sampling and laboratory testing of ballina clay

SAMPLING AND LABORATORY TESTING OF BALLINA CLAY Jubert Pineda1, Laxmi Prasad Suwal1 and Richard Kelly1,2

1

ARC Centre of Excellence for Geotechnical Science and Engineering, The University of Newcastle, Callaghan, NSW, Australia 2 Coffey, Warabrook, NSW, Australia

ABSTRACT This paper discusses some topics related to the sampling and laboratory testing currently ongoing on Ballina clay (NSW). Emphasis is made on particular aspects of natural soft clays frequently neglected in laboratory procedures that may affect its mechanical response. Preliminary results are shown to highlight the importance of sample disturbance, salinity and rate effects in Ballina clay. Ongoing research as well as future activities are discussed in the last section of the paper. Implications for the current state of practice as well as the development of new constitutive models for soft clays are highlighted.

1

INTRODUCTION

A fundamental requirement for a National Field Testing Facility is to perform a state of the art laboratory testing program to characterise the material behaviour of the soils. Three sampling campaigns were planned to this end. The first campaign corresponded to installation of instruments for an embankment constructed using wick drains (PVDs). The purpose of the first campaign was to sample soils using different types of tubes to understand effects of sample disturbance, to develop appropriate laboratory procedures and to obtain preliminary soil parameters with depth. The second campaign sampled soils during installation of instruments when a second embankment was constructed directly on the ground. These samples will be used to perform stress path tests in preparation for the third campaign. The third campaign involves block sampling of soils using a Sherbrooke sampler to minimise soil disturbance. The methods and procedures developed in the first two campaigns should ensure that the highest quality information is obtained from the Sherbrooke samples. These tests can then be used to calibrate an anisotropic, rate dependent, structured constitutive model with confidence. This paper presents some of the initial findings from the first sampling campaign. Factors that affect the soil behaviour are explored and developments to equipment are discussed.

2

SOIL SAMPLING, TRANSPORT AND STORAGE

The first sampling campaign at the Ballina site, exclusively devoted to tube sampling, took place in July 2013. It was aimed at performing detailed soil characterization profiles using specimens retrieved from boreholes spatially distributed along the site. The location of the boreholes corresponds to the distribution of the instrumentation used for monitoring the performance of two embankments, named as PVD and No PVD in Figure 1, built-up after the first in situ testing and sampling campaigns (Kelly et al., 2014). The boreholes were drilled using track-mounted rigs. The holes were started using augers to 0.5 m depth and then continuous samples were taken. After each sample had been obtained, the borehole was cleaned out to the base of sample and the next sample was taken. Three continuous boreholes drilled up to 13 m depth, named INCLO-2, INCLO-3 and STANDPIPE, were chosen for characterization of index and mechanical properties of Ballina clay (see Table 1). Additional tube specimens, retrieved at two particular depths using a wide range of tube samplers, have been used to study soil disturbance. After sampler retrieval, tube ends were properly sealed in situ with several layers of plastic film underlying a 10 mm-thick polystyrene (porexpan) plate covered externally with wax (10-15 mm thickness). The porexpan plate was intended to isolate the clay from the thermal gradient caused by wax. Silicone grease was applied at the interfaces between the tube sampler and the porexpan plate for improving sealing. Tube ends were finally covered with plastic lids prior to packing for transport. Specimens were placed, vertically-aligned, in sealed plastic containers on a 150 mm-thick layer of wet sand in order to maintain a high relative humidity environment (RH≈99%) and minimize moisture losses. Tubes were packed using scraps of polystyrene (porexpan) to induce lateral confinement and absorb vibrations caused during transport (Figure 2). At the University of Newcastle (UoN), plastic containers have been stored in an industrial fridge under constant temperature conditions (T=16ºC). A fridge was preferred in this case to reduce the likelihood for water condensation to occur which could be absorbed by the clay in the long-term starting undesirable microbiological processes. Three additional continuous boreholes were obtained during a second tube sampling campaign carried out in April 2014 (see Table 1 and Figure 1). The new tube specimens have been devoted to complementary characterization tests. Most of the U75, IGS and Osterberg piston sample tubes were made from stainless steel. The P100 piston sample tubes are made from aluminium. U50 and U63 tubes are made from mild steel.

Australian Geomechanics Vol 49 No 4 December 2014

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SAMPLING AND LABORATORY TESTING OF BALLINA CLAY

PINEDA ET AL.

Table 1:Tube specimens obtained during sampling campaigns at Ballina site. Borehole

BH-1 INCLO-1 INCLO-2* INCLO-3* INCLO-4 MEX-1 MEX-2 STANDPIPE* SURVEY VPW-GR VPW-1

Sampler type

Borehole

Sampler type

VPW-2 VPW-3 VPW-4 VPW-5 VPW-6 IGS-1 IGS-2 MEX-5** MEX-6** MEX-9**

U75-Shelby ( =75mm) open sampler P100 ( =100mm) piston sampler Österberg ( =89mm) fixed-piston sampler Österberg ( =89mm) fixed-piston sampler U75-Shelby ( =75mm) open sampler U50-Shelby ( =50mm) open sampler U75-Shelby ( =75mm) open sampler Österberg ( =89mm) fixed-piston sampler P100 ( =100mm) piston sampler U75-Shelby ( =75mm) open sampler U75-Shelby ( =75mm) open sampler

* Continuous boreholes used for characterization tests

U75-Shelby ( =75mm) open sampler U63-Shelby ( =75mm) open sampler U75-Shelby ( =75mm) open sampler P100 ( =100mm) piston sampler Österberg ( =89mm) fixed-piston sampler IGS ( =63mm) fixed piston sampler IGS ( =63mm) fixed piston sampler Österberg ( =89mm) fixed-piston sampler Österberg ( =89mm) fixed-piston sampler Österberg ( =89mm) fixed-piston sampler

** Second tube sampling campaign

6809600

6809550

INCLO 3 (UoW)

6809500 INCLO 2 (UoN)

STANDPIPE (UWA)

6809450

6809400

6809350 551860

551880

551900

551920

551940

551960

551980

552000

Access Track

Instruments

PVD

No PVD

NewSyd SI

Instruments

SBPM

Boreholes (1st campaign)

Boreholes (2nd campaign)

Figure 1: Location of boreholes used for tube sampling at Ballina site. Plastic lid

Plastic container

Tube 4

Tube 3

Tube 2

Tube 1

Polystyrene

Wet sand

Figure 2: Packing of tube specimens for transport and storage.

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ASSESSMENT OF SAMPLE QUALITY AND SELECTION OF SOIL SPECIMENS FOR LABORATORY TESTING

One of the challenges in the study of the mechanical behaviour of natural soft clays lies in the selection of ‘representative’ soil specimens for laboratory testing. Sample disturbance as well as the natural soil variability along the tube, e.g. presence of shells or inclusions with similar sizes as soil elements, may affect the results of laboratory tests and lead to important discrepancies between in situ and laboratory data. Visual inspection of the sample quality is only possible after complete soil extrusion which is not practical since the time required for visual inspection, sample selection and preparation (or sealing) is too long and may affect the initial state of the soft clay (e.g., due to drying). Non-conventional techniques like X-Ray or Computer Axial Tomography (CAT) analysis, on the other hand, are becoming popular in geotechnical engineering due to their non-destructive nature and simple procedure. The main drawback of the X-Ray technique is that all features of the specimen are superimposed into a 2D image. On the other hand, a 3D reconstruction is obtained from CAT analysis. It gives a 3D picture of the sedimentary structures but also natural heterogeneities (fissures, inclusions, etc) and allows selection of high-quality specimens for laboratory testing. CAT imaging is based on Beer’s law, that relates the incident intensity (I0) and transmitted intensity (I) of a X or gamma-ray beam passing over an entire transverse section by means of a linear attenuation coefficient ( ) (e.g., Duliu, 1999):

I

I 0 exp

x

(1) where x is the sample width. Equation (1) implies that for given density and incident intensity (I0), an increase of sample width will decrease the transmitted signal (I). In other words, increasing of the sample width will decrease the quality of the scans. It is worth noting that the resolution obtained from commercial CAT devices is, at least for practical purposes, good enough to evaluate sample quality of tube specimens. The maximum energy of the CT scanner (120-140keV for most medical devices) should be applied to get the maximum resolution. The use of 2 mmreconstruction process (separation between cross-section images) seems appropriate to build-up the 3D view without losing relevant information about the soil structure. Good results have been obtained using medical CT scanners on specimens up to 200 mm in diameter (e.g., Cremer et al., 2002; Pineda et al., 2012) whereas additional image treatment techniques has been required in the case of block samples (e.g., Sau, 2013; Sau et al., 2014). CAT analysis is being employed at UoN, in collaboration with the Radiology Section of the Mater Hospital in Newcastle (NSW), to examine the internal structure of the tube samples obtained from the Ballina site. Particular emphasis is made on the detection of possible heterogeneities and their potential influence on laboratory testing. A CAT scanner (Toshiba Aquilion®) with 135 keV maximum energy has been used. Figure 3 shows the vertical sections of two tube specimens, retrieved from borehole INCLO-2 using an Osterberg fixed piston sampler ( outer=89 mm) at depths of 3.3-3.9 m and 7.5-8.1 m. Image post processing used the free software Gimias® (Gimias, 2011). The attenuation scale shown in Figure 3 varies from white (maximum attenuation or high material density) to black (minimum attenuation or low density). The comparison of the vertical sections clearly shows important differences between tubes. Soil from 7.5 m to 8.1 m seems quite homogeneous whereas the presence of shells of different sizes, indicated as white inclusions, is evident between 3.3 m to 3.9 m depth. High concentration of shells is observed at the bottom half of the tube. Figure 3 also shows cross-section images obtained at different depths along the tubes. There, capital letters F, C and S stand respectively for fissure/heterogeneity, cavity/channel and shell. It can be noted that the presence of shells in the shallowest tube it makes difficult to obtain specimens for laboratory testing, especially from the bottom half of the tube. Shells with sizes up to 35 mm, the same order of magnitude of laboratory specimens, have been detected. Based on the inspection of Figure 3, soil from slice 2 to 7 of the deeper sample could be used for laboratory testing (i.e., oedometer and triaxial tests) whereas top and bottom ends should be employed only for characterization purposes as suggested by Ladd & DeGroot (2003). For the shallowest tube, only soil between slice 2 and 4 could be used for mechanical tests. The application of CAT imaging for studying sample disturbance is described in the next sections.


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PINEDA ET AL.

6

C S

S

C

INCLO-2 ( 3.3 – 3.9m )

7

5

S

S

8

7 8

6

5

4

3

2

F

S

8

7

6

5

4

3

2

1

F

F

INCLO-2 ( 7.5 - 8.1m )

7

F

5

F

S

S

1

3

F

F

6

2

4

1

S

S

C

S

C

C

S

3

1

C

S

S

C

S

C

C

S

2

4


Figure 3: CT scans of two specimens from INCLO-2.

4

LABORATORY TESTING: CHALLENGES AND IMPROVEMENTS

In general, every laboratory program involves two main stages: material characterization and mechanical testing. Both are essential and require appropriate experimental procedures as empirical correlations between index properties (e.g., liquid limit or plasticity index) and mechanical parameters are commonly used in geotechnical practice. The particular characteristics of natural soft clays (very low in situ stresses and undrained shear strength, high compressibility,

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presence of electrolytes in the pore fluid, presence of organic matter and expansive minerals as well as weak cementation) put forward a number of challenges for which most soil mechanics laboratories are not ready to deal with. A crucial aspect, commonly neglected, is related to the chemical interaction between the pore fluid constituents and the solid phase in natural soft clays. Kelly et al. (2013), for instance, pointed out the strong influence of previous soil drying on the index properties of Ballina clay. Differences in liquid limit of about 30% were obtained when soil was dried prior to testing. They remarked on the lack of appropriate standards for these estuarine sediments but also warned about the use of empirical correlations between index properties, determined using current standards and mechanical soil properties of Australian soft clays. Geochemical analysis (electrical conductivity and resistivity, pH, cation/anion analysis, XRD analysis, organic content, specific surface) as well as microstructural studies (scanning electron microscopy -SEM or ESEM-; mercury intrusion porosimetry –MIP-) are rarely included into laboratory programs despite important clues provided for the expected macroscopic response of the soil under study. Some of these techniques, like soil electrical conductivity, may be implemented in soil mechanics laboratories for practical purposes. Measurement and control of the soil electrical conductivity (EC) is relevant in the case of natural clays coming from estuaries and river systems where the pore water is saline. Sampling natural water for use in laboratory tests has limitations because sulphates in the water can oxidise to acid and slimes can form over time, both of which can affect the soil behaviour. A compromise is to prepare artificial saline solutions (homoionic solutions) in the laboratory. Following this approach Pineda et al. (2013) carried out a pilot experimental program to investigate effects of electrolyte salinity on the shear strength parameters of Ballina Clay. Drained direct shear (DST) tests were performed on two tube specimens (U75 Shelby tubes retrieved from 4-4.5m depth). Two different salinities were used to flood the shear box: (i) deionized water, and (ii) synthetic sea water (35gr/l of NaCl). Electrical conductivity measurements performed on intact specimens varied between 31mS/cm (Shelby I) and 41.5 mS/cm (Shelby II). It indicates that natural salinity of the pore water in Ballina clay seems to be closer to the salinity of the synthetic sea water (≈ 48 mS/cm). Consolidation started after 10-15 min of soaking the shear box which means that consolidation process took place under a concentration gradient generated between the external fluid and the pore water of the clay. The test was performed in order to reproduce the experimental procedure followed in conventional laboratory testing where the chemical equilibrium between the soil pore water and the external fluid is rarely checked before testing. Figure 4(a) shows the peak conditions ( peak vs v) for all specimens subjected to shearing. peak varied with the pore fluid salinity but also depending on the specimen origin (Shelby I or Shelby II). Except for one case, only two specimens were available to define (roughly) the peak strength envelope. For specimens from Shelby I, values of strength parameters were equal to =31º (NaCl) and =26º (deionized). Null effective cohesion was obtained for specimens from Shelby I irrespective of the pore fluid electrolyte used during the tests. In the case of Shelby II, they displayed values of c =7 kPa and =32º (NaCl) and c =0.8 kPa and =30º (deionized). Figure 4(b) compares the strength parameters with the average EC obtained in each case. Reference values of EC for intact soil as well as values of reference pore fluids have been included in this figure using vertical lines. The exposure to deionized water induced a decrease in both c ’ and for specimens from both Shelby tubes. However, the change induced seemed to be dependent on two factors: (i) the initial salinity of the specimen and (ii) the concentration gradient induced. It has been assumed here that strength parameters for intact Ballina clay are closer to values obtained from DST using synthetic sea water (NaCl) due to their similar values of EC. The maximum change in was around 6º and 2º in specimens from Shelby I and Shelby II, respectively. Effective cohesion was almost completely erased in specimens from Shelby II once subjected to deionized water whereas negligible values were obtained in the case of Shelby I. Despite the few experimental results it suggests that as salinity increases a larger friction angle develops. Low or null effective cohesion seems to be developed depending of the soil salinity. However, it may easily be degraded if the clay is exposed to deionized water. The preliminary results described above highlight the necessity for appropriate experimental procedures to determine index and mechanical properties of Ballina clay. Modified guidelines have been implemented at UoN for this purpose. An estimation of the electrical conductivity of the pore fluid is the first requirement. It is obtained by squeezing the soil pore water (around 15cm3), using a stainless-steel mould to avoid corrosion effects. A synthetic saline solution is then prepared at the same electrical conductivity as the soil pore fluid. To do this, the relationship between sodium chloride solubility (g of NaCl per 1000 g of deionized water) and the electrical conductivity of the solution has been experimentally obtained as observed in Figure 5(a). Sodium chloride supplied by Chem-Supply®, Australia (molar mass = 58.44 g/mol) was used. This relationship is being used at UoN to prepare the pore fluid used for index and mechanical tests on Ballina clay. The standard practice for the determination of liquid limit using the fall cone device has been modified as follows. The clayey fraction is first separated by passing the soil (at natural water content) through the 425 m sieve. At this stage, the first measure of the fall cone test is performed. Then, synthetic pore fluid prepared according to the relationship shown in Figure 5(a), is added to the soil mass to increase the soil moisture. An equalization period of 24 h is always allowed prior to the second and third measures using the fall cone device. On the other hand, no modifications have been made to the standard practice for determining plastic limit using the thread


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PINEDA ET AL.

rolling method. It means that soil is dried prior to the determination of plastic limit as it seems not very sensitive to changes in pore water salinity (e.g., Wood, 1990; Kelly et al., 2013). 34

10

Empty symbols: Deionized water Full symbols: NaCl

0 0

25

50

'v (kPa)

(a)

75

100

8 6 4

Reference value Shelby II

24

II

I

II

d iz e ion De

26

lb y

lby

20

by el Sh

28

S he

Sh e

30

30 Deionized water

.3 Na < Cl EC - S < he 4 3 l by .5 II m S/ cm

)

40

(4 2

(kPa)

50

Ballina clay (-4