings should be minimized.

The route may also be affected by other considerations which

may be economic, environmental or technical. These would

become apparent in a full site investigation, where such matters

as previous mining activity, underground services, effect on

neighbouring structures, nature of the groundwater table and

watercourses would be taken into account. Guidance is avail-

able in relevant Codes of Practice, such as BS 6031 and BS 5930.

(See also Chapters 9, 1 I and 17.)

25.1.2.3 Soil cultings

Where the slopes are in noncohesive sands and gravels, the

angle of repose is the limiting gradient (with a maximum value

of I : I). Usually, the gradient is shallower than this due to the

presence of finer layers in the soil system or a silt or clay matrix

around the gravel. The non-cohesive soils tend to be self-

draining but erosion can occur when water springs part way up

the slope. At such locations a non-woven geotextile filter can be

placed and covered with uniform coarse stone (away from the

sun’s rays) which will hold the fabric in place.

Drainage measures should take the form of preventing water

reaching the slope and of removing it from the slope. Unlined

ditches behind the crest of the slope increase the hazard.

Drainage trenches, whether behind or below the crest, should be

designed to intercept water and have impermeable membranes

below them and on the downfill face to prevent water once

collected from re-entering the soil. Modem counterfort or

buttress drains differ greatly from the original open forms. Like

all drains they should be lined with a geotextile layer to prevent

erosion behind and fouling within them. The top I or 2m

should be composed of impermeable material, either compacted

clay fill or a system of stone and plastics membrane to prevent

surface water reaching deep into the ground, where it could

cause internal pore water pressure at likely slip surfaces.

In some cases, there is a mantle of more permeable silt or sand

overlying the older clay and, if possible, a lateral interceptor

drain should be placed about 20 m behind the crest to prevent

the fast seepage of water.

In addition to the above rotational slides, translational slides

can occur, usually as a shallow mass moving on a planar

surface. They can take the form of slab or block slides, wedge

failures, debris slides and flow slides. These possibilities are

assessed in the site investigation. Mires are formed as peat is laid

down in a specific sequence with variations in soil content,

dimensions of fibre (roots, trees, etc.) and extent of humifica-

tion. Peats reduce greatly in volume under the effects of loading

and drainage. In making cuttings in a peat system, quite

substantial waterflow can occur and a filter is advised to deal

with fine particles otherwise carried from the slope.

In all cases of cuttings, other engineering works at ground

level should be considered. Urban or industrial construction

involves roads which act as catchments to deliver water to local

drainage systems and also water services: if these are defective

and near the slope, water can flow to increase pore pressure and

precipitate a slip. Similarly, surcharging, especially if accompa-

nied by dynamic loads from construction plant or the placing of

storage containers, can seriously reduce the factor of safety.

Steep rock faces, chalk cliffs, and boulder-strewn hill slopes

may present rockfall or chalkfall problems which may need

special watchmen, signalling provisions, special fence, apron or

tunnel protection.

25.1.1 Site investigation

This will vary according to the extent of the problem. At the

outset, a preliminary study may give adequate information to

specify the route corridor from geological maps and memoirs,

topographical maps and aerial photographs. Earth satellite

imagery with interpretation of selective wavebands by special-

ists in remote sensing can indicate important features. Water

table conditions may vary throughout the year from those

obtaining at the time of exploration.

For more localized investigation, the type of equipment

(augers, percussion and rotary tools, penetration heads, loading

plates, pumps), instruments (piezometers, inclinometer tubes,

seismometers, resistivity meters, gravimeters, etc.) must be

chosen according to conditions. Relevant disturbed or undis-

turbed samples should be procured for testing. According to the

type of ground, the construction and the design philosophy

applied, it may be necessary to carry out full-scale site testing

with long-term monitoring of instruments.

25.1.2 General

In the past, railways have been maintained over poor ground,

with inadequate trackbed materials with a high input of labour

time and at slow or moderate train speeds. Although the

following concepts may be used in modifying old railways, the

basic approach is to obtain a minimum maintenance high-speed

railway accepting normal freight traffic on conventional

sleepers.

25.1.2.1 Cutting slopes

Slopes in natural ground may be constructed at safe angles

according to the properties of the soil.

25.1.2.4 Embankments

The nature of the natural soil to receive the embankment must

2514 Railways

be investigated. If it is too weak to receive the embankment

loading at the rate of placing likely to be used by the contractor,

failure could occur. A total stress analysis is applicable for this.

The conventional technique is to place berms as counterweights

at the position of heave. A modem alternative is the use of

reinforcing geotextiles or meshes (usually of plastics) to resist

tensile forces. The fabric is laid directly on the ground and

covered with a granular layer at least 200 mm thick; this acts as

a filter and permits construction plant to move easily over the

site without sinking into the underlying soft soil. Further fabric

or mesh is laid at higher levels, the number and spacing of layers

depending upon the engineering properties of the specialist type

of material chosen.

Drainage. The control of water in the trackbed is a major

factor in designing the construction layers in relation to the type

of subgrade soil as discussed above. Soils such as sands and

gravels which may be drained fairly easily donot present a great

problem unless there is an artesian head in this case, there can

be a slow upward migration of fine or medium sand under track

vibration combined with water flow, and a geotextile is neces-

sary to hold down this sand. Cohesive soils cannot be drained

easily and the installation of a channel or pipe will only reduce

the pore water pressure to invert level in its immediate vicinity.

It is not practical to attempt to remove water from clay in this

way as such a large number of drains would be required. If

water arrives through precipitation or by flow from adjacent

areas, then a relatively small amount is sufficient to produce

deleterious changes in pore pressure and so it is practical to

provide drains to intercept and remove this free water. Most of

the water in cohesive soil is held in capillary suction.

The relationship between the moisture content of cohesive

soil and the water table is complex, depending upon the over

consolidation ratio (OCR) of the soil. Weathering reduces the

OCR effect at the surface.

One object in designing the system of drains and track is to

produce a maintenance-free system or one needing minimal

attention. Channel drains are readily accessible for cleaning and

deal with rainwater, they can be laid at very slight gradients and

can deliver at catchpits to deeper pipe camers if necessary.

Many pipe drains act both as collectors and carriers, allowing

water to enter at open joints or through perforations. The

various forms of pipe are glazed earthenware, galvanized corru-

gated steel, pitch fibre and, now coming more into use because

of ease of handling, plain or perforated PVC pipe. Geotextiles

are of great use in static drainage conditions and most railways

report satisfactory results using commercially available filtering

non-woven fabric. For normal purposes, a fully heat-bonded

geotextile with a surface density in the range 100 to 200g/m2is

acceptable; needle-punched geotextiles should be of slightly

heavier grade. Fabrics placed in quasi-static conditions near the

track should be of heavier grade up to 350g/m*, or more if

needle-punched. Such a geotextile would be placed in the sixfoot

of a double track line after one track had been cleaned or

blanketed. This would prevent slurry from the dirty adjacent

track flowing across.

A modern standard design for a side drain is a trench lined

with geotextile with a perforated pipe drain running along its

base; above and around the pipe is placed uniform stone such as

ballast, with the top of the geotextile lapped over the stone

about 200 mm below the surface: more stone on the geotextile

protects it from disturbance and from the effect of the ultra-

violet rays of the sun. As perforated drains can release as well as

collect water, possibly at susceptible locations, it is often the

practice to place a polyethylene film to line the trench.

Slope angles. Embankments may be formed at angles varying

from ratios of 1 :1 horizontal to vertical for crushed rock and

gravels to 2.5: 1 or even shallower for clays and silts. The slope

angles depend both on the material and rainfall. For modem

railways, peat should not be used as fill material.

The surfaces of slopes should be protected from erosion. This

may occur naturally as vegetation is established, followed by a

protective topsoil. If surface erosion is a problem, various

systems are available, such as spraying with a seed mulch,

turving, placing filter fabrics held down by gravel layers or using

a honeycomb mesh to hold seeded compost in place.

Rockfalls. Vertical or sloping rock faces may erode or topple

to cause rocks of various sizes to fall towards the running line.

Although vegetation may help by bonding superficially, protec-

tion must involve coping with the energy of rockfall and

removing the debris regularly. If space is available, one or two

berms are constructed between the base of the slope and the

railway to collect scree. For some faces, a plastic geomesh is

adequate.

Reinforced earth. The dimensions of embankments and of

gravity-retaining structures can be reduced by the inclusion of

various strengthening meshes, fabrics, strips and rods of metal,

glass fibre or plastics. The tensile resistance of these elements is

applied to the adjacent granular soil to produce a composite

system permitting the construction of vertical faces in the fill.

These external faces are protected by facing elements usually of

concrete, resulting in a structure which is economic and which

can accommodate settlement. The various qualities of creep,

longevity of reinforcement, corrosion, etc. are still the subject of

study but many such structures exist throughout the world,

including railway environments. There are none so far beneath a

high-speed running line and, in this particular case, such systems

can be installed up to a horizontal distance of 5m from the

running line.

Trackbed designs. The thickness of subsleeper construction

(trackbed) is a function of number and size of axle loadings and

of the subgrade soil. Modern railways assess the subgrade soil in

one of two approaches: (I) the classification of the soil accord-

ing to its physical properties and taking account of the water

table; or (2) correlating some measured strength or modulus of

the subgrade with an empirical design chart. The thickness of

the trackbed may also depend upon its component layers to

protect against frost, water and particle movement. In very

frost-prone areas, such as Central Europe, the thickness of

ballast for frost protection exceeds that which might be required

for prevention of bearing capacity failure.

The thickness of ballast (all dimensions are below bottom of

sleeper) is a minimum of 200 mm to permit tamping machines to

operate. Although new ballast injection machines would permit

this to be reduced, a high-speed or high-axle-load railway would

require 300 to 500 mm thickness for minimum maintenance.

25.2 Ballast

Ballast, the material around and below the sleeper, is placed to

provide support and lateral resistance to the sleeper. It permits

adjustment of level and alignment as required and, if this is done

manually, the maximum size of particles should be about

50 mm. Ballast should be free-draining, mainly of single size, of

cubical shape but, above all, durable so that there is negligible

volume change under track loading. The wet attrition value

(WAV) gives the best correlation, with minimum maintenance

requirements over the long term and this is determined by the

test described in BS 812: 1951. clause 27, which specifies the

exact size and type of sample to be used. If particles of different

size from the 50 to 37.5 mm required in the test are used, then a

different WAV is obtained. The WAV should not exceed 4% for

Ballast 2515

good ballast and it is possible to obtain stone with a WAV down

to 1%. It is rare for synthetic stone, such as slag, to have

adequate wet attrition properties and they may generally be

grouped with most limestones as being unsuitable to be in

contact with the sleeper. Hardness is difficult to define in

relation to other tests but, if the ultrasonic pulse velocity of the

homogeneous mineral is 6000 m/s or greater, the stone will be

suitable. In severe climates, a freezing and thawing test may be

applicable.

The dimensions should conform by weight to the values

shown in Table 25.1; and the 1.18 mm limit effectively minimizes

the amount of dust present.

Table 25.1

nominal 20mm size between the ballast bed and the sleeper,

which is lifted to insert the stone. There is now a track-levelling

machine, electronically controlled, which evaluates cant and

level from an advancing inclinometer trolley and places the

exact quantity of stone by pneumatic injection to obtain proper

level. As stone more than 50 mm below the sleeper is not moved

and brought with its dirty matrix up to bottom sleeper level, as

with a tamper, the likelihood of pumping track is somewhat

lessened; the beneficial effect of high-quality small stone is not

reduced by mixing with existing worn stone and there is less

attrition of the base of the sleeper.

25.2.1 Track profile

Under traffic, rail level, as measured by accurate optical or

inclinometer-based machines, shows a profile which is repeated

even after many successive tamping and loading cycles. This is

related to the care with which the original trackbed layers were

installed. When the subgrade is prepared initially, and as

subsequent layers of blanket and ballast are placed and com-

pacted, it is necessary to provide extra sighting instruments on

compaction and grading plant so that there are no short

variations in level. This can be successfullydone by using a laser

system aimed at the surface a fixed distance ahead and which

causes the equipment to compensate for deviation from the

required level by moving its instrumented blade up or down.

The ends of the sleepers need to be boxed-in with shoulder

ballast to a minimum width of I50 mm for any tracks, 200 to

250 mm for running lines carrying moderate-speed, moderate-

axle-load traffic, 300mm minimum for welded track on the

straight and 350 mm for welded track on curves. Generally,

there is little advantage in extending shoulder ballasting beyond

300 to 350 mm. In the last decade or so, the practice of raising

the shoulder ballast in a slope from about top of sleeper level at

the rail to about top of rail level at the shoulder edge has become

common on European railways. This practice not only increases

the lateral stability of the track but provides a useful reserve of

boxing ballast which can be temporarily utilized to make good

the boxing ballast when the track is tamped. The angle of the

shoulder should be about 55'.

Square mesh

sieve

Yo topass

100

100-97

20-0

1.18

0.3-0

The stone should have a maximum flakiness index of 50%.

For elongation qualities, not more than 2% by weight of

particles should have a dimension exceeding 75 mm.

The effect of many tamping cycles is to break up ballast

particles; stone as hard as possible is required to accommodate

this. When a sleeper is tamped, a horizontal load - the major

principal stress - is applied to the ballast, causing it to deform

vertically and lift the sleeper. The minor principal stress is

vertical with the subsequent arrangement of stone particles in

the least favourable position to support track loading, even

though the rail level is now correct. The maximum rate of rail

settlement occurs after tamping, which reduces as the stone

packs down under traffic, with the major principal stress becom-

ing vertical. The trackbed becomes more stable under vertical

loading and the best relevelling procedure is the manual or

mechanical placing of measured quantities of small stones of

3307

- Id

3307

(Min. dimension)

Pandrol rail.fastening

915

1000-gauge

polythene sheet

Approved sand

where required

Pipe drain

Figure 25.1

rail (CWR) track (dimensions in millimetres)

Components and formation for continuous welded

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