Abstract

The NZ Transport Agency Baypark to Bayfair link upgrade project will provide improvements to two key intersections on State Highway 2 in the east of urban Tauranga in the Bay of Plenty. The project will be constructed in an area which is susceptible to liquefaction and cyclic strain softening. The ground conditions comprise Holocene beach deposit sands overlying swamp deposits, underlain by Pleistocene age volcanically derived alluvium. Preliminary analyses using CPT based methods indicated potential widespread liquefaction (and associated effects) in a 1/2500 APE event. This represented a significant risk to the project and it was recognised that a robust and considered assessment of the liquefaction hazard was necessary. This paper describes the investigation techniques used and the approach taken to undertake that assessment. The assessment was based on an extensive investigation programme comprising boreholes, Cone Penetration Tests (CPTs), seismic CPTs, seismic Dilatometer tests, downhole (Geonor) Shear Vane tests and laboratory testing. The investigation highlighted the importance of, and difficulties with, obtaining good quality data in the field. Both CPT and shear wave velocity based methods were used in the liquefaction assessment with a particular emphasis on the liquefaction potential of the Pleistocene aged soils. Findings from the liquefaction assessment showed good agreement with other recent liquefaction studies carried out within volcanic soils in the central North Island.

1 INTRODUCTION

The NZ Transport Agency Baypark to Bayfair link upgrade project will provide improvements to the Te Maunga intersection at Baypark and to the Maunganui Road/Girven Road intersection at Bayfair, in Tauranga, Bay of Plenty. The project lies at the northern terminus of the Tauranga Eastern Link (TEL) which was completed in 2015. The TEL is located to the east of Tauranga and is a key freight route for transporting goods from the Eastern Bay of Plenty agricultural and forestry areas to the Port of Tauranga and wider markets.

The project layout is shown in Figure 1. The project features a 3 span flyover at the Maunganui/Girven intersection and 2 single span bridges at the Te Maunga intersection, with significant lengths of MSE type approach embankment. The estimated project cost is $100m.

The project will be constructed in an area which is susceptible to liquefaction and cyclic strain softening. Seismic stability of the up to 8m high approach embankments and abutments under the various design events was a key consideration. Preliminary analyses indicated that liquefaction could be initiated in events with an Annual Probability of Exceedance (APE) of as little as around 1/100 (equivalent to a Minor earthquake), with widespread liquefaction (and associated effects) indicated in a 1/2500 APE (Design earthquake) event. This represented a significant risk to the project.

Figure 1: Project layout plan

2 GEOLOGY, GROUNDWATER AND SEISMICITY

2.1 Geology

The site is relatively flat and sits between the coastal barrier system at Omanu Beach to the northeast and the Matapihi peninsula to the southwest. The Tauranga Harbour and associated estuarine environment extends to the south of the site at Rangataua Bay. The city of Tauranga is located almost completely within the Tauranga Basin, a Pleistocene to Holocene (about 2 million years to the present) tectonic sedimentary basin up to 150m thick (Briggs et al, 1996). The basement of the Tauranga Basin consists of variably welded ignimbrites of Upper Tertiary age.

Deposits infilling this basin are termed Tauranga Group and consist of a basal Pleistocene sequence and an upper Holocene unit. The Pleistocene sequence comprises mainly alluvial deposits interbedded with unwelded ignimbrites and tephras, and is commonly referred to as the Matua Subgroup. The Holocene sediments include estuarine (swamp), alluvial, beach and dune deposits. The most recent geological map covering the area (Leonard et al, 2010) shows that the low lying areas of Tauranga and Mount Maunganui, where the site is located, are underlain by Beach Deposits and Swamp Deposits. The Matapihi peninsula, close to the southwest limit of the site, is underlain by the Matua Subgroup. These three units were of particular significance to the study and are described below in stratigraphic order:

2.1.1 Holocene beach deposits

The beach deposits are composed mainly of sands with little fines. Most of these deposits consist of loose to medium dense, fine to coarse-grained sands, composed largely of quartz and, subordinately, of shells and pumice. Trace silt was frequently observed, whilst minor pumice gravel occurs locally.

The sands extend across the whole site with relatively uniform thickness, to depths of about 10m below ground level (bgl). In the southeastern part of the site, a dense to very dense bed of gravelly sands was encountered at the base of the beach deposits. A number of Cone Penetration Tests (CPTs) refused on this bed.

2.1.2 Holocene swamp deposits

The swamp deposits consist of a sedimentary package of clays and silts interbedded with lenses/beds of sands. These deposits are estuarine in nature but the “swamp deposits” terminology was adopted to follow the QMAP series units. The swamp deposits occur throughout much of the site, immediately below the beach deposits, from a depth of about 10m bgl. Apart from the edges where the unit thins out, the thickness of the swamp deposits is reasonably regular, varying between 4 and 8m. The swamp deposits were classified into cohesive and granular sub-units. The cohesive sub-unit was typically described as a soft to firm sensitive clayey silt of high plasticity, the granular sub-unit as loose to medium dense sands with some silt.

2.1.3 Matua Subgroup

This Pleistocene deposit is composed mainly of loose to dense silty sand alluvium and is occasionally pumiceous. In addition to the sands, beds or lenses of fine-grained soils are frequently distributed throughout the unit and are interpreted as likely tephras.

A cross section that presents the typical geology of the site is shown in Figure 2.

2.2 Groundwater

The groundwater level is typically encountered at around 2.5m bgl across much of the site.

Figure 2: Typical geological sectio

2.3 Seismicity

The Tauranga area is located to the north of the Taupo Volcanic Zone (TVZ) and to the east of the Hauraki Rift. Active faults associated with both the TVZ and the Hauraki Rift occur in the region. However no active faults are known to exist within a 20km radius from the site (New Zealand Active Faults Database, 2015). The closest faults to the site are the Kerepehi Fault (38km to WSW) and, possibly, some offshore faults close to Motiti Island (22km to ENE).

The Peak Ground Accelerations (PGAs) and corresponding Representative Magnitudes recommended in the Site specific Seismic Hazard Assessment (SSHA) were used as inputs to the detailed liquefaction assessment (refer Table 1).

Table 1: SSHA Peak Ground Acceleration & corresponding Representative Magnitudes

Note. MCE = Maximum Considered Earthquake

3 LIQUEFACTION ASSESSMENT CHALLENGES

3.1 Pleistocene age volcanogenic soils

A number of researchers and recent studies (refer Clayton et al, 2017) have highlighted challenges with undertaking liquefaction assessment in volcanically derived soils of the central North Island of New Zealand. It is widely considered that conventional penetrometer based liquefaction assessment methods in these soils can over predict liquefaction triggering in these unusual soils.

The over-prediction is thought to result from the presence of crushable pumice grains and/or age effects. Where particle crushing occurs during a static Cone Penetration Test (CPT) the relative density of a soil can be under-estimated. Older deposits (such as the Pleistocene age Matua Subgroup) are considered to exhibit some cementation which are only partially recognised using large strain investigation techniques such as a CPT. Shear wave velocity based liquefaction assessment methods have been suggested as being more appropriate for such soils – refer to Clayton et al (2017) for further discussion.

3.2 Residual shear strength

Correlations associated with conventional penetrometer based methods may under-estimate the undrained and residual shear strength of young sensitive soils, such as the Holocene swamp deposits. This can result in the resistance to cyclic strain softening being under-estimated. Cyclic strain softening was an important consideration for stability given the extent of embankments on the project.

4 INVESTIGATION METHODS

4.1 Shear wave velocity

Paired seismic CPT (sCPT) and seismic Dilatometer (sDMT) direct push tests were carried out to provide down-hole shear wave velocity (Vs) data at five key locations. This enabled a comparison to be made between pseudo interval and true interval methods and gave confidence in the quality of the data obtained. Figure 3 shows the typical setup for the sCPT and sDMT testing.

Some challenges had been experienced with obtaining good quality shear wave velocity data during the initial investigations, particularly in locations where there was significant background noise, such as near the East Coast Main Trunk (ECMT) nearby. Within this investigation close collaboration between the consultant, the investigation contractor (Perry Geotech Ltd) and their specialist geophysics supplier (Baziw Consulting Engineers Ltd) was found to be beneficial as rapid evaluation and interpretation of test results allowed changes in investigation scope to suit conditions identified and evaluation of data reliability allowed interpretation to place appropriate levels of confidence on results.

Tags : #Liquefaction#Volcanic soils