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THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[November 



several other instances where considerable lengths of road, passing 

 over the very worst descriptions of subsoil, have been successfully 

 treated with a concrete foundation, which has ever since kept the clay 

 from rising, and served more than any other contrivance could have 

 done to keep the road in a sound and perfect condition. At the same 

 time it is deplorable to witness the miserable and barbarous expedi- 

 ents which are resorted to, even at the present day, to form a founda- 

 tion for the roads in the neighbourhood of London. It is generally 

 known that the tough blue clay of the London basin, in common with 

 the yellow plastic clavs, on which the former rests, is one of the worst 

 possible, subsoils over which a road has ever to be formed, in conse- 

 quence of its tendency to work up amongst the metalling. When the 

 clay has thus become mixed with the metalling — whether consisting 

 of gravel or broken stones — the most favourable condition for grinding 

 down the crust of the road is at once realized, because the stones are 

 separated from each other by a soft yielding matter, so easily squeez- 

 able and easily set in motion, that every pressure upon the road ne- 

 cessarily unsettles more or less the stability of part of the crust. 

 The constant grinding of the stones against each other of course 

 effects a destruction much more rapid than where the stones contain 

 no more matter mixed up with them than just what is sufficient to fill 

 up (heir interstices, and where this filling up matter is bard and firm 

 instead of being soft and yielding, like the clay which has been 

 spoken of. Few of the surveyors round London are ignorant of this 

 troublesome property of clay, and they have in consequence com- 

 monly adopted some means which they have considered suitable for 

 keeping the clay in its proper position beneath the metalling. For 

 this purpose they have resorted at different times and different places 

 to some one or other of the following expedients — large flat stones, 

 broken bricks, bushes, ur tin eliippings. All these have been tried 

 with various degrees of success, but with the exception of the broken 

 bricks they are all either highly objectionable or perfectly useless. 

 The flat stones become unevenly bedded in the clay, prevent the 

 metalling from setting and binding, and cause the road to wear in a 

 very irregular manner. The bushes are worse than useless, for they 

 cause the road to be spongy and elastic while they continue to retain 

 the least vestige of vegetable life, and when thev decay and rot, the 

 clay becomes Kneaded and worked into them with the greatest fa- 

 cility. The tin clappings are of course well calculated, by the sharp- 

 ness and hardness of their edges, for working and cutting into the 

 clay, so that they actually do harm, and increase the evil they are de- 

 signed to prevent. Of the several expedients, therefore, which hive 

 been mentioned, the broken bricks alone are entitled to any favourable 

 attention. This substance being of a dry absorbent nature will in 

 some degree correct the unctuous and slippery nature of the clay, 

 tend to keep it at rest, and prevent it from working up so rapidly. 

 It is evident, however, that this effect of broken bricks will be only 

 temporary, for they cannot resist the repeated saturation of the clay, 

 which must at length cause the latter to rise up through any thickness 

 of brokerr bricks which may have been laid mi. There are probably 

 some of our readers who maybe sceptical as to the employment of 

 such a substance .is bushes for the foundation of roads at the present 

 day, and in 1 lie immediate neighbourhood of the metropolis. Such 

 barbarous expedients, however, are by no means rare, and as an in- 

 stance, we may mention tin' well known cemeteries on the south side 

 of London at Norwood and Nunhead Hill. The approaches to these 

 cemeteries and the paths mule through them, have been executed 

 according to the must approved system ol bushing, as it is called; 

 that is to say, upon a foundation composed of boshes covered 1 vi 1 

 with a huge, depth of clean sharp angular flints, without one particle 

 of binding matter to unite them together. We need not say bow 

 much cheaper anil more effectual it would have been to lay a concrete 

 foundation, which might have been done for Is. per square yard, and 

 to cover it over with six inches of metalling abuut half the thickness 

 w hich is laid on at present. When a concrete foundation has been 

 mice, formed for a road, the subsoil is effectually cut oil' and for ever 

 prevented from rising. Then fore it is of little consequence whether 

 the top covering be rather clayey in its character, or whether it con- 

 tain only just sufficient clayey admixture to make it hind well to- 

 gether. In the woist case, that is, where it contains too much clay, 

 the latter will soon work through the stones up to the surface, and 

 during wet weather may be scraped off* in the shape of mud. Where 

 the concrete, however, is nut interposed between the subsoil and the 

 metalling, all the labour that can be bestowed in selecting good mite- 

 rials and in reducing them to a clean state, is absolutely thrown away, 

 for it will all be entirely counteracted by the rising of the subsoil, 

 which it must be. remembered is quite inexhaustible, and will con- 

 tinue to rise through sui cessivelaj ers of gravel or broken stone, however 

 thick they may be. Even in the best macadamized roads in London 

 there is more mischief done by the working up of the clay, and by 



the consequent grinding of the stones together than by any other 

 cause. The grinding action on a soft subsoil, is at least double what 

 it would be on a hard foundation of concrete. 



Materials proper for making Concrete. 



There is no part of the country which is destitute of materials fit 

 for this purpose. Amongst them may be mentioned any kind of pit, 

 river, or sea-side gravel, any kind of granite, sandstone, or limestone, 

 broken bricks, fragments of pottery, oyster shells, and in fact every 

 description of hard mineral substance. Should none of these be 

 easilv procurable, a very good substance for concrete may be made by- 

 burning clay in open heaps with any description of refuse coal. Clay 

 burnt in this maimer costs about 2s. 6d. to 3s. per cubic yard, mea- 

 sured in the heap when burnt, and will be found a very good substi- 

 tute for stone, when the latter cannot be procured for making con- 

 crete. 



Sand is an important ingredient which should never be neglected, 

 but there are many substances which will answer equally well for 

 mixing with the lime in concrete. Where sand, properly so called, 

 is made use of, it should be clean, sharp, and not too fine, should feel 

 gritty when rubbed on the palm of the hand, and should not soil the 

 fingers, otherwise we may be sure it contains clay or loam, or some 

 other substance which will injure the concrete. Amongst the sub- 

 stitutes for common sand may be mentioned the scales of iron, 

 pounded iron ore after roasting, brick or tile dust, road drift, or 

 pounded cinders. Any kind of clean sand may be used, whether 

 from the sea shore, from pits, or from the beds of rivers. The 

 Th noes ballast, as commonly used for concrete in London, contains 

 about the pioper proportion of sand, namely, about one of sand to 

 three or four of stones, conceiving all that to be sand which will pass 

 through a sieve with wires one-eighth of an inch apart. 



The varieties of lime, or what is the same thing, of the limestones 

 which yield a lime proper for concrete or mortar, arc so well known 

 in this country, as not to require enumeration. 



There is scarcely a locality in England wdiere lime of some kind or 

 other cannot he procured it the distance of a few miles; and al- 

 though these limes are widely different in their quality, some being 

 much purer and weaker than others, it may be taken as a genera] rub: 

 that the same quantity of sand which it would be pruper to mix with 

 the lime for making good mortar, will be just that same quantity which 

 should be used with the same lime in making concrete. Thus the 

 common white chalk lime, which if properly burnt — as it seldom is — 

 will take 3 or 3* sand, may be used with the same proportion in 

 concrete, while the grey chalk lime should not be used with more 

 than '2 or 2| sand. The common chalk lime should never be used for 

 concrete where it is subject to water or even to moisture in the 

 ground ; but the grey chalk lime may be used with perfect confidence 

 in any situation, however damp. Specifications for concrete should 

 always direct that the lime is to be ground into powder, otherwise it 

 will net go nearly so far in the concrete, as innumerable small frag- 

 ments will remain unslacked. This is of the highest importance, and 

 should never be neglected, because no care, however great, bestowed 

 upon tin' slacking, will so effectually bring out the virtue of the lime 

 as when il is slacked in the state of powder. 



The proportions which we would recommend for concrete in the 

 neighbourhood of London, are the following: — 



1 part bv measure of pounded quick lime, burnt from the lower or 



grey chalk of Dorking, Merstbam, or Hailing. 



2 to 24 parts by measure of clean sharp sand, road drilt, or other 



suitable material, as already described, 

 ti to S parts by measure of gravel or broken stones, &c, none larger 

 than 'J 1 ! inches in its largest dimension. 



Where the river gravel is used, and where it contains a sufficient 



quantity of sand, it may be mixed with lime in the proportion of 



part of lime to 8 or It) of gravel. 



The best way to make the concrete for the foundation of a road, is 

 to spread a stratum of gravel mixed with its proper proportion it 

 sand to the depth of about six inches. This stratum should be 

 formed across the road for the width of about four feet, and a cover- 

 ing of ground lime spread evenly over it about two-thirds of an inch 

 in thickness. Three or four men should then turn the stratum ol 

 gravel and lime several times over, piling into heaps and again 

 spreading it so as thoroughly to diffuse the lime. Lastly, it should be 

 formed into a ridge about two feet high, and the water added only in 

 sufficient quantity to mix with it by degrees, so as to form a thick 

 still' paste, in which every particle is just moist. In this state it 

 should be spread and smoothed to the required depth, which for roads 

 in the neighbourhood of London should be not less than six inches. 



