Determination of Seismic Design Response Spectrum Based on Imai & Tonouchi All Soil Model
1. Input Parameters:
'Imai & Tonouchi All Soil Model' requires information of the thickness, and SPT-N for soil layers in each Borehole.
I. Hazard Design Factor (Z) in term of 'g':
II. Annual Probability of Exceedance and Probability Factor (kp):
Note:
The minimum value of the seismic hazard factor (Z) has been set at 0.08 as stipulated in the 2018 amendments to the national seismic hazard maps in the Australian Standard for seismic actions AS1170.4:2007 (Amdt 2018). This intensity of ground shaking for a notional return period of 500 years can be shown to be consistent with 2/3 of the background seismic hazard in low to moderate seismicity conditions for a return period of 2500 years as derived from a global survey of seismicity information in intraplate regions. The implied kp value for a return period of 2500 years is accordingly equal to 1.5 (as is reciprocal of 2/3).
III. Thickness and SPT-N Values of each Borehole:
(presented in next page)
Note:
D1, D2 ..D20 refer to column nos. 1-20 for inputting the thicknesses (m) of the soil layers whereas N1, N2 ..N20 refer to the input of the SPT-N (blows/300 mm) values.
Table can be filled manually or by performing copy (CTRL+C) and paste (CTRL+V) of data from Excel spreadsheet.
If all entries to a particular “thickness” column have been left blank, the program will automatically fill in a default thickness of 1.5 m.
SPT-N values that have been taken from the testing of very stiff materials have to be corrected to the penetration depth of 300 mm. For example, an SPT-N value of 50 for the penetration depth of 100 mm should be corrected to 150.
Avoid including too many borelogs from too closely spaced boreholes into one analysis as doing this would not necessarily result in obtaining an accurate estimate of the site period.
Must also avoid including borelogs taken from a very large area featuring systematic spatial variation in geology within the area. The maximum number of borelogs to be included into one analysis is capped at 20.
This program which is based on making use of blow counts taken from standard penetration tests (SPT) as input information cannot be used to identify Class Ae (hard rock) sites nor Class Be (rock) sites both of which have much higher shear wave velocity than is measurable by SPT. Sites class Ae, Be and Ce are taken to be site class Ce by default when this program is used.
A borelog indicating exceptional conditions compared to other borelogs taken from the same site is a call for undertaking further investigations to determine the spatial extent of the anomalies. More borelogs need to be taken to map out the site geology to determine the subsoil profile. There is an option to employ geophones to take site natural period on a few locations to help understand the subsoil conditions better. Alternatively, take the more onerous site class for design purposes. With the construction of an important structure in onerous soil conditions, it is worth considering the option of undertaking a more detailed investigation to generate a site-specific response spectrum to override the code specified response spectrum model.
SPT-N Values and Soil Description
Sand1
Clay2
Description
SPT-N Value (blows/300 mm)
Description
SPT-N Value (blows/300 mm)
Very Loose
less than 6
Very Soft
0-2
Loose Dry
6-10
Soft
2-5
Medium Dense
10-30
Firm
5-10
Dense
30-50
Stiff
10-20
Very Dense
>50
Very Stiff
20-40
Gravel
>30
Hard
>40
1AS1170.4:2007. 2BG Look, 2014.
Borehole 1
Borehole 2
Borehole 3
Borehole 4
Borehole 5
Borehole 6
Borehole 7
Borehole 8
Borehole 9
Borehole 10
Borehole 11
Borehole 12
Borehole 13
Borehole 14
Borehole 15
Borehole 16
Borehole 17
Borehole 18
Borehole 19
Borehole 20
D1
N1
D2
N2
D3
N3
D4
N4
D5
N5
D6
N6
D7
N7
D8
N8
D9
N9
D10
N10
D11
N11
D12
N12
D13
N13
D14
N14
D15
N15
D16
N16
D17
N17
D18
N18
D19
N19
D20
N20
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
IV. Energy Ratio (N60/Nmeasured):
Note: Energy ratio is the conversion factor for converting the measured N values to corrected N values corresponding to 60% energy efficiency. A value of 1 is suggested for the case of Australia (Sabatini et al., 2002).
2. Results: Soil Characteristics and Design Response Spectrum
I. Soil Profile with SPT-N Value:
Figure 1: SPT-N Profile
Table 1: Thickness (m) and SPT-N (blows/300mm) Values of Soil Layers of Different Boreholes Note: D1, D2, ..D20 and N1, N2, ..N20 columns in the table are thickness and SPT-N values of soil layers of the respective boreholes.
(This page is left blank because there are no inputs defined for borehole numbers 11-20.)
II. Average Shear Wave Velocity (Vs) and Natural Time Period (Ts) of Soil:
- Imai & Tonouchi all soil model is used to calculate the average shear wave velocity.
$$ {V_s = \frac{H_s}{ \sum^n_{i=1} \dfrac{d_i}{93.7 \times N_{60}^{0.31}} }}\tag{1}$$
$$ {T_s = \frac{4 \times H_s}{V_s}}\tag{2}$$
Where:
Hs = total depth of soil,
n = number of soil layers,
di = thickness of soil layer 'i',
N60i = corrected SPT-N value (corresponding to 60% energy efficiency) of soil layer 'i'
Table 2: Average Shear Wave Velocity and Site Period of Soil of each Boreholes
Borehole No.
Average Shear Wave Velocity (m/sec)
Site Period (sec)
Table 3: Site Characteristics and Ground Type
III. Seismic Design Response Spectrum:
a. Response Spectral Acceleration (RSA) and Acceleration Displacement Response Spectrum (ADRS) Diagram:
Figure 2: RSA Diagram
Figure 3: ADRS Diagram
RSA is Response Spectral Acceleration.
ADRS diagram is Acceleration-Displacement Response Spectrum diagram.
Table 4: Response Spectral Acceleration (RSA) and Response Spectral Displacement (RSD) of soil
T (sec)
RSA-soil (g)
RSD-soil (mm)
3. References:
Standard, A. (2007). AS 1170.4-2007 (Amdt 2018).Structural Design Actions, Part 4: Earthquake Actions in Australia..
Look, B. G. (2014). Handbook of geotechnical investigation and design tables. CRC Press.
Sabatini, P.J., Bachus, R. C., Mayne, P. W., Schneider, James A., Zettler, T. E. (2002). Geotechnical Engineering Circular No. 5: Evaluation of Soil and Rock Properties, Report no: FHWA IF-02-034. Federal Highway Administration, Washington, DC United States.
Wair, B. R., DeJong, J. T., & Shantz, T. (2012). Guidelines for estimation of shear wave velocity profiles. Pacific Earthquake Engineering Research Center.
Disclaimer
The authors assume no responsibility for any injury, damage, liability, negligence and/or otherwise to any individual or property from the use or application of any of the methods, products, instructions, or ideas contained in the material herein.