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Wesley Miller
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Monday, May 16 2016, 10:17 PM
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This was posted several years ago on the old SWS Forum, but have updated a few comments and references.

A copy of a brief updated report concerning piezometer stability in Texas Vertisol hydrology studies with additional details and published references is enclosed (this site rejected the report as an attachment due to unsupported format?). Individual questions or comments are welcomed. Contact me at wmiller2318@gmail.com. Wesley L. Miller, USDA-NRCS, Technical Specialist Wet Soils, Texas (retired). Victoria, Texas.

Piezometer Stability in Long Term Vertisol Hydrology Studies in the Texas Gulf Coast Prairie
Wesley L. Miller
USDA-NRCS Soil Scientist (retired)
Victoria, Texas
May 15, 2016

The high shrink-swell characteristics of Vertisols as they go through seasonal wet-dry cycles, associated deep open cracks during dry periods, and bypass flow along cracks and/or slickensides (Griffin et al., 1992) (Sprecher, 2008) make the measurement and interpretation of Vertisol hydrology field studies challenging. One question in recent years concerns the physical stability of piezometers made of schedule 40 PVC pipe during long term Vertisol hydrology field studies (Jacob et al. 1997). In the past several years piezometers made of schedule 40 PVC pipe were removed from two long term Vertisol hydrology studies in the Texas Gulf Coast Prairie that allowed an evaluation of this possible stability problem.

Between 1989 and 2004 there were two formal USDA Natural Resources Conservation Service and university cooperative sponsored long term soil hydrology studies that included Vertisols in the Texas Gulf Coast Prairie. Piezometers were made of schedule 40 PVC pipe. At each monitoring site (eight total) there were three piezometers at 25 (or 30 cm), 50, 100 and 200 cm depths (total of twelve). Additional piezometers that ranged from 400 to 625 cm deep were placed at selected monitoring sites.

The first long term Vertisol hydrology study in most Texas Gulf Coast Prairie was part of the USDA International Committee on Aquic Soils Study from 1989 to about 1996 (Kimble, 1992). One of the Vertisol study sites was located in Victoria County, Texas, on Laewest clay, fine, smectitic, hyperthermic Typic Hapluderts (Soil Survey Staff, 1999) in an area of native prairie.

The possibility of piezometer failure due to tilting during long term Vertisol hydrology studies was first mentioned by Jacob (Jacob et al., 1997) based on a field observation made at the Victoria County, Texas, Laewest clay site in 1996 in preparation of a chapter on Vertisol wet soil studies. The piezometers had been in the ground seven years at the time the field observations were made. All piezometers (1.9 cm ID schedule 40 PVC pipe) were functional in 1996. Only one 100 cm deep well made of 25 cm diameter PVC pipe (50 cm above ground) on a microhigh had tilted the most from vertical about 5 to 10 cm (W.L. Miller, unpublished data).

In 2005 the piezometers were removed from the Laewest clay site. Piezometers had been in the ground for about sixteen years. The depth of several of the piezometers was checked at each microhigh-microlow location, and the 500 cm piezometer was also checked. We could measure to the designated piezometer depth and all were functional (allowed water flow). Remaining geofabric covering the open end and eight narrow slots at the bottom of the piezometers was in good condition.

The second long term Vertisol hydrology study in the Texas Gulf Coast Prairie from 1996 to 2004 was the Soil Characterization and Hydrological Monitoring Project, Brazoria County, Texas, Bottomland Hardwood Vertisols (Miller and Bragg, 2007) (Bottomland Hardwood Vertisol Study). Soils were Pledger clay (nonponded), very-fine, smectitic, hyperthermic Typic Hapluderts (Soil Survey Staff, 1999) and Churnabog clay (ponded) very-fine, smectitic, hyperthermic Typic Epiaquerts (Soil Survey Staff, 1999). There were three nonponded and four ponded bottomland hardwood monitoring sites. Standard depth piezometers at each monitoring site were made of 4.5 cm ID schedule 40 PVC pipe. At five selected sites an additional piezometer made of 1.9 cm ID schedule 40 PVC pipe was installed. These piezometers ranged from 400 to 625 cm deep into contrasting sandy and loamy layers below the clays.

In 2004 a study was conducted to measure the vertical (heave) and horizontal (throw) movement of the soil and/or piezometers at the Bottomland Hardwood Vertisol Study sites (Bragg and Miller, 2006). The piezometers had been in the ground about eight years. Soil movement above original ground level marked on piezometers when installed ranged from 0 to 9 cm. Tilt of piezometers from vertical ranged from 2 to 5 cm (Miller and Bragg, 2007). All of the piezometers we checked were still functional.

In December, 2010, the piezometers and equipment were removed from the Bottomland Hardwood Vertisol Study sites. The piezometers had been in the ground for about fourteen years. Based on observed piezometer PVC pipe field conditions, none had tilted significantly from the observations made in 2004, or had failed (were not collapsed at depth), including the 400 to 625 cm deep piezometers. When the piezometers were removed, remaining geofabric covering the open end and eight narrow slots at the bottom of the piezometers was in very good condition. Piezometer slots were not blocked and still allowed water flow.

The results of these two long term Vertisol hydrology studies in the Texas Gulf Coast Prairie indicated that piezometers made with schedule 40 PVC pipe did not tilt significantly or fail (collapse) due to shrink-swell cycles during monitoring periods of fourteen to sixteen years. Quality geofabric covering the slots and open bottom end of the piezometers did not deteriorate significantly during the same time periods.

Bypass flow along Vertisol open cracks and/or slickensides to the deeper piezometers was observed several times. Most notable bypass flow usually occurred within a relatively short period after large, short duration precipitation events during dry periods when cracks were open and extensive at the surface (W. L. Miller, unpublished data). We also observed that over time, at ponded sites in both studies, a few deeper piezometers leaked water along the sides due to incomplete bentonite sealing.

Leakage along piezometer sides can be avoided by first digging very straight holes to the required depth. Then, after the piezometer is inserted into the hole and as the bentonite seal is added, tamping the bentonite completely around the piezometer PVC pipe. In later wet soil studies we used a same ID PVC pipe as the piezometer pipe, cut in half lengthwise and slightly longer than the piezometer being installed, to accomplish the tamping of the bentonite completely around the piezometer.

References

Bragg, A. L. and W.L. Miller. 2006. Vertical and horizontal movement of soil and piezometers at a long term wet Vertisol study in Texas Gulf Coast Prairie bottomland hardwoods. In 2006 Agronomy abstracts. ASA, Madison, WI.

Griffin, R.W., L.P. Wilding, and L.R. Drees. 1992. Relating morphological properties to wetness conditions in the Gulf Coast Prairie of Texas. p. 126-134. In Kimble, J.M. (ed.). 1992. Proceedings of the eighth international soil correlation meeting (VIII ISCOM), characterization, classification and utilization of wet soils. USDA-SCS, National Soil Survey Center, Lincoln, NE.

Jacob, J.S., R.W. Griffin, W.L. Miller, and L.P. Wilding. 1997. Aquerts and aquertic soils: a querulous proposition. p. 61-77. In M. J. Vepraskas and S.W. Sprecher (ed.). Aquic conditions and hydric soils, the problem soils. SSSA Spec. Publ. 50. SSSA, Madison, WI.

Kimble, J.M. (ed.). 1992. Proceedings of the eighth international soil correlation meeting (VIII ISCOM), characterization, classification and utilization of wet soils. USDA-SCS, National Soil Survey Center, Lincoln, NE.

Miller, W.L. and A.L. Bragg, 2007. Soil characterization and hydrological monitoring project, Brazoria County, Texas, bottomland hardwood Vertisols. NRCS. Temple, Texas.

Soil Survey Staff, 1999. Soil Taxonomy. A basic system for making and interpreting soil surveys. USDA. Agric. Handbk. 436. U.S. Govt. Print. Office, Washington, DC.

Sprecher, S.W. 2008. Installing monitoring wells in soils. (Version 1.0). USDA-NRCS National Soil Survey Center, Lincoln, NE.
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