【正文】 in 5ft (~m) increments as the dam height increased.A 25yd3 (19m3) dualfeed hopper was designed by Rotec to allow either conveyor system to operate at any capacity at any time without dead storage in the hopper and without segregation. Each of the systems terminated in two 50ft () swingers(可擺動(dòng)的尾管), one for RCC and one for conventional concrete. The belts were controlled by a single operator at the discharge control station.At Middle Fork Dam, a 200yd3(152m3/h) Ross batch plant and drum mixers for RCC were situated upstream of the dam on fill pacted behind a sheet pile tieback wall. For the lower elevations of the dam, a gob hopper discharged RCC onto a stacker conveyor and then into a 36indiameter (), enclosed rock ladder. It consisted of a steel pipe with semicircular steps welded on the inside which kept the aggregate from segregating as the mix was chuted(斜道) down a maximum distance of about 50 ft ( m) to the construction surface.As RCC placement progressed, sections of the rock ladder were removed and the vertical drop decreased until the conveyor was discharging directly onto the dam. For the uppermost elevations, three sections of conveyor were used in a scissored (剪刀狀的)arrangement. The final stacker conveyor was placed atop a tower made of scaffolding（腳手架）. Frontend loaders were used to catch or pick up the RCC from the chute and then to spread the mix for most of the lifts. Near the crest, the conveyor discharged into 10 yd3 ( m3) dump trucks. CompactionOne of the important differences between RCC and conventional mass concrete is in the way the materials bee densified. Mass concrete is densified, or consolidated, mainly by the influence of gravity with some help from the reduced friction between particles that results from externally imposed vibration. RCC is forcedensified with external highenergy paction or tamping. Because there is no intentionally entrained air and because the large aggregate is pacted into its optimum orientation, significantly higher densities can be achieved with RCC than with conventional concrete. As the degree of density required increases, however, the paction effort needed to achieve it grows exponentially.The optimum moisture content for paction is generally in the range of to 7 percent by dry weight of aggregate. Determining the optimum depends on the grading and maximumsize aggregate used in the mix, temperature, humidity, and the plasticity of the fines. Cement content has little effect on the moisture needed for best paction results. Compaction should be done within 10 min of spreading and within 30 min of mixing, although full paction can be achieved at longer intervals with wetter or setretarded mixes. RCC is pacted in lanes. An 8 to 20in (200 to 510mm) external strip of each lane should be left undisturbed and pacted after the adjacent lane is placed to bond the two together.Compaction efficiency is affected by the grading and shape of aggregates, the RCC mix design, and the type of paction equipment. The paction effect depends on the mechanical behavior of the mix as lift thickness and/or maximum particle size in the mix during paction and on the types of forces exerted. Lowfrequency, highamplitude vibration has been considered to be the most effective as lift thickness and/or maximum particle size in the mix gradation increase. Research in Japan has indicated that highfrequency, highamplitude paction may be more suitable for the kinds of mixes used in RCD projects. Each RCC mixture behaves differently under paction, so field tests are the best way to determine the number of passes(碾壓遍數(shù)) and other parameters.The most influential factors in achieving densification with vibratory rollers are static force, centrifugal force, the roller39。s vibrating mass and its acceleration. Amplitude, frequency, and vibrating mass are factors in determining centrifugal force.Because of a high degree of uniformity in design parameters for rollers, the unit weight per unit drum width of the roller enpasses such factors as static loading, vibrating mass, and roller dimensions. The unit weight per unit of drum width is expressed as pounds per linear inch (PLI) (or kg/cm) of drum width. The PLI can be specified along with amplitude, frequency, and rolling speed. Remended guidelines for RCC construction include:Average PLI 115 min for pacted lifts to 6 in ( kg/cm for lifts to 15mm)150 min for pacted lifts over 6 in (27 kg/cm for lifts over 15 mm)Amplitude to in ( to mm)Frequency Not less than 1700 vibrations per minute ( Hz)Rolling speed Not greater than 2 mi/h ( km/h)Four to eight passes of a self propelled, doubledrum, 10ton ()vibratory roller have generally been required. The Japanese have required as many as 12 passes with 7ton () doubledrum vibratory rollers followed by 6 passes with rubbertired rollers. In all cases the trend is toward using heavier vibratory rollers14 and 15ton ( ) machines. Backup rollers are generally required due to breakdowns caused by the near continual use of pactors on RCC projects.At Upper Stillwater Dam, 15ton () Dynapac CC50A asphalt pactors were used for the final season of placement after maintenance problems developed with the contractor39。s original machines. The Dynapac machines were operated at an amplitude of in ( mm) and a frequency of 2400 VPM. The dynamic paction force of the drum was 429 lb/in ( kg/cm) as pared to 400lb/in ( kg/cm) for the original machines. The Japanese have favored Bomag B2000 pactors for their RCD projects.Initially, 10 passes with a 10ton () ) doubledrum vibratory roller were required for each 2ft (~m) lift at Elk Creek. After placement of 156,000 yd3 (119,000 m3) during April and May of 1987, however, the Corps of Engineers Portland District discovered that the dozer action alone produced the required consolidation. Nuclear density tests(核子密度測(cè)試) showed that the extensive reworking of the wet, retarded mix (its Vebe t