INTRODUCTION
This is the definitive guide for gilgai landforms. Because of the length, this is a 2 part blog. I explain why self-mulching and all shrinkage crack theories are wrong in part 2. For the most part, gilgai and hogwallows in the literature are the same thing. They are a naturally occurring temporal landform called microrelief that only occurs in expansive clays mostly in the wet seasons. The higher areas have been called microknolls, mounds, and microridges, and the lower areas are called depressions, micro-lows, or micro-depressions. According to Gustavson (1975) the maximum relief produced by these landforms is about 18 inches. The three basic forms are mounds and depressions, the more mature polygons (with microridges), and linear ones that are elongated in the direction of downslope. The expression "hogwallows" appeared first (in print) in the United States in about 1839 but the Australian expression "gilgai" has become the more commonly used word since 1930's. These landforms are caused by montmorillonitic expansive soils. The study of these landforms is compelling because the resulting micro-topography is anomalous concerning 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 a special mechanism or process that starts and maintains these landforms. At stake is some process that produces extraordinary differential movements in expansive clays. This post was updated on 12/4/2018, 4/25/2020, and again on 2/11/2024.
This is the definitive guide for gilgai landforms. Because of the length, this is a 2 part blog. I explain why self-mulching and all shrinkage crack theories are wrong in part 2. For the most part, gilgai and hogwallows in the literature are the same thing. They are a naturally occurring temporal landform called microrelief that only occurs in expansive clays mostly in the wet seasons. The higher areas have been called microknolls, mounds, and microridges, and the lower areas are called depressions, micro-lows, or micro-depressions. According to Gustavson (1975) the maximum relief produced by these landforms is about 18 inches. The three basic forms are mounds and depressions, the more mature polygons (with microridges), and linear ones that are elongated in the direction of downslope. The expression "hogwallows" appeared first (in print) in the United States in about 1839 but the Australian expression "gilgai" has become the more commonly used word since 1930's. These landforms are caused by montmorillonitic expansive soils. The study of these landforms is compelling because the resulting micro-topography is anomalous concerning 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 a special mechanism or process that starts and maintains these landforms. At stake is some process that produces extraordinary differential movements in expansive clays. This post was updated on 12/4/2018, 4/25/2020, and again on 2/11/2024.
There is a similar class of microrelief that are sometimes called gilgai or hog wallows that are not related to expansive soils. These landforms are distinctly different because they have a flat or tableland area between the microknolls without clearly defined microlows. In other cases, there are apparent sinks or microlows separated by flat or tableland without any defined microhigh areas. These similar landforms can also have a much 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. However, studies of these similar types do not reveal a connection with expansive clays.
In 1994 I published a new theory that shows a connection between these landforms and overconsolidated (OC) expansive clays. If this theory is correct we have to use the language and concepts developed by geotechnical engineers to fully understand these landforms. More specifically the explanations of horizontal stress as used in this text come from the science of unsaturated soil mechanics. With this concept, gilgai are one result of the natural weathering process of OC expansive clays. Before weathering these clays are massive and unfractured. Weathering breaks down the massive structure into smaller and smaller peds. This is the only theory that explains how gilgai develop under pavements and this theory also explains all the cracks, joints, and shapes of these landforms. Since 1994 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. In 2016 a map that I produced was published (The Geology of Mississippi by Dockery and Thompson) showing dozens of gilgai sites occurring inside the Jackson Group (Yazoo clay). This map is located on page 28 of this classic textbook. In my next publication, I will show all the known gilgai sites in Mississippi and their associated geologic formations. Maps like my map in The Geology of Mississippi are important because they show that once these OC clays become buried by silts then gilgai no longer occur. Maps like this of Texas and Alabama are still needed to further show this established connection between OC expansive geologic deposits and these landforms. However, Hilgard without the benefit of Google Earth or aerial photography first made this connection 158 years ago. 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 and Mississippi related to 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 sometimes results in gilgai landforms. Even the more recent Pleistocene era Beaumont Formation of Texas has been shown to have properties of unloading that produce 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 the related (higher) horizontal stress is connected.
Thirty years later I still stand by the solution I proposed in 1994, mainly because there have been no flaws or arguments raised against this concept since it was first published. Structural and geotechnical engineering concepts were used to solve this problem in geomorphology which 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 185 years later! Unfortunately, there are no clear online images. But 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, 3-8-2011, and 12-24-23 images at an elevation of about 2350 feet. These images have poor resolution (compared 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 which includes the Cook Mountain Formation. I located these hogwallows by using Riddell's description which was eighteen miles above Robbins Ferry on the Trinity River. The location of this ferry can be determined from 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 could be once seen at 32.642325, -96.522785. Unfortunately, the updated image does not show gilgai, but they can still be seen in this neighborhood in historic images in Google Earth most years between December and March.
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 in aerial photography. This is fairly typical of aerial photos in the southeastern United States. Higher-resolution images are also a requirement. There is also a chance the ground surface will be dry even it the images are made during the optimum period. Presently the extreme rural areas of Mississippi and Texas do not have adequate photographic coverage to map and to fully define the distribution. This is unfortunate because these landforms could help map unknown areas where damaging expansive clays exist.
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 in aerial photography. This is fairly typical of aerial photos in the southeastern United States. Higher-resolution images are also a requirement. There is also a chance the ground surface will be dry even it the images are made during the optimum period. Presently the extreme rural areas of Mississippi and Texas do not have adequate photographic coverage to map and to fully 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, -87.424533. I could not find these gilgai in Alabama until new aerial photos were taken during the wet season in January of 2013. 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 hilltop, there are polygonal gilgai but it appears that 2 or 3 depressions (darker spots) are usually grouped together. Here linear gilgai are stretched out in the downhill direction (around the hill) but still have the occasional depression in the valleys below. The elongated or linear gilgai were mentioned by Riddell in 1840 and also by Gustavson in 1975 but there are no theories to explain this form of gilgai until now (see PART II of this blog).
E. W. Hilgard (1906) the former state geologist of Mississippi added an element of expansion (from expansive clay) 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, etc. 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 stated that "measured horizontal stresses in OC (overconsolidated) expansive clays are far below a critical buckling load." To me, differential rebound is clearly not the same as buckling, but if this has been misunderstood, I feel I must clarify this concept. So this blog is inspired by this issue 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 to 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: Ko is 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 its geologic history where it is loaded and unloaded. Because of horizontal self-confinement, Ko increases during unloading (created by normal erosion) this along with an increase in shear strength results in 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. The key point here is to recognize that expansive clays are load-reactive much like a spring. When you mix this reactivity with Ko >1 near the ground surface gilgai landforms are the result. A simplified linear model can be produced by compressing a long spring that is confined in the lateral horizontal direction. Here as with gilgai, the self-weight of the material affects the wavelength. As you increase the self-weight the wavelength gets shorter. Eventually, the reaction is suppressed altogether when the self-weight becomes excessive. We see this real-world effect where gilgai appear and disappear when there is excessive material above the expansive clays that is not expansive. In part II of this blog, I will also introduce a more comprehensive elastic gilgai soil model than the spring model used here.
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 at the surface and allow more lateral movement. This causes shear joints to form around gilgai mounds. I will also show how linear gilgai form on slopes from the same process. 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 a telltale negative pore pressure and an altered stress state of the clay deposit that can be measured with geotechnical instruments. 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 "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 modified 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 in the rebound of expansive clays. This chart shows how unloading produces increasing positive values of Ko as unloading progresses. 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.
a.jpg "Simplified Stress History of an Expansive Clay by Britt Maxwell")
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 value of Ko that could produce gilgai, but it might be as low as 1.5 in ideal conditions. It depends on the state of the surface soil. 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 increase 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 between 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.
The Origin of Hogwallows and Gilgai - PART II The conclusion of this blog.
