therefore, S would equal to 1
0·25 to 1
1·10 = 4 to 1 TO 0·91 to 1, say, 1 to 1.
This is the required inclination to prevent movement provided no pressure is exerted upon the surface, and not taking into consideration the disturbing and weakening effects of vibration and all other deteriorating influences, such as the variable degree of moisture of the soil, the irregularity of its character, the destruction of the continuity of the surface by trenches or drains in a slope, the effect of gravity to detach a mass, the process of excavation or deposition, and the expansion and contraction of soils of an argillaceous nature. When any earth becomes suddenly water-charged or deteriorated by any of the agencies previously and subsequently mentioned, movement may be expected, and it should be remembered that in the same soil the resisting powers to disintegration frequently vary, consequent upon inequality in the quantity of moisture, the roughness, evenness, smoothness, compactness, looseness, the degree of fineness of the earth, and also the manner in which strain is applied.
Friction upon a dry surface is almost invariably greater than that upon a wetted surface, and is so beyond all question upon any plane lubricated with an unguent. The disturbing and enfeebling effect of water may be judged from a careful analysis of many reliable experiments to ascertain the frictional resistance in the case of the same material in a dry and in a wet state on an unplaned surface of cast iron and on timber piles. It shows the following results in addition to those given in the table of coefficients of friction of earth upon earth in the next page.
The surface friction of masonry or brickwork upon dry clay is reduced by from 25 to 30 per cent. when the clay is wet.
The frictional resistance of an unplaned surface of cast iron upon wet sand is about 16 per cent. less than the resistance upon the same material when dry. In the case of timber piles, it is about 12 per cent. less, and about 40 per cent. less in sandy clay and gravelly clay soil.
In sandy gravel the difference in the resistances is small, being only from 5 to 10 per cent. less when the earth is wet.
Sand has about 20 per cent. more friction than sandy gravel, both materials being in a wet state; and below a depth of from 10 to 15 feet, the frictional resistance increases little in gravelly sand and gravelly soils.
In lieu of a sufficient example under similar conditions and circumstances, the most reliable method to ascertain the slope of repose of any earth is that of S, the slope of repose, to 1, = 1
F, any cohesion of the soil being disregarded and considered as a margin of stability liable to be much impaired; and, therefore, except in a mass, it is prudent to look upon it as non-existent in the whole of a slope; and when motion has commenced, as even a means of accelerating movement by causing lumps to become displaced instead of mere particles.
The following coefficients of friction during motion are here given in confirmation of the frictional resistance of an earth being an indicator of its slope of repose.
They have been tabulated from different authorities, are the average results of practical experiments, and have been compared with the inclination of slopes actually assumed under the ordinary conditions of work.