(Paper No. 4089.) Experiments upon Mortar, and Diatomaceous Earth as By A. POULSEN, Government Chief Engineer for Maritime [Abridged.1] THE Author, after paying a visit of inspection in 1912 to various defence-works on the south and north-east coasts of England, offers the following brief account of groyne-construction in Denmark for purposes of comparison with English methods of coast-defence, and also presents the results of experiments in sea-water upon various cements and mortar compounds, especially a diatomaceous earth known in Denmark as "Mo-Ler," which, so far, it is believed, has not been used for a similar purpose in this country. GROYNES IN DENMARK. Groynes have been constructed in several parts of Denmark, notably on the north coast of Zealand, facing the Cattegat, as also at Elsinore, Hornbeek, Gilleleje and Tisvilde, and likewise on the east coast facing Øresund. It has been found possible in these cases to make use of comparatively small and cheap structures, with posts of pine timber, packed with boulders gathered from the seabeach. The sea-coast consists mainly of clay and sand, mingled with pebbles and boulders, all belonging to the glacial epoch, and in the course of time, as the land has become washed away, the heavier materials have remained behind. Moreover, on the Baltic coast, as also on the Danish islands of Lolland and Falster, it has The original Paper is filed in the Library of The Institution. been necessary to construct a considerable number of groynes to protect the large dikes, erected after a flood in 1872, which caused great damage to these fertile districts. All these works, however, when compared with those constructed on the North Sea coast, in the vicinity of Thyborón, are insignificant. The term "west coast," as generally applied to the whole of the Jutland coast facing the North Sea and the Skager Rack, is really a misnomer, since part of this coast, at Grenen, the extreme point of the Skaw, actually faces north-east, while the coast between Esbjerg and the Horns Reef faces south-west. Between these extremes there is a difference of 180°, and on this coast it will also be found that the conditions are diametrically opposite, as regards shelter, nature of sea-bottom, tides, movements of sand, etc. At Thyborón the range of tide amounts to about 20 inches, and the movement of sand, both at Thyborón and along the coast to the south of it, is in a southerly direction, whereas at Vorupór and Hanstholm, where sea-walls have recently been constructed, as also along the rest of the coast as far as the Skaw, the sand moves to the north or east. The dividing line lies about 15 miles north of Thyborón, at the village of Stenbjerg, which is, moreover, the southernmost point on the west coast where the sea-bottom is of a calcareous nature. The sea-bed at Thyborón consists of fine sand, sometimes mixed with shingle, and at depths of 16 to 32 feet is found a sandy alluvial clay. This same formation extends some 6 miles north and 12 miles south of Thyborón. Beyond this the soil is composed of clay and sand belonging to the glacial formation, such as occurs in the hills of Lodbjerg and Bovbjerg. It is on this tract, extending over about 20 miles, that the groynes, seventy in all, have been constructed (Fig. 1). The land on which they are built is of little or no value, but the Thyborón Canal constitutes the navigable approach to the Limfjord and its numerous ramifications. The surrounding country is extremely fertile, and many small towns of some commercial importance are situated on the fjord. The canal itself is a North Sea fishing-centre, and fishing is also carried on in the Limfjord. These fisheries, developed since the inundation of 1825, would inevitably be ruined if the coast and canal were left unprotected. In a short time the canal would be silted up, and the Limfjord would again be converted into a fresh-water or brackish lake. At Thyborón a shipping pier for fishing boats has been built, which is a species of groyne, as the piles are secured by a bank of boulders on the sea-bed, forming, in fact, a lower groyne. Works are also projected with a view to render the Thyborón Canal a better entrance to the Limfjord, and to form a proper harbour.1 The amount hitherto expended on the protection of the coast may be stated as follows: Each groyne with a dike of 1,230 feet in length has thus cost about 110,000 kroner, or about 300 kroner per lineal metre of coast (about £5 per foot). When the works were commenced in 1875 there was an entire absence of experience in Denmark concerning these structures, and the Dutch groynes were taken as models, notwithstanding very different conditions. The unsuccessful works carried out in consequence are classed as "experimental works." Details of Groyne Construction.-Fig. 2 gives a vertical section through the groynes, with concrete filling, most recently constructed, namely, groynes A and B at Bovbjerg. The size of the concrete blocks is about 62 cubic feet, each weighing about 4 tons. The same section was employed for the groynes erected about 10 years ago, the only difference being that, in that case, it was possible to use granite blocks. The granite for the purpose was taken from the sea-bottom at the Cattegat, and was brought by the vessels, 1 These works have since been commenced, and will cost about 1,800,000 kroner (£100,000). via the Limfjord, to a pier at Krik. From this point a railway track was laid to the groynes. It will be seen from the Figure that a scaffold formed of piles was employed to construct a track for the trollies. This is raised to such a height that the work can be carried on without interruption at all times when the weather is fairly normal. The height at the shore end is 14 feet above mean water-level, with a fall of 1 in 60 throughout the entire length. The groyne proper is 620 feet long, with a height at the point where it joins the land-or, more properly, the dike-of + 13.12 feet, and at the other extremity of 1·62 foot. There is a bend at the centre of the groyne beyond which the slope is 1 in 80. For the part nearest the shore the slope is 1 in 20. Many of the groynes, for the sake of greater protection, have their dikes placed at a distance of 164 feet to 328 feet behind the end of the groyne, this latter being connected with the main structure by means of a transverse dike. This dike, at the time of its construction, is at least 16 feet in height, but in consequence of deposits of sand such dikes rapidly increase in height. In several places it has even been possible to form the dikes entirely by regulating the drift of sand. This is effected by means of fascine shelters, about 3 feet in height, which are replaced by a new one as fast as the previous one has been filled up with sand. The use of concrete filling in a large proportion of the groynes is due to the circumstance that the conditions south of the Thyborón Canal render it very difficult to obtain the granite for the purpose. The experience gained with respect to the durability of the concrete used for the earlier works was so unfavourable that in 1899, when further groynes were about to be constructed, and the choice lay between granite and concrete, there was no hesitation in selecting the former material. It has been necessary, moreover, to endeavour to improve the quality of the concrete used for the groynes south of the Thyborón Canal. The materials used are very good, as the shingle and the sand taken from the sea-beach between the groynes is washed extremely clean by the waves, and the surface of the pebbles, owing to attrition in the sea-water, is always smooth and free from all soft or impure substances. The shingle employed is a mixture of large and small pebbles, about 95 per cent. of which is composed of black flint, and the residue consists of granite. With respect to size, no pebble is to weigh more than 2 lbs. or less than 4 OZ., and in no case are there to be more than seventy or less than four pebbles to the pound. The sand consists of a white arenaceous |