QUARTERLY JOURNAL OF ENGINEERING GEOLOGY AND HYDROGEOLOGY
Article DOI: 10.1144/qjegh2016-136 Article number: qjegh2016-136 Article-type: research-article Subject: Research article
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7 Anschütz et al. 2015 - please add surname offinal author, and add book editor/s and publisher’s name + city 8 Anschützet al. 2016 - please add fourth author’s surname
9 Aylsworth & Hunter 2004 - please add book editor/s, publisher’s name + city, and page numbers of paper 10 Bazin et al. 2016 - please add book editor/s, publisher’s name + city, and page numbers of paper 11 Calvert & Hyde 2002 - please add book editor/s and publisher’s name + city
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13 Doveton 2001 - please add book editor/s, publisher’s name + city, and page numbers of paper 14 Eggestad & Sem 1976 - please add book editor/s and publisher’s name + city
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24 Longva et al. 2003 - please add book editor/s
25 Lunne et al. 2003 - please add book editor/s
26 Pfaffhuber et al. 2010 - please add book editor/s, publisher’s name + city, and page numbers of paper 27 Pfaffhuber et al. 2016 - please add book editor/s, publisher’s name + city, and page numbers of paper 28 Sandven et al. 2004 - please add book editor/s
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Glacio-marine clay resistivity as a proxy for remoulded shear strength: correlations and limitations
Q1
Michael Long
1*, Andreas Aspmo Pfaffhuber
2, Sara Bazin
2, Kristoffer Kåsin
2, Anders Gylland
3& Alberto Montaflia
31School of Civil Engineering, University College Dublin, Newstead Building, Belfield, Dublin 4, Ireland
2Norwegian Geotechnical Institute, Oslo, Norway
3Multiconsult, Trondheim, Norway
A.A.P.,0000-0002-0885-8349; S.B.,0000-0001-6478-7760; S.B.,0000-0001-6478-7760; K.K.,0000-0002-2397-2342;
A.M.,0000-0002-6428-372X
* Correspondence:Mike.Long@ucd.ie
Abstract: In geotechnical engineering in Norway, Sweden and Canada the presence of sensitive and/or quick clays poses a major challenge. Formation of these clays involves the leaching of salt from the pore fluid. Thus it has been recognized that electrical resistivity measurements could be useful in delineating leached and unleached clays. This paper seeks to assess the applicability, repeatability and reliability of the various geophysical techniques in the study of sensitive clays. It also attempts to understand the factors that control the measured resistivity and in particular to determine the limitations of directly obtaining the remoulded shear strength from the resistivity measurements. It was found that borehole, surface and airborne resistivity measurements are accurate and compatible. For the 30 Norwegian sites studied it was found that resistivity is primarily defined by the porewater salt content, with minor additional influence by clay content and plasticity, and porosity. A relationship exists between resistivity and remoulded shear strength but this is limited to material deeper than the dry crust and a surface weathering zone of about 7.5 m thickness. High resistivity (>10Ωm) may indicate quick or weathered clay but low resistivity (<10Ωm) conclusively points to stable, unleached clay.
Received7 December 2016;revised4 August 2017;accepted12 October 2017
Sensitive glacio-marine clays, so called quick clays, are typically found in Norway, Sweden and Canada (there often referred to as Leda or Champlain Sea clays), and are characterized by a remoulded undrained shear strength (cur) that is considerably lower than the intact undisturbed shear strength (cu). In geotechnical engineering the presence of sensitive clays poses a major challenge. The landslide at Rissa in 1978 (Gregersen 1981) is perhaps the most famous quick clay slide, as the sliding action was captured on camera. More recently in Norway the slides at Tosbotn in April 2016 and Sørum in November (Fig. 1), are devastating reminders of the potential threats related to such soils. The Tosbotn slide carried away three houses and cut the only road connecting Brønnøysund ( population c. 5000) with the mainland, forcing all traffic onto ferries. At Sørum three forestry workers were killed by the slide, which encapsulated an area of 270 m × 420 m and had a runout distance of some 1 km. For the geotechnical engineer in a construction project, or during regional or local hazard assessment, it is hence important to determine if there is sensitive clay present and, if there is, to determine the extent of the deposit.
The Scandinavian post-glacial marine clays were deposited in a marine environment during and after the last ice age some 10 000 years ago, entrapping porewater of high salt content in the voids. Leaching of the porewater by meteoric groundwater flow has diluted the porewater salinity in some clays. Without its salt, the clay structure can easily collapse and the clay becomes quick. According to the Norwegian definition quick clay is one in whichcuris less than 0.5 kPa (NGF 2011). The most reliable method to confirm quick clay is sampling and index testing in the laboratory to measure curand sensitivity (St=cu/cur). However, these tests are costly for systematic quick clay hazard zonation.
The electric resistivity method goes back as far as the early 20th century and was primarily developed to distinguish oil-bearing from
water-bearing layers (Archie 1942). In general, resistivity (the ability to conduct electrical current) of soils and rocks is a function of Q2 porosity, the ion content or salinity of the porewater, clay content, and the presence of charged minerals such as graphite and some sulphides (see, e.g.Rhoadeset al. 1976;Palacky 1987). For clays in general, and for leached clays in particular, it is mainly the salt content that influences the resistivity (Shevnin et al. 2007). The resistivity is normally higher in leached clay than in the intact marine clays. By measuring the soil resistivity, one may hence be able to deduce the potentially leached zones. Results from Canada, Sweden and Norway clearly point towards a relationship between the geotechnical sensitivity and the measured resistivity.Aylsworth & Hunter (2004) showed the resistivity contrast between leached and saline Leda clay, Dahlinet al. (2013)described comparable studies based on quick clay in Sweden, and Rømoen et al. (2010) gave resistivity ranges for various Norwegian soil types (and their significant overlap). Based on a number of Norwegian sites, Solberg et al. (2012) suggested a classification scheme (seeTable 1), a simplified approach that may be applied for a confined region with consistent sedimentation history.
With these principles in mind resistivity measurements have been carried out, using several different techniques, in a significant number of Norwegian marine clay sites over recent years. This paper will review the methods used, assess whether repeatable and reliable values of resistivity can be obtained, evaluate the scale effects and zone of influence pertaining to each of the methods, and determine the usefulness of the techniques in quick clay mapping projects. This will be achieved by assembling data from 30 sites, comparing the results from several methods and examining which parameters control the measured resistivity.
First, some characteristics of a typical Norwegian quick clay site will be presented, followed by a short discussion on the geochemistry of quick clay.
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Research article Quarterly Journal of Engineering Geology and Hydrogeology
Published Online First https://doi.org/10.1144/qjegh2016-136
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Example of typical Norwegian quick clay site
The Tiller quick clay research site, located just south of Trondheim, has been used by researchers at the Geotechnical Division of the Norwegian University of Science and Technology (NTNU, formerly NTH) for research purposes for many years. Full details of the geotechnical properties of the Tiller site have been given by Gyllandet al. (2013). For the site, average water content (w) is about 36%, unit weight (γ) is 18.7 kN m−3and clay content is some 36%, and these parameters remain reasonably constant with depth (Fig. 2). On average, the plasticity index (Ip) is about 5%
(Table 3), and is perhaps a little lower in the quick clay zone. Q3 Details of the measuredSt,curand salt content values are also given inFigure 2. It can be seen that the site comprisesc.8 m of low- sensitivity clay over quick clay. Despite the clear distinction between the low-sensitivity clay and quick clay at about 8 m depth, the salt content of the pore fluid remains more or less constant throughout the profile, with an average value of about 1 g l−1. Considering that the material was deposited in marine conditions, it is clear that the material has been leached throughout the profile.
This finding, which is common for many Norwegian sites, poses a significant challenge in using resistivity to distinguish between non- quick clays that have been‘over-leached’and those that have been leached and are also quick, as both have a similar resistivity. This is generally a shallow phenomenon and further examples (e.g.Fig. 3) Table 1.Classification of Norwegian marine clays according to resistivity value (Solberg et al. 2012)
Material Resistivity range (Ωm)
Unleached marine clay 1–10
Leached, possibly quick clay 10–80/100
Dry crust clay, slide deposits, coarser material such as sand and gravel, and bedrock
>100
Fig. 1.
Q31 Recent 2016 landslides in quick clay. (a) Tosbotn slide, April 2016, showing damaged houses and the blocked regional road in the background ( photograph: Ole-Christian Olsen, Norwegian Broadcasting Corporation/NRK; Brønnøy skred_Tosbotn_Ole-Christian Olsen_NRK.
jpg). (b) Sørum slide, November Colour
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Fig. 2Tiller site: (a) sensitivity; (b) remoulded shear strength; (c) salt content; (d) resistivity.
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M. Longet al.
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show the clearly lower resistivity of deeper, unleached clay units.
Some further explanation of the nature of leached yet non-quick clays is given in the following.
Geochemistry of marine clays
The salt content in the depositional environment for the Scandinavian clays may have been in the range 30–35 g l−1and was highly dominated by sodium (Na+) and chloride (Cl−) (see, e.g.
Moumet al. 1971;Appelo & Postma 2005;Mitchell & Soga 2005).
Na+is the abundant ion both in the porewater and in the adsorbed positions on the mineral surface in clays sedimented in the glacio- marine environment. This high salt content suppresses the diffuse double-layer surrounding the clay minerals, resulting in low repulsive forces and a stable structure.
Following isostatic uplift groundwater can migrate through the clay deposits, diluting and changing the ion composition in the porewater. Flowing groundwater is often dominated by divalent ions such as magnesium (Mg2+) and calcium (Ca2+), both of which are preferred over Na+by the mineral surface (Mitchell & Soga 2005). As Mg2+and Ca2+are absorbed onto the mineral surface Na+ is released into the pore fluid. The salt content is now reduced and thus the thickness of the diffuse double-layer increases, which in turn leads to a rise in the repulsive forces between the clay particles (see, e.g. Penner 1965; van Olphen 1977;Torrance 1983). The material now has a lowcur, a highSt, is easier to remould and may even be quick.
Leaching is a continuing process; more and more divalent ions enter the clay–water system and thus the concentration of Na+in the porewater is further depleted. These divalent cations have a greater impact on the double-layer thickness than the monovalent cations at the same concentration (see, e.g.Helleet al. 2017a,2017b). The salt content is still low but the divalent ions suppress the diffuse double- layer and the repulsive forces now decrease, thus gradually again increasingcurand reducing St. It is also for this reason that the
geotechnical properties of quick clays can be improved by treatment with potassium chloride (KCl). K+is preferred over Na+by the mineral surface and thus has a greater effect in suppressing the diffuse double-layer and reducing the repulsive forces.Eggestad &
Sem (1976) and Helle et al. (2015, 2016) have shown how introducing KCl by salt wells improved the characteristics of quick clays at the Ulvensplitten and Dragvoll sites in Norway.
Helleet al. (2017a)suggested that the clay behaviour changes from quick to non-quick at the threshold value of 20% of the ratio of the sum of K+, Mg2+and Ca2+to the major cations. An example of this idea applied to the Tinghuset site in Drammen, Norway (Moum et al. 1971,1972) is shown inFigure 3. At this site two boreholes 25 m apart showed distinctly different conditions. BH1 had a 3 m zone of quick clay, whereas BH2 had a 10 m quick clay zone (Fig. 3a). However, both had leached clay with very low salt content throughout the sequence (Fig. 3b). The ratio of the sum of K+, Mg2+
and Ca2+to that of the major cations, that is, (K++ Mg2++ Ca2+)/
(Na++ K++ Mg2++ Ca2+), is 20% or less over the quick clay zone in both boreholes (Fig. 3c). An understanding of the geochemistry of marine clays is clearly a key to identifying quick clay zones.
Study sites
A summary of the sites studied as part of this work is given inTables 2and3. The reader is referred to the references given inTable 2for further details of the sites. The sites are distributed over Norway’s most populated, quick clay prone areas in South East and Central Norway (Fig. 4). The sites 1–11 in SE Norway are located in the counties Østfold, Akershus, Buskerud, Vestfold and Telemark, and the Central Norwegian sites, 12–30, are in Trondheim, Sør Trøndelag and Nor Trøndelag. Some soil properties at the study sites are summarized in Table 3. In general, the sites are all underlain by soft or medium stiff lightly overconsolidated slightly silty clays. The range of the measured soil properties is relatively Fig. 3.Tinghuset site, Drammen: (a)St; (b) salt content; (c) ion content of porewater (Moumet al. 1971,1972).
Resistivity and remoulded clay shear strength
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narrow, withwandγvalues being typically in the ranges 30–45%
(mean about 33%) and 1.8–2.0 Mg m−3respectively. Clay content is relatively high, being typically 30–45% (mean at 35%), andIpis more or less always less than 20% and frequently less than 10%.St values vary widely.
Resistivity techniques
A detailed description of the techniques used in this study to measure resistivity in geotechnical boreholes, from the surface and from the air, has been given byPfaffhuberet al. (2016)and these techniques are briefly summarized as follows.
Resistivity cone testing (RCPTU)
The sounding equipment used for RCPTU consists of an ordinary piezocone (CPTU) probe and a resistivity module mounted behind the probe. Scandinavian manufacturers of RCPTU equipment have chosen to equip their resistivity probes with four ring-electrodes.
The two outer electrodes transmit electric current into the soil, whereas the two inner electrodes measure the difference in potential.
The electrodes need to be in contact with the soil volume where the measurements take place. The module is powered by batteries, and it can read, store and transmit measured data acoustically through the rods or via an electric cable to a receiver on the surface. The measured data can also be stored on a digital memory-card mounted
in the probe. The resistivity depth profile is limited only by the maximum borehole penetration depth (of the order of 50–70 m).
The module needs to be regularly calibrated in brine solutions of salt and water to ensure correct readings.
Electrical resistivity tomography (ERT)
ERT is a geophysical ground imaging method in which DC electrical current is injected into the ground via short steel electrodes installed 10–20 cm into the ground. By measuring the differences in electric potential at the ground surface, a measure of the soil resistance is obtained for all electrode locations or a combination of electrode pairs. Typically electrode spacing varies between 2 and 4 m for high-resolution surveys that are needed for quick clay investigations. The measuring profiles are organized in one or more straight lines. Use was made of both the Wenner and Gradient array systems. The Gradient array uses a large number of potential electrode combinations scanning across the electrode layout and can yield up to seven times more data than the Wenner array in a shorter time, and thus can be useful for examining lateral changes in resistivity (Dahlin & Zhou 2006). Present-day equipment can measure potentials on several parallel channels and the total time required for measurements in a profile is under 1 h. The investigation depth is defined by the maximum distance between the current and potential electrodes, and the resolution is defined by the electrode spacing. Generally the investigation depth will be 10– 20% of the profile length, depending on the resistivity distribution in the soil. By processing the data and running an inversion algorithm, a 2D or 3D or even 4D resistivity model of the ground can be obtained. The software RES2DINV was used to invert all of the ERT data acquired in this study. Details of the processing method have been given byLoke & Barker (1996)andLoke (2016).
Usually resistivity is gradually increased or decreased laterally and in depth until the model fits the data, leading to a smooth resistivity model. There exists no unique resistivity model for an ERT measurement, and use of different calculation models can illustrate the uncertainty (Bazin & Pfaffhuber 2013).
Airborne electromagnetics (AEM)
AEM measurements are used to map the electrical resistivity of the ground in a larger area. The sensor (antenna) of the AEM equipment is operated at a height of about 30 m above the ground surface, and is usually lifted by a helicopter. Modern airborne systems have sufficient resolution to allow use in geotechnical applications.
Different AEM systems are available, some adapted to the need for large penetration depths for mineral exploration, others for more shallow applications in hydrogeology and geotechnical engineer- ing. All systems have in common that a magnetic field generated by the antenna induces current in the ground, which propagates downward and outwards. The rate of change in the electromagnetic field produced by these currents is recorded by a secondary coil. By inversion of the measured data points, the resistivity distribution in the ground can be modelled. Interpretation of AEM resistivity data with regard to sediment properties has so far been done manually and is an advanced task. The possible investigation depth may vary from 50 m to about 500 m, depending on the geology and type of soil in the area, the AEM system and the influence of noise from surrounding infrastructure.
Laboratory resistivity measurements
Both horizontal and vertical (relative to the direction of sampling) laboratory resistivity can be measured by cutting the samples into cubes ofc.4 cm side and measuring the resistance between two copper plates. Measurements are usually taken using a sinusoidal Fig. 4.Map showing location of study sites in Norway. (Jean-Seb
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M. Longet al.
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Resistivity and remoulded clay shear strength
661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726
727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 Table3.Summaryofsoilpropertiesatstudysites −3−Sitew(%)ρ(Mgm)Clay(%)I(%)SOCRQ34V(ms)Resistivity(Ωm)pts 1Onsøy60–651.63540–6033–404.5–61.5–1.380–1401–36 2Kløfta(BH2284)28–451.71–2.0012–191–903–51 Kløfta(BH3043)32–491.78–1.8715–225–73 3E16Sandvika(BH1306)38–501.69–1.949–178–2621–37 4DobbeltsporSkøyen-Asker22–401.8–2.1310–200110–18020–49 5RVIIHilleren30–401.82–1.8928–458–187–1351.2–2.6170–20021–76 6Museumpark50–551.72–1.7848307–81.5100–1704–37 7Smørgrav35–451.80–1.9336–609–225–77105–2303–62 8Vålen35–471.85–2.0137–3915–225–151.2–1.8100–2403–65 9Hvittingfoss22–361.91–2.0917–364–6>100150–25033–95 10MånejordetQ35 2.5–5.528–501.83–2.092014–16<104.5–5.5110–18035–165 5.5–14.525–401.83–2.0024–276–950–350 11Skienselven26–331.95–2.03110–24086–15063–169 12Tiller30–451.8–2.035–402–85–10002–475–23029–102 13Esp30–501.75–1.9530–403–1510–1152–4100–2206–192 14Klett(South)22–361.92–2.0428–352–1010–2401.5–3120–2504–108 15Klett(North)30–401.92–2.0427–366–1010–3001.2–2.44–52 16Dragvoll30–421.88–2.028–484–1216–1521–2110–19034–113 17Leire25–2931–385–75–11524–102 18Rissa(Reinkirke)28–401.85–2.042–477–1210–602–4100–2804–170 19Nidarvoll25–451.78–2.041–205–200+32–80 20Melhus,Fallan30–371.88–1.9732–343–1210–1443–?15–68 21Melhus,Kaldvelladalen28–421.92–1.97357–126–1442–100 22Buvika30–391.87–1.9728–4712–1715–250+7–66 23Rødde26–331.95–2.0130–473–2352–107 24Levanger,Rinnan25–321.99–2.044255–4031–88 25Levanger,Fleskhus21–311.89–2.0450108–10030–188 26Levanger,Hove20–301.94–1.9850102–2011–92 27Grong26–331.98–2.126–133–51041–129 28Kattmarka–Gullvika28–381.92–2.0124–356–94–6711–66 29Kattmarka29–361.91–2.0442–448–127–351.45–7.0105–1905–27 30Finneidfjord28–401.86–2.099–173–1137–212 792
M. Longet al.
793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858
859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 Fig. 5.Resistivity measurements for sites in Trondheim area: (a) Dragvoll; (b) Nidarvoll; (c) Klett South; (d) Klett North.
Colour online= colour hardcopy
Resistivity and remoulded clay shear strength
