Loessification and hydroconsolidation: there is a connection

i.m. Vaclav Ambroz

Ian J. Smalley

Giotto Loess Research Group, Geography Department,

Leicester University, Leicester LE1 7RH, UK

(ijs4@le.ac.uk)

Slobodan B.Markovic

Department of Geography, University of Novi Sad, Trg

Dositeja Obradovica 3, RS-21000 Novi Sad, Serbia

(slobodan.markovic@dgt.uns.as.rs)

Abstract

Loessification can be defined as the acquisition of loessic

characteristics by ground systems. Hydroconsolidation, in

this context, is the collapse of the loess ground structure

under the influence of loading and wetting. Loess, on aeolian

deposition, is metastable- a pre-requisite for eventual

collapse. The actual collapse mechanism is dependent on

the presence of a critical amount of clay mineral material at

major particle contacts. This clay accumulates via post-

depositional processes; processes which can be described as

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part of loessification and as a contribution to collapsibility.

Studies at Ospringe in Kent, England support observations

made in the Bohemian Massif in the Czech Republic about

the nature of the loess ground system and the role of clay

minerals and calcite crystals. Fragipan formation, another

important post-deposition event, could depend on

hydroconsolidation in loess ground.

Keywords: Loess, loessification, hydroconsolidation, post-

depositional events in loess, fragipans

Introduction

Loess is a widespread continental sediment which had its

origin during the glacial phases of the Pleistocene Epoch.

Within the loess deposits multiple loess-palaeosol sequences

are recognised as key continental archives of Quaternary

climate and environmental dynamics(e.g. Ding et al 2002,

Markovic et al 2009, Roberts et al 2007).

In spite of current developments of different aspects of loess

research our understanding of the nature of loess material

and the processes of loess formation is still relatively poor.

This study aims to incorporate results of loess

hydroconsolidation studies by geotechnical investigators into

a discussion of the fundamental nature of the loessification

process.

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Loessification: Berg’s idea (Berg 1916), and his term; the

conversion of non-loess ground into loess ground by

processes of weathering and soil formation. Loessification: a

term much derided by sedimentologists who know that the

critical moment in the formation of a loess deposit, the

moment that confers the classic loessic properties, is the

aeolian deposition of the silty material to form that well-

sorted open structure. Incompatibility: it appeared that the

two approaches to loess deposit formation, loessification or

aeolian deposition, were completely and totally

incompatible, and at one fundamental level they are. Berg

wildly overstated his position and his well-known denial of

any contribution to loess formation by aeolian action is well

known (Berg 1964, p.22 ). But there is more to loess

formation, and to the nature of loess, than this simple

confrontation allows. Our position in this paper is that the

key event in loess deposit formation is the aeolian deposition

of the silty material and we have expounded on this

elsewhere (Smalley et al 2010b); but it should be

acknowledged that loessic properties and aspects continue

to develop after that key moment of deposition. There were

key events in loess history before the aeolian events;

material had to be formed and long river transportation

moved it across the landscape(Smalley et al 2009). In the

same way that important events prefigure the aeolian

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apotheosis, there are post-depositional events worthy of

study and discussion.

We focus in particular on to the development of collapsibility

in loess ground. The aeolian deposition event yields a

metastable deposit, but collapsibility develops via post

depositional activity. This is what might be called the

dynamic defining aspect of loess. The mechanical property

of loess which has a defining role is the capacity for

structural collapse when loaded and wetted. This has been

studied for many years in a geotechnical context (see

Rogers et al 1995); a critical moment was the observation by

Denisov (1953) of the failure of irrigation canals in the loess

of Uzbekistan. These canals, dug in classic Central Asian

loess, showed the phenomenon of self-weight collapse. The

load provided by the wetted ground was sufficient to cause

the loess ground structure to collapse. There has been much

discussion of the mechanism of loess collapse, of

hydroconsolidation, and it can now be seen that much of the

hydroconsolidation capacity is delivered in post-depositional

times. In fact a loessification process enables

hydroconsolidation, and this is a loessification process

already described by Cilek (2001).

Loessification

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A recent study on L.S.Berg (Smalley et al 2010a) has

attempted to assess his contribution as a great geographer,

and to place his studies on loess into context. In the world of

loess scholarship Berg is famous for his theory of loess

formation, variously called the ‘soil theory’, the ‘eluvial

theory’, the ‘in-situ’ theory, the ’pedological theory’, and the

‘loessification theory’. This Berg idea, first published in 1916,

proposed that loess deposits were essentially formed by

processes of weathering and soil formation. Non-loess

ground was turned into loess ground by a process of

loessification.

Berg was very forthright in support of his theory, and equally

forthright in condemning alternative ideas of loess deposit

formation. In particular he was very dismissive of the aeolian

theories of Richthofen and Obruchev. This set up an

unfortunate dichotomy with each side claiming and neither

side listening (see Rozycki 1990 ). There were also echoes of

a political dimension (see Smalley 1980, Blackburn 1980),

there were certainly impediments in place which prevented

sensible comparative discussions and reconciliations. It is

only recently that the parts of the aeolian aspect and the

loessification idea that actually fit together have been

brought together (Smalley et al 2010b).

It has been proposed that the best way to study

loessification is to divide it into two parts (Smalley et al

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2006c). There shall be ‘grand’ loessification(gL) and ‘petit’

loessification(pL); the gL concept is the old Berg 1916 idea in

which all loess is formed by weathering and soil formation,

and this appears to be defunct- it denied any role for aeolian

deposition, now seen as a key event in loess deposit

formation. So gL is essentially abandoned, but pL has

continued relevance. It was the pL concept which Pecsi

(1990) was heading for when he claimed that ‘Loess is not

just the accumulation of dust’. This was the concept that

Ambroz (1947) was proclaiming in his studies of the loesses

of the hill countries (as Cilek 2001 pointed out). After aeolian

deposition the basic structure of the loess deposit is in place

and the open structure is established and the material is

draped over the landscape, but the story continues. Soil

forming processes begin, clay starts to migrate towards

particle contacts. Lozek (1965) proposed that the aeolian

deposition and the onset of pL loessification were almost

contemporaneous (“Die Windaufschuttung und Verlossung

erfolgen etwa gleichzeitig.”) suggesting that Obruchev’s

approach to the problem of loess formation was essentially

correct. In fact the Lozek observations are very similar to

those of Cilek(2001). Here we might highlight the influence

of sharp glacial climates characterised by cold winters with

long frosty periods, a humid transitional season, and short

and relatively warm, dry summers on ground processes at

the surface of silty(proto-loess) deposits. Simultaneously to

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these seasonal environmental changes the atmosphere was

permanently gusty (McGee et al 2010).

Possibly processes of gL weathering for soil formation could

be simultaneous with intensive aeolian deposition. An

example might be the Surduk section in northern Serbia

Antoine et al 2009). Grain size variations reflect cyclic

periods of weak to moderate pedogenesis and intensive

loess formation.

Hydroconsolidation

Hydroconsolidation in loess has been reviewed by Rogers et

al (1995). In the geotechnical world loess is classified as a

collapsing soil; the whole range of collapsing soils has been

reviewed by Derbyshire et al(1995). Loess, when tested in

an oedometer (the classic consolidation testing machine),

displays typical collapse behaviour. The loess sample resists

the initial applications of stress, but collapses when wetted

(see e.g.Feda 1995). Loess can appear to be a strong and

brittle material but can lose most of its strength when

wetted. This has led to some large scale construction

failures, in particular the collapse of part of the Atommash

factory in Volgodonsk in 1989. Even remoulded loess, which

will have lost its intial structural status can fail

catastrophically as was demonstrated in the Teton Dam

failure in Idaho in 1960 (Smalley & Dijkstra 1991, Smalley

1992).

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There has been considerable discussion on the nature of

loess collapse; much of this is in the Russian literature

because it was in the Soviet Union that there was a

coincidence of reasonably high population densities and the

need for construction on loess ground (see Kriger 1986,

Trofimov 2001, Jefferson et al.2003) There are two main

aspects to the study of loess collapse; an assessment of the

packing structure and status in the original loess deposit,

and the nature of the collapse dynamics. The problems have

been tackled via model studies (Assallay et al 1997, Dijkstra

et al 1999).

Two types of model study have been undertaken (i) making

loess samples in oedometer sample rings by a simulated

aeolian deposition (Assallay et al. 1997), and (ii) generating

two-dimensional representations of loess deposits by simple

Monte Carlo methods (Dibben et al 1998). The direct

deposition model allowed one very critical set of

observations to be made. Once it had been established that

the system produced was a reasonable representation of a

real loess deposit it became possible to make samples with a

carefully controlled range of clay mineral contents. This

allowed a clear demonstration of the dependence of

collapsibility on clay content. Samples with very low (~0%)

and high (~30%) clay contents tended not to collapse. An

intermediate clay content was required, which, concentrated

at main particle contects could soften and weaken on

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wetting allowing collapse to occur. The variation of collapse

with clay mineral content is shown in Assallay et al (1998).

Observations from Ospringe

A recent comprehensive study of loess collapsibility has

been carried out at Ospringe in Kent, England (see

Milodowski et al 2010). Testing of the loess(brickearth) has

revealed some insights into the collapse mechanism. The

metastable, collapsible calcareous loess/brickearth is

characterised by an open packing arrangement of pelletised

aggregate grains of compacted silt/clay, supported by an

inter-ped matrix of loose-packed silt grains, in which the

grains are held in place by a skeletal framework of illuviated

clay. This clay forms meniscus clay bridges and pillars

separating and binding the dispersed component silt grains.

There is little direct grain-grain contact, and the resultant

fabric has a very high void ratio.

Any applied load is largely supported by these delicate

meniscus clay bridge and pillar microfabrics.

Hydroconsolidation and collapse of this brickearth fabric can

be explained by a sequence of processes involving: (i)

dispersion and disruption of the grain-bridging clay on

saturation, leading to an initial rapid collapse of the loose-

packed inter-ped silt; (ii) rearrangement and closer stacking

of the compact aggregate silt/clay peds; (iii) with increasing

stress further consolidation may result from the deformation

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and break up of the peds as they collapse into the inter-ped

regions. Smectite is a significant part of the clay component

at Ospringe and will swell on wetting, which will further

encourage the disruption and breaking of the clay bonds.

Minor calcite and dolomite mineralisation may also form

meniscus cements between silt grains. These have either

acted as scaffolds on which illuviated clay has subsequently

been deposited, or have encrusted earlier-formed grain-

bridging clay. In either case the carbonates may help to

reinforce the clay bridge fabrics. It appears that fine needle-

like carbonate crystals can form meshes which help to trap

clay mineral particles at particle contacts, and thus enhance

collapsibility. The development of collapsibility depends on

an initial formation of a network of fine calcite crystals; these

trap the clay mineral particles and build up the clay mineral

concentration at the structural contact points. Thus a

collapsing loess will contain critical amounts of calcite and

clay mineral material. At Ospringe two types of

brickearth/loess are observed, the upper loess is not

calcareous, and it is not collapsible. The lower loess is

calcareous, and it is collapsible.

The Bohemian Massif

Cilek (2001), in his study of loess in hilly regions, has

investigated loess on the Bohemian Massif and discussed the

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nature of loessification. He pointed out that loessification

must be a very rapid process, particularly if the very open

structure (voids ratio around 1.0) of the loess deposit is to be

preserved. He noted that clay bridges among the quartz

grains were often impregnated with calcium carbonate and

Al-Si hydroxides. He proposed that three important types of

cementation might be recognised: (i) calcitic bonds, (ii)

allophane bonds, and (iii) siderogel bond. He also suggested

that the rapid impregnation of clay particles between quartz

grains and the general consolidation of porous ‘dust

accumulations’ by the three types of cement probably

constitute the key factor in loess formation.

He noted that the most common authigenic mineral was

calcite in the form of needle like crystals (very like the

situation at Ospringe). He reported some experiments by

Ambroz (1947) which showed that the calcite can be

supplied by bicarbonate ground waters. He performed a

simple experiment in which de-calcified soil was moistened

from below and, after two weeks, the fine-grained calcite

could be found within the soil and a carbonate crust formed

at the surface. Relative to this Cilek proposed that the

evaporation transport of the capillary soil solutions during

hot and short continental glacial summers seems to be the

most likely mechanism involved in the sudden internal

hardening of the loess structure by way of impregnation of

clay bridges between silt grains.

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Commentary

In a loess deposit, after the aeolian deposition event, an

open-structured system exists, which has several of the

most important characteristics of loess. The high voids ratio

means that it is a metastable deposit, an open packing

capable compacting down into a less-dense, more stable

deposit. The aeolian deposition disperses the material across

the landscape, the ‘draping across the landscape’ property is

observed. But, immediately post-deposition, the deposit

appears to lack collapsibility; it has metastability but it lacks

collapsibility. This will develop via a post-depositional

process, a pL loessification process.

The collapsible nature of the loess deposit has implications

for further processes. It appears that structure collapse may

be a key to fragipan formation in loess ground. The hard

fragipan horizon appears to favour loess ground and Bryant

(1989) proposed that a hydroconsolidation process led to

fragipan formation. The fragipan is a dense horizon which, by

and large, forms at a constant depth below the surface.

Assallay et al (1998) produced some experimental support

for the Bryant hypothesis and it does appear that fragipan

formation could fit into the sequence of post-depositional

events involving clay movement and structural collapse.

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There is even a stratigraphical implication. Zhou &

Shackleton(1999) have proposed that certain dating

anomalies in some loess stratigraphic sections could be due

to ground collapse causing positional measurement

difficulties.

Conclusions and proposals

Loessification and hydroconsolidation are connected. The

collapsibility of loess is one of the key defining factors, and

while other defining factors stem directly from the action of

aeolian deposition, collapsibility accrues in a post-

depositional manner. The acquisition of collapsibility is an

event of pL loessification; the ground takes on a more loessic

character. Not quite the dramatic event that Berg envisaged

but definitely falling within the purview of loessification.

The collapsibility of loess appears to depend on the presence

of clay minerals and calcite. Meshes of fine needle-like

calcite crystals grow quickly, soon after the aeolian

deposition event. These serve to trap clay minerals in the

regions of particle contact and these clay minerals control

the deformation properties of the contact zones. An excess

of clay mineral material (perhaps as observed in the New

Zealand loess) prevents structural collapse by filling the pore

spaces and not allowing the initial loess packing to collapse.

The correct amount of clay allows the contact to be mobile;

the clay softens on wetting and collapse (hydroconsolidation)

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can occur. The operating model for loess hydroconsolidation

is essentially the ‘small clay’ model as described by Rogers

et al (1994).

Finally, this study carries the loessification term across into

the geotechnical world. The Berg idea requires large

modification but the observation of the development of

collapsibility clearly indicates a post-depositional change in

loess ground. There is a space for the idea of loessification.

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