tailieunhanh - Systematic Drilling and Blasting for Surface Excavations Part 12

Tham khảo tài liệu 'systematic drilling and blasting for surface excavations part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | EM 1110-2-3800 1 Mar 72 optical lever arms is usually fixed at some value between 25 and 75. The displacement seismograph is generally mounted on three leveling screws that rest on the ground or floor of a structure and the center of gravity of the instrument is above the level of the surface on which the instrument rests. A permanent trace of the ground displacement as a function of time is made on 2- to photographic paper traveling at a speed of 4 to 5 ips. The useful frequency range is from about 5 to 50 cps the dynamic recording range which is the ratio of maximum signal deflection to minimum readable deflection is about 20 and the maximum acceleration allowable is approximately g .4 5 46 For a sinusoidal vibration of frequency f the relation between peak acceleration a and peak displacement u is .2-2 a 4ir I u Thus the allowable range of displacement that can be recorded depends upon the frequency of the vibration. Displacement seismographs can be used to measure the peak particle velocity V of the ground motion. The maximum slope of the displacement-time record is the peak particle velocity. If sinusoidal vibrations are assumed the particle velocity can be calculated from V 2irfu 3 As damage criteria are usually based on particle velocity it is recommended that particle velocity be measured directly rather than be inferred from displacement or acceleration. Velocity seismographs for recording vibration from blasting consist of three mutually perpendicular coils free to move in a magnetic field. These are mounted in a box which may be buried in the ground or placed on the surface of the ground or floor of a structure. Associated with the seismometer box is another box containing amplifiers galvanometers a multiplechannel paper recorder and a d-c power supply. The sensitivity of these seismographs is adjustable in the range from 20 to in. of record motion per 1 ips ground motion and their useful frequency range is from 2 to 300 cps. The .