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If the revetment after construction be not broken by extraneous causes its life will depend upon the durability of the material of which it is constructed, especially of the brush and wire of the mattress, as the lasting quality of compact limestone riprap need not be questioned.
In the early work brush was used as a covering for the upper bank, but, subject as it was here to alternate submergence in the water in exposure to the atmosphere, it decayed rapidly and possessed little strength after about two seasons, and it was this rapid decay of the brush, permitting it to be easily broken up and exposing the bank to erosion, that led to its disuse and the substitution for it of an all-stone paving.
It is commonly thought that brush constantly submerged will not deteriorate, but observations in these districts have shown this not to be true. It does lose its strength, but at a very much slower rate than when exposed to the air. This rate of decay also varies with the kind and size of the brush, hardwoods deteriorating much less than soft and the heart of the wood less than the sap.
Some investigations as to the life of the willow brush have recently been made by taking samples from subaqueous mats which were sunk at different times from six to eighteen years back.
The samples of brush from mats sunk six years ago show but little signs of deterioration, and the bark still adheres to them in many places.
From samples submerged for nine years the bark had disappeared entirely, as have also all small twigs or branches, and the sap of the wood shows considerable deterioration, it being soft and spongy when saturated and cracking, in the small sizes, in drying. The deterioration of the heart, however, appears to be slight. The brush, in general, shows a decrease in strength, more marked in the small sizes where the sap predominated.
The samples of brush submerged sixteen and eighteen years showed no practical difference in condition. Pieces about 112 inches in diameter and from 15 to 200 feet long could, when grasped at the butt by the hand, be readily broken in two by giving them a sudden jerk. This was done while they were thoroughly saturated with water and were quite heavy. When dried, their strength was largely increased. An examination showed that the sap had so badly deteriorated that it resembled a pulp, and its cohesion, especially when saturated, was practically nil, so that the
only strength was in the heart, which was small. The larger brush, or that above 2 inches in diameter, still exhibited fair strength, all due to the heart, but this was somewhat softer than brush of half its age of service. The deterioration of the heart was greater in some brush than in others, due probably to the species of the willow, for while in some samples it was almost as firm as the nine-year-old brush, in others it was much softer.
A sycamore pole used as a binder, taken from one of the eighteenyear-old mats, had undergone but little change, the fiber being still quite strong and pliable.
The conclusions drawn are that eighteen-year-old mats are still quite effective as a non-erosive bank covering, if they are undisturbed, but that they will not, when composed of small brush, stand bending, and should their outer edge be undermined even to a moderate extent it is probable that the brush would break up instead of bending to a slope, especially as these mats are of the old woven type, with but one thickness of brush. If, however, considerable of the brush is large the mat might bend somewhat without rupture. Brush in fascines would offer more resistance to breaking, but even in this form, with small brush in the condition of the older samples, the resistance would not be great; it would increase, however, materially as the brush increased in size. The investigations show that the sap quickly deteriorates and that the heart possesses considerable durability and, therefore, indicates the advisability of using as much large brush as practicable. The durability of a fascine might also be largely increased by using hardwood poles in the core. Even if the deterioraton of the heartwood of the willow should continue at the ratio of the past nine years, the brush will serve its purpose for many years to come.
Samples of wire and wire strand were taken from the same mats as the brush and all showed deterioration, increasing with their age. Strands of seven wires, only six years old, have had the galvanizing removed from the exposed outside surface, evidently by sand scour, while in the core, where the wires are protected, the galvanizing is still intact. The sections of the wires have also been slightly decreased by corrosion. Where the wire has been in service for a longer period the decrease in both area of section and galvanizing has been greater. This decrease in section is not uniform but in spots, with intervals of wires of nearly full gage size between