Professor Ansted has classed clay soils as under:—

When combined with 30 or 40 per cent. of sand they are CLAY LOAMS.

When combined with 40 to 70 per cent. of sand they are TRUE LOAMS and LOAMY SOILS.

But not until they have 90 per cent. of sand in them are they SANDY SOILS.

When combined with from 5 to 20 per cent. of lime, the soils become MARLY.

But not until they have more than 20 per cent. of lime in them are they CALCAREOUS SOILS.

With respect to the slope of repose in clay earths, it may be anything from 1 to 1 TO 12 to 1, and when the clay is mere dried mud and becomes saturated it may not be at rest until it is horizontal.

In cuttings, strong compact clay, if not seriously impaired by air and moisture, will stand at from,

Ordinary clay, such as the blue clay, with protected slopes and proper drainage, and firm boulder clay,

Plastic clay from,

Lias clays of doubtful character,

Superficial beds of London clay,

and not less for any weak clay.

In embankments, much depends upon the height, mode of tipping, state of the soil when deposited, the protection afforded and uniform character of the mass, the larger it is the more difficult it will be to drain.

Although firm clay may for a short time stand at nearly a vertical slope for any height of face not exceeding about 10 to 12 feet, few clays will permanently stand at a less slope than,

although they may for a time do so at 1 to 1 in unsubmerged work, but if the slope is covered with water, as in a canal, and subject to wash or wave action, however shallow the depth, it is seldom they repose at a less inclination than 1½ to 1.

Should an embankment be more than about 30 feet in height

Firm clay from,

Plastic-clay, consequent upon the percolation and pressure of water in cuttings, may stand at a steeper slope in a surface protected embankment, and if carefully tipped, it will repose at from

but from hydrostatic pressure the same clay may require from 2 to 1 TO 3 to 1 slopes in cuttings, and even an inclination of 5 to 1 has been found to be necessary.

The varying slope-system is referred to in Chapter VI., as also the range of slopes.

It is always well to bear in mind that in the case of clay, loamy or marly soils, or any possessing soluble particles, the quantity of water in them governs the required flatness of the slope, and as it may be variable, a clay that will repose at a certain angle may slip upon receiving an additional amount of water, hence the importance of drainage and protection. The cohesive strength of clay also varies greatly; and as it is likely to be impaired, and, perhaps, destroyed by water and other causes, it should be considered as of fluctuating value, and therefore as generally unreliable.

The slopes of repose required in river-banks have a wide range according to the degree of exposure and opposition to the free flow of water, and the protection given to the surface. In canals, and drainage-channels, or ordinary rivers, they range from 1½ to 1, when protected, to 5 to 1; but there are numerous small, shallow, and sluggish streams with almost vertical banks. As the earth of river-banks is frequently of a clayey nature the slopes are referred to under the head of Clay; usually it is mixed in character, and, therefore, the slope of stability cannot be determined from that of any particular and unalloyed soil. Towards the mouth the land often consists of detritus liable to be washed away by a stronger flood than that by which it was deposited; it may also be eroded by constant or increased wave action. Ordinary clayey, loamy, tenacious, or not easily moved, silt and sand river-banks, in a current that will not erode them at moderate depths, if the face is protected, will stand at an inclination of about 1½ to 1, when the channel is freshly cut.

If not covered or protected at from 2 to 1 TO 3 to 1, but when in clay soil of a doubtful character and non-homogeneous, or boulder-clay, about 4 to 1.

2 to 1 TO 3 to 1 is the most general slope; but in marsh land, unless the surface is protected, they may not permanently stand at 5 to 1. Fascine-work may be the easiest and most convenient protection, or a gravel or turf-covering may suffice, and should new channels have to be created, the question to decide is, will it be cheaper to make the slopes sufficiently flat so that they can be left unprotected, or is it preferable to have a steep inclination, such as 1½ to 1, and to carefully cover and protect the surfaces, the probable cost of maintenance of the protecting cover being duly considered?

As a rule in homogeneous earth, such as clay, underground excavation is the most free from water, and therefore from slips; but the enormous force clay possesses upon swelling—and this disturbance of the soil may occur many times by the action of air and moisture—and its contraction upon drying, cause tunnels in clay to be risky undertakings. The chief precautions against failure are to allow plenty of space for the expansion of the clay, reduce the uncovered lengths to the least dimensions, have dry, yet hard and compressible, filling between the arch and the walls and the clay, use the strongest bricks, or material with great compressive strength for the lining, adopt a form giving as equal support as possible in every direction, i.e., one closely approaching the circular, leave plenty of weep-holes to prevent any accumulation of water, and thoroughly drain and gently conduct to the proper drainage channels all water as it exudes.