September i, 1892] 



NATURE 



415 



B^ 



ment, though this would be corrected by reference to the 

 chapter on the blood. 



Taking a general survey of the whole, we see that the 

 book is far from perfect. Few books are when they first 

 appear, and much that is faulty can be corrected in sub- 

 sequent editions. We must, however, congratulate Dr. 

 Griffiths on being the first to break new ground by pro- 

 ducing a work on the subject, as well as on the good 

 points that the book exhibits, and to which allusion has 

 already been made. 



W. D. H. 



THE DESIGN OF RETAINING WALLS AND 



RESERVOIR DAMS. 

 A Text- Book on Retaining Walls and Masonry Dams. 



By Prof. Mansfield Merriman, (New York : John 



Wiley and Sons, 1892.) 



I EFORE entering upon the investigation of retaining 

 walls and their design, the author devotes two chapters 

 to the consideration of earthwork slopes and the lateral 

 pressure of earth. Owing to the changeable condition of 

 earth under the influence of moisture, and the variable 

 nature of any stratum, it is impossible to obtain strictly 

 exact expressions for the forms of slopes of cuttings and 

 embankments, or definitely accurate values for the lateral 

 pressure of earth ; but, nevertheless, the formulae deduced 

 by the author from general principles are useful in serving 

 as a guide to correctness of design. It is indicated that 

 theoretically an earthwork slope should be curved, be- 

 coming flatter towards the base ; and though a straight 

 slope is always adopted for cuttings and embankments, 

 the curved form left by slips is somewhat in accord with 

 this theory. The inclination of slopes must indeed depend 

 on the nature of the soil, and must be flatter in made 

 ground than in cuttings ; whilst efficient drainage and 

 protection of the surface of the slopes from the weather 

 are equally important for ensuring stability. 



The pressure of earth is the basis of all theoretical 

 principles relating to retaining walls, and it has formed 

 the subject of numerous experimental investigations in 

 England and on the Continent which might have been 

 advantageously referred to in this book. The author 

 adopts the view that the pressure is normal to the back of 

 the wall ; but as this theory is not universally accepted, 

 he has also obtained a formula for inclined pressure. A j 

 retaining wall may fail by sliding or rotation, and the j 

 masonry is assumed to be laid dry, owing to the uncertain j 

 amount of cohesion in mortar joints. In practice, how- | 

 ever, retaining walls of any height are built with cement i 

 mortar ; and sliding occurs at the base, or even some- 

 times on detached slippery surfaces of clay below the ', 

 base ; whilst rotation is due to excessive pressure on i 

 yielding foundations at the front of the wall. Stability 

 largely depends upon the nature of the foundation and 

 the backing behind the wall. A clay foundation is far 

 less trustworthy than gravel, and sliding is most effectually 

 prevented in slippery soils by carrying down the founda- 

 tions well below the surface ; whilst careless backing up 

 with bad materials, not brought up in their layers, may 

 push over the wall. Efficient drainage, moreover, at the 

 back of the wall, and outlets for water at intervals 

 NO. II 92, VOL. 46] 



through the wall to prevent its accumulation behind, are 

 almost as important considerations as the design of the 

 wall. A wall leaning over backwards is shown to be the 

 most economical ; but though this form might be adopted 

 for building against an embankment, it would not be con- 

 venient for a wall built in a timbered trench to retain the 

 side of a cutting. The four chapters relating to earthwork 

 and retaining walls, which comprise the main portion of 

 the book, will be very useful for practical engineers who 

 desire to extend their theoretical knowledge on these 

 subjects ; but students should bear in mind that an 

 almost exclusive treatment of the theoretical aspect of 

 these questions must be supplemented in actual design 

 by practical experience. 



The theory of the strains on masonry dams, considered 

 in the concluding chapter, is more precise, owing to the 

 exactness of our knowledge of the laws of water pressure 

 as compared with the uncertain and variable pressure of 

 earth. The well-known condition of stability, that the 

 lines of resultant pressures, with the reservoir empty and 

 full, should fall within the middle third of the cross 

 section, is explained, as well as the uncertainty which 

 exists as to the actual distribution of the pressures 

 throughout the dam. The lines of resultant pressures for 

 any given section are easily obtained graphically, for the 

 line of pressure with the reservoir empty is the locus of 

 the centres of gravity of the sections above a series of 

 base lines taken down the dam, and the actual pressure is 

 the weight of these successive sections ; whilst the line 

 of pressure with the reservoir full is the modification pro- 

 duced in the former line by the addition of the water 

 pressure at the successive depths. The theoretical 

 section given on page 1 10 resembles the section of the 

 Furens dam in France, the highest masonry dam hitherto 

 erected, the form of which was determined by elaborate 

 analytical calculations. The principles laid down con- 

 cerning masonry dams require to be supplemented by 

 two practical considerations, namely, that high masonry 

 dams must be founded on solid rock to secure them 

 against undermining and settlement, which would be 

 fatal to their stability ; and that their inner face should be 

 coated with an inpervious material, to prevent the in- 

 filtration which otherwise takes place through their joints 

 at great depths. In taking the pressure due to waves on 

 the top three or four feet of the dam below the water as 

 equivalent to the greatest observed pressure exerted by 

 waves on the sea-coast, the author far exceeds the 

 probable limit; for ocean waves, owing to the great 

 extent of the exposure and the depth, are impelled with a 

 much greater force than the waves of a comparatively 

 small and sheltered reservoir. The additional strength 

 given to a dam by arching it towards the reservoir is very 

 properly neglected in the calculation of its stability, for 

 besides being difficult to estimate precisely, this increase 

 in strength is inappreciable in a long dam, and even in 

 the short Furens dam the arched form was merely 

 regarded as an extra safeguard. 



The book is clearly and concisely written ; it is illus- 

 trated by numerous diagrams in the text ; and problems 

 to be worked out are given at the end of most of the 

 articles, each of which deals with a subject under a 

 special heading. 



