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The Effects of Urmieh Lake Salt Water on the Proctor Compaction and CBR Test Results of Well Graded Gravel-Sand Mixed With Clay (GSCW) Soil Samples
M. Reza Emami Azadi Assistant Professor, Department of Civil Engineering, Tel/Fax: 0098-412 3340311 |
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The Effects of Urmieh Lake Salt Water on the Proctor Compaction and CBR Test Results of Well Graded Gravel-Sand Mixed With Clay (GSCW) Soil Samples
M. Reza Emami Azadi Assistant Professor, Department of Civil Engineering, Tel/Fax: 0098-412 3340311 |
Abstract
In this experimental work, we investigated the effect of salty water of Urmieh Lake during the proctor compaction tests on the rather well graded gravel-sand-clay mixture(GSCW) soil samples in the soil mechanics Lab of Azarbaijan University near by the Urmieh Lake. The clay content varied from about 1/6 to 1/3 of total weight of the test samples. The clay was obtained from area nearby the lake with about 5-10% bentonite which had higher water absorption capacity. The proctor test results on these soil samples indicated that using the Urmieh lake water has reduced the optimum moisture content of the soil samples by about 16-27% compared to the samples tested using tap water in the Lab. The results also showed that the maximum unit weight of the soil samples has slightly changed using the Urmieh lake's salty water. Plasticity index of the soil samples has reduced from 25.6% to about 3.66% using the salty water of the Urmieh Lake. CBR value has also varied for the tested samples from 62.24% to 68.96%
Keywords: Proctor Compaction Tests; Optimum moisture content; Maximum unit weight; Plasticity Index (PI), California Bearing Ratio (CBR)
Introduction
In the past, Several studies have been conducted on improving the properties of sensitive soils using some additives such as Ca2CO3, Gypsum, Cement such as works by Pyne (1955), Chen (1981), Ghafoori et al., (1997), Ghafoori (2000), Lopez et al. (2001), Muntohar et al. (1999, 2000).
Form the previous studies, it is expected that metal oxides, hydroxides, or halides additives can enhance soil stabilization. However, none of these studies investigates the use of a mixture of these compounds as a soil stabilization agent.
More recently a study by Basam Mahaseneh (2005) showed the effects of the dead sea water near Jordan on the soil properties as an improving agent. In particular, he studied the optimum moisture content, maximum dry density, PI and the unconfined compression strength of various soil samples from base, sub-base material etc. His findings paved the way for the current experimental study on the effect of Urmieh Lake water on the GSCW soil samples. Urmieh Lake is perhaps a pair for the dead sea in the middle east in terms of its salty water content.
The Urmieh Lake is one of the most prominent geomorphologic features in Azarbaijan and its water contains about 346.2 grams of salt per liter of water. In this paper the idea of using Urmieh Sea water as soil properties improvement agent comes as a result of understanding what the others did regarding usage of calcium chloride, metal oxides etc, and the idea comes to investigate the ability of using the Urmieh Lake water because it contains several kinds of salts (Sodium chloride, Potassium, etc.) as soil improvement agent comes to reveal the benefit of using Urmieh Lake water in soil stabilization, pavement design, and foundation construction.
EXPERIMENTAL WORK
About the Urmieh Lake
Urmieh Lake has been located in the border between east and west Azerbaijan provinces of Iran. Urmieh Lake is about 1276 m above the sea level and it is located on the south-west of Tabriz city at East-West Azerbaijan border (35o, 58' -39o, 48'N and 44o, 14' 47o, 19'E)-. Its area is about 5000-5500 km2 and its water volume ranges between 12 and 32 billion m3 in high water and low water seasons. Its dissolved salt content varies between 300 and 400g/lit seasonally. The temperature of the Lake region varies between -10 and 38 in Jan. and July-Aug. The recent activities involving build up of a cross-over road/bridge by Norwegian NGI, Jacobsen A.S and Danish companies in the lake in the vicinity of our site at Azarbaijan T.M. University. The road sub-base has been build on the lake bed using the rock boulders from nearby Azarshahr region and the base has been built of gravel/sand/clay mixtures from this area. On the Urmieh Lake road superstructure however, a well graded gravel/sand with small portions of clay mixtures according to AASHTO have been used. The mostly observed dissolved minerals in the Urmieh Lake are NaCl, MgCl2, KCl, CaCl2. In terms of Ions Na+ and Cl- have the largest concentration in the Urmieh Lake as we obtained from Azaruniv. Lab. in Sept. 2007.
Table 1 below shows the ionic (mineral) contents of the water sample from Urmieh lake and tap Water in the Lab. which was determined at Azarbaijan University Lab. in September 2007 during our test program. It can be seen that the total amount of ionic content is about 345.902 gr/lit in the Urmieh lake water while the minerals in the tap water is far less only about 1.326gr/lt. It is also seen that the amount of Na+ (sodium ions) is 33.6% and Cl- (chloride ions) is about 57% of the total ionic content of the Urmieh Lake water used during our tests.
Table 2 shows the mineral (ionic) content of the Bentonite particles used in soil specimens during the proctor compaction tests as will be described in the following sections. As seen the amount of Na+ ions is about 60-70% of the total clay Bentonite minerals. However, the Table 2 shows the overall mineral content of the clay with Bentonite. Bentonite clay used during these tests had a reddish brown color with a very fine particle sizes with a greasy and very soft appearance in touching by fingers. The used Bentonite clay has high water absorption potential and large cation exchange capacity.
Table 1: Chemical Tests results of Ions in the Urmieh Lake Water
and Azaruniv. Lab.(g/lit)
Table 2: Chemical Test results of Ionic Content in the Bentonite Clay(%)
Test Procedure
Our experimental work was carried out at the soil mechanics Laboratory of Faculty of Eng., Azarbaijan T.M. University from early Sept.2007 until late Nov.2007. The gravel-sand mixtures were obtained from the Azarbaijan T.M. University site nearby the lab. located about 35Km from Tabriz City. We first performed a sieve analysis on the gravel-sand mixture to check the soil grain size diagram and obtain the Cc and Cg parameters. Then the clay soil samples which had to be used in the compaction tests were obtained from a near the Urmieh Lake site which had an about 8-10% bentonite minerals. This particular clay had a great water absorption capacity and also high liquidity limit (40-50%). In our experiments, we added between 1/6 to 1/3 weight ratio clay to the well grained gravel sand mixture. For the obtained GSCW soil samples for tests we then added both Urmieh Lake water and also Lab tap water to conduct the Modified Proctor compaction tests. We used light weight test with 3 kg weight hammer dropping from 30cm, 27 times at each soil layer in a uniform manner in circles around the sample center which was placed each time in a modified compaction cylinder(manufactured by ELE company in Britain) containing the soil sample. After each test we had taken 2 small samples from the top and bottom of the cylinder to determine the moisture content of soil in the electrical Oven with 110\oc. We repeated the test at least 5 times for each soil sample with different Urmieh Lake and tap water content in our lab.
We subsequently performed the Casagrande Liquidity limit test to obtain LL and also the plasticity limit test to find the PL of clay soil samples used in our experiments.
To evaluate the bearing index of the GSCW soil samples in particular used as base or sub-base material in the road superstructure in the Urmieh Lake region, we conducted California bearing(CBR) tests on the GSCW soil samples with 1/3 clay content with both Urmieh Lake water and the tap water in our Lab.
To evaluate further, the bearing index results, we conducted recently several more tests(Jan.2008) with 1/4, 1/6 ratios of clay content with both Urmieh Lake and tap water. The results will be discussed subsequently.
Summary of Results
The sieve analysis results on the gravel-sand samples used in our tests has been shown on Fig.1 below.
Figure 1:
Grain Size Diagram For Sieved Gravel-Sand Mixture used in the Test Samples
(before mixing with different Bentonite clay proportions)
As seen in the grain size diagram (Fig.1), the sample contains about 45.82% percentage finer than sieve no.4 and less than 0.94% finer than sieve size no.200 hence according to USGS classification it is considered as well graded gravel-sand mixture(GW or GSW). The percentage finer than sieve size 37.5mm is about 100% (i.e. largest grain size in the sieved GS sample is 37.5mm)
Figure 2:
Dry Unit Weight (kN/m3) vs. Moisture Content (%) for 1/3 ratio of clay to GS mixture
in the sample tested with the Urmieh Lake Water
Figure 3:
Dry Unit Weight (kN/m3) vs. Moisture Content (%) for 1/3 ratio of clay to GS mixture
in the sample tested with the Tap Water
In general each tested sample requires an amount of water(moisture) to lubricate between the soil particles and hence the soil particles could moves gradually under the hammer action (compaction) to a new and denser structure so that the void spaces within the sample is reduced and hence the dry unit weight is also increased. As the moisture content increased and reaches an optimum value the void spaces reach to a minimum (compaction 95% or higher) and so the sample is very compacted. As we added more water in to the soil, it can not help to further compact the sample and by adding more water the void spaces increase and hence the relative density of the soil decreases and it becomes less compact because the hammer action is also less useful at this stage and it is taken partly by the water particles instead of of compacting the soil. We can usually see the descending part of diagrams after reaching the optimum moisture.
Fig. 2 shows that the optimum moisture content for the GSCW sample tested by adding the Urmieh Lake water is about 15.5% and the corresponding Dry Unit Weight is about 20.86kN/m3. While Fig.3 shows the result of 5 modified proctor tests carried on the GSCW sample with 1/3 clay ratio using tap water. As seen on Fig.3, the optimum water content in these tests were obtained as about 18.5% while the corresponding dry unit weight of soil sample is about 21.57kN/m3. Comparing Figs.2 and 3 indicates that adding the Urmieh lake water into the GSCW sample has resulted in about 16% reduction in optimum moisture content while dry unit weight has changed only by 3.3%(i.e. decreased).
Figure 4:
Dry Unit Weight (kN/m3) vs. Moisture Content (%) for 1/4 ratio of clay to GS mixture
in the sample tested with the Urmieh Lake Water
Figure 5:
Dry Unit Weight (kN/m3) vs. Moisture Content (%) for 1/4 ratio of clay to GS mixture
in the sample tested with the tap Water
Figs.4 and 5 show the results of 10 modified proctor tests carried out on 1/4 clay ratio in the GSCW samples using Urmieh Lake and tap water, respectively. As seen the optimum moisture content has decreased from 17.3% to 12.85% by adding the salty water of Urmieh Lake. While the maximum dry unit weight has increased from 20.34kN/m3 to 20.62kN/m3 (i.e. only 1.4% increase).
Figure 6:
Dry Unit Weight (kN/m3) vs. Moisture Content (%) for 1/5 ratio of clay to GS mixture
in the sample tested with the Urmieh Lake Water
Figure 7:
Dry Unit Weight (kN/m3) vs. Moisture Content (%) for 1/5 ratio of clay to GS mixture
in the sample tested with the tap water
Figs.6 and 7 show the modified proctor compaction results on the GSCW samples with 1/5 clay ratio using the Urmieh lake water and tap water, respectively. As seen the optimum moisture content has decreased from 14.06% to about 11.5% by adding the Urmieh lake salty water instead of tap water in the lab. However, the maximum dry unit weight has just changed by 0.2%(i.e. 0.2% decrease).
Figure 8:
Dry Unit Weight (kN/m3) vs. Moisture Content (%) for 1/6 ratio of clay to GS mixture
in the sample tested with the Urmieh Lake Water
Figure 9:
Dry Unit Weight (kN/m3) vs. Moisture Content (%) for 1/6 ratio of clay to GS mixture
in the sample tested with the tap water
Figs.8 and 9 show the results of modified proctor compaction tests on GSCW samples with the clay ratio of 1/6 to the total sample weight using the salt water of Urmieh lake and the tab water, respectively. The results show that the optimum moisture content has decreased from 12.22% to about 9.68% by adding the salty water of Urmieh Lake instead of tap water in the lab. While the maximum dry unit weight has increased from 19.04 kN/m3 to about 19.95kN/m3 by using the salty water additives (i.e. 4.5% increase).
Liquidity and Plasticity Test Results
A sample of about 200 gr of the clay used in our tests which contained Bentonite minerals was taken to be added with Urmieh Lake water and also the tap water in the lab. It was carefully mixed with the water samples in the successive tests. The first series were carried out on the tap water with the standard Casagrande apparatus. Three tests were performed around 25number of drops of soil sample during the Casgrande test, 27, 22 and 17 drops were observed. Then the initial weight and the weight after drying 24 hrs in the oven with 110oc were recorded.
Table 3: Liquidity Limit Test Results in Casagrande Apparatus
Figure 10:
Liquidity Limit Test Results in Casagrande Apparatus
Table 4: Liquidity Limit Test Results in Casagrande Apparatus
Table 4 shows the LL and PL limits for the clay specimens tested by mixing the salty Urmieh Lake water and tap water. It can be seen that the plasticity index(PI) for the clay specimen has reduced from 25.57% to about 3.66% by adding the salty lake water. Similar results indicating more than 50% decrease have been reported in the tests carried out by Mahsneh(2005).
CBR Test Results
CBR tests have been performed on the GSCW samples with the 1/3 clay ratio to the total weight of the test specimen mixed once with salty water of Urmieh Lake at optimum moisture of 15.5% and another time with the optimum moisture content of 18.5% tap water of lab. The soil was poured into the standard CBR test compaction cylinder and compacted uniformly in 5 layers with equal thickness with the 62 drops of 3Kg. hammer from 30cm (light compaction method). Two samples were again taken from top and bottom of the soil specimen after compaction for determining the moisture content.
The compacted soil specimen then placed on the loading plate of the CBR test equipment of ELE company. Then the overload disks are placed on the top of the specimen and the loading jack was activated by upwards rotational movement of the loading plate under the specimen until the loading gauge has started in a dial movement. At the mean time the displacement recording gauge has began recording the fixed overhead piston's penetration into the soil specimen in the cylinder. The loading was continued until 6.0mm of fixed piston penetration into the soil specimen. It is often recommended to continue the loading until 7.5mm penetration. The aim was to determine the loading vs. penetration. curve for the given soil. From which we can obtain the strength of soil specimen (i.e. soil bearing capacity) compared to the standard california crushed stone specimen under static loading which is used as an important index for design of the flexible road superstructures near the Urmieh Lake area.
Figure 11:
CBR test Results on GSCW specimen (1/3 clay ratio) using Urmieh Lake Water
Figure 12:
CBR test Results on GSCW specimen (1/3 clay ratio) using tap Water
Figs. 11 and 12 show the results of CBR tests on the GSCW sample with 1/3 clay ratio to total sample weight. These results indicate that CBR for the sample test mixed with the salty water of Urmieh Lake at optimum moisture 15.5% and then compacted is 62.24% while the CBR value for the same clay proportion compacted with the optimum moisture of 18.5% of tap water in our lab is obtained as 68.96%.
Discussion
As seen the optimum moisture content of all tested GSCW soil samples using the salty water of Urmieh Lake has decreased considerably (in the range of 15.7%-27%) with the largest decrease observed for the highest clay size particle to gravel-sand size particle ratio of 1/3. This may be due to substitution of more H+ O- ions of water molecules with the Na+, Cl- in th salty water causing a strong bonding with the silicates and aluminates plates which reduces the double layer system and hence the moisture content. This phenomenon is called cagolition in the clayey soils with salty water.
Although, a slight increase was observed in the dry unit weight of soil using salty Urmieh Lake water which may be only partly due to this decrease in the double layer system and the void spaces in the clay system. But in overall due to substitution of salt particles with the water/air molecules in the void spaces between gravel and sand particles the wet unit weight slightly increases.
On the other hand as seen the plasticity index (PI) of the tested GSCW soil specimens have drastically decreased from 25.57% to 3.66% using the Urmieh Lake water instead of tap water in the lab. This sharp decrease which is mainly due to decrease in liquidity limit shows that adding the salty lake water has reduced the soil plastic properties due to reduction of double layer system in clay dipolar bonding affected by Na+ K+ Mg2+ Cl- ionic content of water, making soil structure more stable, reducing its water absorption and hence its swelling potential when it is used at road base/subbase material and exposed to wet conditions. Hence, using Urmieh Lake water has improved the GSCW soil samples compaction and swelling/shrinkage properties.
More recent examinations have shown that the gravel-sand-clay mixture in Azarshahr at the Azarbaijan University site may contain some percentage of carbonate Calcium. This has given rise to a chemical reaction in the GSCW samples under compaction test with slaty Urmieh Lake water produces Hydroxide Calcium and hence Chloride Calcium which enhances the soil structure hrdening properties (Lopez et al.,2001) This may also increase the soil shear strength (Mahasneh, 2005).
Although our CBR tests are limited here due to the main purpose of this study. But the resulted diagrams showed a slight decrease in the CBR of the sample with 1/3 clay ratio tested using Urmieh lake water compared to the that using tap water. This may be due to the fragility of the bonding of the silicate sheets by Na+ Cl- K+ Mg2+ H+ O- ions.
Conclusion
In this article we studied the effects of Urmieh Lake water on the soil compaction, plasticity and bearing characteristics such as optimum moisture content, maximum dry unit weight, plasticity index, California Bearing Ratio(CBR).
During these experiments, we observed that the optimum moisture content (w%) of the rather well graded gravel-sand mixture with a variable portion of clay (with 5-10% Bentonite) has decreased to the range of 15.7%-27%. It is also observed that the optimum moisture content has decreased from 18.5% to about 14% for the tested GSCW samples with varying the clay content ratio from 1/6 to 1/3.
It was seen that the maximum dry unit weight of GSCW soil samples during these tests changed slightly by 0.2% to 4.5%. The trend observed during these experiments showed a slight increase of dry unit weight on the tested soil samples using the Urmieh Lake water.
Another particular feature observed during these experiments was the range of the maximum dry unit weight which was 19.04-21.57kN/m3. This might be due to the complete compaction of soil samples with gravel, sand and very fine clay filling most the void spaces and compacted to a very dense form in the mould.
The plasticity index of the GSCW soil samples tested during this work showed a drastic change using the Urmieh Lake water from 25.57% to 3.66% due to the Na+ and Cl- ions. This can also be considered as an improvement of the soil properties for using in road construction in the region. It showed that the Urmieh lake water could be used as an improving agent.
Soil CBR is an important index to be determined for usage as the road base or sub-base material. Our CBR tests gave values in the range of 62.24%-68.96% for the tested GSCW soil samples. Due to the limited number of CBR tests, we can not draw specific conclusion at this stage on the effects of Urmieh Lake Water. Further study of CBR effects on the GSCW soil samples using the Urmieh Lake and tap water will be presented in another article.
Acknowledgment
This research work was carried out at Azarbaijan T.M University Soil Mechanics Lab. by help of Mr. Habibollah Kabiran. Assistance of Mr. Ziayee at Soil Mechanics Lab. and Mr.Habibi at Chemical Lab. of Azarbaijan University are greatly appreciated.
References
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