The depths to which some puddle trenches are carried has been objected to by some engineers, and among them Sir Robert Rawlinson, as excessive and unnecessary, and, in the opinion of the latter, the same end might be obtained by going down to a depth say of 30 ft. only, and putting in a thick bed of concrete, and also carrying up the concrete at the back of the puddle trench, with a well for collecting water, and a pipe leading the same off through the back of the dam to the down stream side.  An arrangement of this kind is shown in the Yarrow dam, Fig. 4.

The thickness of the puddle wall varies considerably in the different examples given in the diagrams before you, a fair average being the Row bank of the Paisley Water Works, Fig. 6; and although in instances of dams made early in the century, such as the Glencorse dam—­Fig. 5—­of the Edinburgh Water Works, the puddle was of very considerable thickness, and it would appear rightly so.  This practice does not seem to have been followed in many cases, as, for instance, again referring to the Dale Dyke dam, Fig. 2, where the thickness of the top was only 4 ft., with a batter of 1 in 16 downward, giving a thickness of 16 ft. at the base.  For a dam 95 ft. in height this is very light, compared with that of the Vehui dam at Bombay, of which the engineer was Mr. Conybeare—­Fig. 7—­where the puddle wall is 10 ft. wide at the top, with a batter downward of 1 in 8, the Bann reservoir—­Fig. 8—­of Mr. Bateman’s design, where the puddle is 8 ft. broad at the top, and other instances.  The same dimension was adopted for the puddle wall of the Harelaw reservoir, at Paisley, by Mr. Alexander Leslie, an engineer of considerable experience in dam construction.

There appears to be a question as to what the composition of puddle should be, some advocating a considerable admixture of gravel with clay.  There is no doubt that clay intended for puddle should be exposed to the weather for as long previous to use as possible, and subject to the action of the air at any rate, of sunshine if there be any, or of frost.  When deposited in the trench, it should be spread in layers of not more than 6 in. in thickness, cut transversely in both directions, thoroughly watered, and worked by stamping.

The position of the puddle wall is, as a rule, in the center of the bank and vertical; but laying a thickness of puddle upon the inner or up stream slope, say 3 ft. thick, protected by a layer of gravel and pitching, has been advocated as preventing any portion of the dam from becoming saturated.  There are, however, evident objections to this method, as the puddle being comparatively unprotected would be more liable to damage by vermin, such as water rats, etc.; and in case of the earthwork dam at the back settling, as would certainly be the case, unless its construction extended over a very lengthened period, the puddle would be almost certain to become fissured and leaky; in addition, the comparative amounts of puddle used in this manner, as compared with the vertical wall, would be so much increased.  With the puddle wall in the position usually adopted, unequal settlement of the bank on either side is less liable to affect the puddle, being vertical.