ABSTRACT:
Palmerston North City Council (PNCC) have identified a number of earthquake-prone buildings requiring seismic strengthening located along the ‘Priority Route’ in the City Centre. Seismic assessment (i.e. estimation of %NBS) and the subsequent design of strengthening schemes is heavily influenced by the Seismic Site Classification (most commonly NZS1170.5:2004 Site Sub-Soil Class). The subject area has very little available ground investigation data and structural seismic assessments are often based on crude assumptions of the ground conditions.
This paper presents the findings of the initial stage of a project currently being undertaken collaboratively between PNCC and Miyamoto International NZ Ltd. (Miyamoto) with the main objective to provide PNCC, building owners and practitioners with additional information, at a preliminary level, to assist with seismic assessment and / or seismic strengthening, repair or maintenance works to building along the Priority Route.
1. INTRODUCTION
Palmerston North City Council (PNCC) have identified a number of earthquake-prone buildings along the ‘Priority Route’ in the City Centre (Figure 1). PNCC encourage the active protection and management of the natural and cultural heritage of Palmerston North City, a significant portion of which is directly related to Heritage buildings. PNCC offers targeted financial assistance to property owners and other relevant parties to address the conditions imposed on the owners of private properties. Miyamoto International NZ Ltd (Miyamoto) was commissioned by PNCC initially to undertake high-level structural assessments for the buildings and subsequently to undertake a geotechnical assessment for the area encompassing the priority route.
As part of the commission we have undertaken a Multi-channel Analysis of Surface Waves (MASW) geophysical survey of the entire Priority Route and collated available information from the surrounding area including information held in the council archives.
This paper summarises the findings of our investigation and presents a discussion on the Seismic Site Classification for the area, alongside a number of useful parameters suitable for preliminary assessment and design.
The MASW data will be uploaded to the New Zealand Geotechnical Database.
Figure 1: Site and MASW Survey Location Plan
2. Geology
Palmerston North is located to the eastern extent of the Whanganui sedimentary basin, as shown in Figure 2.
Figure 2: Whanganui Basin (extract from P. Balance, 2009)
The local GNS Geological Map (Lee & Begg, 2002) shows the surface geology of the Priority Route to be Late Pleistocene river deposits, classified as “poorly to moderately sorted gravel with minor sand or silt underlying terraces (unit Q2a)”. The surface geology just south of the Priority Route is mapped as Holocene river deposits, classified as “alluvial gravel, sand, silt, mud and clay with local peat; includes modern river beds (unit Q1a)”. An extract of the geological map is shown in Figure 3.
Within the local area, the depth to the ‘actual’ greywacke bedrock may be in the order of 1000 m or greater distance from ground level, however, ‘effective’ or engineering bedrock with shear wave velocity (Vs) values greater than 700 or 800 m/s can be considered at significantly shallower depths (as discussed later in this paper).
Figure 3: Geological Map Extract (Lee & Begg, 2002)
3. GROUND INVESTIGATION
As part of the initial phase of the investigation and to guide the following phases, available existing ground investigation information was collated, including a rigorous search of the Council archives (refer to Section 4.2).
A MASW geophysical survey was selected for the initial phase of the investigation in order to provide the most coverage over the entire Priority Route for a relatively low cost compared to other investigation methods. The MASW survey was undertaken by Southern Geophysical Ltd. on behalf of Miyamoto and included the collection of data over 15No. lines located within the road carriageways (Figure 1 & Table 1).
Thirteen of the lines are located along the Priority Route with an additional 2No. lines (MASW 9 & 15) undertaken at the location of existing machine boreholes to facilitate correlation of parameters and ‘ground truthing’.
The resulting data has been processed into 2D shear wave velocity (Vs) subsurface profiles (to >25 m depth) identifying changes in stratigraphy and lithology along the survey lines, a typical example of which is shown in Figure 4. The full survey will be uploaded to the New Zealand Geotechnical Database (NZGD).
A number of engineering boreholes and well bores, and a GNS microtremor study available from the subject area have also been utilised in our assessment and for correlation purposes.
Figure 4: Indicative Shear Wave Velocity Profile from MASW Survey
4. Data Review and Assessment
Using the measured Vs, and the resulting velocity profiles from the MASW survey as a baseline, a preliminary understanding of the ground conditions has been developed through correlation with the existing available information, and correlation with geotechnical properties through published correlations.
4.1. MASW and Shear Wave Velocity Data
The in-situ soil Vs measurement taken in the field is a small-strain soil property related to the ‘undisturbed’, small-strain or maximum shear modulus (Go) of the soil and the soil-mass density (ρ).
As Vs is dependent on the stiffness and density of a soil generally, as soil depth increases so too does stiffness, thus Vs also increases. Table 2 shows the dependency of Go and Vs on varying geological / geotechnical parameters.
The shear wave velocity was recorded to depths in excess of 30 m over the majority of the Priority Route, reaching maximum depth of ~44 m. Whilst there is some variability in the Vs measurements across the site, in general the data shows a fairly consistent underlying profile for such a large area, with:
- A time-average Vs,5 around 310 m/s (~250-420 m/s) for the upper 5 m across the site;
- Vs,10 around 350 m/s (~280-440 m/s), for the upper 10 m across the site;
- Vs,30 around 400 – 480 m/s (SD) for the top 30 m.
Exceptions to the above generalisation are MASW lines 12 and 15 which show a greater thickness of material with lower shear wave velocities, indicative to softer/looser soil layers. Both MASW 12 and 15 are fairly isolated (~300 m southwest and ~250 m southeast respectively) from the remainder of the Priority Route and located close to an apparent historic stream (named “Terrace” Stream as shown in GNS, 2011), so this may be expected. In addition, MASW 15, with lower Vs,5 and Vs,10 values, is located in a different mapped geology and does not form part of the Priority Route.
Vs,30 is a widely used parameter in international practice for the classification of site class (discussed in more detail later in this paper) and is defined as the time for a shear wave to travel from a depth of 30 m to the ground surface. The time-averaged Vs,30 is calculated as 30 m divided by the sum of the travel times for the shear waves to travel through the individual layers as follows:
Vs,30 = 30 / ∑ (d/Vs) [Equation 1]
where: d = individual layer thickness and Vs = the shear wave velocity of the individual layer
As part of our assessment we have assessed Vs,30 for each of the MASW lines including minimum, maximum and mean values as shown in Figure 5.
Where Vs data was not recorded to 30 m depth (i.e. standard Vs,30 calculation invalid) minimum and maximum values of Vs,30 have been estimated by:
- Minimum: assuming the minimum value recorded for the missing data to 30 m depth;
- Maximum: assuming the deepest value recorded extends to 30 m depth (considered more probable than the minimum estimation).
Figure 5: Vs,30 Interpretation (95% confidence interval shown in parenthesis)
4.2. Calibration / Correlation with Available Data
As part of the initial stages of the project the following existing information was sourced:
- 3No. machine drilled boreholes with associated engineering logs in proximity of the Priority Route;
- GNS Science, Palmerston North City Council Buildings – microtremor analysis report (dated 1 February 2013);
- GIS data referenced in the GNS liquefaction assessment (Beetham et all, 2011) including gravel and groundwater depth information;
- A list of PNCC boreholes / well bores in the wider area, including some with lithology descriptions.
In general the, SPT N values from the machine boreholes correlated well with the Vs profiles in that they provide an indication of the transitions between denser and looser layers, however, there was a reasonably large spread in the data. Additionally, comparison of Vs with SPT N values is limited for dense soils such as the underlying gravel as the majority of the SPTs refused resulting in a value of 50+ and therefore no differentiation between denser soils.
GNS undertook a microtremor study (including HSVR and SPAC analyses) for a building on the Priority Route as part of an assessment of the NZS1170.5 Site Class for the building. A reasonable correlation can be seen between the data sets below ~4 mbgl, however, there is a fairly large spread in the data and GNS have derived higher Vs values (300 to 320 m/s) than those recorded during the MASW survey (~200 to 350 m/s) above 4 mbgl (refer to Figure 6).
Figure 6: Vs Profiles – GNS 2013 Microtremor & Miyamoto 2020 MASW Surveys
The GNS study incorporated the lithology descriptions from the above referenced PNCC well bores, derived Vs profiles and estimated the depth to ‘effective bedrock’ (also referred to as ‘engineering bedrock’) where theVs increases above a certain threshold (commonly 700 m/s). The depths to engineering bedrock presented by GNS are 76 m and 83 m for the geological and microtremor (SPAC) models respectively, the latter being GNS’s preferred model. The study concluded that the site may be classified as NZS1170.5:2004 Site Sub-soil Class D, however, with a natural site period (Tn) of less than 0.7 seconds, the site falls very close to Class C.
A number of well bores were obtained from PNCC / GNS archives, however, the boreholes only have information related to lithology and are located in or close to the boundary of an area with a different mapped geology. As such, these boreholes have been used with caution in the assessment and as an indication of the deeper ground conditions.
Several GIS files were obtained from GNS Science, including contour plans of ‘depth to groundwater’, ‘depth to gravel’ and ‘thickness of cover material above gravel’. The contour plans indicate the following:
- The depth to groundwater is indicated to range from 5.0 m to the north of the site to 7.0 m at the southern extent, while the depth to saturated soils above the gravels ranges from 2.0 to 5.0 mbgl which is closer to the recorded water levels in the boreholes along the priority route (1.2 m to 4.4 mbgl);
- The depth to gravel (and thickness of cover material) across the site ranges from 0.0 m (i.e. ground level) to the north of the site increasing to 4.0 m to 5.0 m depth to the south of the site. This loosely correlates with the other available information.
5. Evaluation and Assessment
5.1. Preliminary Ground Model
Based on our review of all available data, the ground conditions underlying the majority of the Priority Route are considered to be fairly consistent with dense to very dense gravel with varying sand and silt content and relatively thin isolated layers of silt / sand present from between ground level and ~4.0 mbgl. Where present, the material overlying the gravel is considered to comprise predominantly fine-grained material (silt & clay) with varying sand content.
The ground conditions as per the current study can be summarised as:
- Soil formations of dense to very dense sand – sand gravel and/or stiff to very stiff cohesive soils (silts and clays), of great thickness (> 60.0 m), whose mechanical properties and strength are constant and/or increase with depth.
With the exception of MASW 12 and 15 where:
- Soil formations of great overall thickness (> 60.0 m), interrupted by layers of recent soil deposits with soft and/or fairly loose materials of a small thickness (5 – 15 m), within soils of evidently greater strength and Vs≥ 300 m/s.
5.2. Seismic Site Classification
Of critical importance in earthquake engineering and engineering seismology are the underlying soils and site characterisation. Existing seismic codes ‘cover’ ordinary structures for ground shaking characteristics for ‘normal’ soil-site conditions (i.e. not necessarily covering special soil-site conditions such as liquefaction, slope stability, etc).
For seismic design of structures, and / or for seismic strengthening projects, using the current seismic codes, the site of interest must be classified into one of the soil categories adopted by the code. Based on the soil class, the appropriate site-dependant design spectrum can be defined and used.
5.2.1. International Methods of Seismic Site Characterisation
Site characterisation schemes of the seismic codes use different descriptions of geological and geotechnical parameters to define soil classes.
As discussed earlier in this paper, a widely used parameter in the classification of site class is Vs,30. The U.S. seismic code (or International Building Code) proposes six soil types using the Vs,30 values as the main characterisation parameter, together with the SPT N (standard penetration test blow count) and Su (undrained shear strength) values as ‘secondary’ index parameters.
Similarly, the current version of Eurocode 8 (EC8), Vs,30 is used as the main classification parameter, following the U.S. practice, along with SPT N, plasticity index (PI) and Su as ‘secondary’ index parameters, for the five main and two special defined soil classes.
It is not the intention of this paper to discuss these methods in any detail, rather to touch on the different assessment criteria therein.
For comparison purposes, Figure 7 presents an extract of the Eurocode 8 and ASCE 7-16 site classification based on Vs,30 (Note – other classification criteria omitted), and Figure 8 presents a comparison of several seismic codes.
Figure 7: Eurocode 8 & ASCE 7-16 Site Classification Criteria (extract only)
Figure 8: Site Soil Classification in Seismic Codes Comparison (Pitilakis et. al., 2015)
5.3. NZS1170.5:2004 Site Sub-Soil Class
Although the most commonly used parameter is the time-averaged vertical shear wave velocity in the uppermost 30 m (Vs,30), NZS 1170.5 adopts a mixture of parameters for different soil classes including but not limited to To (fundamental period), Vs, Vs,30, SPT N (standard penetration test blow count) and Su (undrained shear strength) values.
The general classification criteria as per NZS1170.5 are shown in Figure 9.
Figure 9: NZS1170.5:2004 Site Sub-soil Classification Definition
The preferred methodology for assessment of the NZS1170.5 Site Sub-soil Class is based on measurements of shear wave velocity (Vs) or shear wave travel times through estimation of the site period, Ts using the following (general) equation:
Ts = 4H/Vs [Equation 2]
where: H = depth to bedrock (m)
As can be seen from Equation 2, the site period is highly dependent on the depth to bedrock, and with the depth to ‘actual’ greywacke rock understood to be in the order of 1 km at the site, NZS1170.5 Site Sub-soil Class D (Deep or soft soil sites) may seem an appropriate classification. However, as discussed earlier the underlying very dense gravels may be considered ‘engineering bedrock’ at a certain depth where the Vs increases above a threshold (700 m/s commonly adopted) and consideration as such may give rise to much shorter site periods which may be more in line with Site Sub-soil Class C (Shallow Soil Sites).
Currently there is a considerable increase in amplification between the NZS1170.5 design spectra for Site Class C (shallow soil) and D (deep or soft soil) sites, leading to significantly different design criteria when undertaking seismic performance assessment of buildings (i.e. estimation of %NBS) and seismic structural design (i.e. design of a seismic strengthening scheme). As a generalisation, the adoption of Class D over Class C results in an increase of up to 63% (for a certain period range) the design response spectra leading to more onerous structural demands and ultimately increased development costs (i.e. overall building, building renovations, seismic strengthening schemes etc.).
With the Priority Route being assessed as very close to or below the boundary between Class C and D, i.e. close to a 0.6 s site period, potential benefit may be realised through more thorough investigation and assessment, of course depending on the proposed development.
To demonstrate the potential benefits of further site-specific investigation and more rigorous assessment, a ‘hypothetical’ sensitivity assessment of the site period along the Priority Route has been undertaken. The assessment incorporated the Miyamoto MASW measured Vs data, where available (i.e. up to 30 – 44 m depth), in combination with the GNS derived Vs profiles (Section 4.2), and slight variations of such, below. The following three scenarios were assessed:
- Scenario A: MASW measured Vs data to max. 44 m depth over the GNS ‘preferred’ Vs profile (i.e. Vs = 509 m/s to ‘engineering’ bedrock at 83 m).
- Scenario B: MASW measured Vs data to max. 44 m depth over the GNS ‘geological model’ Vs profile (i.e. Vs = 550 to 600 m/s to ‘engineering’ bedrock at 76 m).
- Scenario C: MASW measured Vs data to max. 44 m depth and Vs = 600 m/s to effective bedrock at 83 m.
The results of the assessment are plotted in Figure 10 and demonstrate how sensitive the assessment is to fairly small changes in the underlying ground conditions (Vs and / or depth to bedrock), with the ‘possible’ ranges of site period for the three scenarios straddling 0.6 s (Site Sub-soil Class C / Class D).
Figure 10: ‘Hypothetical’ Assessment of Potential Site Period (95% confidence interval shown in parenthesis)
Furthermore, in the event that further site-specific investigation and assessment resulted in NZS1170.5 Site Sub-soil Class D for a specific development project, as opposed to full compliance with the Class D design criteria, alternative assessment may be feasible such as site specific response analysis or the procedure put forwards by McVerry (2011).
To reiterate, this assessment is hypothetical (albeit potentially realistic) and is provided to demonstrate the significant potential for benefits to be realised through site-specific investigation and more rigorous assessment.
6. Conclusions and recommendations
The main objective of this study was to provide PNCC, building owners and practitioners with additional geotechnical information, at a preliminary level, to assist with seismic assessment and / or seismic strengthening, repair or maintenance works to the buildings along the Priority Routes.
To provide the most coverage over the entire Priority Route for a relatively low cost compared to other investigation methods, a MASW geophysical survey was undertaken and calibrated with available existing data. The data from which (shear wave velocity, Vs profiles) in itself is an extremely valuable indicator of dynamic soil properties and a useful parameter for seismic assessment.
Through interrogation and assessment of the data it is demonstrated that there may be significant benefit in further site-specific investigation and assessment of the NZS1170.5 Site Sub-soil Class, and that Class C may be appropriate for sections of the route. Furthermore, for sites that do fall into Class D, alternative methods of assessment may be warranted so that full compliance with Class D design criteria may not be required. However, the level of additional investigation and assessment will be dependent on the scale of the proposed development and potential cost savings.
The adoption of Site Sub-soil Class C as opposed to Class D will have significant beneficial effects to seismic performance assessment of buildings (i.e. estimation of %NBS) and seismic structural design (i.e. design of a seismic strengthening scheme).
Although this study provides significantly more geotechnical information to that previously available, further targeted investigation and assessment is required.
The project is ongoing we plan on publishing the findings of the future stages.
7. Acknowledgements
The authors would like to acknowledge PNCC for the positive impact this project will have for the city centre building owners and the wider Palmerston North community. We would also like to thank and dedicate this work in memory of our late colleague and friend Brent Neale for his encouragement and support. Lastly, we thank Clem Gibbens for his collaborative work on data interpretation also informed this study.
8. References
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Dobry, R., Vucetic, M. 1987. Dynamic Properties and Seismic Response of Soft Clay Deposits. Proceedings of the International Symposium on Geotechnical Engineering of Soft Soils. August 1987.
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Institute of geological and Nuclear Science, Palmerston North City Council Buildings – Microtremor analysis, 1 Feb 2013
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Palmerston North City Council Local Maps Viewer. Accessed via https://geosite.pncc.govt.nz/MapGallery/
Pitilakis et al., 2015. Site characterization and seismic codes, Orfeus Annual Observatory Coordination Meeting
Semmens et al., 2011. NZS 1170.5:2004 site subsoil classification of Wellington City. Proceedings of the Ninth Pacific Conference on Earthquake Engineering, Auckland, New Zealand