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EARTHWORK SLIPS ***




Produced by Richard Tonsing, Chris Curnow and the Online
Distributed Proofreading Team at http://www.pgdp.net (This
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by The Internet Archive)









EARTHWORK SLIPS AND SUBSIDENCES UPON PUBLIC WORKS:
Their Causes, Prevention, and Reparation.
_ESPECIALLY WRITTEN TO ASSIST THOSE ENGAGED IN THE CONSTRUCTION OR
MAINTENANCE_
OF
RAILWAYS, DOCKS, CANALS, ROADS, WATERWORKS, RIVER-BANKS, RECLAMATION
EMBANKMENTS, DRAINAGE WORKS, &c.


BY

JOHN NEWMAN, _Assoc. M. Inst. C.E._

AUTHOR OF

“NOTES ON CONCRETE AND WORKS IN CONCRETE;” “IRON CYLINDER BRIDGE PIERS;”
“QUEER SCENES OF RAILWAY LIFE.”

[Illustration]

LONDON:

E. & F. N. SPON, 125, STRAND,

NEW YORK: 12, CORTLANDT STREET.

1890.

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PREFACE.


The absence of any but fragmentary information on EARTH-SLIPS AND
SUBSIDENCES UPON PUBLIC WORKS, one of the most annoying and expensive
occurrences in engineering construction, has prompted the author to
write this book as a vade-mecum for those in charge of such undertakings
as _Railways_, _Docks_, _Canals_, _Roads_, _Waterworks_, _River-banks_,
_Reclamation embankments_, _Drainage works_, &c., and also to fill,
however imperfectly, somewhat of an hiatus in engineering literature.

The theory of the lateral pressure of earthwork is not examined, as it
is well understood; the intention being to concisely describe the chief
causes of slips and subsidences in different earths and many points
requiring attention, to call to remembrance some soils especially
treacherous and unstable, and to name various preventive measures and
effectual remedies.

A reference to the table of contents and the index will demonstrate the
comprehensiveness of the subject, for it involves in the various
practical applications the science of geology, physical geography,
meteorology, the laws of pressure of earth, some chemical and botanical,
and other scientific knowledge.

It is scarcely necessary to observe that no exhaustive treatise is
herein attempted, for that would indeed be an Herculean task; but in
this volume the endeavour has been made to present reliable information,
the result of experience, research, considerable labour, and lengthened
observation.

J. N.

LONDON.
_March, 1890_.




CONTENTS.


CHAPTER I.

PAGES

INTRODUCTION—GENERAL CONSIDERATIONS—ENUMERATION OF THE PRIMARY
CAUSES OF SLIPS AND SUBSIDENCES IN CUTTINGS AND EMBANKMENTS,
AND EARTHWORKS CONSTRUCTED TO CONTAIN OR EXCLUDE WATER—SOME
DOMINANT PRINCIPLES TO BE REMEMBERED IN DETERMINING THE
LOCATION OF EARTHWORKS 1–12

CHAPTER II.

THE PROBABILITY OF A SLIP—TIME OF THE MOST FREQUENT
OCCURRENCE—SOME CONDITIONS UNDER WHICH SLIPS AND SUBSIDENCES
IN CUTTINGS AND EMBANKMENTS MAY BE EXPECTED IN DIFFERENT
EARTHS, SUCH AS ROCK, CHALK, SAND, GRAVEL, CLAY, &C.,
&C.—NOTES ON THE SLOPES OF REPOSE 13–49

CHAPTER III.

THE GENERAL EFFECT OF A SLIP IN A CUTTING OR AN
EMBANKMENT—ENUMERATION AND CONSIDERATION OF SOME PROTECTIVE
AND REMEDIAL WORKS—TREATMENT OF THE SLIPPED EARTH 50–64

CHAPTER IV.

NOTES ON THE PERCOLATION OF WATER—SYSTEMS OF DRAINAGE OF
CUTTINGS AND EMBANKMENTS IN DIFFERENT KINDS OF EARTH AND
UNDER DIVERSE CONDITIONS—THE CONSTRUCTION OF CULVERTS,
PIPE-DRAINS, TRENCHES, DITCHES, AND CATCHWATER DRAINS 65–86

CHAPTER V.

APPROXIMATE SAFE MAXIMUM LOAD UPON DIFFERENT EARTHS—NORMAL
PRESSURE OF THE EARTH—THE SAFE MAXIMUM LOAD UPON DEPOSITED
EARTH—APPROXIMATE SAFE MAXIMUM HEIGHT OF AN EMBANKMENT 87–97

CHAPTER VI.

SLOPES, GENERAL CONSIDERATIONS—TABLE SHOWING THE GENERAL RANGE
OF SLOPES—TABLE OF COEFFICIENTS OF FRICTION—NOTES ON THE
COHESION OF EARTH—FORM OF A SLOPE—SOME CONDITIONS GOVERNING
THE NECESSARY INCLINATION—WIDENING EARTHWORKS WITHIN THE
ORIGINAL FENCES 98–114

CHAPTER VII.

NOTES UPON THE PRESERVATION OF THE FOOT OF A SLOPE—VARIOUS
METHODS OF COVERING AND SUPPORTING A SLOPE—PROTECTION FROM
SNOW-DRIFTS—THE FORMATION WIDTH OF CUTTINGS AND
EMBANKMENTS—THE DELETERIOUS EFFECTS OF VIBRATION 115–141

CHAPTER VIII.

EARTHWORKS IN OR UPON SIDELONG GROUND—SOME INSECURE
CONDITIONS—PRECAUTIONARY MEASURES—EMBANKMENTS UPON SOFT
GROUND—EMBANKMENTS COMPOSED OF SOFT EARTH—THE PROMOTION OF
STABILITY AND CONSOLIDATION 142–155

CHAPTER IX.

THE DEPOSITION OF AN EMBANKMENT—PREPARATION OF THE GROUND UPON
WHICH AN EMBANKMENT HAS TO BE DEPOSITED—METHODS OF
PROCEDURE—CONSIDERATION OF SOME OF THE DIFFERENT SYSTEMS—THE
EFFECT OF THE HEIGHT OF A TIP AND THE LENGTH OF A LEAD—THE
STEAM NAVVY AND EMBANKMENTS 156–170

CHAPTER X.

NOTES UPON THE LOCATION, PRESERVATION, AND PROTECTION OF SEA,
ESTUARY, RECLAMATION, CANAL, AND RESERVOIR EMBANKMENTS OF
EARTH CONSTRUCTED TO CONTAIN OR EXPEL WATER 171–197

CHAPTER XI.

NOTES UPON THE FAILURE OF DOCK AND OTHER WALLS FROM A FORWARD
MOVEMENT OF THE EARTH FILLING OR BACKING—CONSIDERATION OF THE
CAUSES OF SUCH ACTION AND SOME PREVENTIVE AND REMEDIAL
MEASURES 198–215

CHAPTER XII.

NOTES UPON SLIPS OF EARTH, SUBSIDENCES, AND MOVEMENT IN
FOUNDATIONS CAUSED BY “BOILS,” OR AN UPWARD RUSH OF WATER IN
LOOSE EARTHS—CONSIDERATION OF SOME PRECAUTIONARY AND REMEDIAL
OPERATIONS 216–225




EARTHWORK SLIPS AND SUBSIDENCES UPON PUBLIC WORKS.




CHAPTER I.

INTRODUCTION—GENERAL CONSIDERATIONS—ENUMERATION OF THE PRIMARY CAUSES
OF SLIPS AND SUBSIDENCES IN CUTTINGS AND EMBANKMENTS, AND EARTHWORKS
CONSTRUCTED TO CONTAIN OR EXCLUDE WATER—SOME DOMINANT PRINCIPLES TO
BE REMEMBERED IN DETERMINING THE LOCATION OF EARTHWORKS.


Earthslips and subsidences may be caused by the terrible power of an
earthquake or other dreaded subterranean destroying force, upheaving,
cracking, and shattering the earth’s crust and dealing death and havoc
in its awe-inspiring course. They may also originate from the untiring
efforts of the meanest rodents or the most minute crustaceous animals
burrowing passages for aqueous action, the chief agent of the
instability of the surface soils of the earth.

The ceaseless mutability of the created elements has been thus
magnificently described:—

“For know, whatever was created needs
To be sustain’d and fed; of elements
The grosser feeds the purer, earth the sea,
Earth and the sea feed air, the air those fires
Ethereal, and as lowest first the Moon;
Whence in her visage round those spots, unpurg’d
Vapours not yet into her substance turn’d.
Nor doth the Moon no nourishment exhale
From her moist continent to higher orbs.
The Sun, that light imparts to all, receives
From all his alimental recompense
In humid exhalations, and at even
Sups with the ocean.”

It is the constant absence of peace and rest in the earth that produces
instability, however great, however small, for the disastrous landslips
that have occurred in Switzerland and other parts of Europe, in India,
and recently at Quebec, and in all regions of the world were caused by
the same disintegrating operations as those which generate an earth-slip
of comparative insignificance. In proceeding to a practical
consideration of earthslips and subsidences, it may be well to call
attention to the complexity of the subject, the character and conditions
of earth and the impairing elements being so very variable and numerous
that it is impossible to determine any rules even for a particular soil;
and, moreover, it is necessary to separately consider slips in cuttings
and those in embankments, as movement is somewhat differently created,
and it does not necessarily follow because earth stands well in a
cutting that it will do so in an embankment, or _vice versâ_.

It may be said every kind of earth will slip or weather under certain
conditions, even the hardest rock if superimposed upon an unstable
stratum; therefore, some of the main questions to be considered are:—

I. THE PROBABILITY OF THE OCCURRENCE OF A SLIP.

II. THE EFFECT OF A SLIP.

III. SHOULD EVERY PRECAUTION BE TAKEN TO PREVENT A SLIP WHEN A
CUTTING IS BEING EXCAVATED OR AN EMBANKMENT BEING DEPOSITED; OR IS
IT BETTER TO REPAIR A SLIP AS IT HAPPENS?

It is obvious in railway cuttings and embankments a mere crumbling of
the surface may be disregarded, but in dock, canal, or any works
containing or expelling water, the smallest movement, crack, or aperture
must immediately receive due attention.

In order to effectually remedy a disease it is necessary to ascertain
its character. Many of the primary causes of slips in cuttings and
embankments are, therefore, here enumerated; but, of course, they are
not named in their order of importance, which cannot be established.


HEADS OF THE CHIEF CAUSES OF SLIPS AND SUBSIDENCES IN CUTTINGS.

1. The want of uniformity of the earth, particularly as regards
percolation, cohesive power, and resistance to change by the action of
water or meteorological influences.

2. The temporary or permanent exposure of the earth to the effects of
the atmosphere, rain, frost, and snow.

3. The opening to the air and weather, &c., of thin seams of an unstable
character, which, when unsupported, gradually crumble away and cease to
support superimposed strata.

4. The tapping of springs.

5. The lower portion of a slope being impaired or undermined through an
infiltration and flow of water.

6. The erosion of the slope.

7. The earth having intermediate unstable seams.

8. The unprotected surface of a cutting in light soil being loosened and
blown away by a storm of wind, especially when it is accompanied by
rain.

9. The slopes being honey-combed and disturbed by rodents, particularly
in clay soils, clay marls, and clay loams; and upon submerged earthwork,
and in certain districts in the tropics by a mollusk which will
penetrate and even destroy rocks.

10. From one portion of a cutting being more exposed than another to
disintegrating meteorological influences.

11. By the discharge of water from land drains following the old
drainage course, and by the localisation of the surface or land water
flow.

12. The improper or imperfect drainage of land outside a railway fence,
causing land water to accumulate in and discharge itself through the
slope, thus disturbing the established equilibrium.

13. By an interference with the natural flow of any underground waters.

14. By allowing water to accumulate in the gullet, or upon the formation
during the process of the excavation of a cutting.

15. The local percolation of water through the slopes of a cutting from
the defective construction, wrong location, or permeability of the
surface of a drain upon the cess.

16. An accumulation of water caused by the unevenness of the slopes.

17. The acceleration and inducement of a flow through the slope of any
water contained in land outside a railway fence, and consequent
incitement to the land-water to exude.

18. Vibration.

19. Insufficient flatness of a slope either at the time of excavation or
after exposure to meteorological influences.

20. The strain upon the face from lumps of earth being allowed to remain
upon a slope during the construction of the works, or the gullet being
excavated for a considerable distance in advance; the cohesion of the
soil being thereby unduly and unequally strained.

21. By overweighting.

22. By unequal loading.

23. The establishment of spoil banks upon the cess, or the additional
loading of the ground near and outside a railway fence, in soft soils
having practically no cohesion or tenacity.

24. The excavation removed destroying the continuity of support,
especially in soils partaking of a semi-fluid character.

25. The want of uniformity of the covering of a slope causing unequal
percolation or exudation of water.

26. The neglect to fill up, or otherwise remedy, cracks or fissures in
the slopes or cess.

27. By artificial or irregular consolidation either of the formation or
slopes superinducing movement and weathering in any portion not so
compacted.

28. From an accumulation of water behind a retaining wall at the foot of
a slope, resulting in the stability of the wall being overcome by
pressure.

29. By unequal pressure upon the foundations of a retaining wall at the
foot of a slope caused by lateral over-pressure tilting it, or by its
unequal settlement.

30. In sidelong ground, by the removal of support against the action of
sliding, which, without artificial aid, may not be arrested until the
slope of a cutting on the higher side approaches the steepest
inclination of the face of the hill.

31. By blasting laminated rock dipping at a considerable angle towards a
cutting in the side of a hill; the result sometimes being that a cavity
is made depriving the upper beds of support and causing them to
overhang, and a mass extending to the top surface of the hill to slip
along the unsupported stratum.

The first ten “heads of the chief causes of slips in cuttings” might be
classed as NATURAL, _i.e._ produced or effected by nature and,
therefore, beyond the power of man to entirely prevent; the remaining
heads as ARTIFICIAL, and therefore, in some degree to be prevented,
unless obviously the result of the unavoidable exigencies of
construction.


HEADS OF THE CHIEF CAUSES OF SLIPS AND SUBSIDENCES IN EMBANKMENTS.

1. The percolation of surface water into the toe and under an embankment
upon the original surface of the ground, and also downwards through the
formation.

2. Unequal percolation of water through the formation or the slopes.

3. The surface of the ground upon which an embankment is tipped
inclining in one direction, or falling on each side from the centre.

4. The effects of rain, frost, snow, and the atmosphere on the deposited
earth.

5. By a crumbling of the lower portion of a slope.

6. By a hurricane or extreme wind force, especially when accompanied by
rain and the location is that of a narrow, steep valley, blowing away or
dissipating the top and the surface of an uncovered embankment.

7. By a slope becoming honey-combed by rodents, and in some few
countries by an embankment in a river or the sea being bored and
disturbed by crustacea.

8. Insufficient slope for permanent stability at the time of or after
deposition.

9. The want of adhesion between the surface of the ground upon which an
embankment is tipped and the deposited material.

10. By the accumulation at the foot of an embankment of boulders or
lumps having no cohesion, and no adhesion to the surface of the ground
upon which they are deposited.

11. The weight being too great upon the ground upon which an embankment
rests.

12. By unequal loading.

13. An accumulation of water, caused by the unevenness of the surface of
an embankment, or by the ground not being prepared so as to prevent a
lodgment of water.

14. By obstructing the established discharge of land-water or by
attempting to divert the natural flow of underground waters.

15. The localisation of the surface drainage.

16. Water percolating into a benching trench, made to receive the toe of
a slope, thereby impairing the cohesion of the soil and reducing its
weight-carrying capacity and stability.

17. Vibration.

18. The different nature and state of the earth tipped into an
embankment and the consequent localisation of water in the more pervious
soil, causing unequal settlement, subsidence and movement.

19. An embankment being tipped of material in a different state of
dryness, moistness, hardness, softness, or in a frozen condition.

20. The size and character of the earth, as excavated in the cuttings;
for instance, whether picked and shovelled soil, or lumps of excavation
simply barred away and deposited in small masses with earth approaching
a state of dirt or mud.

21. Overpressure upon the material forming an embankment.

22. The different conditions of the weather, when an embankment is
tipped, causing portions to become dry, wet, or frozen.

23. The lead or distance from a cutting to the place of deposition being
of considerable length.

24. The earth being loosened from vibration and concussion during
transit in the wagons, and in the process of deposition, thus causing it
to be non-homogeneous.

25. By the earth being tipped with greater impetus at one part than at
another place.

26. An embankment not being tipped to its full width as it progresses,
whether in one or more wagon roads.

27. First tipping the central portion of an embankment, and completing
the width and slope by side deposition after some time has elapsed.

28. Tipping the contents of earth wagons from a considerable height,
thereby loosening and separating the soil, causing the larger and
heavier material to be near the foot of a slope and in lumps, and an
embankment to have interstices and be temporarily or permanently
unstable.

29. Irregular consolidation, artificial or otherwise.

30. Unequal exposure, particularly in embankments upon sidelong ground.

31. Insufficient width of the formation, especially in high and exposed
embankments.

32. The junction of two embankments tipped from cuttings in different
kinds of earth.

33. No time being allowed for subsidence or consolidation before the
deposited earth is subject to varying loads and vibration.

34. By allowing water to collect upon the formation and to form channels
down the slopes.

35. The neglect to fill, or otherwise remedy, cracks or fissures.

36. The want of uniformity of the covering of the slopes or top of an
embankment causing unequal percolation of water.

37. By a retaining wall at the toe of a slope preventing the discharge
of water that has percolated through the formation and the slope.

38. By an abnormal increase of the load upon the foundation of a wall
caused by lateral thrust and tilting forward, or fracture of the
footings or the concrete bed.

The first seven heads of “the chief causes of slips in embankments”
might be classed as NATURAL, _i.e._, produced by nature, and, therefore,
beyond the power of man to entirely prevent; the remainder as
ARTIFICIAL, and, therefore, more or less to be prevented.

In the case of earthworks made to contain or exclude water for the
purposes of docks, canals, waterworks, reclamation of land, irrigation
or drainage, &c., may be added, without reference to their
construction:—

1. Leakage along a discharge culvert, sluice, or tunnel in an
embankment, or through the earthwork.

2. Erosion of the land slope or backing of a retaining wall by waves, or
spray falling thereon and necessarily passing over the top of an
embankment.

3. From abrasion and damage, caused by vessels or barges rubbing or
colliding against a slope.

4. From erosion caused by wave action produced by the passage of boats
or ships propelled by machinery; or by wind waves.

5. Variation in the water level, causing unequal pressure.

6. In the case of a reclamation embankment, it being closed from the
ends, and not by raising in layers, from the ground level.

7. Variation of the submerged area, and consequent change in the degree
of exposure to deteriorating influences.

* * * * *

To sum up, the principal causes of slips in earthwork may be stated to
be air, water, frost and thaw, over-pressure, and vibration; the chief
agent both in cuttings and embankments being water, which forces forward
the surface of the slopes and destroys the cohesion of the soil, and
impairs its frictional resistance until the earth is unable to sustain
the weight upon it; vibration aiding and completing the movement, as it
not only tends to loosen the soil, but may disturb the equilibrium of
earthwork which is almost moving, and only requires a slight shock to
set it in motion: in fact, vibration is frequently the complementary
agency that causes a slip, and is obviously felt most in loose soils;
but if there should be fissures in earth of a tenacious character, or
boulders in clay to disconnect it, the effect of vibration will be more
serious in the latter case, as whole masses of earth may become
detached, instead of an equal settlement proceeding as with soils, such
as sand and gravel, which may become consolidated by shaking, owing to
wedging of the particles, should the slopes be sufficiently flat to
prevent lateral movement; but it may detach portions of the slopes in
soil having little or no cohesion, and thus initiate a slip.

In the following chapters the chief causes of slips in cuttings and
embankments are considered, together with others bearing upon a solution
of the subject, which is so interwoven that it is impracticable to
preserve a successive order, but an endeavour has been made to
separately indicate the cause and some remedies that may be adopted: but
before proceeding to particularise, it may be well to name a few
dominant principles of the alignment of public works, which if duly
regarded may tend to prevent slips of serious importance.

Consequent upon financial and other causes, an engineer is usually
required to so quickly prepare the necessary parliamentary plans and
sections of public works, more particularly for railways, that it is
beyond the power of the most experienced to set out in a few hours the
best line of railway, &c., across a country, giving due consideration to
the parliamentary, constructional, economical working, district and
through traffic, and financial requirements of the undertaking. There
are, however, a few points which he may be able to regard respecting the
stability of earthworks, some of which are now enumerated.

1. Avoid cuttings or embankments in drift soil in or upon the side of a
hill.

2. Avoid all damming back or flow of the natural drainage waters, or
heaping of snow by the erection of an embankment, especially in
mountainous, hilly, or sidelong ground, and in an undrained district.

3. When any excavation is in the side of a hill, observe the natural
configuration of the ground in the wettest part, and remember that the
slope of a cutting may not stand unless at the same inclination, or the
toe of the slope is supported by a massive retaining wall and extensive
draining, and that any disturbance may cause it to require a flatter
inclination.

4. Avoid river or stream diversions in earth of a very porous character;
and should an embankment have to be erected near to a deep river having
a steep bank, locate the line a sufficient distance from the edge that
the slope of the river bank may be flattened when required, as an
extraordinary flood may cause it to be unstable and to fall in, and in
order to restore it to a condition of stability, it may be necessary to
widen the river and reduce the angle of inclination of its banks.

5. In treacherous earth do not locate a railway close to a road, except
at a station, as if the line be placed above the road, a slip upon the
railway may result in a slip upon the road; and should the line be below


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