Friday, November 15, 2013

The Origin of Hogwallows and Gilgai Landforms - PART II

You can scroll down to see my most recent blogs including Part I of this blog or you can click on this link:
The Origin of Hogwallows and Gilgai - PART I10/31/2013. This is a closer look at a theory I published in 1994 to explain these landforms. Included in this blog is a great photo of gilgai at ground level taken from Google street view in Waxahachie Texas.


PREBUCKLING STRESS AND DIFFERENTIAL REBOUND IN OVERCONSOLIDATED EXPANSIVE CLAYS

It maybe that there has been some confusion about what buckling is and how it is applied in this case. Generally buckling is deformations produced by an axial load. There is a critical load at which buckling produces a sudden failure. However, prior to the critical buckling load there are internal stress reactive changes that resist the applied load as the load builds. It is important to understand the difference between a pre-buckling load (axial) that is not critical and a critical buckling load. The internal stresses associated with critical buckling do not just suddenly appear when the load becomes critical. So prior to a critical buckling load the normal internal stresses are altered. Expansive soils react to this because they react to changes in confining pressure. That is they swell more with less confining pressure and swell less as the confining pressure increases.

A deeply buried expansive soil will not expand if the confinement pressure (from the weight of the soils above) equals or is greater than the expansion pressure. As this confining pressure is decreased, the clay can expand more and more. The key idea is that prior to a critical buckling load there are internal changes in stress that alters the normal confining pressure (in the horizontal direction) that then triggers differential rebound. Where mounds will appear the normal confining pressure is decreased and where depressions will occur the normal confining pressure is increased. These internal changes in stress, less than the critical buckling load can trigger differential heave and create the appearance of buckling. The evidence that gilgai are not caused by a critical buckling load are:

1. Gilgai do not appear suddenly. Critical buckling happens suddenly. Farmers that have plowed gilgai flat report that gilgai will reappear in a few years. I.e. if the movement rate is more like normal shrink-swell movements, then this is not a critical (sudden) buckling issue.
2. Gilgai only happen in expansive soils. If critical buckling was involved it would happen in other clays like Kaolinite, Illite and Chlorite.

So this means that the normal shrink-swell behavior of expansive clays must be the energy that is creating the gilgai. If you were to place some heavy lead plates on the ground in a pattern resembling gilgai depressions, in a few years there would be a differential rebound that would mimic gilgai with the plates in the depressions. In this case Ko >1 had nothing to do with the movement. However, Ko >1 but less than the critical buckling load can take the place of the lead plates. This is because an elastic member can carry a significant axial load up the critical buckling load but not without reacting to the load internally. This is what I call prebuckling. When this happens expansive soil senses a change in vertical load just like there were lead plates on the surface of the ground. So in one sentence Ko >1 and shrink-swell behavior work together to produce differential rebound. I think we can say that once the buckling load represented by Ko approaches 25 to 50% of the critical buckling failure load then differential heave kicks in to create gilgai. But that is just a guess as to those percentages. The true analytics of the problem are quite complex because there are a lot of related properties that change with depth such as Ko, expansion potential and soil strength and others.

I will now show you in more detail how this works using a simplified model constructed from a long strip of elastic material that has self weight and can buckle like a continuous sine wave. Think of this as a heavy very long column (an axial loaded member) that is lying down. It is fully braced in the z direction (in and out of the page) due to self confinement. Oddly in this case the material self weight helps to brace the column, so for true buckling to occur it has to overcome the buckling strength of the material and it also has to lift the material as well. But there is a lifting influence prior to the critical buckling load.

Simplified Elastic Model showing prebuckling
I zoom in to look at a random segment of this longer strip in Figure 2. I have made several simplifications to show what happens internally and more importantly how the weight of the material is supported at the base of the material. A technically correct drawing would have roller supports along the sides and below. I have removed these for clarity. This drawing shows three levels of Ko in the material. This is the ratio of horizontal stress with the vertical stress. In the first drawing the model has Ko =1 the horizontal loading on the side matches the weight of the material. In the second drawing Ko =Kcr (the critical buckling load) and the strip of material has buckled due to the higher horizontal loading. We don't know the actual value of Ko in this case, but we assume it is higher than the Rankin failure I show in Figure 1. So in this 2nd case, the material is a stronger material than a clay deposit for this behavior to occur. In the third drawing there is less horizontal load but it is still higher than the vertical (self weight due to gravity). Although the load is not high enough to produce buckling, changes in the internal stress are still there. You could say that the material it trying to buckle and producing a differential load (see support reactions) on the subsurface layers. This should trigger a differential rebound that would only occur in an expansive soil. The effect is the same as if you placed some heavy lead plates in locations of future depressions.

HOW MOUNDS TRANSFORM INTO MICRORIDGES (POLYGONS)

Figure 3 shows how mounds and depressions (a) change into more commonly seen polygon (c) shapes. My photo in PART I shows gilgai that are in transition (b) in-between. In mature gilgai mounds are replaced by interconnected micro-ridges to create polygons. If you look at the aerial image at location (32.419913,-96.77876) dated 2-27-2001 you can see better developed polygonal gilgai in the yard behind the house. You can also see where contours have been plowed through the gilgai to help retain water and where the gilgai have been plowed flat in adjacent fields.

The Three Stages of Gilgai Development
In 1994 I defined the term "shear joint" to describe the special subsurface fracturing that forms around gilgai mounds and micro-ridges. In the literature you find these joints also called slickensides. However, the term slickensides has been used to describe lots of different shiny surfaces with different origins.  This is a special kind of fracture related only with gilgai and I think a special name should be used for these fractures. In Jackson Mississippi I have seen shear joints that indicate that gilgai existed thousands of years ago and were eroded away leaving only the shear joints as remnants of gilgai. Joeckel (1999) calls this kind of fracture synformal slickensides. Williams etal (1998) excavated two sections through gilgai and described the subsurface fractures. These are the best descriptions I have seen for subsurface gilgai fractures.

The key concept of this final part of this theory shown in Figure 3 is that once mounds and depressions develop there is a slight loss of horizontal self confinement. This then promotes additional lateral movement toward the mound. This squeezes the mounds into narrow ridges that eventually connect to create a continuous micro-ridge system or polygons. The development of shear joints around the perimeter of the mounds enables additional horizontal movement. The formation of linear gilgai on slopes occurs for the same reason that polygons form; the slope allows downhill creep to occur, this releases self confinement in the downhill direction. This means that Ko >1 becomes directional occurring only perpendicular with the slope. This also means that Ko >1 is providing some of the energy into downhill creep that is well known in expansive clays. Linear gilgai prove that there are strong horizontal forces that are behind the formation of gilgai.

Mound squeezing explains several phenomenons associated with gilgai mounds. Some of these are:

1. Tightly wrinkled shear joints inclined at steep angles under the mound perimeter. A photo of this can be seen in Figure 3-32 in Chapter 3 of the Soil Survey Manual. They do not relate these fractures with gilgai but these look the shear joints I have seen in the local Yazoo clay. Another image of this can be seen in Joeckel(1999). Search for image "0728_f10.jpg" in Google images.
2. Features called first called chimneys where the subsurface strata appears to be pushed up under the mounds. These features have been compared with diapirs(Gustavson 1975).

Nature "turns off" or switches off gilgai when the Ko >1 soils become too deep or when expansive soils are replaced with non expansive soils. This means that when you find a large field with polygons (c) around the perimeter you will occasionally see less mature forms (a) or (b) where nature is preventing the mature forms from developing. The transition is most often represented by the micro-ridges being wider. My guess is that this is most often happens when Ko >1 soils are buried by soils where Ko is equal to or less than one. Because the rate of erosion is slow, there is no reason for gilgai to be actively forming, at least in the southeastern United States unless surface soils have removed by human activities and elevated Ko >1 expansive soils nearer to the surface. This means that the gilgai we see have been there for at least hundreds and maybe thousands of years. So if we see the less mature forms of gilgai that simply means that there is not enough expansive energy to push the gilgai into the mature stage.

The development of salt polygons in desert environments appears to occur by an almost identical process as I describe here, except in the case of salt polygons the horizontal force is produced by the growth of salt crystals (see the textbook Geomorphology of Desert Environments (2009). This parallel process maybe the best supporting evidence for Ko >1 theory of gilgai. 

Surface cracks vs Ko>1 theory

1. Surface cracks or shrinkage theories can't explain polygons or linear gilgai; the two most common forms. The big problem is that the surface crack theories only seem reasonable in 2 dimensional section drawings. If you try to create a plan view, you can not do it and make something you would expect to find in nature. So these theories do not work in 3 dimensions.
2.  If gilgai resulted from surface cracks, it could be established by mapping the surface cracks as related to the topography or maybe by filling cracks with soil or by running water in the cracks in the field or by correlating moisture content with cracks. These kinds of studies have not shown correlations that support these theories. Two examples are Spotts (1974) and Kishne, Morgan and Miller (2009).
3. Surface cracks theories were first proposed from 1840-1930 before modern soil mechanics discovered that horizontal stress naturally occurs in overconsolidated expansive clays. I think this negates these awkward attempts to generate horizontal thrust with a shrinkage crack.
4.  Shrinkage crack theories would not function under pavements, and pavement undulations have been associated with gilgai.
5.  Not all gilgai have surface cracks. There are two sites in Mississippi on public land that don't have shrinkage cracks even during drought conditions. What causes these gilgai?
6.  The Ko>1 origin of gilgai can create gilgai of all forms with or without surface cracks. This is the only gilgai theory discussed in a peer reviewed engineering journal.There are no valid published criticisms that have not been answered since this theory was introduced in 1994. l would be happy to respond any questions or criticisms.
7. The Ko>1 condition is well established in overconsolidated clays, if in the case of expansive clays, that gilgai are not produced, then what does happen? How could an expansive clay ignore the presence of Ko>1 instability without reacting in some way that is out of the ordinary?

A FEW FINAL THOUGHTS

If you want to see gilgai in the field based on Google Earth images, you must realize that gilgai you see in aerial photos may not always be visible on the ground. The relief is typically less than 1 foot so if the grass or brush is higher than 1 foot, you may not be able to see the gilgai on the ground. So you need a fresh mowed field, yard or an actively used pasture to see gilgai on the ground. Your best bet is a pasture or land that is mowed regularly. Also occasionally owners plow the gilgai flat. In this case the gilgai may come back but you may have to wait a few years. If you find this subject interesting and wish to read more please check out Riddell's two publications. His hogwallow descriptions are most interesting. I don't think his work has ever been referenced in recent gilgai research, so his work is herein being brought back in the history of gilgai research. Also the paper by Mayne and Kulhawy is considered to be a classic in geotechnical engineering.

REFERENCES:

Gustavson, T. C. (1975). "Microrelief (gilgai) structures on expansive clays of the Texas coastal plain--Their recognition and significance in engineering construction." Bur. Econ. Geology--Geological Circular No. 75-7, University of Texas, Austin, Tex.

Hilgard, E. W. (1906). "Soils" New York, The Macmillan Company. 593 p.

Howard, A. (1932). "Crab-Hole, Gilgai and Self-Mulching Soils of the Murrumbidgee Irrigation Area" Pedology (Pochvovedeni), p.14-18.

Joeckel, R.M. (1999). " Paleosol in Galesburg Formation (Kansas City Group, Upper Pennsylvanian), Northern Midcontinent, U.S.A.: Evidence for Climate Change and Mechanisms of Marine Transgression." J Sedimentary Research. Vol. 69, No.3, p720-737.

Kishne, A. S., Morgan, C. L. S. and Miller W.L. (2009) "Vertisol Crack Extent Associated with Gilgai and Soil Moisture in the Texas Gulf Coast Prairie" SSSAJ: Vol. 73: No. 4. p. 1221-1230.

Maxwell, B. (1994). "Influence of Horizontal Stresses on Gilgai Landforms." J Geotech Eng., ASCE 120:1437–1444.

Mayne, P.W. and Kulhawy, F.H. (1982). "Ko-OCR Relationships in Soil." J Geotech Eng. ASCE 108 No. GT6:851-872.

Michalowski, R. L. (2005) "Coefficient of Earth Pressure at Rest." J Geotech Geoenv Eng., ASCE 131:11, 1429-1433.

Riddell, J.L. (1839) "Art. II.- Observations on the Geology of the Trinity Country, Texas. The American Journal of Science and Arts, Vol. 37, Nov., p. 211-217.

Riddell, J.L. (1840) "37. Hog Wallow Prairies.*--Extract of a letter to the Editors…May23, 1840." The American Journal of Science and Arts, Vol. 39, Oct., p. 211-212.

Spotts, J.W. (1974) "The role of water in gilgai formation." P.H.D. Dis. Texas A&M University, College Station, TX.

Williams, D. Wilding L., Lynn, W. Kovda, I. and Chervenka, G. (1998) "Slickenside arrangement in Burleson clay - a udic haplustert" 16th World Soils Congress. Vol.1, p. 89