Thursday, October 31, 2013

The Origin of Hogwallows and Gilgai Landforms - PART I

INTRODUCTION

Gilgai and hogwallows are the same thing. They are naturally occurring landforms that are called microrelief. The higher areas have been called microknolls, mounds, and microridges and the lower areas are called depressions, microlows or microdepressions. According to Gustavson (1975) the maximum relief produced by these landforms is about 18 inches.The three basic forms are mounds and depressions, polygons (with microridges) and linear that are elongated in the direction of slope. The expression "hogwallows" appeared first (in print) in the United States but the Australian expression "gilgai" has become the more commonly used word. These landforms are caused by montmorillonitic expansive soils. The study of these landforms is compelling because the resulting micro-topography is anomalous with regard to surface moisture and the expected shrink-swell behavior. Low spots collect water, this should make these areas heave and disappear. So we would expect expansive soils to dampen out low areas, but the exact opposite occurs with these landforms. This means that there must be some special mechanism or process that starts and maintains these landforms. At stake is an unknown process that produces extraordinary differential movements in expansive clays.

There is a similar class of microrelief that are sometimes confused with gilgai that are not related with expansive soils. These landforms have a flat or tableland area between the microknolls without clearly defined microlows. These landforms can have higher relief than gilgai and are more appropriately called pimple mounds, mima mounds, hillocks, prairie mounds, nebkhas, and hummocks. In Texas there are pimple mounds in the same general area as gilgai.

In 1994 I published a new theory that shows a connection between these landforms and overconsolidated (OC) expansive clays, since then I have found several hundred sites in Mississippi and Alabama and several thousand sites in Texas using Google Earth, Bing Maps and sometimes Yahoo Maps. But Hilgard without the benefit Google Earth first made this connection. In 1860 he wrote"..the Hog-Wallow prairie region, in which only the clay marls of the Jackson Group are to be looked for." Most of the expansive clay deposits from Texas to Alabama related with gilgai are either Eocene or Cretaceous age. This results in an approximate age in the range of 34 to 145 Million years. These deposits have had hundreds of feet of sediments eroded from above. This creates the condition of overconsolidation that gets gilgai started. Even the more recent Pleistocene era Beaumont Formation of Texas has been shown to have properties of unloading that produces overconsolidation and a naturally occurring horizontal stress. The result is instability that promotes weathering, shearing, and the formation of gilgai landforms. This blog (Part I) is a close look at the meaning of overconsolidation and how high horizontal stress is connected.

In the solution I proposed, structural and geotechnical engineering concepts are used to solve a geomorphology problem that has been scientifically recognized since 1840. That year John Leonard Riddell published the first surface crack theory about hogwallows in Texas. Riddell speculated that rains would wash earth into shrinkage cracks and convert "them into little valleys, and leaving intermediate hillocks."Amazingly the hogwallows that Riddell first saw in April and May of 1839 are still there 173 years later! Unfortunately there are not clear online images. If you carefully study other sites in Texas and this area you can tell that there are gilgai at this site. I direct you to Google Earth at Lat. 31.338337 and Long. -95.729792.  To see these landforms look at the 1-22-1995 and the 3-8-2011 images at an elevation of about 2350 feet. These images have poor resolution (compare with other locations below) but those are Riddell's hogwallows. You can find several clusters of hogwallows totaling over 200 acres in this neighborhood that he might have seen. This site is associated geologically with the Claiborne Group that includes the Cook Mountain Formation. I located these hogwallows by using Riddell's description that was eighteen miles above Robbins Ferry on the Trinity River. This ferry has a historical marker at 31.074917, -95.701623. Note you can copy and paste these locations into the GE search box.
In Texas gilgai are so common, they sometimes appear in Google Street View as on Broadhead Road in Waxahachie Texas. This amazing photo was taken at 32.419913,-96.77876. Another good street view image of gilgai can been seen at 32.642325, -96.522785. To find these landforms you usually have to look at historical images taken between December and March because during this period gilgai may be holding water and this makes them more visible. This is fairly typical of aerial photos in the southeastern United States. Higher resolution images are also a requirement. There is also a chance it will be dry even it the images are made during the optimum period. Presently the extreme rural areas of Mississippi and Texas does not have adequate coverage to map and define the distribution. This is unfortunate because these landforms could help map unknown areas where damaging expansive clays exist.

An example of gilgai near Macon Mississippi can be seen at 33.118497,-88.507111 and near Marion Alabama at 32.613375, -87424533. I could not find these gilgai in Alabama until new aerial photos were taken during the wet season this past January. Both of these sites are associated with the same geological units, the lower Demopolis and the Mooreville which are chalky Cretaceous deposits. Maybe the most interesting gilgai are near Denton Texas like at 33.179328, -97.220062 (see image 2-27-2001). At a hill top there are polygonal gilgai but it appears that 2 or 3 depressions (darker spots) are usually grouped together. Here linear gilgai run downhill around the hill but still have the occasional depression in the valleys. The elongated or linear gilgai were mentioned by Riddell (1840) and also by Gustavson (1975) but there are no theories to explain this form of gilgai I have seen until now (see PART II of this blog).

E. W. Hilgard (1906) the former state geologist of Mississippi added the element of expansion with "the heavier and more continuous rains wet the land fully, also causing the consolidated mass in the crevices to expand....the result being that the intermediate portions of the soil are compelled to bulge upward, sometimes for six or more inches." In 1932 or maybe 1939 A. Howard wrote "…further penetration of rain will only take place in isolated points where root channels, burrows, ect. have broken down the natural impermeability. When the B1 and B2 horizons become saturated in these isolated spots, the swelling due to the high clay content causes a mound to form."  My 1994 paper was titled Influence of Horizontal Stresses on Gilgai Landforms. In this paper I proposed a whole new concept to explain the origin of these landforms. This paper was referenced in a textbook titled Geomorphology of Desert Environments (2009). J.C. Dixon wrote the chapter that includes gilgai. This is the best review of gilgai theory that I have read. But there was a paper with the title: Structure development in surficial clay soils: A synthesis of mechanisms by Kodikara, Barbour and Fredlund (2002). These writers stated" Maxwell (1994) suggested that continuum buckling due to lateral swelling pressures might be responsible for gilgai undulations." I was surprised when I read this because in my paper I state "It is proposed that at a lower level of stress (than buckling stress), there are changes in vertical stress in a prebuckling mode… This effect could then induce a differential rebound with a buckled appearance." I also state that "measured horizontal stresses in OC (overconsolidated) expansive clays are far below a critical buckling load."  To me differential rebound is clearly not buckling, but if this has been misunderstood, I feel I must clarify this concept. So this blog is inspired by this and I will attempt to fill in the gaps and expand this theory as I published in 1994 in the Journal of Geotechnical Engineering. Because this is a long blog I have left out some of the general information about gilgai and focused on aspects related with this theory and split the blog into two parts. I will explain the three parts of this theory in detail, but here is a quick summary:

1.  THE ORIGIN OF Ko >1: Kis called the (at rest) lateral earth stress coefficient or the ratio of horizontal (effective) stress with the vertical (effective) stress. OC expansive clays have naturally occurring horizontal stress (Ko >1) that creates the instability needed to form gilgai. I will explain the origin of this stress state as a result of it's geologic history where it is loaded and unloaded. Because of horizontal self confinement and shear strength, Ko increases during unloading created by normal erosion, this creates a pre-buckling stress (item 2) that alters the natural confinement pressure in the soil profile.

2.  DIFFERENTIAL REBOUND IN A PREBUCKLING MODE: In part II of this blog, I will contrast the difference between a prebuckling internal stress and a buckling internal stress and show how a prebuckling stress creates differential rebound and the initial mounds and depressions.

3.  HOW POLYGONS AND LINEAR GILGAI FORM: I will then show how mounds and depressions change into the more commonly seen polygons. This happens because the initial mounds and depressions cause a release of self confinement and allows more lateral movement. I will also show how linear gilgai form on slopes. I have developed this since 1994 so this is new information.

THE ORIGIN OF Ko>1; I.E. THE HORIZONTAL STRESSES THAT CREATES GILGAI

An OC clay is a clay that has had more stress (or load) applied to it in the past than presently exists. This creates telltale changes in pore pressure and stress state of the clay deposit that can be measured. In my paper I explained the origin of Ko >1 as follows: "horizontal stresses larger than the vertical in OC clays are primarily residual consolidation stress" and that "when these clays rebound from unloading, internal friction causes a lag in the release of horizontal stress induced during consolidation."  To show this in more detail, I have altered the classic stress history chart that shows the origin of Ko >1.The following interpretation of the chart is my own understanding of what is happening. This chart shows how unloading produces the actual values of Ko. There are ways to measure the horizontal stress of a clay during compression or consolidation and then under a later cycle of erosion or unloading as the clay becomes overconsolidated. Mayne and Kulhawy (1982) state that "any reduction of the effective overburden stress results in overconsolidation of the soil." The vertical stress or the effective overburden stress is simply the effect of the weight of the material above corrected for the effect of pore pressure. Thus the changes in horizontal and vertical stress created by changes in load can be plotted as a stress history. The combined cycles of virgin (first) loading and subsequent unloading are referred to as the stress history of a clay.
Simplified Stress History of an Expansive Clay by Britt Maxwell
A simplified stress path or stress history of an expansive clay is shown in Figure 1. An expansive soil deposit like in the Eocene or the Cretaceous has a much more complex loading and unloading history than is shown by this chart. Whenever there is an unconformity in the deposits above an expansive clay we know that there was a period of unloading, reloading and then a final unloading. This creates loops in the path, but the end effect when the clay is finally unloaded should be essentially the same as the unloading path shown in this chart.

Any straight line through the origin (0) is a line of constant Ko with Ko increasing counterclockwise. In the middle of the chart is the Ko =1 line. This would be the path of a hypothetical frictionless material (like a compressible fluid) during any loading or unloading. Above this line Ko >1, below this line Ko <1. The stress path varies from Ko =1 only because of internal friction. So this chart shows how internal friction causes values of Ko to vary. The internal friction does not change during the loading cycle so Ko is a constant value (a straight line). This value is called the virgin loading and is expressed as Ko here but Konc  has also been used in the literature where the nc stands for "normally consolidated." At the end of the virgin loading cycle (point B) there is an increase in internal friction or shear strength caused by a dropping pore water pressure. This then causes Ko to increase throughout the unloading cycle. So this plot contrasts the difference between the virgin loading cycle and the unloading cycle under the influence of internal friction that begins to change when unloading starts at point B. These two cycles of the stress history are described in more detail as follows:

THE CONSOLIDATION OR LOAD PATH (A TO B): Initially a deposit of clay is a soft mixture of clay and water particles. As more deposits accumulate above, the clay is subject to more compression. The clay becomes denser because water is squeezed out with positive pore pressures. This effect causes the transfer of the vertical compressive force into the horizontal direction at a constant rate. Ko is less than one and constant during the whole loading cycle. The actual number usually varies between .4 and .8 and the value depends on the clay.

THE UNLOADING PATH (B TO C): The overconsolidated coefficient of earth pressure at rest has been represented by different symbols in the literature. Some of these are Kou , Ko(oc), and Kooc. An Equation was proposed for this coefficient by Mayne and Kulhawy (1982). More recently Michalowski (2005) reviewed the proposed Ko functions for loading and unloading. Note that this paper is available online. As the clay is unloaded it becomes unsaturated and overconsolidated and as the vertical load decreases a negative pore pressure increases. This causes the clay to gain strength and develop more resistance against the particles from sliding past each other. If you could freeze the material and lock the particles together at point B the unloading path would be horizontal and the horizontal stress would not change. So any drop in horizontal stress during unloading is allowed by interparticle shifting. This is what I call “unloading shear.”

Because of the increase of internal friction at the beginning of unloading, the stress path shifts horizontally when unloading begins and the horizontal stress drops slower than then the vertical stress. This causes Ko to steadily increase as unloading progresses. Ko >1 and a negative pore pressure now identifies the clay as being overconsolidated. As Ko increases the clay becomes more unstable. The dryer inter-particle shifting must create fractures or distort the clay to relieve horizontal stress. The process of losing horizontal stress during rebound is herein named “unloading shear” and it is this process that produces gilgai and the associated shear joints in expansive clays.

In my paper I use Rankine theory to compute a maximum value of Ko. This is shown as point C (the end of the unloading path) in Figure 1. For an average friction angle Ko is limited to a value of about 3. I don't know the smallest values of Ko that could produce gilgai, but it might be as low 1.5 in ideal conditions. It depends on the state of the surface soils. I think often the surface soils would be highly weathered and have Ko close to unity. When this happens these soils can't participate in forming gilgai and surface cracks can appear. Two gilgai sites in Mississippi on public land do not have any significant surface cracks even in drought. In the photo above there are no surface cracks visible, but the ground is dry. I interpret this as meaning that Ko must be greater than one very near the surface. These are sites where a surface crack origin would not work and provide evidence that surface crack theories are not correct. It is important to realize that Ko and instability steadily increases during the unloading cycle and the formation of gilgai with the related shear joints is an important (horizontal) stress release mechanism. This process contributes to the weathering of expansive clays. Shear joints initially form in the subsurface around the perimeter of gilgai mounds and are explained in more detail in part II of this blog. I will also perform a more detailed comparison with this theory and the older surface crack theories.

Here is a link to part II of this blog:

The Origin of Hogwallows and Gilgai - PART II  The conclusion of this blog.