Remoulded Shear Strength Research Articles

Potassium chloride wells used as quick-clay landslide mitigation: installation procedures, cost–benefit analysis, and recommendations for design

Retrogressive development of landslides in highly sensitive clays (quick clays) may extend several hundred metres upslope from an initial landslide, and liquified slide-debris may impact buildings or infrastructure in the run-out zone. By installing wells filled with potassium chloride (KCl) in quick clays, the salt migrates into the surrounding clay and increases its remolded shear strength, reducing its sensitivity. The salt-stabilized, nonquick clay volume may act as a barrier preventing backward retrogression, thereby contributing to reducing both the area susceptible for being involved in a quick-clay landslide, and the area of the run-out zone. Installation procedures and design guidelines for salt stabilization are examined herein. Installation procedures generating temporary, very small excess pore-water pressures were tested at National Geo-Test Site Tiller–Flotten. Although the benefit-to-cost ratios related to these procedures are small compared to conventional landslide mitigation measures, reducing the installation costs to less than 30 USD per m well and increasing the center distance between the wells may justify salt-stabilization as a landslide mitigation measure. This paper describes experience from testing safe installation procedures, evaluations of cost–benefit and environmental impact, and proposed design guidelines, introducing KCl as an alternative to conventional landslide mitigation-measures in slopes with highly sensitive quick-clay deposits.

Experimental investigation of the impact of salinity on Champlain Sea clay

This paper presents an experimental investigation of the impact of leaching on the geotechnical properties of Champlain Sea clay. Champlain Sea clay has a high salinity level in its pore fluid. A leaching apparatus was developed to leach a series of 15-cm diameter undisturbed Champlain Sea clay samples to different salinity levels. A series of geotechnical tests were conducted on leached samples. The salinity levels investigated in this study ranged from an initial salinity value of 9.50 g/l to as low as 0.35 g/l. Based on test results, it was found that leaching causes profound and significant property changes in Champlain Sea clay. It results in a greater reduction for the liquid limit than that of the plastic limit. In the study, the plasticity index was reduced by 14% to 31% due to leaching, while the liquidity index reduced by 4% to 26%. Leaching also causes an increase in the sensitivity of the clay since leaching reduces more the remolded shear strength of the clay than that of undisturbed shear strength. Within the studied salinity levels, leaching increased the compressibility of Champlain Sea clay by 20% and reduced the preconsolidation pressure of the clay sample by one-third. At the same time, leaching reduced the shear strength by more than 40% and undrained shear modulus of Champlain Sea clay by about 50%.

Chapter 7 Quick clay behaviour in sensitive Quaternary marine clays – a UK perspective

Abstract The term quick clay has been used to denote the behaviour of highly sensitive Quaternary marine clays that, due to post depositional processes, have the tendency to change from a relatively stiff condition to a liquid mass when disturbed. On failure these marine clays can rapidly mobilise into high velocity flow slides and spreads often completely liquefying in the process. For a clay to be defined as potentially behaving as a quick clay in terms of its geotechnical parameters it must have a sensitivity (the ratio of undisturbed to remoulded shear strength) of greater than 30 together with a remoulded shear strength of less than 0.5 kPa. The presence of quick clays in the UK is unclear, but the Quaternary history of the British islands suggests that the precursor conditions for their formation could be present and should be considered when undertaking construction in the coastal zone.

Predicting required time for stabilising Norwegian quick clays by potassium chloride

Salt wells filled with potassium chloride (KCl) were installed in a highly sensitive, low-salinity, glaciomarine quick clay at Dragvoll, Trondheim, Norway, to investigate the improvement in geotechnical properties and the required time to stabilise the clay volume sufficiently to inhibit retrogressive landslide development. Improving the remoulded shear strength (c ur) beyond 1 kPa reduces the risk for successive back-scarp failures. When more than 20% of the cations in the pore water consist of potassium (K+), magnesium (Mg2+) and calcium (Ca2+) ions, c ur is improved beyond 1 kPa. Optimisation of the well grid is necessary prior to installation to ensure that c ur > 1 kPa within 2–3 years. PHREEQC simulations predicted a diameter of stabilised clay of minimum 1·5 m within 3·3 years after installation, which fits reasonably with observed pore-water compositions and improved geotechnical properties in situ. Salt wells may be used as landslide mitigation, preventing backward successive, retrogressive development, and PHREEQC can be used to give a rough estimate of the required time to stabilise the quick-clay volume around the wells.

Storage Duration Effects on Soft Clay Samples

Abstract The effects of storage duration on the sample quality of 54-mm piston samples were investigated by an extensive laboratory study. A soft clay deposit in Central Norway was used as a research site—the Tiller site. The soft clay deposit consists of two types of leached marine clay, a low-sensitivity and high-sensitivity soft clay. In total, 21 boreholes were drilled, and four samples were retrieved from each borehole between 5 m and 11 m in depth, two samples from each layer. Immediately after the sampling, the thin-walled stainless steel sampling tubes were sealed airtight and stored at the temperature of 4°C, slightly lower than the ground temperature. The storage duration varied between 0 days to 603 days, and the laboratory testing was conducted periodically, using the same equipment and test procedures. Results indicate a reduction of undrained shear strength, preconsolidation pressure, and constrained modulus with increasing storage duration. The reduction in the material properties and sample quality is rapid for the high-sensitivity soft clay, but marginal for the low-sensitivity soft clay samples. The effects of storage duration seem to be caused by physical processes taking place within the sample, as no significant changes were observed in the measurements of the water content, Atterberg limits, remolded shear strength, and pore water chemistry during the storage of the samples. In light of the laboratory results and the existing literature, an attempt has been made to explain the cause of the observed changes of the material properties and sample quality during storage of soft clay samples.

Glacio-marine clay resistivity as a proxy for remoulded shear strength: correlations and limitations

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 (>10 Ω m) may indicate quick or weathered clay but low resistivity (<10 Ω m) conclusively points to stable, unleached clay.

Effectiveness of resistivity cone penetration tests in salt-treated highly sensitive clay

Salt wells filled with potassium chloride may be used as landslide mitigation in highly sensitive quick clays, as the changed pore-water composition permanently improves the remoulded shear strength. To verify improvements, resistivity cone penetration tests were conducted around salt wells installed at Dragvoll, Trondheim, Norway. The cone-test measurements for electric resistivity (or conductivity), tip resistance and pore pressure were combined with the results from laboratory tests on piston-core samples extracted around the wells. Salt content, soil and pore-water conductivity, pore-water composition and geotechnical properties were studied. Quick clay is defined as that having a remoulded shear strength of <0·5 kPa, and the soil conductivity is often <100 mS/m. Increased tip resistance was detected at soil conductivities exceeding 200 mS/m as a result of increased salt content, corresponding to a laboratory-determined remoulded shear strength of 3·5 kPa. At Dragvoll, improved non-quick clay is detected by a normalised tip resistance of 3·5 and a pore-pressure parameter of 0·9. Improved geotechnical properties around salt wells may be verified by cone testing, but a site-specific interpretation model based on geotechnical properties from laboratory tests correlated to cone-test results may be needed.

Integrated scanning for quick clay with AEM and ground-based investigations

Near shore, high latitude lowlands throughout Canada, Norway, Sweden, Finland and Russia are prone to a particular geohazard – quick clay. The key geotechnical properties of quick clay, also called Leda clay in Canada, are a very small remoulded shear strength (< 0.5 kPa in Norway after NGF, 2011; and <0.4 kPa in Sweden after Rankka et al., 2004) and consequent high sensitivity (= undisturbed/remoulded shear strength). These properties stem from the fact that this formerly marine clay has been leached by ground water owing to postglacial uplift, losing salt ions that stabilized the flocculated clay structure. When quick clay fails, it liquefies and leads to retrogressive landslides that have caused massive damage and claimed lives both in North America and Europe. Quick clay can be found in sedimentary areas close to the coast and below the marine limit, areas that are attractive for human settlements. Two of Norway’s four most populated urban areas (Oslo and Trondheim) are located in quick clay areas. Detailed study and mapping of areas prone to quick clay slides relies on geomorphology, quaternary geology and point information from geotechnical boreholes and laboratory tests. This is a labour intensive and long lasting process that, in Norway, has been going on since the 1980s and only the most vulnerable parts of the country are mapped so far. The detailed hazard mapping continues, at a pace of a hand full of municipalities a year (Figure 1).

Displacement rate effects during T-bar cycling in remoulded Gulf of Mexico clay

The influence of the displacement rate of a T-bar on the remoulded shear strength of a soft, natural, lightly overconsolidated marine clay is investigated. A series of T-bar cyclic tests was carried out in 12 box cores collected from the Gulf of Mexico. One general equation relating the displacement rate to the remoulded strength is presented, which provides insight on rate effects for remoulded clays and which can be used to correct for remoulded penetration resistance data for natural soft clays for a T-bar recorded at displacement rates other than the commonly used displacement rate of 0·020 m/s.

A new attraction-detachment model for explaining flow sliding in clay-rich tephras

Abstract Altered pyroclastic (tephra) deposits are highly susceptible to landsliding, leading to fatalities and property damage every year. Halloysite, a low-activity clay mineral, is commonly associated with landslide-prone layers within altered tephra successions, especially in deposits with high sensitivity, which describes the post-failure strength loss. However, the precise role of halloysite in the development of sensitivity, and thus in sudden and unpredictable landsliding, is unknown. Here we show that an abundance of mushroom cap–shaped (MCS) spheroidal halloysite governs the development of sensitivity, and hence proneness to landsliding, in altered rhyolitic tephras, North Island, New Zealand. We found that a highly sensitive layer, which was involved in a flow slide, has a remarkably high content of aggregated MCS spheroids with substantial openings on one side. We suggest that short-range electrostatic and van der Waals interactions enabled the MCS spheroids to form interconnected aggregates by attraction between the edges of numerous paired silanol and aluminol sheets that are exposed in the openings and the convex silanol faces on the exterior surfaces of adjacent MCS spheroids. If these weak attractions are overcome during slope failure, multiple, weakly attracted MCS spheroids can be separated from one another, and the prevailing repulsion between exterior MCS surfaces results in a low remolded shear strength, a high sensitivity, and a high propensity for flow sliding. The evidence indicates that the attraction-detachment model explains the high sensitivity and contributes to an improved understanding of the mechanisms of flow sliding in sensitive, altered tephras rich in spheroidal halloysite.

Development of cyclicp–ycurves for laterally loaded pile based on T-bar penetration tests in clay

Offshore pile foundations are always subjected to cyclic lateral loads, which can result in the remolding and softening of the surrounding seabed soil. Cyclic T-bar penetrometer testing provides a rapid and effective method for assessing the remolded shear strength. It is widely believed that the soil strength degrades with the accumulation of plastic strain, but the strain cannot be measured. Numerical analysis described in this paper shows that the accumulated plastic displacement of the T-bar in a cyclic range of two diameters is approximately equal to its accumulated displacement. By using T-bar test data, a cyclic degradation model based on the accumulated (plastic) displacement is developed to describe the soil strength degradation at a given depth. Furthermore, an improved p–y curve model based on the cyclic degradation model is proposed to estimate the lateral response of pile under cyclic loads. The improved p–y curve model was embedded into the OpenSees program to investigate the cyclic lateral responses of soil elements and the pile. A case study was conducted to verify the improved p–y curve model by comparing it with published centrifuge experiment data. Results indicate that the improved p–y curve model based on T-bar test data is highly precise and practicable.

Long-term effect of potassium chloride treatment on improving the soil behavior of highly sensitive clay — Ulvensplitten, Norway

Re-establishing high salt concentration in leached low-saline, highly sensitive clays significantly improves their mechanical properties. Long-term effects on quick clay exposed to diffusion of potassium chloride (KCl) from salt wells installed in 1972 at Ulvensplitten, Oslo, Norway, are investigated. The increased undrained and remolded shear strengths, as well as increased Atterberg limits, remain 30 to 40 years later. The undisturbed shear strength increased from less than 10 to 25–30 kPa, and the remolded shear strength increased from less than 0.5 to more than 6 kPa. The liquid limit increased beyond the natural water content. Adding KCl to quick clay improved the properties to such an extent that it no longer appears as quick. Recent ground investigations in the area suggest a permanent effect on an engineering time scale. Consequently, the method may be suitable to prevent large flow slides in quick clay areas.

The 1994 landslide at Sainte-Monique, Quebec: geotechnical investigation and application of progressive failure analysis

In 1994, a landslide occurred in the municipality of Sainte-Monique, Quebec. The debris of the landslide had graben and host shapes, typical of spreads in sensitive clays. The geotechnical investigation shows that the soil involved is a firm to stiff, sensitive, nearly normally consolidated grey silty clay of high plasticity. This soil exhibits a high sensitivity and a high brittleness during shear and is therefore susceptible to progressive failure. Traditional stability analysis cannot explain this landslide. This gives the opportunity to examine the applicability of progressive failure analysis to this spread. Using the finite elements method, it is demonstrated that the initiation and observed extent of the failure surface are explained by a soil having high brittleness during shear and a large-deformation shear strength close to the remoulded shear strength of the soil. The dislocation of the soil mass can also be explained by the active failure occurring in the soil mass above the failure surface during or shortly after failure propagation. It is therefore numerically demonstrated that progressive failure explains the initiation and the extent of the failure surface of this spread.

Compression and undrained shear strength of remoulded clay mixtures

Clay mixtures are a by-product from open pit mining during the excavation of various soil layers. The mechanical behaviour of landfills composed of these clay mixtures is complex. In the present study, two clays were used to produce soil mixtures artificially in the laboratory. The physical properties, compression behaviour and remoulded undrained shear strength of the resulting materials were investigated. The test results show that the actual measured values of the liquid limit were close to the expected theoretical value (the constituents are supposed to contribute proportionally to their amounts), but there was a non-negligible difference in the plasticity indices. The constants of intrinsic compressibility are affected by both the initial void ratio and the liquid limit. The remoulded shear strength of the clay mixtures can be also normalised with the liquid limit or the consistency index.

Discussion of “Atterberg Limits and Remolded Shear Strength—Water Content Relationships”

Abstract The present discussion tries to bring out the importance of clay mineralogical composition of fine-grained soils on their liquid limit behaviour. It reinforces the author's observation that the undrained shear strengths at liquid limit water content and at plastic limit water content are not unique.

Closure to Discussion by K. Prakash and A. Sridharan of “Atterberg Limits and Remolded Shear Strength–Water Content Relationships” [Geotechnical Testing Journal, Vol. 36, No. 6, pp. 939–947

Abstract This article presents the closure to the discussion on the Geotechnical Testing Journal, GTJ20130012, Vol. 36, No. 6, which had three stated objectives: (i) a critical assessment of empirical methods for predicting remolded undrained strength (sur) at different water contents on the basis of measured Atterberg limit values; (ii) a review of sur values mobilized at the Casagrande liquid limit (LL) and plastic limit (PL) conditions and hence the strength variation over the plastic range; and (iii) a new approach was propose for predicting sur values mobilized at different water contents. The original article stated the importance of clay mineralogy on clay behavior. A referenced comment was made regarding discourse by other researchers in recent literature on the link between mineralogy and differences in strengths at LL determined using the Casagrande cup and fall cone approaches. Although outside the immediate scope of the original article, this closure presents the opportunity to elaborate on this discourse since specifically raised by the discussers. The discussers also raised an unrelated but important point, namely attempts by several researchers of late to determine the PL using variants of the fall cone method. The author concurs with the discussers that these approaches deduce a strength-related reference water content that is fundamentally different from the lower water content limit for soil workability, which the Casagrande PL can be considered.

Quick Clay Development and Cation Composition of Pore Water in Marine Sediments from the Ariake Bay Area, Japan

By analyzing the cation composition of pore water in the soil samples of Ariake Bay sediments, the present study assesses the development of quick clay by leaching in both the original and seawater-saturated soil samples. Divalent cations were dominant in the pore water of the original soil sample, whereas Na+ was the major cation in that of the seawater-saturated soil sample. The cation proportion in the pore water for both soil samples remained the same after leaching. The difference in pore water cation composition between the original and seawater-saturated soil samples affected how their geotechnical properties changed through leaching. The undisturbed shear strength of both soil samples remained almost the same, but a large disparity between the soil samples was observed in the remolded shear strength: it remained almost the same in the original soil sample after leaching. Hence, sensitivity was not increased and quick clay was not formed. However, in the seawater-saturated soil sample, the remolded shear strength decreased to a great extent, and quick clay with a sensitivity exceeding 700 developed. The lack of development of quick clay in the original soil sample is attributed to the dominance of divalent cations in the pore water, and the development of quick clay in the seawater-saturated soil sample is ascribed to the dominance of Na+ in the pore water.

Discussion: Remoulded shear strength at plastic and semi-solid states

The undrained shear strength of remoulded soils is of great concern in geotechnical engineering applications. This study aims to develop a reliable approach for determining the undrained shear strength of remoulded fine-grained soils, through the use of index test results, at both the plastic and semi-solid states of consistency. Experimental investigation and subsequent analysis involving a number of fine-grained soils of widely varying plasticity and geological origin have led to a two-parameter linear model of the relationship between logarithm of remoulded undrained shear strength and liquidity index. The numerical values of the parameters are found to be dependent to a lesser extent on the soil group and to a greater extent on the soil state. Based on the values of regression coefficient, ranking index and ranking distance, it seems that the relationship represents the experimental results well. It may be pointed out that the possibility of such a relationship in the semi-solid state of a soil has not been explored in the past. It is also shown that the shear strength at the plastic limit is about 32–34 times that at the liquid limit.

Atterberg Limits and Remolded Shear Strength—Water Content Relationships

The remolded undrained shear strength (sur) is of importance in many geotechnical applications. This paper uses published regression analyses for strength variation with water content (w) to draw some generalized conclusions on soil strength behavior, including values of sur mobilized for the Casagrande liquid limit (LL) and plastic limit (PL) conditions and, hence, the shear strength variation occurring over the plastic range. Reported methods of deducing strengths from measured values of water content (w) at the Atterberg limits are reviewed, many of which assume a strength variation of 100 over the plastic range. Regression analysis of reported log w: log sur correlations for 14 mineral soils and four organic sediments indicated that the strength variation is generally not 100 and can vary significantly between soils. Hence, a new approach is proposed for predicting values of remolded undrained strength mobilized for different water contents. Whereas existing formulations are based on empirical sur values associated with the Atterberg limits, the proposed approach is based on direct strength measurements along with water contents for two test specimens.

Remoulded shear strength at plastic and semi-solid states

The undrained shear strength of remoulded soils is of great concern in geotechnical engineering applications. This study aims to develop a reliable approach for determining the undrained shear strength of remoulded fine-grained soils, through the use of index test results, at both the plastic and semi-solid states of consistency. Experimental investigation and subsequent analysis involving a number of fine-grained soils of widely varying plasticity and geological origin have led to a two-parameter linear model of the relationship between logarithm of remoulded undrained shear strength and liquidity index. The numerical values of the parameters are found to be dependent to a lesser extent on the soil group and to a greater extent on the soil state. Based on the values of regression coefficient, ranking index and ranking distance, it seems that the relationship represents the experimental results well. It may be pointed out that the possibility of such a relationship in the semi-solid state of a soil has not been explored in the past. It is also shown that the shear strength at the plastic limit is about 32–34 times that at the liquid limit.