_Outline Study of Soils. Puddle._
The following study of soils is merely suggestive and is here given
to emphasize the importance of the subject, at the risk of being
considered a digression. Soil formations are made in one of three ways:
1. By decomposition of exposed rocks.
2. By transportation or sedimentation of fine and coarse
materials worn from rocks.
3. By transformation into humus of decayed organic matter.
The transforming agencies by which soils succeed rocks in geological
progression have been classified as follows:
1. Changes of temperature.
2. Water.
3. Air.
4. Organic life.
_Heat_ and its counter agent frost are the most powerful forces in
nature, their sensible physical effects being the expansion and
contraction of matter.
_Water_ has two modes of action, physical and chemical. This agent is
the great destroyer of the important forces, cohesion and friction.
_Cohesion_ is a force uniting particles of matter and resists their
separation when the motion attempted is perpendicular to the plane of
contact. _Friction_ is a force resisting the separation of surfaces
when motion is attempted which produces sliding. The hydrostatic
pressure and resultant effect upon submerged surfaces need to be
kept constantly in mind. When the surface is impermeable the line of
pressure is normal to its plane, but when once saturated there are also
horizontal and vertical lines of pressure. Since the strength of an
earth dam depends upon two factors, namely, its weight and frictional
resistance to sliding, the effect of water upon different materials
entering into an earth structure should be most carefully considered.
This will therefore occupy a large place in these pages. An earth
embankment founded upon rock may become saturated by water forced up
into it from below through cracks and fissures, reducing its lower
stratum to a state of muddy sludge, on which the upper part, however
sound in itself, would slide. The best preliminary step to take in such
a case is to intersect the whole site with wide, dry, stone drains,
their depths varying according to the nature of the ground or rock.
_Air_ contains two ingredients ever active in the process of
decomposition, carbonic acid and oxygen.
_Organic Life_ accomplishes its decomposing effect both by physical
and chemical means. The effect of organic matter upon the mineral
ingredients of the soil may be stated as follows:
1. By their hydroscopic properties they keep the soil moist.
2. Their decomposition yields carbonic acid gas.
3. The acids produced disintegrate the mineral constituents,
reducing insoluble matter to soluble plant food.
4. Nitric acid results in _nitrates_, which are the most
valuable form of nutritive nitrogen, while ammonia and the
other salts that are formed are themselves direct food for
plants.
_Vegetable Humus_ is not the end of decomposition of organic matter,
but an intermediate state of transformation. Decay is a process almost
identical with combustion, where the products are the same, and the end
is the formation of water and carbonic acid, with a residue of mineral
ash. The conditions essential to organic decomposition are also those
most favorable to combustion or oxidation, being (1) access of air, (2)
presence of moisture, and (3) application of heat.
Now the coöperation of these chemical and physical forces, which are
ever active, is called “weathering.” Slate rock, for instance, weathers
to clay, being impregnated with particles of mica, quartz, chlorite and
hornblend. Shales also weather to clay, resulting often in a type of
earth which is little more than silicate of aluminum with iron oxide
and sand.
In the vicinity of the Tabeaud Dam, recently built under the personal
supervision of the author, the construction of which will be described
later, there is to be found a species of potash mica, which in
decomposing yields a yellow clay (being ochre-colored from the presence
of iron), mixed with particles of undecomposed mica. This material
is subject to expansion, and by reason of its lack of grit and its
unctuous character it was rejected or used very sparingly. Analysis of
this material gave, Silica, 54.1 to 59.5%; potash, 1.5 to 2.3%; soda,
2.7 to 3.7%.
Soil analysis may be either mechanical or chemical. For purposes of
earthwork, we are most interested in the former, having to deal with
the physical properties of matter. Chemical analysis, however, will
often afford information of great value regarding certain materials
entering into the construction of earth dams. The most important
physical properties are:
(1) Weight and specific gravity.
(2) Coefficient of friction and angle of repose.
(3) Structure and coloring ingredients.
(4) Behavior toward water.
There are two distinct methods of mechanical analysis: (1) Granulating
with sieves, having round holes. (2) Elutriating with water, the
process being known as silt analysis.
It would require a large volume to present the subject of soil analysis
in any way commensurate with its importance. Experiments bearing upon
the subjects of imbibition, permeability, capillarity, absorption and
evaporation, of different earth materials, are equally interesting and
important.[1]
The permeability of soils will be discussed incidentally in connection
with certain infiltration experiments to be given later.
Puddle.
_Puddle_ without qualification may be defined as clayey and gravelly
earth thoroughly wetted and mixed, having a consistency of stiff mud
or mortar. Puddle in which the predominating ingredient of the mixture
is pure clay, is called _clay puddle_. _Gravel puddle_ contains a much
higher percentage of grit and gravel than the last-named and yet is
supposed to have enough clayey material to bind the matrix together and
to fill all the voids in the gravel.
The term _earthen concrete_ may also be applied to this class of
material, especially when only a small quantity of water is used in
the mixture. These different kinds of puddling materials may be found
in natural deposits ready for use, only requiring the addition of the
proper amount of water. It is usually necessary, however, to mix,
artificially, or combine the different ingredients in order to obtain
the right proportions. Some engineers think grinding in a pug-mill
absolutely essential to obtain satisfactory results.
Puddle is handled very much as cement concrete, which is so well
understood that detailed description is hardly necessary. Instead of
tampers, sharp cutting implements are usually employed in putting
puddle into place. Trampling with hoofed animals is frequently resorted
to, both for the purpose of mixing and compacting.
As has been stated, clays come from the decomposition of crystaline
rocks. The purest clay known (kaolin) is composed of alumina, silica
and water. The smaller the proportion of silica the more water it will
absorb and retain. Dry clay will absorb nearly one-third of its weight
of water, and clay in a naturally moist condition 1-6 to 1-8 its weight
of water. The eminent English engineers, Baker and Latham, put the
percentage of absorption by clayey soils as high as 40 to 60%. Pure
clays shrink about 5% in drying, while a mixture by weight of 1 clay
to 2 sand will shrink about 3%. It follows, then, that the larger the
percentage of clay there may be in a mixture the greater will be both
the expansion and the contraction.
Clay materials may be very deceptive in some of their physical
properties, being hard to pick under certain conditions, and yet when
exposed to air and water will rapidly disintegrate. Beds of clay,
marl and very fine sand are liable to slip when saturated, becoming
semi-fluid in their nature, and will run like cream.
The cohesive and frictional resistances of clays becoming thus very
much reduced when charged with water, a too liberal use of this
material is to be deprecated. The ultimate particles forming clays,
viewed under the microscope, are seen to be flat and scale-like, while
those of sands are more cubical and spherical. This is a mechanical
difference which ought to be apparent to even a superficial observer
and yet has escaped recognition by many who have vainly attempted a
definition of _quicksand_.
Mr. Strange recommends filling the puddle trench with material having
three parts soil and two parts sand. After the first layer next to bed
rock foundation, which he kneads and compacts, he would put the layers
in dry, then water and work it by treading, finally covering to avoid
its drying out and cracking.
Prof. Philipp Forchheimer, of Gratz, Austria, one of the highest
authorities and experimentalists, affirms that if a sandy soil contains
clay to such an extent that the clay fills up the interstices between
the grains of sand entirely the compound is practically impervious.
Mr. Herbert M. Wilson, C. E., in his “Manual of Irrigation
Engineering,” recommends the following as an ideal mixture of materials:
Cu. yds.
Coarse gravel 1.00
Fine gravel 0.35
Sand 0.15
Clay 0.20
––
Total 1.70
This mixture, when rolled and compacted, should give 1.25 cu. yds. in
bulk, thus resulting in 26½% compression.
Mr. Clemens Herschel suggests the following test of “good binding
gravel:” “Mix with water in a pail to the consistency of moist earth;
if on turning the pail upside down the gravel remains in the pail it is
fit for use, otherwise it is to be rejected.” For _puddling material_
he would use such a proportion as will render the water invisible.