Operation of the Freezing Process.—The freezing is done by passing double pipes (one within the other) at suitable distances apart through the material to be frozen, and by causing brine—previously chilled in a reser voir by a refrigerating-machine—to circulate through these pipes. The chilled brine passes through the inner pipes and returns through the space between the inner pipes and outer pipes, the former being open at the ends and the latter closed. The heat is thus abstracted from the soil surround ing the pipes until it is frozen to the desired thickness. The returning brine flows into a tank, where it is again chilled. In Germany, by this process, several shafts have been successfully sunk through quicksand and other troublesome materials, and one in Belgium (at Houssu) has been sunk to a depth of 224 feet, all below a depth of 3o feet from the surface being quicksand which had defied everv effort to accomplish the result by other methods. Figures r, 2 (pi. 5o) exhibit, respectively, the application of the Poetsch system to shaft excavation, and to the construction of a pneumatic caisson for the foundation of a bridge pier.
Fascines.—In other cases, where the nature of the soil is unstable or Yielding to a considerable depth (as in the case of soft mud, bog, or quick sand.bottom), numerous layers of fascines, or bundles of stout twigs and small branches, sunk by weighting with earth, stones, etc., are often deposited over a considerable space beyond that to be occupied by the base of the structure. The interlocking parts of these fibrous masses serve as an excellent means for distributing the pressure over a great area when the weight of the intended structure comes upon it. They not only fulfil a useful purpose in the construction of railway embankments over treacherous ground, but have also been applied with the best results to subaqueous foundation-work for bridge piers, jetties, etc. An example of this expedient applied to bridge engineering is afforded by the great suspension bridge of Kieff, in Russia (1852), with spans of 44o feet, of which the piers and abutments were founded in this manner on a shifting quicksand. " There the fascine mattresses extend too feet beyond the
bases of the masonry which rests upon them." This expedient proved of great value in the work of improving the channel of the South Pass of the Mississippi River by the system of jetties successfully built by the late Captain James B. Eads (1/. 55, figs. 2, 3).
South Pass bed of the AIississippi at the South Pass afforded an exceedingly unstable foundation of soft sediment, into which any work of stone would speedily sink and disappear; so that the task of devising a method of securely and permanently founding the masonry of these extensive lines of jetties was made extremely difficult. These jetties are simply dykes or levees tinder water, intended to act as artificial banks to the river to confine its waters within a narrow channel, so that the increased velocity of flow shall cause the stream to scour out for itself a channel of a required depth. It was found that piles alone or crib-work would not answer, as they' were quiekly undermined and washed away by the scour of the current. The plan which proved successful consisted in driving piles along the inside of the line for the intended artificial banks. Then great mattresses of willow-brush were made and securely joined together by cross-ties and pins. These were towed into position adjoining the piles and fastened to them. If placed in this manner at night, by the next morning it was found that the deposit of sediment from the current had so filled the interstices as to cause the mattresses to sink to the river bottom. Each of these mattresses was not only fastened to others adjacent to it and to the piles, brit was also anchored in place by a layer of stone. It was found that by the continued deposit of sediment in and about them they had become more solid than the natural bank. They thus serve to protect the row of piles from the current, and these, in turn, aid in hold ing the mattresses in place. Upon this foundation the solid stone paving of the jetties was subsequently built. This work will be more fully described in the next section.