The in-situ measurement of the seismic shear wave velocity in the top 30-m of the earth is referred to as a Vs30 survey by Finn and Wightman (2003) and the National Building Code for Canada – 2005 (NBCC-2005).
The measurement of shear wave velocities in soils is described by Hoar & Stokoe (1978) in ASTM STP654. Improvements in technology and field procedure are described by Crice (2002). Shear wave velocity surveys are performed in general conformance with the above referenced documents. It should be noted that there is no ASTM or CSA standard available yet that describes the methodology to be used to meet the requirements of the NBCC-2005 for seismic characterizations of the top 30-m of the earth. The methodology has been developed in consideration of the above documents, the requirements of NBCC-2005 and by consultation with experts at the Geological Survey of Canada.
A Geostuff model BHG-3 three-component geophone package is normally used for the downhole seismic recordings (see Figure 1 below). The geophones are mounted with their vibration axis in an orthogonal x-y-z geometry to help monitor the wave motion orientation in the earth. The x and y components in the probe are oriented at right angles to each other and perpendicular to the probe axis, while the z-component geophone is mounted with its vibration axis parallel to the probe axis. Normally these surveys are recorded in approximately vertical boreholes so the x and y components are then approximately horizontal and the z-component is approximately vertical. A motorized bow-spring provides strong mechanical clamping of the probe to the borehole wall. A fluxgate magnetometer provides a consistent orientation for the geophone package with respect to the local geomagnetic field using a servo-motor with feedback from the y-component. The y-component in the downhole package is then oriented approximately parallel to the geomagnetic north. SH-waves are generated and recorded for the probe at 0.5-m intervals.
Figure 1. Photograph of the 3-component clamping geophone probe.
| NBCC-2005 Site Classification for Seismic Site Response (Finn & Wightman, 2003) | ||||
|---|---|---|---|---|
| Site Class | Soil Profile Name | Average Properties in Top 30-m | ||
|
Shear Wave Velocity Vs(m/s) |
Standard Penetration Resistance N60 |
Undrained Shear Strength Su |
||
| A | Hard rock | Vs > 1500 | n/a | n/a |
| B | Rock | 760 < Vs ≤ 1500 | n/a | n/a |
| C | Very dense soil and soft rock |
360 < Vs > 760 | N60 > 50 | Su > 100 kPa |
| D | Stiff soil | 180 < Vs < 360 | 15 ≤ N60 ≤ 50 | 50 < Su ≤ 100 kPa |
| E | Soft soil | Vs < 180 | N60 ≤ 15 | Su < 50 kPa |
| E | Any profile with more than 3-m of soil with the following:
|
|||
| F | Others | Site specific evaluation required |
||
Figure 2. NBCC-2005 Site Classification for Seismic Site Response (Finn & Wightman, 2003).
Figure 3. Schematic plan and section view of the 3 component geophone.
Figure 4. Apparent shear wave velocity vs. depth profile.
Figure 5a. SH-wave velocity-depth model. 5b. Velocity depth profile derived from the model in 5a.
Figure 6. Borehole to borehole & borehole to surface seismic response.
References:
Adams, J and S. Halachuk. 2004. A review of NBCC 2005 Seismic Hazard Results for Canada – the interface to the ground and prognosis for urban risk mitigation. Canadian Geotechnical Conference Quebec City.
Crice, D. 2002. Borehole Shear-Wave Surveys for Engineering Site Investigations. Geostuff Inc. 14p.
Finn, W.D. and A. Wightman, 2003. Ground motion amplification factors for the proposed 2005 edition for the National Building Code of Canada. Canadian Journal of Civil Engineering v30, p272-278.
Hoar, R.J. and Stokoe, K.H. II. 1978. Generation and Measurement of Shear Waves In Situ. Dynamic Geotechnical Testing. ASTM STP 654 pp3 – 29.
Nakamura, Y. 1997. Seismic Vulnerability Indices for Ground and Structures using Microtremor. World Congress on Railway Research, Florence - Italy, 16-19 Nov.1997 pages 1 – 9.