Usage of superabsorbent geocomposite in erosion protection

Transcription

1 Usage of superabsorbent geocomposite in erosion protection Pawłowski A. Lejcuś K. Garlikowski D., Orzeszyna H., Institute of Environmental Engineering, Wroclaw. University of Environmental and Life Sciences, pl. Grunwaldzki 24, Wroclaw, Poland KEYWORDS: Geocomposite, superabsorbent, water retention, slope protection ABSTRACT: Vegetative cover in slope erosion protection is widely used in earth works. Plants, usually grass, in many places have very difficult environmental conditions. Fast downhill flowing of rainfall water, small infiltration rate, direct sun radiation are some of the most important factor influencing actual amount of water available for plants. To retain the water in the subsoil water from precipitation or from watering superabsorbent (SAP) can be used. In agriculture superabsorbent is applied directly by mixing with soil. Such method of application on a slope can cause stability problems because strength parameters of soil are negatively influenced by the presence of the SAP in the pores between grains. To solve this problem a simple form of a geocomposite containing SAP was developed. It can be placed on a slope under humus layer, pinned if necessary, keeping SAP apart from the soil while water is still available for plants root system. It eliminates the risk of cover soil stability failure. Some tests made on soil mixed with superabsorbent have confirmed important loss of shear strength if compared with soil itself. Investigation of geocomposite with superabsorbent application has proven despite of additional water retention possibility, its positive influence on roots system development. Roots penetrating geocomposite were creating a kind of anchorage even in the most unfavorable soil conditions. 1. INTRODUCTION In earth works, embankments slopes are usually protected against erosion with grass or other plants, sometimes called a biotechnical protection. The plants on inclined surfaces have more difficult condition for growth and accessibility of water is one of the most important problems. On a steep slope rainfall water is quickly flowing down and even infiltrating water retained in soil will have tendency to move down. So storing of water available from precipitation or watering is much more important than on plane, horizontal areas. There are different ways of increasing water retention of the subsoil. One of them is to use superabsorbent (SAP), known also as hydrogel, mixed with soil. Superabsorbent is a polymer, which can absorb big amount of water up to of its own weight. Retained water, can be later used by plants. The process is reversible, so after giving up water or drying it can be absorbed again. Recently, research on the use of superabsorbent polymers as water managing materials for the renewal of arid and desert environment has attracted great attention, and encouraging results have been observed as they can reduce irrigation water consumption, improve fertilizer retention in soil, lower the death rate of plants, and increase plant growth (Buchholz & Graham 1998, Lan Wu et al. 8). 562

2 Using of soil-sap mixture on a slope could be dangerous for stability of the surface layer. It was found, that this method of application decreases strength parameters of soils. (Sojka et al. 1998). It reduces soil permeability and in extreme case can make impossible to use machinery cultivation. Some simple tests made by the authors have shown that one should use another method of SAP application on slopes. Geocomposite containing SAP could be one of the solutions. It eliminates most of the disadvantages of SAP direct mixing with soil. This solution supports, with its retention possibilities, growth of plants on inclined surfaces and development of their root system. Good developed roots system is positively influencing stability of cover layer (Wu 1994, Schmidt et al. 1). 2. TESTS ON SOIL MIXED WITH SUPERABSORBENT To investigate SAP influence on soil shear strength one have to consider dosage usually used for testing. In laboratory scale it is not a crucial factor. When SAP is used in full scale it can affect not only surface layer stability on a slope, but can make very difficult all the soil cultivation works. E.g. Leciejewski (8) in his tests is using g of SAP for 1 dm 3 of soil and Martyn & Szot (1) 1 6 g in 1 m 3 of soil. It means, even assuming the smallest absorption rates - 1 : 300, that using the full water absorbing capacity 1 dm 3 of soil should retain cm 3 of water (0,3 1,5 dm 3 ). This theoretical volume of hydrogel far exceed possible volume of pore space between grains of soil, which is usually not bigger then % and, it is clear, that even in very extreme cases pore volume cannot exceed 100 %. On the site there is of course many factors, which are influencing the process of swelling, like overburden pressure, soil granulation, rate of water infiltration, presence of different chemical substance, which can disturb adsorption process etc. Nevertheless, by great dosage of SAP reported by investigators, one can expect changes of soil s properties. It means, that after full swelling, soil grains will be moved apart or/and hydrogel will migrate toward free pore space or toward surface. This phenomenon can be observed in very simple test on dry SAP / sand mixture (medium sand was used in experiment, SAP water adsorption ratio was 1:300, equivalent of 1 g of SAP added to 1 dm 3 of loose sand 0,059 % by weight was used to prepare a sample). Both materials were placed in shale or in a glass cylinder. They were carefully mixed and then water was added in such amount to make possible using maximum adsorption capacity of SAP. At the beginning sedimentation of sand particles was observed. Bearing capacity of sand/superabsorbent mixture was very low. A ruler placed vertically on soil surface was sinking under its own weight. After several hours mixing of both materials in shale was repeated, after the process of swelling has fully occurred. This time SAP viscosity was high enough to keep soil particles suspended. Particles were not falling down. There was no contact between grains, so the hydrogel was the only material deciding on the strength parameters of the mixture. Of course in natural application so extreme phenomenon does not necessary happen, but one have to take into consideration the possible effects of direct soil and SAP mixing negatively influencing strength parameters. After making this quality testing some tests on shear strength of SAP/soil mixes were performed. Experiences, described above, make it clear, that SAP dosage used in many laboratory investigations, are reasonable for increasing water retention capacity of soil but must drastically diminished its shear strength. So, for shear strength tests SAP amount was so chosen, that after using its full water absorbing potential volume of hydrogel was not bigger, then pore volume. Porosity n of loose medium sand is equal ca. 40 % - to have some reserve it was assumed that after swelling 36 % of the whole volume will be filled with water retained by SAP. It is equivalent to the dosage of 0,9 g of SAP in 1 dm 3 of soil. SAP and sand in dry state were carefully mixed and then water was added. Amount of water was so calculated, that the whole adsorption potential of SAP could be used. Then the mixture was placed, without compaction, in 60x60 mm shear box. Shear test were made only under small pressure of 12.5 kpa, because investigated problem concerns cover soil layer. Tests were made on air dry medium sand and sand mixed with SAP right after adding of water and after 2 hours. Shearing speed was chosen as for non-cohesive soils v = 0.1 mm/min. Test s results are presented on Figure 1. It was found, that just after adding water to dry mixture of medium sand and superabsorbent in proportion mentioned above, one can observe almost 50 % loss of shear strength. After 2 hours of absorbing process strength loss was smaller, but still was equal 1/3 of the shear 563

3 Usage of superabsorbent geocomposite in erosion protection Pawloski A., Lejcuś K., Garlikowski D. & Orzeszyna H., strength determined for medium sand. In certain circumstances surface landslide can occur as a result of negative influence of superabsorbent on tested soil shear strength. By higher dosage, after taking in maximal possible amount of water, SAP volume exceeds soil s pore volume. In this case one can expect much higher shear strength loss approaching 100 %, when soil grains are separated by hydrogel film. τ (kpa) Ps Ps+SA Ps+SA Figure 1. Shear strength of medium sand (Ps) mixed with superabsorbent tests made in shear box apparatus 3. CONCEPT OF GEOCOMPOSITE WITH SUPERABSORBENT Taking into consideration all disadvantages of direct SAP and soil mixing a conclusion was made, that both components should be kept apart. A new kind of geocomposite was realization of this idea. It has got a form of two long strips made of non woven geotextiles, ca. 100 mm wide (length of the strips is limited only by production possibilities) and SAP small grains are placed evenly along the strip. Both edges of strips are bonded together by needle-punching (Fig. 3). This way controlled amount of SAP can be placed on a chosen depth. It protects swollen SAP grains from drying, as can happen, if they are placed near a ground surface and stimulates roots system to penetrate subsoil to the depth where water retained in geocomposite can be found. So the roots system is anchoring surface layer to a geocomposite embedded in soil. SAP dosage can be regulated by changing distance between straps. Figure 2. A section of geocomposite with superabsorbent after full saturation 564

4 Geocomposite is not blocking infiltration because of free space between elements, while in case of direct mixing, by higher dosage and full saturation, space between grains is filled with hydrogel forming a barrier for infiltrating water. 4. INFLUENCE OF USING GEOCOMPOSITE WITH SAP ON ROOT SYSTEM AND GREEN COVER DEVELOPMENT To investigate influence of geocomposite containing SAP on roots systems both quality and quantity tests were made. At the beginning it was simple system in rectangular, plastic buckets. Three buckets were filled with sand covered with thin, 20 mm thick, humus layer and grass was seeded. On the bottom of two containers a strip of geocomposite with SAP was placed, the other one was left for reference. The tested pots were placed on the flat roof of the Institute building. After 4 months, hot summer months including July and August, the contents of containers were controlled. Much better developed roots system and better plants condition were observed in the containers, where geocomposite was present. Roots were reaching the 300 mm depth, where geocomposite was placed, in the container without geocomposite average roots depth was equal ca. 100 mm (Orzeszyna et al., 6). The next step was to investigate more precisely geocomposite with SAP influence on grass roots system on different subsoils and the depth of geocomposite placement (Lejcus et al, 8). More comprehensive study was made with several different subsoil models. The humus layer was ca 50 mm thick, underneath, in a test bucket, was 150 mm thick layer of medium sand, fly ash from hard coal combustion or copper ore flotation waste. On the surface grass was seeded ca. 25 g/m 2. Geocomposite was placed on the bottom of the pot, but in tests with copper ore flotation waste geocomposite was installed just above or just underneath contact surface between humus and subsoil. Tests arrangements are summarized in Table 1. Table 1 Test arrangements to investigate influence geocomposite with superabsorbent placed in different soil condition on development of grass roots system Test No Layer thickness mm H H H H H H H H 140 mm MS MS MS PFA CuFW CuFW CuFW CuFW Depth of geocomp. installation mm mm* * Two dimensional geocomposite Abbreviations: Without geocomposite mm H humus MS medium sand PFA fly ash from hard coal combustion CuFW - copper ore flotation waste mm 70 mm 55 mm* Without geocomposite Tested containers were placed on an inclined platform modeling an embankment slope. Inclination was equal 1:2. The whole installation was placed outdoor subjected to all natural weather influences like rains, winds, sun radiation. There was no additional water supply except water from precipitation. The installation was dismantled after 15 months test had been initiated in April and ended July the next year. Degree of roots system development was evaluated visually and some quantity test determining roots density distribution with depth were made. Bigger roots density was observed in the proximity of the geocomposite in comparison with roots density, when geocomposite has not been used. The summary of visual evaluation of roots system development is presented in Table 2. For most of soil/geocomposite arrangements roots zone shear strength test were made. Shear box for 60x60 mm specimen was used. Tests were performed for 3,125 and 12.5 kpa normal stresses. As a result of application of geocomposite with SAP, an increase of angle of internal friction and cohesion occurred if compared with roots system in the same soil but without geocomposite. Shear 565

5 Usage of superabsorbent geocomposite in erosion protection Pawloski A., Lejcuś K., Garlikowski D. & Orzeszyna H., strength on the contact zone between geocomposite and soil was tested, either. Roots penetrating geocomposite increased shear strength of kpa (Lejcus et al. 8) Table 2 Visual evaluation of root system development for different subsoils with and without geocomposite. Test nr Description of root system development 1 Roots are penetrating sand subsoil reaching geocomposite 2 Roots are penetrating sand subsoil reaching geocomposite 3 Roots developed mainly in humus layer weakly penetrating deeper into the subsoil 4 Roots are penetrating sand subsoil reaching geocomposite 5 Roots developed only in humus layer. Grass could not profits from water retained in geocomposite 6 Roots developed only in humus layer. Grass could not profits from water retained in geocomposite 7 Roots developed in humus layer, but penetrating into geocomposite superabsorbent zone 8 Roots developed only in humus layer 5. POSSIBILITIES OF GEOCOMPOSITE APPLICATION ON SLOPES Geocomposite with superabsorbent can be used in every place, where water retention in the subsoil is very small or where water retained in geocomposite can positively influence plants growth and condition reducing negative influence of longer dry periods. The places where water from precipitation is more difficult to be caught and retained are eg. dams and embankments slopes and soil cover over high permeable layers like buttresses or drainage, including inclined areas exposed on direct sun radiation. Despite of water retention function, geocomposite with SAP increases roots system development, what is very important for stability of grass cover layer protecting slope against erosion. Two examples of possible arrangements of geocomposite on a slope are presented on Fig. 3. HUMUS GRASS COVER NON VOWEN GEOTEXTILE LIVE BRANCH CUTTINGS GEOKOMPOSITE WITH SUPERABSORBENT GABIONS MATTRESS FILLED WITH STONES AND SUPPLEMENTED WITH HUMUS RO OTS SYSTEM GEOCOMPOSITE WITH SUPERABSORBENT Figure 3 Some possible arrangements of geocomposite with superabsorbent in protecting systems against erosion using plants Observation of functioning of geocomposite on existing slope of river levee (embankment) has shown, meanwhile without more detailed tests, that geocomposite placement effectively supports growth of anti-erosion grass cover, making it more resistant against unfavorable weather condition eg. long dry periods during summer. 6. CONCLUSIONS 566

6 A new form of the geocomposite containing superabsorbent was proposed. It can retain water for further use by plants forming protecting cover against erosion, thus improving their condition particularly during dry periods. Geocomposite can be fastened to the subsoil and roots penetrating it in seeking water are forming additional anchorage of cover layer. It makes possible to benefit from superabsorbent retention properties even in places, where direct mixing with soil, because of reduction of soil s shear strength, could negatively affect stability of surface layer eg. on slopes or inclined areas. REFERENCES Buchholz F.L. & Graham A.T. (1998). Modern superabsorbent polymer technology. Wiley-VCH. New York p. Lan Wu, Mingzhu Liu, Rui Liang Bazaraa (8). Preparation and properties of a double-coated slow-release NPK compound fertilizer with superabsorbent and water-retention. Bioresource Technology 99 (8) Leciejewski P.( 8,). Wpływ wielkości dodatku hydrożelu na zmiany uwilgotnienia i tempo przesychania gleby piaszczystej w warunkach laboratoryjnych (The Influence Of The Hydrogel Addition On The Changes Of The Sandy Soil s Moisture And The Dynamics Of Soil Drying In The Laboratory Conditions). Studia i Materiały Centrum Edukacji Przyrodniczo-Leśnej R. 10. Zeszyt 2 (18) / 8, Lejcuś K., Garlikowski D, Orzeszyna H., Pawłowski A. (8). Geocomposite with superabsorbent in landfill recultivation and slope protection. Monografia. Management of Pollutant Emission from Landfills and Sludge. Taylor&Francis. London 8, Martyn W. & Szot P. (1). Influence of superabsorbents on the physical properties of horticultural substrates. Int. Agrophysics, 1, 15, Orzeszyna H., Garlikowski D. & Pawłowski A. (6). Using of geocomposite with superabsorbent synthetic polymers as water retention element in vegetative layers. Int. Agrophysics, 6, 20, Schmidt K.M., Roering J.J., Stock J.D., Dietrich W.E., Montgomery D.R. & Schaub T. (1). The variability of root cohesion as an influence on shallow landslide susceptibility in the Oregon Coast Range. Canadian Geotechnical J.ournal38: (1) Sojka R., Lentz R. & Westermann D. (1998). Water and erosion management with multiple applications of polyacrylamide in furrow irrigation. Soil Science Society of America Journal. Vol. 62, Issue Wu T.H. (1994). Slope stabilization using vegetation. Geotechnical Engineering. Emerging Trends in Design and Practice. Editor K.R.Saxena. A.A. Balkema/Rotterdam, pp ,