Vibration monitoring

Vibrations

In simple words, a vibration is a rapid back and forth motion. Construction activities causes vibrations which generate different body and surface waves. These waves travel through the soil or rock medium, affecting adjacent structures and activities.

Natural vibrations can be generated by earthquakes, landslides, avalanches, wind and others.

Some sources of construction vibrations are:

• Pile driving
• Sheet pile installation
• Compaction
• Excavation
• Blasting
• Tunneling
• Demolition
• Pavement applications
• Deep dynamic compaction

Therefore, vibratory pile driving, and vibratory compaction are characterized as steady-state vibrations. On the other hand, impact pile driving, deep dynamic compaction, and blasting are transient vibrations.

Vibrations can cause structural damage on buildings, such as cracking of foundation walls and masonry elements. Several cases are reported in literature concerning failure of railway embankments due to train induced ground vibrations (Pando et al. 2001). A unique failure of a road embankment occurred on Highway 94 in Michigan's Upper Peninsula during a deep reflection study. The seismic signals were generated by six 22-ton trucks and caused ground vibrations that liquefied the embankment soil, causing a flow slide and the collapse of a 91-m section of the embankment (Hryciw et al., 1990).

Vibration monitoring

Vibration monitoring can be implemented for:

• Risk Management
• Legal Defense
• Protection of sensitive structures
• Liquefaction
• Construction Control
• Compliance
• Community Relations
• Performance validation 
• Design verification
• Safety and failure warnings
• Problem recognition / mitigation
• Research projects
• Observational method

Vibration monitoring process

The first step for a successful monitoring process is to identify the key parameters of the problem under consideration. This will allow to plan the monitoring campaign; define the required instrumentation, the position of the measurement points, and the measurement schedule.

The next step is to install the monitoring system, check its functionality and perform a pre-construction condition survey. The pre-construction condition survey should be performed after excavation and dewatering in order to separate damage from construction activities and dynamic sources.

A pre-construction condition survey is a crucial to further assess the effect of the construction vibrations to the adjacent structures. It consists of the documentation of existing cracks and all other damage; analysis of the probable causes of existing damage; addressing the damage susceptibility of adjacent structures; determination of the mitigation measures of the effects on adjacent structures; and measurement of the background vibrations.

During the implementation of the monitoring campaign, the measurements are collected, processed and reported at predefined intervals. With the data in hand, it is important to assess the problem under consideration and evaluate the monitoring campaign.

The raw signal, measured in vibration monitoring, shows the instantaneous vibration velocity, which fluctuates positively and negatively about the zero point.

The most common parameter used in vibration monitoring is the Peak Particle Velocity (PPV), which is the maximum speed of a particular particle as it oscillates about a point of equilibrium. PPV indicates whether damage is likely to occur to surrounding structures, since the strain induced in the ground during vibration is proportional to the particle velocity.

Vibrations propagate through the ground predominantly by means of Rayleigh (surface) waves and secondarily by body (shear and compressional) waves. The amplitude of these waves diminishes with the distance from the source. The attenuation is attributed to the expansion of the wave front (geometrical attenuation), and to the energy dissipation within the soil itself (material damping).

Material damping is attributed to energy loss due to internal sliding of soil particles. It depends on soil type and increases linearly with frequency of vibration. Fluid motion in pores may also produce attenuation. Clays tend to exhibit higher damping than sandy soils. Wet sand attenuates less than dry sand because the combination of pore water and sand particles in wet sand does not subject compressional waves to as much attenuation by friction damping as does dry sand. Propagation of R-waves is moderately affected by the presence or absence of water. Frozen soil attenuates less than thawed soil (Amick, 1999).

Wave attenuation can be modelled with Wiss (1981) equation:

where:

• K is the velocity at one unit of distance
• D  is the distance from the vibration source
• n is the attenuation rate (or slope), ranging between 1 and 2

Wiss equation can also be found normalized, combining the effects of distance from the source and imposed energy posed by impact or blasts. The attenuation rate, n, is a pseudo-attenuation coefficient that accounts for both geometric and material damping. Woods and Jedele (1985), from field construction data, proposed n=1.5 for component soils that can be dig with a shovel (most sands, sandy clays, silty clays, gravel, silts, weathered rock) and n=1.1 for hard soils that need a pick to break up (dense compacted sand, dry consolidated clay, consolidated glacial till, some exposed rock).

An alternate approach to describe the attenuation of ground vibration amplitude with distance is with Bornitz equation:

where:

• w₁ and w₂ are vibration amplitudes at distances r₁ and r₂ from the source 
• γ is the geometric damping coefficient 
• a is the material damping coefficient

References

Amick, H. (1999). Frequency-dependent soil propagation model. In Optomechanical Engineering and Vibration Control (Vol. 3786, pp. 72-80). SPIE

California Department of Transportation, CALTRANS. (2020). Transportation and Construction Vibration Guidance Manual.

Hryciw, R. D., Vitton, S., & Thomann, T. G. (1990). Liquefaction and flow failure during seismic exploration. Journal of Geotechnical Engineering, 116(12), 1881-1899.

Pando, M. A., Olgun, C. G., & Martin II, J. R. (2001). Liquefaction potential of railway embankments. (2001). International Conferences on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics. 16.

Wiss, J. F. (1981). Construction vibrations: state-of-the-art. Journal of the Geotechnical Engineering Division, 107(2), 167-181.