Such dams were usually built for heads of 6 to 16 feet, though the Connecticut River Dam at Holyoke had a height of 30 feet. This dam was gradually undermined by...die:erosive action of the water falling olt.._as it had Better heads were often produced by a, dam at the head of some rapid, with ,a race leading down stream.
-- ' Timber dams were lit a. at variety of forms, much depending on` . the site and the material available. A eomilichi ' type was the crib or structure with the pockets filled with stone, having a broad base, sheeted up-stream and banked with earth on the down-stream side; the spillwayandapron: dropping in, steps. Piling. was sometimes driyen, capped and sheeted 'to ,an apex like the roof of ' arhoitse, the spaces between piling being filled with stone and the'tap-stream toe banked With earth. Frame dams were Tulsa, built, a Sheeting of timber being supported by triangular frames resting on i. amdsille ;the) up- stream' slope 'was flat, usaidlyttwb'trilone, the Me being covered by a,ibatilvibf (Leg' dams were tribbed up and , OfitetER, I: aptdreitin ulike i franivdrans 7 land &Many/ere •bisitt/ of dpilinglandliheeting:alone; driven into the river bed.
!dams' hive • well. : *beim . the hertgitchas not been excessive, 'the 'upper-parts ; been! easilY;•:rmiewed' 'when; decayed or worn. out) !been 'exempt from failureliwith disastrotet: results. ' Most of the failuites!'oti- record Intik been'due! to lack of a proper apron, ' running -oiler gradually cutting! away:the; fottndation. ...They will always 'be ;used in titnber regions for manu facturing Inmber. and with - sliticew4s for: driv ingi logs. No material eqtials timber ing water shock and .vibratiori,.aridlits will be iiref erred by Many. . Under miatc4 !thither is evalasting,rand it will. always; ma terialito:the 'engineer. i!.; .: •', •• • -IA !large !'proportion of • ethe• small water powers midi:with the de velopment of the turbine higher heads have been 'Sought.: The 'electric zgerienator 'arid trans mission has greatly enlarged the water-poWer field,,_and _this has accentuated the: tendency toward higher dams and larger units. In recent years Portland cement has become cheap and reliable, .and timber less available;. and concrete dame with higher heads are coming i se.
' • Brushwood dams are useful on 1 streams hailing a soft-!bottom. Sairigs yen tr - s :of considerable size-areJnied for the larger co •Struefroits, the branches being laid up-strea . ' t lo and gravelly soil is thrown on to fill' 4 ts"slks. These dams are carried up in several alternate courses of brush and gravel, and are either planked on the face or covered !notable examples of the timber dam are: .The NeW?Hartford dam, 232 feet long and 1 feet high, witlilla width at the bottom of 68 feet.- • The timbers -41ap,.,9 to 12 inches thick and
6 feet apart, the spaces It?.tweeli,being filled with shone. oth faces are phatkelf"fth oak, and the sitle-leA a long graveled earth lope: This dam has stood a freshet of water 10 feet deep flowing over the top of the dam.
The Columbia Dam across the Susquehanna :Meer. is;..62491 feet long and feet high. It is 16. feet. wide at the topaazid,,304etrt at the bot tom. It has withstood many severe ice freshets since it was built, in '... !: • i ' 7 The' Canyon !Perry Dim is 485 feet long and 29 feet- high.••I Itsis built of tim ber cribs' 'filled with 'stone! The' 'downstream face is Constructed in two long Slopes, the lower one flatter. These' slopes 'take the place' Of 'an apron.' The up-stream slOpe •ia plankedimid protected by a 'gravel1fili covered with 1 riprac.
Masonry 'Dams.— Madonry . darns it eLlienit after three general types: ' (1) gravity dams, in which the' weight of the Material ofi which-the dam is built is depended on to'hicild the pressure of the 'impounciett'witer; !arch dams, in which 'theiprinciplei Pt the larch is opposed to 'the water pre4itire; !(3) 'hollow dams, in -which-, the water! the , dim is supported 1 !series [of - buttiesite& In! the third dais belong the! ((multiple archf .thants, in which the ismall arches are sprung from the buttresses. - .'-' - ' ''' -; '.! !I - 'The first 'requisite tion Ilk an expleirationtri-ithe!sitet !Whereithere iscim• oiarth the locatidn rand gad! itai of :the: Inecesdady rock formation; and exposed f rock is searched . for existing seams. . After diversion works to turn Aside the stream while the dam -is -being built, -a trench is dig down tor the rock, large- enough to , accommodate, the base of the dam, And room • for'carrying•on the. work. The rock surface its , ckaned. of . earth; with. wire brooms ; large seams ..are filled, *WI. concrete., -and stnall. 'seams' are grouted under pressure:-1 up' to -MO- pounds, per Aquart inch. iGroeves,'are cut in ahe rock for the toe -of the dant arid-if the structure is..a high one, additional qtricreives: are cut: to ensure 'against the, diun's!slidmg- biz its foundation.-, • The former practice of : building: the , . two faces of tut stone and filling betWeen with rub 1.ble has -beeti • largely:: skipersedetl by cyclopean masotrry,: in ;which great Masses of rock are placed by the derrick, and the filling !out to • the faced r done • with • concrete. With ,present-day _machinery- cytIopean .masonry- can be laid -foils-tint:es as - fast. as, rubble ImasonrY. -Another: development in construction is the -:reduction of the proportion of -stone used in the -.Concrete. Where formerly -the stone: amottated to 50-per Cent of it:is now only about :25 per cent.. , • .