Effect of Treated and Raw Wastewater on the Behavior of Unsaturated Soil
and
Civil Engineering Department, Faculty of Engineering Mutah University, Karak, Jordan
E-mail abponkm@mutah.edu.jo Husdam64@yahoo.com
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Effect of Treated and Raw Wastewater on the Behavior of Unsaturated Soil
and
Civil Engineering Department, Faculty of Engineering Mutah University, Karak, Jordan |
ABSTRACT
Many tests have been carried out to investigate the effect of mixing Karak clay soil with treated and raw domestic wastewater on the behavior of unsaturated soil. The specimens have been prepared by mixing the soil with three kinds of water; distilled water, treated wastewater and raw domestic wastewater. A specific experimental programme has been planned and completed on a range of degree of saturation and different normal stress. The shear strength test and swelling test results using distilled water have been compared with the presence of contamination in Karak clay. The test results show that there is not too much difference between the unsaturated shear strength tests on specimens that mixed with distilled water and that mixed with treated wastewater. Also, the use of treated water will not affect the swelling of the soil. On contrast, raw wastewater has a great influence on the behavior of unsaturated soil.
The results obtained from this study are expected to be of great benefit and practical use for carrying out stability analysis of structures like embankments and foundations supported by clay particularly in a country that suffers from water shortage like Jordan. However, experimental evidence to support this study is still limited.
Keywords: Shear strength, unsaturated soils, degree of saturation, treated wastewater.
INTRODUCTION:
Jordan is facing challenges in its water natural resources. Rainfall is too low and unpredictable in the majority of the country. Jordan has and will be facing challenges in its water sector due not only to the insufficient water recourses, but also due to the abnormally high population growth and migration. Jordan’s population of around 5 million is growing and will double within generation. Rapid increases in population and industrial development have placed unparalleled demands on water recourses. Total demand is approaching one billion cubic meters per year (Ministry of Water and Irrigation in Jordan Manual 2002). Current water use exceeds the renewable water supply. Water recourses in Jordan are limited to surface and ground water. Renewable groundwater recourses have been withdrawn at an unsustainable rate in order to meet the increase demeaned. However, surface and ground water quality in some areas is failing. Therefore, the Jordan should pay high attention to find new water resources and to mange the water use.
Wastewater management can be considered one of the most important factors to develop the water resources. This can be achieved because the wastewater may contribute as an integral part of renewable water resources. Also the treated wastewater can be used in irrigation. Jiries (2001) found that the treated wastewater quality from Mutah university treatment plant (MUTP) is suitable for irrigation purposes in terms of salinity and its chemical parameters especially sodium content.
Treated wastewater is a perennial water source and shall form an integral part of renewable water resources and the national water budget. Collection and treatment of raw wastewater is a necessity to circumvent hazards to the public health and the environment, which it becomes imperative when contamination of freshwater resources with raw wastewater is eminent. Then, existing levels of raw wastewater services shall be maintained and upgraded where necessary to enhance public health and the environment. Treatment of raw wastewater shall be targeted towards producing an effluent fit for reuse in irrigation in accordance with World Health Organization (WHO) guidelines as a minimum. Reuse of treated wastewater in other purposes such as in civil engineering works should be investigated. The optimum treatment and reuse of wastewater helps in mitigating the problem of water crises in Jordan. On the other hand, it participates in producing a large part of food and increasing the income of farmers without any negative effect on the environment. The presence of nutrients in the treated wastewater mainly nitrogen and phosphor Increase crop productivity.
The object of the current research is to explore the possibility of using treated and raw wastewater in civil engineering works such as compaction in roads. To achieve that the soil mixed with treated and raw wastewater and then experimental program was carried out to measure shear strength, free swelling and swelling pressure. As the soil in Jordan is unsaturated due to long summer, the second purpose of the present paper is to evaluate the effect of changing in water content of the soil with and without contamination on the shear strength of unsaturated soil.
The behavior of unsaturated soil is presented here in terms of degree of saturation rather than suction because the suction measurement is costly, needs high expertise and is time consuming. The treated and raw wastewater, which is investigated in the current paper, comes from Mutah University Treatment Plant (MUTP). Many tests are carried out to investigate the properties of treated and raw wastewater. The results show that the treated wastewater is within the safe limit, has no health hazards, and has no noticeable effect on shear strength and swelling (Maaitah and Tarawenh 2003). On contrast, raw wastewater has a bad effect on the behavior of unsaturated soil.
BEHAVIOR OF UNSATURATED SOIL
Jordan lies in arid to semi arid zone where long summers exist, and the evaporation exceeds infiltration. The surface layer in this area has a very low saturation. The soil in Jordan, particularly in winter may have a high saturation or become fully saturated due to rainfall. It may also become saturated in summer due to irrigation. The variation in soil volume due to changes in the degree of saturation under a building footing or any earth structure will sometimes cause damage.
In Jordan, buildings constructed on soils, which have apparent strength due to negative pore water pressure in summer, will settle in the winter due to wetting. On the other hand, in wet areas the suction that is applied to the soil from trees and vegetation will change due to wetting. Climate, therefore, plays an important role in whether a soil is saturated or unsaturated. In unsaturated soil the problem is not associated with the shear strength at certain degrees of saturation, but arises from changing shear strength due to changes in the degree of saturation. The shear strength of unsaturated soils is required to determine the bearing capacity of shallow foundations, slope stability, stability of vertical excavations, lateral earth pressure, and many other geotechnical structures.
Understanding the behavior of unsaturated soil mechanics plays a significant role in the practice of geoenvironmental engineering and Geotechnical. The vast majority of civil infrastructure systems are founded on and in soils above the groundwater table where pore-water pressures are negative particularly in arid and semi arid regions. The solution to a large number of contaminant transport problems depends on an understanding of surface flow boundary conditions and the movement of fluids through the unsaturated zone where soils have negative pore water pressures. Rational unified approaches for dealing with unsaturated soils have recently emerged, providing the Geotechnical engineer with enhanced tools for dealing with the challenges of unsaturated soils.
Compacted soils used in earth structures are in an unsaturated state during construction and, in several cases, during operating conditions. Their behavior is thus affected by the simultaneous presence of water and air in the pore spaces, which makes the pore fluid mixture compressible and influences the stress state through the water and air pressures. Despite the clear evidence of this behavior, the response of soil used in construction has rarely been explored under controlled-suction conditions (Rampino et al. 1999a, 1999b; Vinale et al. 1999). Furthermore, despite the key role of small strain behavior in predicting the performance of earth structures (Wu et al. 1989; Quin et al. 1991), rather little effort has been devoted to understanding unsaturated soil. Some contributions have recently been made to the investigation of the more general topic of the small strain behavior of unsaturated soils.
Unsaturated soil behavior cannot be predicted using either Terzaghi’s effective stress principle or any single stress variable combining pore-air pressure (ua), pore-water pressure (uw), and total stress tensor |(| (Aitchison 1961; Bishop 1959; Jennings 1961). It is well known that two independent stress variables are required to describe unsaturated soil behavior (Bishop and Blight 1963; Fredlund and Morgernstern 1977; Fredlund et al. 1978), the most common choice being net stress tensor |( – ua | and matric suction (ua – uw) (Bishop and Blight 1963; Fredlund and Morgernstern 1977). In fact, suction and external loads have independent effects on stress state (Burland and Ridley 1996). The net stress induces both normal and tangential forces at inter-particle contact points, which has the same role as the effective stresses of saturated soils. In other words, in most cases |( – ua| is entirely carried by the soil skeleton, independent of drainage conditions, and often causes particle slippage and specific volume changes. Unlike |( – ua|, variations in matric suction only induce changes in the normal forces at particle or cluster contact points, affecting soil skeleton stability.
On this scale, suction only influences the available slippage strength. In other words, wetting-induced collapse results from external load actions, with suction variation only responsible for a decrease in shearing resistance at contact points. Other plastic deformations, such as irreversible swelling upon wetting or irreversible shrinkage upon drying, result from volume changes of saturated clusters, having induced plastic rearrangement of an unsaturated macrostructure; suction changes are merely equivalent to effective stress variations for cluster behavior (Alonso et al. 1994). Under axially symmetric conditions the stress variables for unsaturated soils reduce to
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PROPERTIES OF TREATED AND RAW WASTEWATER
Water, in general, contains some type of impurities as a solid or gases state. The amount of such impurities determines the adequate water use. Firstly, the treated and raw wastewater were investigated only and then the treated and raw wastewater were mixed with the soil under question. For the purpose of comparison the soil was mixed with distilled water and then tested. All results are presented in Table 1, Table 2, Table 3 and Table 4. Anions were analyzed by ion chromatography. Major cations were measured with flame photometer, the Electrical conductivity was measured by conductivity meter and pH was measured with pH meter. All the chemical parameters for treated wastewater (HCO-3, CO3-2 , Ca+2, Mg+2 , K+ , Na+, F-, Cl-, Br-, NO3-, PO4-3 and SO4-2) are within the range of wastewater quality according to WHO and Jordanian standard. The effect of raw wastewater is illustrated in the tables below
Table 1. Concentration of cation composition in ppm of treated wastewater
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| Parameter | Treated wastewater only | Soil mixed with distilled water | Soil mixed with Treated water |
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| HCO–3 | 261.24 | 48.123 | 302.49 |
| Ca+2 | 66.24 | 15.456 | 66.24 |
| Mg+2 | 26.827 | 6.71 | 34.88 |
| E.C (in mS/cm) | 1100 | 60.3 | 1108 |
| PH | 7.08 | 7.73 | 8.00 |
| K+ | 23.6 | 4.4 | 26.8 |
| Na+ | 10.8.46 | 3.5 | 111.54 |
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Table 2. Concentration of Anion composition in ppm of treated wastewater
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| Parameter | Treated wastewater only | Soil mixed with distilled water | Soil mixed with Treated water |
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| F– | 0.3467 | 0.9203 | 0.4887 |
| Cl– | 84.392 | 1.7463 | 60.763 |
| Br– | 0.82738 | Non-deductive | 0.1507 |
| NO3– | 28.063 | 0.08135 | 18.53 |
| PO4–3 | 23.078 | Non-deductive | 20.646 |
| SO4–2 | 65.861 | 1.3873 | 69.991 |
| Parameter | Raw wastewater only | Soil mixed with raw water | |
| NH+4 | 46 | 0 | |
| Ca+2 | 43.6 | 82.7 | |
| Mg+2 | 22.5 | 29 | |
| PH | 8.6 | 9 | |
| K+ | 35.6 | 12 | |
| Na+ | 74 | 108 | |
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Table 4. Concentration of Anion composition in ppm of raw wastewater.
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| Parameter | Raw wastewater only | Soil mixed with raw water |
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| F– | 2.75 | 2.97 |
| Cl– | 115.6 | 194 |
| Br– | 0.69 | 1.6 |
| NO3– | 15.8 | 55.9 |
| PO4–3 | 14 | 0 |
| SO4–2 | 58.68 | 189.7 |
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EXPERIMENTAL WORK
The soil used in the tests was silt-sandy with significant amount of clay, which is typical from the south of Karak. Results of standard Proctor compaction test on soil mixed with distilled water, with treated wastewater and with raw wastewater are shown in Figure 1. Liquid and plastic limits are 40% and 20%, respectively. The specific gravity is of 2.7. The chosen dry unit weight was 1.6 gr/cm3 at moisture content of 8%, which is coincident with initial void ratio of 68%. The tests were conducted at room temperature. The surface tension for both kinds of water is the same and is equal to 0.0735 Nm–1.
Figure 1. Standard Proctor Test
There is a need to undertake further laboratory research studies on unsaturated soils, particularly to obtain a fundamental understanding of the shear strength of unsaturated soils by testing soil specimens with simple soil structures. Three groups of tests have been conducted by using simple direct shear device. The first group was tested by using pure water, and treated wastewater on fully saturated test to obtain the effective angle of friction, f/, which is found to be 23°±0.5°. The results of consolidated drained (CD) tests show no differences in the angle of friction. On the other hand, the effective angle of friction, f/, is found as 20°±1° when the raw wastewater have been used. This reduction in friction may be explained as the wastewater increases the lubrication between the soil particles.
The second group was carried out with different initial degrees of saturation (i.e. Sro = 2, 5, 10, 15, 20, 30, 40, 50, 60, 70, 80, and 90)% and constant normal stress, which is equal to 0.09 kg/cm2. The third group was conducted at the same condition of second group but the normal stress is of 50 kg/cm2. The same procedures were repeated by using pure water, treated wastewater and raw wastewater.
Free swell test have been conducted by using odometer apparatus and at the end of each test the swell pressure was measured on soil mixed with pure water, treated wastewater and raw wastewater. The tests were carried out with different initial degrees of saturation (i.e. Sro = 10, 20, 25, 30, 35, 40, 50, 60, 70, 80 and 90)%.
RESULTS AND DISCUSSION
Many tests were carried out on unsaturated soil by using distilled, treated wastewater and raw wastewater. The samples have been sheared under drained condition in direct shear apparatus. The stress-strain behavior is nearly ductile for all the tests, with some strain softening taking place. Drained condition at low saturation will not affect the amount of water because the capillarity is greater than gravity force. The rise in pore air pressure for low normal stress is neglected which in this paper, it is atmospheric.
Figure 1 confirms that using treated water has no effect on the result of compaction test (Maaitah and Tarawenh 2003). It can be concluded from Figure 1 and Table 1 that the treated water has no effect on the compaction. However, this is not the case when raw wastewater is used. The wastewater play lubricant role between soil particles, which as the higher contamination content the higher negative on the soil behavior, is produced.
Figure 2. Effect of saturation and normal stress on the behvior of unsaturated soil
Figure 2 shows the behavior of unsaturated soil as the degree of saturation and normal stress change. The figure shows that as the normal stress increases the shear strength increases. As the degree of saturation increases up to 30 % the shear strength increases and then it becomes nearly constant between saturation 30% to 50%. In terms of suction, the shear strength increases as the suction increases and then decreases to a fairly constant value. This pattern is also seen in the results of other researchers (Donald 1956, Towner et al., 1972, and Fredlund et al.1993). The shear strength may increase, decrease or stay constant as the suction increases beyond a certain value. As the saturation increases more than 60 % the shear strength decreases.
At saturation less than 30% the behavior is governed by the role of menisci water. The presence of a curved interface only implies that pore-water pressure is less than surrounding air pressure (i.e., unsaturated case). Hence, changes in suction or unsaturated case, a constant net stress correspond to pore-water pressure reductions and to equal changes in mean effective stress (p¢ = p – pw). A force between the soil particles varies linearly with variations in suction s, which is null at zero suction. The shear strength of unsaturated soil, at saturation less than 30%, depends on its behavior on degree of saturation, number of contact or void ratio, surface tension, and normal stress (Maaitah, 2002). As the previous parameters have not changed in all tests, therefore the shear strength does not vary when using treated wastewater or distilled water. This result has been proved here.
Samples could be prepared at a high saturation (greater than 50%), in which the air is initially uniformly distributed, but this will be a temporary state, and the case with which air can move through the pore spaces determines how long this temporary state will last. This means that the air in such a soil will tend to aggregate, so that the soil becomes separated into regions of full and low saturation. Therefore, the experiments here are conducted under drained conditions to avoid any kind of segregation. Also, this is the main reason for choosing simple direct shear test.
The relationship between the degree of saturation and shear strength for soils that mixed with raw wastewater, treated wastewater and distilled water show in Figure 3. Differences between the soils that were mixed with distilled and treated water could be due to differences in initial saturation, void ratio, instrument accuracy and repeatability of test. However, there is no difference between the strength of the soil that is mixed with raw wastewater with soil the mixed with pure water at saturation less than 20 %. On the other hand, the difference in strength increases as the saturation increases. This can be attributed due to the increase of contamination in the sample. The organic mater reduces the friction significantly because it increases the slippage between the soil particles. This means that the raw wastewater is unsafe to be used in civil engineering works such as compaction. Also, using such water in irrigation or the seepage from septic tank will affect the strength of the soil beneath building footing.
Figure 3. Unsaturated shear sterngth versus degree of saturation
Figure 4. Result of swell pressure tests
Mixing soil with treated water has no effect on free swell and swell pressure as shown in Figure 4. There are somewhat differences, which may be explained by the repeatability of tests. At low saturation the soil exhibits higher swell pressure as shown in Figure 4. Also as the degree of saturation increases the swell pressure decreases. Furthermore, as the degree of saturation increases the suction decreases and the swelling pressure decreases. The volume of change increases as the suction decreases and that the volume change became more significant at relatively low suction values (Escario and Saez 1973). The soil swelling depends on many factors as the soil wetted under a constant pressure such as the degree of saturation (Cox 1978). However, there are differences between the free swelling in the soil sample that is mixed with raw wastewater with soil the mixed with pure water. Also, the swelling pressure is greater in the soil samples that mixed with raw wastewater. The increase in swelling can be explained due to the increase of contamination in the sample.
Figure 5. Free swell test
Figure 6. Free swell test
Figures 5 and 6 show the comparison between the free swell for soil mixed with distilled water and raw wastewater. The figures show clearly that the soil mixed with raw water swells higher than the soil mixed with distilled water. The free swell increase as the degree of saturation decreases as illustrated in Figure 7.
Figure 7. Free swell test for soil mixed with raw wastewater
CONCLUSIONS
In order to avoid hazards to the public health and the environment central treatment plants should be built to serve semi urban and rural communalities, and collection of wastewater can be made initially through trucking until collection systems are justified. Particular attention shall be paid to the protection of underlying aquifers. It is highly imperative that a section in the Water Authority be responsible for the development and management of wastewater systems as well as the treatment and reuse of the effluent.
It is clear that the degree of saturation and normal stress play a significant role in determining the shear strength and swell pressure of unsaturated soil. Also, shear strength increases as the normal stress increases. Important conclusions can be drawn from the experimental program that has been conducted in this research:
We recommend using treated water from MUTP for civil engineering earthwork. This conclusion will have a great benefit in solving the water shortage in Jordan.
ACKNOWLEDGMENT
The authors wish to express their gratitude for the considerable support give to this work by soil laboratory staff at Mutah University, specially, for Mr. Abd-Ablest Abu-Khaled and Mr. Yousef Tarawneh.
REFERENCES
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