Soil Dynamics (lect 4-5)

Soil Dynamics (lect 4-5)

1. Sources of dynamic loads

   • Monotonic
     • Explosions
     • Load tests (pile testing)
     • Dynamic Compaction - drop hammer
     • Creep - constant load
     • One direction only/ Soil behaviour depends on strain and rapidness of its application
   • Cyclic
     • Earthquakes
     • Oscillating structures (e.g. wind)
     • Construction activity (vibrations)
     • Machine foundations (virbations affect hospital sensitive apparatus)
     • Site investigation (e.g. shear wave)

2. Types of waves

   • Body waves
     • Pressure waves
       • Displacements are PARALLEL to the direction of motion
     • Shear waves
       • Displacements PERPENDICULAR to the direction of motion
   • Surface waves
     • Less energy than a body wave/travel at the interface between materials/ Translational or circulatory movement RAYLEIGH (circ.)/ LOVE (transit.)
       • If soil is homogenous - only P, S and Rayleigh waves
       • Transverse waves -Love- if 2 or more strata (e.g. soil/air)

3. Transmission and reflection at interface

   • At interface a wave can either REFLECT or CONTINUE (e.g. when at ground surface the wave is obligated to reflect - LOVE)
   • Transmission from a POINT SOURCE (e.g. machine vibrations) spread outwards

4. Vibrations limits

   • 'Peak particle velocity' measurements/ velocities do not affect structures/high soil strains affect the structures/ Serviceability of structures/ 5-10mm/s maxium ppV for humans
     • Pile Driving
       • up to 2/3 of energy can be converted in Rayleigh waves (e.g. energy of pile hammer) EC for disturbance limits!
     • Railway Vibration
       • Permanent!/Mitigated by trenches of suitable impendance material fill
     • Blast loading
       • Very brief (e.g. demolition, mininig,etc.) Pressure wave followed by a longer duration smaller amplitude negative pressure pulse

5. Superposition

   • Two waves travelling in a medium will SUPERIMPOSE (sum contribution of both)
     • Meet up at the l/4 interface reflected + incident
       • Frequency of soil formula/ dynamic amplification depends on the Frq. of material and the one imposed