The four-lane, 21.8 kilometre long, Hamilton Section of the Waikato Expressway is the largest roading project undertaken in this region’s history and one of the larger projects currently being undertaken in New Zealand. Many of the seventeen expressway bridges in the Hamilton Section are underlain by Pleistocene soils assessed as having a high liquefaction potential. Typically the soils encountered are volcanic in origin either as primary tephra deposits or reworked volcanically derived material. A number of researchers have noted the potential for misclassification of such soils by the CPT, therefore it was decided to undertake co-located borehole/SCPT/SDMT and laboratory classification testing for use in conjunction with a site wide geologic model to develop soil specific correction factors for use in liquefaction assessment. Soil specific correction factors were investigated for the Ic sand like/clay like cut-off (B&I 2014) and the fines content (Cfc). The specific correction factors derived are presented along with examples from the project showing the consequence, some of which are significant, of adopting soil specific correlations. Given the widespread occurrence of some of the soils considered e.g. Hamilton Ash, within the Waikato and Bay of Plenty region, the authors suggest that the database of testing presented in this study could be readily supplemented by results from other sites and the proposed soil specific correction/correlations extended.
This paper presents a case study of the development of site specific CPT derived soil behaviour/composition correlations within volcanogenic sediments of the Waikato Region of the north island of New Zealand. Two correlations are considered:
- Soil Behaviour Index (Ic) to fines content (FC) and;
- Soil behaviour Index (Ic) to Plasticity Index (Ip) as a proxy of liquefaction susceptibility
These two correlations are key to the assessment of liquefaction susceptibility and so the authors have chosen to investigate how they may vary from the norm in volcanogenic sediments, in particular those containing pumice (NZ Geotechnical Society, 2010).
This study uses data sourced from the extensive investigation undertaken for the Hamilton Section of the Waikato Expressway, a recently designed 21.8km long highway project including 17 bridges. The project area lies in the centre of the Hamilton Basin, a large alluvial plain approximately 40km wide by 90km long.
Many of the soil types encountered in this study are widespread across the region. In the future the authors would like to extend the database by incorporating suitable investigation data from elsewhere within the Waikato and Bay of Plenty region, where similar soils are known to exist.
2 GEOLOGY & SEISMICITY OF AREA
The Hamilton Basin can generally be divided into two geological terranes, the Hamilton Hills and the Lowlands. The Hamilton basin was infilled with sediments derived from volcanic activity within the Taupo Volcanic Zone, located approximately 100 km southeast of Hamilton, during the Quaternary period (last 2 Million years). Most of the basin is a broad alluvial floodplain (lowland) with widely spaced rounded hills (Hamilton Hills) that protrude some 20 m to 70 m above the plain surface. Most of the primary (airfall and pyroclastic deposits) and reworked (alluvial fans and lacustrine) soils in this region are rhyolitic in composition sourced from volcanic events within the Taupo Volcanic Zone, located approximately 100 km southeast of Hamilton. The soils contain significant amounts of volcanic glass, pumice, other rhyolitic lithic gravels, as well as crystalline minerals.
2.1 Hamilton Hills
The Hamilton Hills comprise older volcanic ignimbrites mantled by volcanic airfall deposits whose upper surface forms a characteristic stiff to very stiff orange-brown weathered silt-clay crust (Hamilton Ash Formation). These hills comprise a sequence of at least three distal ignimbrites (pyroclastic density currents) deposited approximately 1 to 1.2 Million years ago interfingered with alluvium. The hills have since been eroded by the ancestral Waikato River into a series of rounded hills and valleys, which in turn deposited reworked volcanic sediments. Subsequently the hills were mantled by ash fall beds known as the Kauroa and Hamilton Ash Formations and undifferentiated younger tephra beds (cover beds). The surfaces of the Hamilton Hills have been deeply weathered with the highly reactive volcanic glass fragments altering to sensitive clay minerals such as allophane and halloysite (Moon et al, 2015).
The Lowlands are the valleys between the Hamilton Hills that have been infilled with younger (late Quaternary, last 25,000 yrs) alluvial sediments of the Piako Subgroup. The Lowlands typically have a very gently tilting (south to north) topographic surface, known as the Hinuera Surface. Deposits underlying the surface are known as Hinuera Formation. Interfingering with the thick and often pumice rich sand beds are silt layers comprising silicic fragments (volcanic glass with limited weathering) as well as silty/sandy and occasionally organic alluvium.
3 LIQUEFACTION SUSCEPTIBILITY
3.1 Potential for Misclassification
A number of researchers have reported concerns with the use of liquefaction assessment procedures based on penetrometer testing in rhyolitic (or specifically pumiceous) soils as the structure, shape and composition of the sediment varies significantly from the more typical alluvial sediments derived from sedimentary rocks which form the basis for empirically derived liquefaction assessment tools.
Murashev et al (2014) stated in NZTA research Report 553 that conventional interpretation (i.e. penetrometer tests such as SPT and CPT) are less effective in assessing liquefaction resistance in pumice soils. Numerous researchers also have concluded that penetration tests can underestimate liquefaction resistance in pumice soils. Clayton et.al (2017), Murashev et al (2014), Clayton & Johnson (2013). Orense & Pender M. (2013)
The referenced works generally refer to the potential for underestimation of the relative densities and hence liquefaction resistance in rhyolitic soils. Should this be the case then it follows that there may be the potential for CPT tests to also misclassify such soils.
3.2 Assessment of Susceptibility
Soil response to earthquake shaking and hence liquefaction potential has been widely related to soil plasticity. Idriss and Boulanger (2006, 8, 14) suggests that all soils may be divided, based on their plasticity index into the following categories:
- Sand like – soil that may be subject to classical cyclic liquefaction if sufficiently loose and;
- Clay like – soil not subject to classical liquefaction, but may be subject to cyclic softening if sufficiently soft.