Autobiography of Sir John Rennie, F.R.S., Past President of the Institute of Civil Engineers / Comprising the history of his professional life, together with reminiscences dating from the commencement of the century to the present time.

by Sir John Rennie

In fact, geology combined with mineralogy he will find to be of most essential service in almost every department of civil engineering.

Embankments.—This is another department of engineering which requires a good deal of skill and judgment, particularly along an exposed open coast, where lowlands are to be protected against the encroachments of the sea. The first point is to select the line of embankment in such a manner that there shall always be in front of it a good foreshore, so that the force of the sea may be broken before it reaches the embankment; that is to say, where practicable, to have a certain extent of green or outlying marsh in front of it, so that the embankment when completed will seldom have a head of water to contend with at high tide of above six or seven feet. And even with this moderate depth at high water, when exposed to the action of a heavy gale of wind, there will for three or four hours be a considerable broken sea, calculated to do a great deal of damage, if the embankment be not properly constructed. Now, if the embankment have a good green foreshore in front, with sea slope of about 5 or 6 to 1, well sodded up, a facing of clay about 18 inches thick, 6 feet above the highest level of spring tides, the top being 6 feet wide, with back slopes of 2 to 1, with a back ditch 10 feet from the foot of the inner slope, the interior of the embankment being composed of sound earth well rammed or pressed together, so as to make it solid—an embankment of this kind will be able to resist such a pressure as we may ordinarily expect it to be exposed to.

There may be extraordinary cases where this will not be sufficient. When these occur it will be necessary to pave the surface with stone, about 9 inches thick, or with fagots. The former is, however, decidedly the best plan, as it will be permanent, whereas fagots are constantly rotting, and require renewal.

If the sea shows a tendency to carry away the foreshore, it must be prevented, by means of jetties so disposed as to collect the alluvial matter held by the sea water in suspension. These, if properly designed and constructed, will generally have the desired effect.

In cases where the water outside is deep and the sea face of the embankment may be exposed to a head of water of 12 feet and upwards, much greater precautions must be taken to guard against accident. The sea slopes of the embankment must be increased to 7 or 9 to 1, well faced with clay and paved with stone, having the foreshore in front well protected with jetties. In fact, no two cases will be alike: each must be treated separately according to the particular local circumstances, and therefore it is impossible to design a proper plan for any embankment without knowing all the local circumstances. The general principle is that the sea face of the embankment should never be less than from 4 or 5 to 1. In some particular cases a less slope will do, say 3 to 1. This, however, certainly depends upon local circumstances. The base of the outer slope should be particularly watched, and if any crack appears to be forming, it should be immediately stopped by jetties carried out as far as necessary. In forming embankments it is usual, when it can be done, to take the earth from the outside of the sea slope, but this should never be done within less than 10 yards from the base of the slope, and these “floor pits,” as they are termed, should generally not exceed 12 to 18 inches in depth, and be increased in width in proportion to the quantity of earth required for the bank; at every 10 or 15 yards, in the longitudinal direction, the earth should not be removed, but left to form small cross banks between the floor pits, so as to prevent any current being formed in them; thus these floor pits will soon be filled up by the alluvial matter brought in by the tide, when the outside slopes of the bank are neither exposed to the heavy lash of the waves nor to strong currents. Then if they are covered with good grass sods properly laid on and beaten into the face of the bank it may suffice, but not otherwise. If this should not answer the slope must be increased and, if necessary, paved with stone as above mentioned. When good clay cannot be obtained to face the bank, then the best of the earth that can be got must be employed, mixed with straw, well puddled with water, and laid upon the surface of the bank in a moist state about 18 inches thick, and then faced with stone about 9 inches thick, well rammed edgeways into it. In cases where it is necessary to protect any line of coast against the ravages of the ocean, the measures to be adopted will depend upon the form and geological character of the coast to be so protected, whether low flats and alluvial, or cliffs composed of rocks more or less hard, and easily acted upon by the waves, rain, and atmosphere. In the former case it will generally be found that the coast is surrounded by extensive flat sands, and that the water holds a large quantity of alluvial matter in suspension. The great object, therefore, should be to cause this alluvial matter to be deposited in such form and in such places as are best adapted to our purpose. Now this may generally be effected in an inexpensive manner, considering the object to be attained, by a series of jetties, either composed of stakes wattled together with fagots, or lines of loose stones disposed in such a manner that they shall break the rising and falling waters, and make them stagnant between the jetties, so that they may deposit their alluvial matter. In the first instance these jetties need not be raised more than two feet above the level of the sand, and when the sand or alluvial soil has accumulated up to the top, they may be again raised to a similar height, and so on until the soil in front of the coast has been converted into a green marsh; thus there will not only be formed a protection to the coast invaded by the sea, but fresh land may be gained in front of it and embanked from the sea. It is impossible to explain the precise disposition and direction of these jetties and works without a thorough knowledge of the locality, and such circumstances as its exposure to winds, tides, and currents. The principle however is to check the currents gradually, and in such a way as to prevent any strong current from being formed; for if a new and strong current should be created, not only will the alluvial matter not be deposited, but the works themselves will be carried away, and all the labour and expense will be wasted. It is generally advisable that such works should be commenced near the shore, and worked downwards towards the sea; thus, if they are properly managed, no deep pools or strong currents will be formed behind them; and the required process of filling or silting up will proceed regularly seaward, always increasing the protection required, and obtaining additional land as they proceed.

In some cases, where the sea is heavy, it may be necessary to have stronger jetties or works to relieve and protect the minor ones above described; but these should only be resorted to in places where the others are insufficient, or in greatly exposed situations; wherever the minor works will suffice, as they will in most cases if properly applied and constructed, the less heavy works are resorted to the better, as the great object is to lead not drive Nature; that is, to work with her instead of against her. By this means a few bricks and stakes will do a great deal more than far greater and more expensive works. So far as regards low alluvial coasts, these, if properly managed, will be found comparatively easy to deal with.

When we come to rocky coasts that are wearing away by the combined action of the sea below and the rains and atmosphere above, and where there is little or no alluvial matter held in suspension by the waters, that might be collected so as to form a protecting deposit at their base, then we must adopt a different system, but not altogether ignoring the other when it can be made useful. In this latter case we must secure the bases of the cliffs at least up to high-water mark by means of retaining walls, where the rock itself is not hard enough to resist the action of the sea. These walls need not be carried higher than absolutely necessary. In some cases a mere footing will do; in others, the wall may be carried up to half tide; and in others up to the full high-water mark; and although the rock may be naturally soft, yet if its surface be protected by harder stone, even of a very moderate thickness, it will be quite sufficient to resist further encroachment by the sea. As these retaining walls will be founded upon a base of solid rock, there is very little fear of their being undermined; therefore, when I said before that it would be necessary to carry these retaining walls up to high-water mark, it must be understood as applying only to those rocks that are easily abraded by the sea.

There is another point to be attended to. The base being secured, we must look to the cliff above. Here, from the effect of rains, the water frequently cannot get away, accumulates behind at the top, and sinks through the fissures, when partly by hydraulic pressure and partly by the effects of frost, large masses are detached and fall below; and as this is continually occurring, the progress of decay goes on increasing. Having secured the base, the next thing, where practicable, is to slope off the upper surface of the cliff, so as to prevent it from overhanging, and then to make a drain at the back to carry off any water that may lodge there. By these means, if properly carried into effect, the base of the cliff being protected against the sea from below and rainwater from above, there is every probability that it will be preserved, in all ordinary cases. In extraordinary cases additional measures must be taken to meet them upon the same principles. With regard to retaining walls of brickwork or masonry, these should be always in excess of strength beyond the pressure, whether vertical or lateral, that they may have to resist. When the pressure is simply lateral, then the mean thickness of wall built of masonry and brickwork—the mean thickness, generally speaking, of the main body of the wall—should be about one-fourth of the height, besides counterforts at the back at certain distances from each other, regulated according to the particular circumstances. These, upon the average, including the thickness of the main wall, will make the total mass to be equal to nearly one-third of the total height. My father frequently made these walls curved in the front as well as at the back, the front being struck from a radius whose centre was level with the top of the wall, and of such a length that the face of the wall should batter one-fifth of the total height; the back of the wall should be struck from a centre at the same level as the other, but a little longer, so that at the base the wall might be about 2 feet thicker than at the top, in addition to two or three footings of 6 inches each; and the base of the wall was made to incline backwards, according to the radius from whence it was struck.

These walls, when they are to rest upon alluvial soil, must be founded upon a platform composed of piles of a sufficient length and thickness, driven at right angles to the line of the foundation, until with the blow of a ram weighing 15 cwt. and falling 20 feet they will not move one-eighth of an inch. These piles should be driven in regular rows, longitudinally and transversely, about 3 feet apart, and hooped and shod with wrought-iron hoops and shoes. At the front, immediately under the tie of the wall, there should be a row of grooved and tongued sheeting piles driven close together, and to the same depth as the others, about 6 inches thick, having a waleing or longitudinal brace 6 inches thick and 12 inches wide, well bolted in each side of the top of the sheeting piles. The loose earth should be taken out to about a foot in depth, and the space filled in with stone or brickwork to the level of the pile heads, which should be carefully trimmed, then covered with sills about 12 inches square, well spiked down to them. The spaces between the sills should be well faced with brickwork, and the whole surface should then be covered with 6-inch plank, properly spiked down to the sills below. Upon this platform the masonry and brickwork of the wall should be built. The wall should be carefully backed up as it proceeds with sound earth or clay, or clay mixed with one-sixth of gravel or concrete, as shall be deemed most advisable. These curved walls, if properly constructed, are stronger and more economical than the ordinary walls.

In some cases, as in that of Sheerness, for example, the foundation is so bad that a totally different plan must be adopted. At Sheerness it was necessary that the base of the walls should be increased, distributing the weight over a wider area, so that each superficial foot of the superincumbent mass should have a larger bearing, thus greatly relieving the pressure over every part.