Abstract

The near surface geology of the Auckland area generally comprises alluvial soils, volcanic tuff/ash and/or residual soils. These soils may be expected to display different engineering characteristics given their different geological origins. The soils are mostly cohesive and the undrained shear strength, s_u, is of geotechnical interest. The cone penetration test (CPT) allows an estimate of s_u to be made via application of bearing capacity theory using a cone factor, N_kt. Typically, N_kt varies from 10 to 18, depending on the soil type, so it is often necessary to compare with a reference test to determine the N_kt factor that is appropriate for the soil type on a site-specific basis. In this study, we have undertaken flat dilatometer tests (DMT) next to CPT tests at various sites in Auckland with the DMT acting as the reference test. In this way, typical N_kt factors for the different geological units have been suggested.

1 INTRODUCTION

The undrained shear strength of a cohesive soil can be estimated using the cone penetration test (CPT). This is usually done by utilising a cone factor, N_kt. The N_kt value is generally not known and can vary over a large range depending on geology and soil type. This often leaves the geotechnical engineer having to guess what N_kt value to use, if there are no reference tests to help establish a site specific correlation. The purpose of this study is to help provide an indication of what N_kt values may be applicable to the different soil types in Auckland by using the DMT test as a reference test.

In this study, the authors have selected 50 side-by-side CPT and DMT tests from various sites of various geological units across Auckland. The results have been compared to help determine suitable N_kt values.

2 BRIEF GEOLOGY OF AUCKLAND SOILS

The geology of Auckland is shown on the GNS Geological Map and described in the associated booklet (Kermode, 1992). In general terms, the near surface soils in the Auckland area are predominately cohesive (silts and clays) but have been formed by different geological processes. The most recent deposits (Holocene) are marine sediments in the harbours and under reclaimed land, and localised areas of stream and flood plain alluvium. A large proportion of the surface geology of Auckland is alluvium of the Tauranga Group (mostly Pleistocene), which comprise mostly firm to stiff silts and clays, but also contains Pumice material from the Taupo Volcanic Zone as well as significant deposits of peat in places. More recent volcanic deposits from the Auckland Volcanic Field provide a coverage of tuff and ash (as well as Basalt lava flows and scoria) over parts of Auckland. The tuff and ash have generally weathered to form soft to very stiff sandy or silty clays. The volcanic soils can be interbedded or interspersed with the sedimentary alluvial deposits. The Waitemata Group sandstones and siltstones (Miocene), which underlie most of the Auckland area, form a mantle of residual soil (silts and clays, some sand) over a deep weathering profile. The Waitemata Group residual soils often present at, or near the surface. To the east of Auckland, uplifted Greywacke (Mesozoic) through the line of the Hunua ranges, is exposed in Waiheke and Motutapu Islands of the Hauraki Gulf, weathering to clays at the surface.

Although the near surface soils are mostly clay-like, they vary in plasticity and silt content, are sandy in places, are usually layered and almost always non-homogeneous. This adds a further complication to the varying geological origins, which provide different clay minerology and (macro and micro) structural effects. This makes establishing correlations to geotechnical soil parameters difficult and standard correlations that are based on well behaved ‘text book’ soils may not be applicable.

3 UNDRAINED SHEAR STRENGTH

The undrained shear strength, s_u, is not a unique soil property. It varies depending on the mode of failure, the stress state of the soil, anisotropic effects and rate of failure. Furthermore, in situ tests may not necessarily be fully drained, as may be the case for silty soils, and so not truly represent undrained shear strength.

Kamei (1996) showed that anisotropy is a significant factor by comparison of isotropic consolidated triaxial tests (CIU) with K₀ (coefficient of earth pressure at rest) consolidated triaxial tests (CK₀U). Figure 1 shows that the CK₀C tests give lower undrained shear strengths than the CIU tests, both in compression and extension. This figure also shows that tests in extension are significantly less than tests in compression, which illustrates variation due to mode of failure. Mayne (2016) also illustrated the effect of failure mode on s_u, as illustrated in Figure 2. There may be a factor of 6 between the highest and lowest measured undrained shear strengths – of the same soil! The figure also illustrates the hierarchy of s_u from highest to lowest, being: field vane test, triaxial compression, direct simple shear (DSS) and triaxial extension.

Figure 1: s_u variation between isotropic and K₀ consolidated triaxial tests (Kamei, 1996)

Figure 2: s_u variation between tests of different failure mode (Mayne, 2016)

This presents a problem as to which test to use as a reference test when correlating to another test, in this case, the CPT. Mayne (2016) suggests that the DSS mode is most appropriate. This mode presents s_u results that fall more-or-less mid-way between the other test modes and thus provides an ‘average’ result. It is also the mode that best relates to the SHANSEP method (Stress History and Normalised Soil Engineering Properties) (Ladd et al 1977) and theory of critical state soil mechanics (Wroth, 1984). The DSS represents the simplest form of shearing.

By the SHANSEP method (Ladd et al. 1977):

 (1)

where

The terms 0.22 and 0.8 in equation (1) are averaged values of a narrow range of variables determined experimentally. A similar relationship can be derived theoretically using critical state soil mechanics (CSSM) (Wroth, 1984):

where

and

c_s and c_c = swelling and compression index, respectively

Both the SHANSEP method and the CSSM methods are related to the DSS mode of undrained shear strength.

3.1 Undrained shear strength from CPT

The undrained shear strength of clays can be estimated from CPT results using the following equation (Lunne et al.):

s_u = q_net/N_kt (3)

where

Equation (3) is a direct application of bearing capacity theory, where N_kt is then the bearing capacity factor for the cone. N_kt is determined empirically, usually by correlation to a reference test. Typically, N_kt is between 10 and 18, with an average of 14 (Robertson and Cabal, 2015).

There are other methods of estimating s_u, such as by using effective cone resistance, by using excess pore water pressure, by cavity expansion, or by critical state soil mechanics (Mayne, 2016). However, equation (3) is the most common method.

3.2 Undrained shear strength from DMT

A correlation for undrained shear strength from DMT was established by Marchetti (1980), via the relationship with OCR:

OCR = (0.5.K_D)^1.56 (4)

Where

K_D = horizontal stress index (strongly correlated to K₀)

By applying the SHANSEP method, represented by equation (1), the relationship for s_u becomes:

s_u = 0.22.(0.5.K_D)^1.25 (5)

This relationship has been found to provide a reliable estimation of undrained shear strength by numerous researchers (e.g. Lacasse & Lunne, 1988 and Powell & Uglow, 1988). However, most research has been in soft to firm normally to moderately overconsolidated sedimentary clays. Marchetti (2015) stressed that equation (5) is applicable only to ‘text book’ clays.

4 SIDE-BY-SIDE CPT AND DMT RESULTS

25 sets of CPT-DMT sounding pairs have been selected for this study from the Ground Investigation Ltd’s database. These have been supplemented by borehole information from the New Zealand Geotechnical database to assist with identifying geological units. The approximate locations where the tests were performed are shown on Figure 3. The pairs were selected based on distance between tests (less than 10m), availability of nearby borehole information and uniformity of ground conditions between tests. It was difficult to find tests that met the last criteria as most of the sites display layered soils and lateral variation.

Tags : #CPT#Dilatometer