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Loess and fragipans: Development of polygonal-crack-network structures in fragipan horizons in loess ground
Quaternary International 399 (2016) 228e233
Contents lists available at ScienceDirect
Quaternary International journal homepage: www.elsevier.com/locate/quaint
Loess and fragipans: Development of polygonal-crack-network structures in fragipan horizons in loess ground Ian J. Smalley a, *, Stephen P. Bentley b, Slobodan B. Markovic c a
Department of Geography, University of Leicester, Leicester LE1 7RH, UK School of Engineering, Cardiff University, Cardiff CF2 3AA, Wales, UK c Department of Geography, University of Novi Sad, Trg Dositeja Obradovica 3, RS-21000 Novi Sad, Serbia b
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 26 February 2015
One of the defining features of the fragipan horizon is the presence of a blocky polygonal network structure. In loess soils, this network structure can be explained by contraction forces (due to drying) operating after hydrocollapse due to loading and wetting, as in the Bryant hypothesis for fragipan formation. Three stages are identified in the formation of a fragipan horizon in loess ground. There is a deposition phase in which the aeolian deposition of loess material produces certain ground properties. A collapse stage allows the soil structure to deform under the influence of loading and wetting. This collapsed material develops internal tensile forces as drying contraction proceeds and these cause the development of a characteristic crack network. The crack network can be modelled using a very simple Monte Carlo approach and the two dimensional structure produced gives a good representation of fragipan cracking. The collapse-contraction process for fragipan formation offers explanations for the strength and hardness of fragipans, the constant depth to fragipan horizons, the slaking in water (predominance of short range contact bonds) and the mineralogical similarities throughout the system. The fragipan horizon impedes drainage, and this becomes increasingly important as land use becomes more widespread. © 2015 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Fragipan Contraction crack network Loess soils/ground Bryant hydroconsolidation hypothesis Fragipan properties Loess structures and bonds
1. Introduction A fragipan is a diagnostic soil horizon (USDA Soil Taxonomy, 2010, p.7). It is a dense horizon with limited permeability and it tends to develop in loess soils. The fragipan is a much discussed phenomenon (see reviews by Grossman and Carlisle, 1969; Smalley and Davin, 1982; Smeck and Ciolkosz, 1989; Witty and Knox, 1989; Bockheim and Hartemink, 2013). Many defining characteristics have been listed including: the high density relative to other parts of the soil system, the disaggregation reaction when plunged into water, the fairly consistent depth from surface to top of fragipan, the mineralogical similarity to the adjacent soil horizons, the formation where the ratio of rainfall to evapotranspiration is quite high. Witty and Knox (1989) listed 16 points which are required to be considered in the definition of fragipan. Point 10 is the most relevant to this discussion:
* Corresponding author. E-mail addresses: [email protected] (I.J. Smalley), [email protected] (S.P. Bentley), [email protected] (S.B. Markovic). http://dx.doi.org/10.1016/j.quaint.2015.01.034 1040-6182/© 2015 Elsevier Ltd and INQUA. All rights reserved.
It (fragipan horizon) has few or many bleached, roughly vertical planes that are faces of coarse or very coarse polyhedrons or prisms. We discuss the formation of these polyhedrons or prisms, the whole large-scale structural network (see Fig. 1 for sketch impression, from Van Vliet and Langohr, 1981). Some other Witty and Knox points have relevance to the current discussion: 4. Compared to the horizons above it, the bulk density is high. 7. When a dry fragment is placed in water, it slakes or fractures. 11. Most commonly, it has an abrupt or clear upper boundary at a depth of 33e100 cm below the original surface. (At about 70 cm in Fig. 1). The purpose of this paper is to discuss the development of the macro-structure within the fragipan layer, the network of characteristic polygonal units. In Keys to Soil Taxonomy (USDA, 2010, p.7), some requirements are listed for the definition of fragipan horizons. Requirement no.3 demands that the fragipan layer has a very coarse prismatic, columnar, or blocky structure of any grade, has weak structure of any size, or is massive. This is the large-scale structure discussed, and it is hoped that providing a realistic and
I.J. Smalley et al. / Quaternary International 399 (2016) 228e233
Fig. 1. The fragipan in position, after Van Vliet and Langohr (1981). See Smalley and Davin (1982 p.80) for discussion. A version of this figure appears on the cover of Smalley and Davin (1982).
sensible explanation of this characteristic fragipan structure will be a positive contribution to the discussions on the mode of formation of the fragipan itself. 2. Densification Consideration of changes in density will form an important part of this discussion so some critical density related terms need to be defined. Soil is a particulate system and in such a system relative packing density can be defined as the ratio of solids present in the system; the range is from all solid, PD ¼ 1, to all empty, PD ¼ 0. Voids ratio e, the relevant term used in soil mechanics, is defined as the ratio of space in the system to the solids in the system:
ðe ¼ ½1 PD=PDÞ: A voids ratio of 1.0 is equivalent to PD ¼ 0.5. The relationship of e to PD is shown in Fig. 2, which provides a convenient framework to demonstrate changes in the loess soil system as deposition is succeeded by collapse and then by contraction. 3. Deposition The loess deposit is formed by aeolian deposition of largely silt sized material, much of which is quartz. The modal particle in a loess deposit might be considered to be a 30um quartz particle with a definite blade shape. Shape studies are not definitive or conclusive, but there are indications that certain shapes will be favoured. Rogers and Smalley (1993), using a very simple Monte Carlo
229
Fig. 2. The relationship between e voids ratio and PD packing density. Some regular packing densities are indicated, for 600, 402 and 204 packings (see Rogers et al 1994a for explanations). See Dijkstra et al., 1995 for discussions of packing transitions.
method, calculated that the modal shape should be defined by a side ratio of 8:5:2. They called this a Zingg 3 m particle (see Smalley, 1966a for shape terminology) and it does have a remarkably pronounced blade shape (blade is simply defined as a > b > c). The blade shape is the least well-defined of the four straightforward particle shapes. In simple terms it might be called ‘flattish’. These particles, on aeolian deposition, have an open structure with a high porosity, a low packing density, a high voids ratio. It is a structure dominated by short range contacts, a relatively rigid structure. In Fig. 2, the deposition process is given a speculative and imaginative presence. There is no evidence for the initial formation of a very open structure with a voids ratio of around 2, but it is believed that the initial deposition process would involve a modest amount of ‘tamping’ while the system settled to the eventually observed e value of around 1.0. This would be the classic initial loess, the Ur-Loess of Smalley and Krinsley (1981). From this position, the denser structures are developed. In Fig. 2 some very ideal packings are indicated, with e values. Applying packing ideas to soil systems has never been very successful. The best attempt was probably by Morrow and Graves (1969), an attempt which was appreciated and discussed by Dijkstra et al. (1995). The 600 packing is the cubic packing and the 204 packing is the close rhombohedral packing. The transition from 600 to 204 is an interesting collapse manoeuvre, and Dijkstra et al. (1995) made some attempts to relate it to loess collapse. Packing problems in general in geo-science settings have been discussed by Rogers et al. (1994a). In a loess deposit, there is a packing of particles and a set of interparticle bonds. The particles, in place, define the structure; the bonds control the post-depositional
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processes. Two types of bond might be defined to facilitate loessic discussion. There are short range bonds which link primary mineral particles. These are short range because when they are broken all strength is lost. The long range bond connects clay mineral particles, electrically charged particles, which have complex interactions. The long range bond involves charged particles and charged ions dissolved in soil water. The long range bond allows plasticity because strength is retained after the bond is disturbed. In the ideal Ur-Loess deposit, soon after deposition, short range bonds can be expected to predominate. This is an open structure, a rigid structure, a metastable structure, but not a collapsible structure. The bonds need to be modified before collapse can occur. Within the loess soil system, short range and long range bonds have roles to play. For a discussion of short/long range bonds in a geotechnical setting, see Cabrera and Smalley (1973). 4. Collapse Collapse is usually measured in an oedometer, a consolidation testing machine (Fig. 3). The specimen is loaded, then wetted, and the collapse amount is measured. This collapse is central to the Bryant hypothesis for fragipan formation (Bryant, 1989), which was investigated by Assallay et al. (1998). The Bryant hypothesis to explain fragipan formation was distinctive in that it involved a physical/mechanical collapse mechanism rather than a chemical diffusion and deposition mechanism for densification. For the Bryant mechanism to occur, a necessary prerequisite is a collapsible soil system. Bryant (1989 p.147) explained the requirements for a collapsible system: “Preconditions for the formation of a collapsible sediment are: (i) a loose granular contact structure. (ii) a metastable condition, and (iii) bonding agents that reduce upon wetting. The process of collapse occurs on wetting to saturation or near saturation because intergranular bond strength, due to capillary tension, is reduced … The desiccation process following collapse would cause cracking and formation of prismatic structure associated with fragipans, and fabric reorientation would result in an interlocked granular contact structure of greater density.”
The geotechnical problem with loess soils is that they collapse when loaded and wetted in a construction situation, they suffer from hydrocollapse or hydroconsolidation (see Rogers et al., 1994b; Smalley and Markovic, 2014). The collapse process is going to produce a more dense system, a marked reduction in voids ratio is observed, but the initial structural bonds are broken. The mechanism of the collapse process has been discussed for many years (see Rogers et al., 1994b) and it is only recently that a realistic mechanism has been proposed (see Smalley and Markovic, 2014). It is necessary not only to explain the process of collapse but also the ways in which collapsibility developed in the soil system. It has been shown (Assallay et al., 1997, 1998) that collapsibility in a loess soil system depends to a large extent on the clay mineral content. A very low clay mineral content inhibits collapse, the particle contacts are too rigid, too short-range. A large clay mineral content also prevents collapse because the pores are filled, as there is no collapse space available. A clay mineral content of around 15e20% is optimal. In a developing loess soil system, this clay can accumulate and render the system available for collapse. Some downward flow of water through the system is required and it appears that the clay accumulation process is assisted by the formation of fine carbonate mineral needles at major particle contacts. The carbonate mineral network traps clay mineral particles and these accumulate in critical zones until the necessary amount has been emplaced. Then, if the overburden pressure is sufficient and enough water is available in the system, collapse can occur. There is a link between the various factors contributing to collapsibility developing and fragipans (see Smalley and Markovic, 2014 for more discussion). 5. Contraction Some contracting systems form a network of cracks when critical stress levels are reached. The classic examples in the geosciences are basalt flows such as the Giants Causeway or the Dashing Rocks basalts (see Fig. 6). Drying mud often exhibits
The Bryant hypothesis had many attractive aspects and was carefully investigated by Assallay et al. (1998). Collapse occurs when a necessary amount of overburden material has accumulated, and this particular load requirement accounts for the generally observed constancy of depth of the fragipan horizon; this is self-weight collapse. This type of collapse can be characteristic of loess systems.
Fig. 3. The collapse event. A typical oedometer test result (from Smalley and Markovic, 2014). The sample is loaded, and then wetted; considerable collapse strain is observed. The actual collapse requires a major structural reorganization and densification.
Fig. 4. Looking down on the ideal fragipan. This is a contraction crack network by Boots (1977) after Smalley (1966b). Boots generalized the contraction crack simulation process to produce ideal divisions of space in two-dimensions; the so-called ‘Smalley process’.
I.J. Smalley et al. / Quaternary International 399 (2016) 228e233
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Fig. 5. Contact types. 100 contacts in an ideal particulate world. The particles are Rogers-Smalley Zingg 3 type blades and they assume all the contact positions possible. 05, 94, 40 and 59 are archetypical table-leg contacts; 00, 55 and 99 are typical parallel contacts. In the collapse process table-legs decrease and parallels increase, in contraction parallels become closer and contact bonds strengthen.
contraction cracking. Drying or cooling appears to deliver the required stress to cause the contraction crack network to develop. In the fragipan setting, we imagine a situation where hydroconsolidation has occurred in a loess deposit. The
hydroconsolidation has occurred because enough loess has been deposited to provide a sufficient overburden pressure and the system is wet enough to allow deformation at the interparticle contacts. The contracting system is a wet system but one in which
Fig. 6. Fragipan and basalt at the Dashing Rocks section in Timaru, South Canterbury, New Zealand. In the Timaru loess there are three fragipans and the bottom one is nicely exposed on top of the Mount Horrible basalts. This was a pahahoe type lava and has formed distinctive contraction crack structures. The fragipan shows similar contraction crack patterns. The continued existence of the lowest fragipan is a testament to the strength of the compacted fragipan structure.
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the rigid soil structure has been destroyed by structural collapse. It is dense (relatively dense) and weak (relatively weak). Loses moisture on drying tensile stresses develops. A certain amount of loess has to accumulate to cause the structural collapse of hydroconsolidation (i.e. a certain load has to develop to cause the stresses causing compaction). This is why there is this relative constancy of depth to fragipan horizons-the default depth represents the default load. The wetness required for hydroconsolidation accounts for the occurrence of fragipans in wet systems: dryness inhibits fragipan formation. The collapsed system is homogeneous and of low strength, and as the drying cycle begins tensile stresses develop. The situation is essentially the same as in a cooling, solidifying basalt flow, and it can be discussed as a two dimensional problem, basically a twodimensional packing problem. There is a consistent tensile stress across the two dimensional surface, the stress increases as the system cools/dries and eventually at random places the tensile strength of the system is exceeded and cracking develops. The cracks develop at random and the phenomenon can be modelled by using a simple Monte Carlo system. Modelling works well for basalt flows (Smalley, 1966b) and a typical result is shown in Fig. 4. The random division of space in two dimensions can be produced and the technique has been compared to the random division of space by other statistical methods by Boots (1977). His ‘Smalley process’ division is shown in Fig. 4. This is the default division for natural contracting systems-such as basalt flows, and fragipans. A structure of this nature would be expected in a fragipan, if the hydrocollapse process is part of the fragipan formation mechanism. The collapsed weak wet system will contract on drying. 6. A speculative set of interparticle contacts Fig. 5 shows a possible set of interparticle contacts, in an ideal deposit of Ur-Loess. This is speculative and is unsupported by observation. All it can really do is clarify the discussion of the structure development and change in loess ground. The 100 structures shown represent the possible particle contacts in the ideal ground. They might be classified into ‘table-leg’ structures and ‘parallel’ structures, and intermediate structures. In the Gedankenexperiment approach to loess formation and deformation, the table-leg contacts would predominate in the freshly deposited UrLoess and the parallel contacts would predominate in the totally deformed, remoulded loess. The table-leg structures give some rigidity to the initial structure, and ensure a high voids ratio. The decoupled parallel contacts give increased density to the deformed structure but provide little strength. There are 100 imaginary contact structures shown in Fig. 5, so simple counting will reveal percentages of structural types. 04 is a default table-leg but it is exactly the same as 59; 55 is a default parallel structure, and so is 00. Obvious table-legs are about 30% of the contacts shown: these are the contacts which give space and strength to the loess in its deposition state. These are deformed on collapse. The collapsed system will be dominated by parallel type contacts, but initially the particles will be well dispersed. As water films evaporate, the parallel contacts draw closer and contraction occurs. Fairly efficient packing structures will be formed: the stepped face-to-face structure (Smalley and Cabrera, 1969) can be expected to predominate, sustained by simple, short-range contact bonding. A dense strong system is produced (but chop off a piece and put it in water and it will disperse-no chemistry is involved). This is the fragipan and it contains a blocky network structure.
7. Discussion Fig. 6 shows a fragipan layer in place on top of a basalt accumulation. Both the basalt and the fragipan show characteristic contraction crack networks. The basalt is a Pliocene material from Mount Horrible and the loess deposit with fragipans is the Dashing Rocks loess at Timaru, South Canterbury, New Zealand. Three fragipan horizons are visible in the Dashing Rocks loess, indicating three distinct periods of loess deposition and the lowermost fragipan has become exposed and visible because it has resisted erosion rather better than the softer weaker layers above. This is a good illustration of the hardness and strength of a collapsed contracted fragipan. It is also a good illustration of the development of contraction crack networks in fragipans and basalts-developed by drying and by cooling. Fig. 5 leads to a very speculative position, and there is some distance between the positions shown in Figs. 5 and 6. However, action at the interparticle bond level will have to be considered if the relatively complex sequence of actions envisaged in the collapse of a loess deposit and the formation of a fragipan horizon are to be elucidated, compared, and explained. There is a complex combination of forces and phenomena which have to interact to provide the coincident and reinforcing activities. Fig. 5 shows the whole range of possible contacts in an airfall loess deposit (in an ideal situation). The particles are the ‘flattish’ blade shape particles which are believed to be the modal form of the predominant silt-sized quartz particles. The ‘table-leg’ type of structure provides the potential for structural collapse. It can be ‘folded’ down into a more compact form. Volume is diminished, a step towards collapse. The presence of a modest proportion of table-leg contacts means that the system has a fair potential for structural collapse. The ‘parallel’ type of structure is also important for the events under consideration. In a wet system the particles are separated by a water film, which will diminish and disappear as the system dries. In the wet system, this substructure allows deformation, allowing the contraction stresses to develop. When the system is dry, this structure, formed into the classic stepped-face-to-face (Smalley and Cabrera, 1969) provides great rigidity and strength. It provides the strength for a fragipan. 8. Conclusions It appears to be possible to reconcile the contraction crack forming process in loess ground with the Bryant hydroconsolidation hypothesis for fragipan formation. The Bryant process does allow an explanation of the presence of the blocky network which characterizes fragipans. Also, other characteristics fit comfortably with this approach. The hardness and strength of the fragipan horizon fit with the contracted structure. This is not a compacted structure, as compaction implies the external application of stress. In the contraction phase of fragipan formation the stress is internally generated. Powerful short range bonds are generated in the contraction process and they confer considerable strength on the fragipan layer. Modest drainage is possible via the cracks in the fragipan and this allows the faces of the unit blocks to become bleached. The crack forming mechanism can be reconciled with the requirements of the Bryant hydroconsolidation hypothesis. It might be seen as a test for the hypothesis. Testing theories of fragipan formation is difficult: it is difficult to test any pedological process. One advantage of the Bryant hypothesis is that aspects can be tested, as Assallay et al. (1998) demonstrated.
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Acknowledgements This paper has been prepared as a contribution to the celebrations of the loessic life of George Kukla and his contributions to loess science. We thank the organizers of the conference at the University of Wroclaw in September 2014 where the Kukla life was celebrated, and the nature of loess was discussed. We thank Monica Smalley for assistance with the illustrations. References Assallay, A.M., Rogers, C.D.F., Smalley, I.J., 1997. Formation and collapse of metastable particle packings and open structures in loess deposits. Engineering Geology 48, 101e115. Assallay, A.M., Jefferson, I.F., Rogers, C.D.F., Smalley, I.J., 1998. Fragipan formation in loess soils; development of the Bryant hydroconsolidation hypothesis. Geoderma 83, 1e166. Bockheim, J.G., Hartemink, A.E., 2013. Soils with fragipans in the USA. Catena 104, 233e242. Boots, B.N., 1977. Contact number properties in the study of cellular networks. Geographical Analysis 9, 379e387. Bryant, R.B., 1989. Physical processes of fragipan formation. In: Smeck, N.E., Ciolkosz, E.J. (Eds.), Fragipans: Their Occurrence, Classification, and Genesis, Soil Science Society of America Special Publication 24, pp. 141e150. Cabrera, J.G., Smalley, I.J., 1973. Quickclays as products of glacial action: a new approach to their nature, geology, distribution and geotechnical properties. Engineering Geology 7, 115e133. Dijkstra, T.A., Smalley, I.J., Rogers, C.D.F., 1995. Particle packings in loess deposits and the problem of structural collapse and hydroconsolidation. Engineering Geology 40, 49e64. Grossman, R.B., Carlisle, F.J., 1969. Fragipan soils of the Eastern United States. Advances in Agronomy 21, 237e279.
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Morrow, N.R., Graves, J.R., 1969. On the properties of equal sphere packings and their use as ideal soils. Soil Science 108, 102e107. Rogers, C.D.F., Smalley, I.J., 1993. The shape of loess particles. Naturwissenschaften 80, 461e462. Rogers, C.D.F., Dijkstra, T.A., Smalley, I.J., 1994a. Particle packing from an Earth Science viewpoint. Earth Science Reviews 36, 59e82. Rogers, C.D.F., Dijkstra, T.A., Smalley, I.J., 1994b. Hydroconsolidation and subsidence of loess: studies from China, Russia. North America and Europe Engineering Geology 37, 83e113. Smalley, I.J., 1966a. The expected shapes of blocks and grains. Journal of Sedimentary Research 36, 626e629. Smalley, I.J., 1966b. Contraction crack networks in basalt flows. Geological Magazine 103, 110e114. Smalley, I.J., Cabrera, J.G., 1969. Particle association in compacted kaolinite. Nature 222, 80e81. Smalley, I.J., Davin, J.E., 1982. Fragipan horizons in soils: a bibliographic study and review of some of the hard layers in loess and other materials. New Zealand Soil Bureau Bibliographic Report 30, 122. Smalley, I.J., Krinsley, D.H., 1981. The Urloess concept and changing ideas of loess formation. New Zealand Soil News 29, 57e59. Smalley, I.J., Markovic, S.B., 2014. Loessification and hydroconsolidation: there is a connection. Catena 117, 94e99. Smeck, N.E., Ciolkosz, E.J. (Eds.), 1989. Fragipans: Their Occurrence, Classification, and Genesis. Soil Science Society of America Special Publication 24, p. 153. USDA, 2010. Keys to Soil Taxonomy, eleventh ed. United States Department of Agriculture, p. 338. Van Vliet, B., Langohr, R., 1981. Correlation between fragipans and permafrost with special reference to silty Weichselian deposits in Belgium and northern France. Catena 8, 137e154. Witty, J.E., Knox, E.G., 1989. Identification, role in soil taxonomy, and worldwide distribution of fragipans. In: Smeck, N.E., Ciolkosz, E.J. (Eds.), Fragipans: Their Occurrence, Classification, and Genesis, Soil Science Society of America Special Publication 24, pp. 1e9.
Contents lists available at ScienceDirect
Quaternary International journal homepage: www.elsevier.com/locate/quaint
Loess and fragipans: Development of polygonal-crack-network structures in fragipan horizons in loess ground Ian J. Smalley a, *, Stephen P. Bentley b, Slobodan B. Markovic c a
Department of Geography, University of Leicester, Leicester LE1 7RH, UK School of Engineering, Cardiff University, Cardiff CF2 3AA, Wales, UK c Department of Geography, University of Novi Sad, Trg Dositeja Obradovica 3, RS-21000 Novi Sad, Serbia b
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 26 February 2015
One of the defining features of the fragipan horizon is the presence of a blocky polygonal network structure. In loess soils, this network structure can be explained by contraction forces (due to drying) operating after hydrocollapse due to loading and wetting, as in the Bryant hypothesis for fragipan formation. Three stages are identified in the formation of a fragipan horizon in loess ground. There is a deposition phase in which the aeolian deposition of loess material produces certain ground properties. A collapse stage allows the soil structure to deform under the influence of loading and wetting. This collapsed material develops internal tensile forces as drying contraction proceeds and these cause the development of a characteristic crack network. The crack network can be modelled using a very simple Monte Carlo approach and the two dimensional structure produced gives a good representation of fragipan cracking. The collapse-contraction process for fragipan formation offers explanations for the strength and hardness of fragipans, the constant depth to fragipan horizons, the slaking in water (predominance of short range contact bonds) and the mineralogical similarities throughout the system. The fragipan horizon impedes drainage, and this becomes increasingly important as land use becomes more widespread. © 2015 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Fragipan Contraction crack network Loess soils/ground Bryant hydroconsolidation hypothesis Fragipan properties Loess structures and bonds
1. Introduction A fragipan is a diagnostic soil horizon (USDA Soil Taxonomy, 2010, p.7). It is a dense horizon with limited permeability and it tends to develop in loess soils. The fragipan is a much discussed phenomenon (see reviews by Grossman and Carlisle, 1969; Smalley and Davin, 1982; Smeck and Ciolkosz, 1989; Witty and Knox, 1989; Bockheim and Hartemink, 2013). Many defining characteristics have been listed including: the high density relative to other parts of the soil system, the disaggregation reaction when plunged into water, the fairly consistent depth from surface to top of fragipan, the mineralogical similarity to the adjacent soil horizons, the formation where the ratio of rainfall to evapotranspiration is quite high. Witty and Knox (1989) listed 16 points which are required to be considered in the definition of fragipan. Point 10 is the most relevant to this discussion:
* Corresponding author. E-mail addresses: [email protected] (I.J. Smalley), [email protected] (S.P. Bentley), [email protected] (S.B. Markovic). http://dx.doi.org/10.1016/j.quaint.2015.01.034 1040-6182/© 2015 Elsevier Ltd and INQUA. All rights reserved.
It (fragipan horizon) has few or many bleached, roughly vertical planes that are faces of coarse or very coarse polyhedrons or prisms. We discuss the formation of these polyhedrons or prisms, the whole large-scale structural network (see Fig. 1 for sketch impression, from Van Vliet and Langohr, 1981). Some other Witty and Knox points have relevance to the current discussion: 4. Compared to the horizons above it, the bulk density is high. 7. When a dry fragment is placed in water, it slakes or fractures. 11. Most commonly, it has an abrupt or clear upper boundary at a depth of 33e100 cm below the original surface. (At about 70 cm in Fig. 1). The purpose of this paper is to discuss the development of the macro-structure within the fragipan layer, the network of characteristic polygonal units. In Keys to Soil Taxonomy (USDA, 2010, p.7), some requirements are listed for the definition of fragipan horizons. Requirement no.3 demands that the fragipan layer has a very coarse prismatic, columnar, or blocky structure of any grade, has weak structure of any size, or is massive. This is the large-scale structure discussed, and it is hoped that providing a realistic and
I.J. Smalley et al. / Quaternary International 399 (2016) 228e233
Fig. 1. The fragipan in position, after Van Vliet and Langohr (1981). See Smalley and Davin (1982 p.80) for discussion. A version of this figure appears on the cover of Smalley and Davin (1982).
sensible explanation of this characteristic fragipan structure will be a positive contribution to the discussions on the mode of formation of the fragipan itself. 2. Densification Consideration of changes in density will form an important part of this discussion so some critical density related terms need to be defined. Soil is a particulate system and in such a system relative packing density can be defined as the ratio of solids present in the system; the range is from all solid, PD ¼ 1, to all empty, PD ¼ 0. Voids ratio e, the relevant term used in soil mechanics, is defined as the ratio of space in the system to the solids in the system:
ðe ¼ ½1 PD=PDÞ: A voids ratio of 1.0 is equivalent to PD ¼ 0.5. The relationship of e to PD is shown in Fig. 2, which provides a convenient framework to demonstrate changes in the loess soil system as deposition is succeeded by collapse and then by contraction. 3. Deposition The loess deposit is formed by aeolian deposition of largely silt sized material, much of which is quartz. The modal particle in a loess deposit might be considered to be a 30um quartz particle with a definite blade shape. Shape studies are not definitive or conclusive, but there are indications that certain shapes will be favoured. Rogers and Smalley (1993), using a very simple Monte Carlo
229
Fig. 2. The relationship between e voids ratio and PD packing density. Some regular packing densities are indicated, for 600, 402 and 204 packings (see Rogers et al 1994a for explanations). See Dijkstra et al., 1995 for discussions of packing transitions.
method, calculated that the modal shape should be defined by a side ratio of 8:5:2. They called this a Zingg 3 m particle (see Smalley, 1966a for shape terminology) and it does have a remarkably pronounced blade shape (blade is simply defined as a > b > c). The blade shape is the least well-defined of the four straightforward particle shapes. In simple terms it might be called ‘flattish’. These particles, on aeolian deposition, have an open structure with a high porosity, a low packing density, a high voids ratio. It is a structure dominated by short range contacts, a relatively rigid structure. In Fig. 2, the deposition process is given a speculative and imaginative presence. There is no evidence for the initial formation of a very open structure with a voids ratio of around 2, but it is believed that the initial deposition process would involve a modest amount of ‘tamping’ while the system settled to the eventually observed e value of around 1.0. This would be the classic initial loess, the Ur-Loess of Smalley and Krinsley (1981). From this position, the denser structures are developed. In Fig. 2 some very ideal packings are indicated, with e values. Applying packing ideas to soil systems has never been very successful. The best attempt was probably by Morrow and Graves (1969), an attempt which was appreciated and discussed by Dijkstra et al. (1995). The 600 packing is the cubic packing and the 204 packing is the close rhombohedral packing. The transition from 600 to 204 is an interesting collapse manoeuvre, and Dijkstra et al. (1995) made some attempts to relate it to loess collapse. Packing problems in general in geo-science settings have been discussed by Rogers et al. (1994a). In a loess deposit, there is a packing of particles and a set of interparticle bonds. The particles, in place, define the structure; the bonds control the post-depositional
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processes. Two types of bond might be defined to facilitate loessic discussion. There are short range bonds which link primary mineral particles. These are short range because when they are broken all strength is lost. The long range bond connects clay mineral particles, electrically charged particles, which have complex interactions. The long range bond involves charged particles and charged ions dissolved in soil water. The long range bond allows plasticity because strength is retained after the bond is disturbed. In the ideal Ur-Loess deposit, soon after deposition, short range bonds can be expected to predominate. This is an open structure, a rigid structure, a metastable structure, but not a collapsible structure. The bonds need to be modified before collapse can occur. Within the loess soil system, short range and long range bonds have roles to play. For a discussion of short/long range bonds in a geotechnical setting, see Cabrera and Smalley (1973). 4. Collapse Collapse is usually measured in an oedometer, a consolidation testing machine (Fig. 3). The specimen is loaded, then wetted, and the collapse amount is measured. This collapse is central to the Bryant hypothesis for fragipan formation (Bryant, 1989), which was investigated by Assallay et al. (1998). The Bryant hypothesis to explain fragipan formation was distinctive in that it involved a physical/mechanical collapse mechanism rather than a chemical diffusion and deposition mechanism for densification. For the Bryant mechanism to occur, a necessary prerequisite is a collapsible soil system. Bryant (1989 p.147) explained the requirements for a collapsible system: “Preconditions for the formation of a collapsible sediment are: (i) a loose granular contact structure. (ii) a metastable condition, and (iii) bonding agents that reduce upon wetting. The process of collapse occurs on wetting to saturation or near saturation because intergranular bond strength, due to capillary tension, is reduced … The desiccation process following collapse would cause cracking and formation of prismatic structure associated with fragipans, and fabric reorientation would result in an interlocked granular contact structure of greater density.”
The geotechnical problem with loess soils is that they collapse when loaded and wetted in a construction situation, they suffer from hydrocollapse or hydroconsolidation (see Rogers et al., 1994b; Smalley and Markovic, 2014). The collapse process is going to produce a more dense system, a marked reduction in voids ratio is observed, but the initial structural bonds are broken. The mechanism of the collapse process has been discussed for many years (see Rogers et al., 1994b) and it is only recently that a realistic mechanism has been proposed (see Smalley and Markovic, 2014). It is necessary not only to explain the process of collapse but also the ways in which collapsibility developed in the soil system. It has been shown (Assallay et al., 1997, 1998) that collapsibility in a loess soil system depends to a large extent on the clay mineral content. A very low clay mineral content inhibits collapse, the particle contacts are too rigid, too short-range. A large clay mineral content also prevents collapse because the pores are filled, as there is no collapse space available. A clay mineral content of around 15e20% is optimal. In a developing loess soil system, this clay can accumulate and render the system available for collapse. Some downward flow of water through the system is required and it appears that the clay accumulation process is assisted by the formation of fine carbonate mineral needles at major particle contacts. The carbonate mineral network traps clay mineral particles and these accumulate in critical zones until the necessary amount has been emplaced. Then, if the overburden pressure is sufficient and enough water is available in the system, collapse can occur. There is a link between the various factors contributing to collapsibility developing and fragipans (see Smalley and Markovic, 2014 for more discussion). 5. Contraction Some contracting systems form a network of cracks when critical stress levels are reached. The classic examples in the geosciences are basalt flows such as the Giants Causeway or the Dashing Rocks basalts (see Fig. 6). Drying mud often exhibits
The Bryant hypothesis had many attractive aspects and was carefully investigated by Assallay et al. (1998). Collapse occurs when a necessary amount of overburden material has accumulated, and this particular load requirement accounts for the generally observed constancy of depth of the fragipan horizon; this is self-weight collapse. This type of collapse can be characteristic of loess systems.
Fig. 3. The collapse event. A typical oedometer test result (from Smalley and Markovic, 2014). The sample is loaded, and then wetted; considerable collapse strain is observed. The actual collapse requires a major structural reorganization and densification.
Fig. 4. Looking down on the ideal fragipan. This is a contraction crack network by Boots (1977) after Smalley (1966b). Boots generalized the contraction crack simulation process to produce ideal divisions of space in two-dimensions; the so-called ‘Smalley process’.
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Fig. 5. Contact types. 100 contacts in an ideal particulate world. The particles are Rogers-Smalley Zingg 3 type blades and they assume all the contact positions possible. 05, 94, 40 and 59 are archetypical table-leg contacts; 00, 55 and 99 are typical parallel contacts. In the collapse process table-legs decrease and parallels increase, in contraction parallels become closer and contact bonds strengthen.
contraction cracking. Drying or cooling appears to deliver the required stress to cause the contraction crack network to develop. In the fragipan setting, we imagine a situation where hydroconsolidation has occurred in a loess deposit. The
hydroconsolidation has occurred because enough loess has been deposited to provide a sufficient overburden pressure and the system is wet enough to allow deformation at the interparticle contacts. The contracting system is a wet system but one in which
Fig. 6. Fragipan and basalt at the Dashing Rocks section in Timaru, South Canterbury, New Zealand. In the Timaru loess there are three fragipans and the bottom one is nicely exposed on top of the Mount Horrible basalts. This was a pahahoe type lava and has formed distinctive contraction crack structures. The fragipan shows similar contraction crack patterns. The continued existence of the lowest fragipan is a testament to the strength of the compacted fragipan structure.
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the rigid soil structure has been destroyed by structural collapse. It is dense (relatively dense) and weak (relatively weak). Loses moisture on drying tensile stresses develops. A certain amount of loess has to accumulate to cause the structural collapse of hydroconsolidation (i.e. a certain load has to develop to cause the stresses causing compaction). This is why there is this relative constancy of depth to fragipan horizons-the default depth represents the default load. The wetness required for hydroconsolidation accounts for the occurrence of fragipans in wet systems: dryness inhibits fragipan formation. The collapsed system is homogeneous and of low strength, and as the drying cycle begins tensile stresses develop. The situation is essentially the same as in a cooling, solidifying basalt flow, and it can be discussed as a two dimensional problem, basically a twodimensional packing problem. There is a consistent tensile stress across the two dimensional surface, the stress increases as the system cools/dries and eventually at random places the tensile strength of the system is exceeded and cracking develops. The cracks develop at random and the phenomenon can be modelled by using a simple Monte Carlo system. Modelling works well for basalt flows (Smalley, 1966b) and a typical result is shown in Fig. 4. The random division of space in two dimensions can be produced and the technique has been compared to the random division of space by other statistical methods by Boots (1977). His ‘Smalley process’ division is shown in Fig. 4. This is the default division for natural contracting systems-such as basalt flows, and fragipans. A structure of this nature would be expected in a fragipan, if the hydrocollapse process is part of the fragipan formation mechanism. The collapsed weak wet system will contract on drying. 6. A speculative set of interparticle contacts Fig. 5 shows a possible set of interparticle contacts, in an ideal deposit of Ur-Loess. This is speculative and is unsupported by observation. All it can really do is clarify the discussion of the structure development and change in loess ground. The 100 structures shown represent the possible particle contacts in the ideal ground. They might be classified into ‘table-leg’ structures and ‘parallel’ structures, and intermediate structures. In the Gedankenexperiment approach to loess formation and deformation, the table-leg contacts would predominate in the freshly deposited UrLoess and the parallel contacts would predominate in the totally deformed, remoulded loess. The table-leg structures give some rigidity to the initial structure, and ensure a high voids ratio. The decoupled parallel contacts give increased density to the deformed structure but provide little strength. There are 100 imaginary contact structures shown in Fig. 5, so simple counting will reveal percentages of structural types. 04 is a default table-leg but it is exactly the same as 59; 55 is a default parallel structure, and so is 00. Obvious table-legs are about 30% of the contacts shown: these are the contacts which give space and strength to the loess in its deposition state. These are deformed on collapse. The collapsed system will be dominated by parallel type contacts, but initially the particles will be well dispersed. As water films evaporate, the parallel contacts draw closer and contraction occurs. Fairly efficient packing structures will be formed: the stepped face-to-face structure (Smalley and Cabrera, 1969) can be expected to predominate, sustained by simple, short-range contact bonding. A dense strong system is produced (but chop off a piece and put it in water and it will disperse-no chemistry is involved). This is the fragipan and it contains a blocky network structure.
7. Discussion Fig. 6 shows a fragipan layer in place on top of a basalt accumulation. Both the basalt and the fragipan show characteristic contraction crack networks. The basalt is a Pliocene material from Mount Horrible and the loess deposit with fragipans is the Dashing Rocks loess at Timaru, South Canterbury, New Zealand. Three fragipan horizons are visible in the Dashing Rocks loess, indicating three distinct periods of loess deposition and the lowermost fragipan has become exposed and visible because it has resisted erosion rather better than the softer weaker layers above. This is a good illustration of the hardness and strength of a collapsed contracted fragipan. It is also a good illustration of the development of contraction crack networks in fragipans and basalts-developed by drying and by cooling. Fig. 5 leads to a very speculative position, and there is some distance between the positions shown in Figs. 5 and 6. However, action at the interparticle bond level will have to be considered if the relatively complex sequence of actions envisaged in the collapse of a loess deposit and the formation of a fragipan horizon are to be elucidated, compared, and explained. There is a complex combination of forces and phenomena which have to interact to provide the coincident and reinforcing activities. Fig. 5 shows the whole range of possible contacts in an airfall loess deposit (in an ideal situation). The particles are the ‘flattish’ blade shape particles which are believed to be the modal form of the predominant silt-sized quartz particles. The ‘table-leg’ type of structure provides the potential for structural collapse. It can be ‘folded’ down into a more compact form. Volume is diminished, a step towards collapse. The presence of a modest proportion of table-leg contacts means that the system has a fair potential for structural collapse. The ‘parallel’ type of structure is also important for the events under consideration. In a wet system the particles are separated by a water film, which will diminish and disappear as the system dries. In the wet system, this substructure allows deformation, allowing the contraction stresses to develop. When the system is dry, this structure, formed into the classic stepped-face-to-face (Smalley and Cabrera, 1969) provides great rigidity and strength. It provides the strength for a fragipan. 8. Conclusions It appears to be possible to reconcile the contraction crack forming process in loess ground with the Bryant hydroconsolidation hypothesis for fragipan formation. The Bryant process does allow an explanation of the presence of the blocky network which characterizes fragipans. Also, other characteristics fit comfortably with this approach. The hardness and strength of the fragipan horizon fit with the contracted structure. This is not a compacted structure, as compaction implies the external application of stress. In the contraction phase of fragipan formation the stress is internally generated. Powerful short range bonds are generated in the contraction process and they confer considerable strength on the fragipan layer. Modest drainage is possible via the cracks in the fragipan and this allows the faces of the unit blocks to become bleached. The crack forming mechanism can be reconciled with the requirements of the Bryant hydroconsolidation hypothesis. It might be seen as a test for the hypothesis. Testing theories of fragipan formation is difficult: it is difficult to test any pedological process. One advantage of the Bryant hypothesis is that aspects can be tested, as Assallay et al. (1998) demonstrated.
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Acknowledgements This paper has been prepared as a contribution to the celebrations of the loessic life of George Kukla and his contributions to loess science. We thank the organizers of the conference at the University of Wroclaw in September 2014 where the Kukla life was celebrated, and the nature of loess was discussed. We thank Monica Smalley for assistance with the illustrations. References Assallay, A.M., Rogers, C.D.F., Smalley, I.J., 1997. Formation and collapse of metastable particle packings and open structures in loess deposits. Engineering Geology 48, 101e115. Assallay, A.M., Jefferson, I.F., Rogers, C.D.F., Smalley, I.J., 1998. Fragipan formation in loess soils; development of the Bryant hydroconsolidation hypothesis. Geoderma 83, 1e166. Bockheim, J.G., Hartemink, A.E., 2013. Soils with fragipans in the USA. Catena 104, 233e242. Boots, B.N., 1977. Contact number properties in the study of cellular networks. Geographical Analysis 9, 379e387. Bryant, R.B., 1989. Physical processes of fragipan formation. In: Smeck, N.E., Ciolkosz, E.J. (Eds.), Fragipans: Their Occurrence, Classification, and Genesis, Soil Science Society of America Special Publication 24, pp. 141e150. Cabrera, J.G., Smalley, I.J., 1973. Quickclays as products of glacial action: a new approach to their nature, geology, distribution and geotechnical properties. Engineering Geology 7, 115e133. Dijkstra, T.A., Smalley, I.J., Rogers, C.D.F., 1995. Particle packings in loess deposits and the problem of structural collapse and hydroconsolidation. Engineering Geology 40, 49e64. Grossman, R.B., Carlisle, F.J., 1969. Fragipan soils of the Eastern United States. Advances in Agronomy 21, 237e279.
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