A Treatise On The Principles And Practice Of Dock Engineering

DOCK ENGINEERING. 158 earth. But the values attaclied to these angles can only be regarded as of an approximate nature, as will be evident from a glance at the following table comprising maximum and minimum results obtained by different experimentalists :— TABLE XV. Material. Range of Angle of Repose. From To Gravel and shingle, 35° 48° Dry sand, 21° 37° Vegetable earth, .... 28° 55° Compact earth, .... 40° 50° Well-drained clay, 40° 45° Peat, 14° 45° Ranges so extensive render it an exceedingly difficult matter to assign any angle to a variety of soil, however spécifie, èspecially in view of a further modification due to its degree of humidity. The amount of moisture présent in the sample under considération very materially influences the experi- mental result obtained for its angle of repose. A slight quantity, just sufficient to occupy the interstices between the grains of solid matter, has been found to inerease the frictional resistance to movement, and, accord- ingly, to produce a correspondingly greater angle of repose. Any excess of moisture, however, over and above this trifling amount, results in a diminution of the frictional resistance ; and if the humidity be indefinitely inereased, the material eventually acquires a muddy consistency to which there is no angle of repose worth noting. Ordinary clay, for instance, in the dry condition crumbles at 40° ; moderately moist, its inclination may be inereased to as much as 50°; allowed to become saturated, it subsides at an angle of 10°. Argillaceous earths are most susceptible to the deteriorating influences of moisture, and any admixture of sand with the clay only produces an accentuation of the evil, because the impermeability of the clay offers an obstacle to the escape of water which has entered through the pores of the sand. A striking instance of this is afforded in a notable landslip behind a quay wall at Altona, to be dealt with at a later stage. The foregoing considerations distinetly emphasise the necessity for the prompt and adéquate drainage of earthwork, and particularly so in the case of dock and river walls, where the earth backing is generally in a state of intermittent immersion. Under the head of a rising tide, water penetrates to an equal height behind the wall, and, unless there be adequate means for its withdrawal with the ebb, the volume of water thus confined will prove a serious augmentation of the overturning forces.