Lab Experiment No - University of New Mexicomichael0/RA/lab/Drainage for Unsaturated... · Web...

Laboratory Report for CE 494

DRAINAGE FOR UNSATURATED SOILS UTILIZING TRANSPORT LAYER

Test Period: June 8 to July 30, 2009

Date of Report: July 31, 2009

Tested and Reported by Chi Bui

Instructed by Dr. Stormont

CE 494 – Summer 2009 Laboratory Report

Overview:

The Geocomposite Capillary Barrier Drain (GCBD) is a system to help drain the soil when it is not saturated. The GCBD consists of three layers: a transport layer, a geonet, and a separator layer. Under unsaturated conditions, coarse-grained materials are not conductive. The geonet layer of the GCBD material has pores large enough to resist water from the soil coming through under negative pressure. Instead, the moisture in the soil is drawn into the hydrophilic transport layer of the GCBD material. If the GCBD is on a grade, then water in the transport layer can drain laterally. The separator layer prevents soil intrusion into the large open pores of the geonet.

An issue with the GCBD system is how to get the water out of the transport layer. Conventional collection systems (i.e., pipes) require positive pressures to be effective. As an alternative, the idea of having the atmosphere itself pull water from the transport layer is under consideration.

The experiment has been conducted on both small scale by using 266-ml plastic cups and large scale by using 38-liter aquariums. The experiment is intended to test the effectiveness of using the transport layer to draw water from a soil under negative pressure, and release the water to the atmosphere. This experiment only involves the transport layer, and not the entire GCBD system.

Methods and materials for small-scale experiment:

Materials:1. 10kg of oven-dried silty sand2. Transport layer material3. 17 plastic cups (266ml)4. Bottle of water5. Table salt6. Scale7. Spatula8. Mixing bowls9. Aluminum foil to seal the plastic cup tops10. 2000-Watt growth light system

Procedure:1. Prepare 2 full (no headspace) cups of silty sand soil with 4% of moisture content with a

strip of transport layer which is placed at the bottom of the cup and extends 4cm above the top of the cup.

2. Seal the cups with aluminum foil and rubber bands. Only the 4-cm tip of the transport layer is exposed into the atmosphere.

3. Repeat steps 1&2 for 6%, 8%, 10%, 12%, 30%, and 50% of moisture contents.4. Weigh each beaker, and record the initial mass. Record the time as time zero.5. Put all beakers 2m below the 2000-Watt growth light system for 24 hours/day for the

entire testing period.

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6. Weigh each cup every 24 hours, and record the mass and time. Take the average of the mass of the cups for each water content and convert it into the remaining water contents in the soil. Details of calculations can be found in the Sample Calculations for Small-Scale Experiment part.

7. Continue weighing the cups every 24 hours until the remaining water content stays constant for one week.

8. Plot the water content in the soil versus time to detect water drained from the soil to see how effective it was for the transport layer material to remove water from the soils with different initial water contents and transmit it into the atmosphere.

9. Have a reference of a water-filled cup in the same configuration with all other cups to give a measure of the maximum rate the transport layer can draw water to the atmosphere. Weigh the water-filled cup periodically in the same manner with all other soil-filled cups to calculate the mass of water drained. Refill the cup after a couple of days as needed to continue monitoring the water drained.

10. Have a reference of brine-filled cup in the same configuration with the water-filled cup to see the drainage effectiveness difference between the water system and the brine system.

11. Have a control cup with no transport layer to ensure that system loss is identified. Only do this for one water content (12%).

12. Determine the moisture characteristic curve of the soil. Pack the 12% water content soil in a metal ring with the density very similar to those of the cups. The sample is placed on top of a saturated porous plate. The underside of the plate is connected by water-filled tubing to a reservoir. Lower the reservoir to let the water in the sample to equilibrate with the water in the porous plate. Weigh the sample after every 48 hours to measure the volumetric water content in the sample. Details of calculations can be found in the Sample Calculations for Small-Scale Experiment part. The difference of water levels in the reservoir and the porous plate is the suction head of the sample.

Sample calculations for small-scale experiment:

1. Remaining Water Content: Sample Calculations for 12% Cups after 3 days: Initial water content = 12% Mass of moist soil at t0 = 363.2g Mass of initial water = 0.12*363.2g = 38.91g Mass of dry soil (constant for the entire testing period) = 363.2g – 38.91g = 324.29g Mass of moist soil at t3 = 361.3g Mass of water lost = 363.2g – 361.3g = 1.9g Mass of remaining water = 38.91g – 1.90g = 37.01g

Remaining water content = 37.01g

324.29g * 100% = 11.41%

2. Moisture Characteristic Curves: Sample Calculations for Mitchell Hall soil at zero suction: Initial packing conditions:

Mass of mold + moist soil = 249.6g

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Mass of mold = 69.5gInitial water content = 12%Mass of moist soil = 249.6g – 69.5g = 180.1gMass of initial water = 0.12*180.1g = 19.30gMass of dry soil (constant for the entire testing period) = 180.1g – 19.3g = 160.8g

Volume of the metal ring = π4 * (6.20cm)2 * (3.81cm) = 115.03cm3

Conditions at zero suction: Mass of mold + moist soil = 286.6gMass of moist soil = 286.6g – 69.5g = 217.1gMass of water = 217.1g – 160.8g = 53.3g

Volume of water = 53.3g

1g /cm3 = 53.3cm3

Volumetric water content = 53.3cm3

115.03cm3*100% = 48.94%

Small-scale experiment results and discussions:

1/ Remaining Water Content: the soil cups have been periodically weighed to calculate the remaining water contents. Water content is the quotient between mass of water and mass of soil in the sample. While mass of soil remains constant, mass of water changes over time as the result of drainage. The summary of the results can be found in the following Figure 1 and Table 1:

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 422468

101214161820222426283032343638404244464850

Time in days

Rem

aining

Wat

er C

onte

nt(%

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Plot of Remaining Water Content in Cups versus Time

4% initial water content6% initial water content8% initial water content10% initial water content12% initial water content30% initial water content50% initial water content

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 423

3.54

4.55

5.56

6.57

7.58

8.59

9.510

10.511

11.512

Time in days

Rem

aining

Wat

er C

onte

nt(%

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Plot of Remaining Water Content in Cups versus Time

4% initial water content6% initial water content8% initial water content10% initial water content12% initial water content

Figure 1: Plot of Remaining Water Content versus Time

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Table 1: Remaining Water Content

Initial Moisture Content

Remaining Water Content

After 1 week After 2 weeks After 3 weeks After 4 weeks After 5 weeks4% 3.58% 3.52% 3.52% 3.49% 3.42%6% 5.16% 4.87% 4.68% 4.49% 4.27%8% 6.87% 6.25% 5.86% 5.45% 5.06%10% 8.64% 7.72% 7.09% 6.29% 5.62%12% 10.53% 9.48% 8.68% 7.71% 6.83%30% 24.73% 23.45% 21.87% 20.36% 19.13%50% 28.48% 25.20% 23.16% 21.35% 19.74%

There were two cups for each water content that were tested simultaneously. The results in Table 1 are the average of the two cups, which reflects the most likely trend of that particular initial water content.

From the results from Table 1 and Figure 1, water was drained more effectively in the first week than in the following weeks. The strip of transport layer in each cup was initially dry. Therefore, there was a significant difference in water pressure heads between the soil next to the strip and the dry wick. The higher the water pressure head difference between two bodies is, the faster water moves to the area with lower head. The initial pressure difference was much greater than that after the wick got wet, resulting in the faster drainage rate from the first week.

After the first week, there has been little change in water content for the 4% cups; while there have been still moderate changes in water contents for all other cups, especially for the 50% cups. It is interesting to notice that after 5 weeks, the remaining water content in the 50% cups is almost identical with that of the 30% cups as seen in Table 1.

It takes longer for water in the 50% water content soil to equilibrate with the dry wicks and the atmosphere. Therefore, the drainage period in higher initial water content cups will be longer than those in lower water content cups. If the data could be collected for a longer period of time, it would appear that all other water content curves would flatten out while the 50% curve would still change over time.

Another observation from this experiment is that the soils in the 50% cups cracked after two weeks, while the soils in all other cups did not. The water in the 50% cups dropped to 28% in the first week, which resulted in abrupt and significant shrinkage of the volume of the soil, and eventually the soils in the cups with 50% initial water content cracked. This indicates that the transport layer could be an effective method to drain high water content out of unsaturated soils; however, it could lead to cracking foundations and weakening structures. The cracks were found along the length of the wicks. Since there was not much bonding between the transport layer and the soil around it, cracks occurred in the area most vulnerable to being separated as seen in the following Figure 2.

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Figure 2: Cracks Can Be Detected from the 50% Water Content Cups

2/ Maximum Drainage Rate: the mass of water drained from the water-filled and brine-filled cups have also been measured. The brine system was prepared with 80g of table salt mixed with 800ml of de-ionized water. The cups have been refilled with water and brine as needed to avoid unnecessary head spaces. After time, salt particles can be seen at the tip of the wick of the brine-filled cup. The results are summarized in Table 2, and Figures 3&4:

Table 2: Cumulative Mass of Liquid Drained

Cumulative Mass of Liquid Drained (g)After 1 week After 2 weeks After 3 weeks After 4 weeks After 5 weeks

Water system 115.0 199.5 268.4 372.1 479.4Brine system 42.1 94.9 148.2 191.6 231.3

The water system is twice as effective as the brine system. The principal reason is that brine has lower energy state compared to pure water, which reduces its evaporation rate. Besides, salt particles trapped in the transport layer could block the liquid to be drained out of the cup, resulting slower drainage rate from the brine system. Though the brine system started a week later than the water system, it could be anticipated that the drainage rate in the brine system will get slower, since more and more salt particles would be trapped in the wick, while the water in the water system is free to flow up the transport layer.

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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 420

20406080

100120140160180200220240260280300320340360380400420440460480500520540560580

Time in days

Cum

ulat

ive

Mas

s (g

) of L

iqui

d D

rain

edPlot of Cumulative Mass of Liquid Drained versus Time

Water SystemBrine System

Figure 3: Plot of Cumulative Mass of Liquid Drained versus Time

Figure 4: Salt Particles Are Visible at the Wick Tip of the Brine System

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3/ System Loss: the mass of a theoretically sealed cup has also been periodically weighed to detect system loss. The system started with 12% water content. After 6 weeks, the water content is 10.44%. The plot of system loss can be found in Figure 5:

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 4210.4

10.5

10.6

10.7

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Time in days

Moi

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nt (%

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Plot to Detect System Loss versus Time

Figure 5: Plot to Detect System Loss

The “closed” system has lost about 13% of its initial water content. The plot looks quite linear, indicating that loss has happened regularly and at almost the same rate. Compared to the other 12% cups with wicks which have drained about 48% of its initial water content within 6 weeks, the system loss is significant. This indicates that sealing cups with aluminum foil and rubber bands is not a wholly effective method.

4/ Moisture Characteristic Curves:

The moisture characteristic curve is the plot between volumetric moisture content and suction head. Suction head is the positive term for negative pressure head. Volumetric moisture content is the quotient between volume of the water in the sample at a particular time and volume of the metal ring that holds the sample. The layer of soil above the water table is called the capillary fringe which maintains negative pressure head. The finer the soil above the water table is, the higher the capillary rise zone is; whereas the larger size of the voids a soil has, the less water will rise above the water table level. From the moisture characteristic curve, with a certain volumetric moisture content, one can tell how high water can rise in that soil under unsaturated conditions. Water in the capillary fringe under the effect of freeze-thaw could cause serious problems to foundations and pavement materials.

The following Figure 6 summarizes the unsaturated hydraulic properties of three different soils. The soil used in the small-scale experiment is clayey sand taken from the Bobcat well. The soils

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used in the large-scale experiment are the clayey sand from Mitchell Hall and the coarse-grained “beach sand” from Bosque. From the plot, it is clearly seen that at zero suction, “beach sand” has the lowest water content. With the curve trend as in Figure 6, and if the data could have been collected for six weeks or longer for all three soils, it could happen that the soil from Mitchell Hall could have the highest capillary zone.

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 520

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Volumetric Moisture Content (%)

Suc

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(cm

)

Moisture Characteristic Curves for Tested Soils

Soil from Bobcat WellSoil from Mitchell HallBeach Sand from Bosque

Figure 6: Moisture Characteristic Curves for Tested Soils

Methods and materials for large-scale experiment:

Materials:1. 60kg of oven-dried beach sand (taken from Bosque)2. 60kg of oven-dried clayey sand (taken from a construction site by Mitchell Hall)3. Two 38-liter aquariums4. Scale that can weigh up to 200kg5. Transport layer6. Water to make 27% water content for the beach sand and 27% water content for the

clayey sand7. Plexiglass and sealant to seal the top of the aquarium8. Moisture cans to collect soil at a specific depth to measure water content at different

locations.9. 2000-Watt growth light system.

Procedure:1. Cut two strips of transport layer with dimensions: 22.5cm width * 37.5cm height.2. Prepare 27% initial water content for “beach sand.”

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3. Place the wick at the middle of the aquarium with one end touching the bottom and the other end extending 7.5cm above the top. Fill the tank with the prepared soil and seal the top.

4. Weigh the initial mass of the “beach sand” system. Record the initial mass and mark the instant as time zero.

5. Place the aquarium 3m below the 2000-Watt growth light system for 24hours/day for the entire testing period.

6. Periodically weigh the “beach sand” system every 24 hours and convert the mass difference into remaining water content. Details of calculations can be found in the Sample Calculations for Large-Scale Experiment part.

7. Plot the remaining water content versus time.8. Prepare 27% initial water content for clayey sand.9. Repeat step 3 through step 7 for the clay system.10. Before taking down both systems, collect the soils at different depth locations to

determine the water contents. Details of calculations can be found in the Sample Calculations for Large-Scale Experiment part. Tabulate the distribution of water contents versus depth.

11. Prepare a cup of “beach sand” with 25% water content and a cup of clayey sand with 25% water content. Put the cups 2m under the 2000-Watt growth light system for 24hours/day for the entire testing period. Monitor the drainage in the same manner for the small-scale experiment.

Sample calculation for large-scale experiment:

1. Remaining Water Content: Sample Calculations for “Beach Sand” Aquarium after 1 day: Initial mass of aquarium, moist soil, and cart = 56.7kg Initial water content = 27% Mass of oven-dried soil (constant for the entire testing period) = 44.6kg Mass of water = 44.6kg*0.27 = 12.1kg Mass of aquarium, moist soil, and cart at t1= 56.6kg Mass of water lost = 56.7kg – 56.6kg = 0.1kg Mass of remaining water = 12.1kg – 0.1kg = 12.0kg

Remaining water content = 12.0k g44.6k g * 100% = 26.78%

2. Moisture Content at Different Depth Locations: Sample Calculations for Clayey Sand at 10cm depth: Mass of moisture can (including lid) = 48.9g Mass of moisture can + moist soil before being oven-dried = 294.2g Mass of moist soil = 294.2g – 48.9g = 245.3g Mass of moisture can + soil after being oven-dried = 244.3g Mass of water = 294.2g – 244.3g = 49.9g Mass of oven-dried soil = 245.3g – 49.9g = 195.4g

Water content of the sample = 49.9 g195.4g * 100% = 25.54%

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Large-scale experiment results and discussions:

1/ Sieve Analysis: since there was not enough classified soil for the large-scale experiment, the soil was taken from a construction site by Mitchell Hall. The soil was taken from 1-m depth to avoid fine particles on the surface being blown away over time. The ASTM D422-63(2007) sieve analysis results are summarized in Table 3 and Figure 7:

Table 3: Sieve Analysis Results:

Sieve No.

Sieve Opening

(mm)

Mass of soil Retained on

Each Sieve (g)

Percent of Mass Retained on Each Sieve

Cumulative Percent

Retained

Percent Finer

4 4.750 3.8 0.7% 0.7% 99.3%10 2.000 61.8 11.6% 12.3% 87.7%20 0.850 82.3 15.4% 27.7% 72.3%40 0.425 72.3 13.5% 41.2% 58.8%60 0.250 73.4 13.7% 54.9% 45.1%140 0.106 138.1 25.8% 80.7% 19.3%200 0.075 42.4 7.9% 88.7% 11.3%Pan N/A 60.6 11.3% 100.0% 0.0%

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Grain Size (mm)

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Plot of Percent Finer versus Grain Size

99.3%

87.7%

72.3%

58.8%

45.1%

19.3%

11.3%

Figure 7: Percent Finer versus Grain Size for Soil Taken from Mitchell Hall

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Based on USCS specifications, the soil can be classified as well-graded sand with silt or with clay. Since the percent of fines is between 5% and 12%, further experiment should be conducted to identify the PI. From the moisture characteristic curves in Figure 6, the soil from Mitchell Hall should be coarser than the soil from Bobcat well. While the soil from the Bobcat well was classified as well-graded sand with clay (SW-SC), the Mitchell Hall soil could be well-graded sand with silt (SW-SM).

2/ Remaining Water Contents in Aquariums versus Cups:

0 1 2 3 4 5 6 7 8 99.510

10.511

11.512

12.513

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14.515

15.516

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17.518

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20.521

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26.527

27.528 Plot of Water Content versus Time

Time in days

Wat

er C

onte

nt (%

)

Beach Sand Aquarium with 27% initial water contentClayey Sand Aquarium with 28% initial water contentBeach Sand Cup with 25% initial water contentClayey Sand Cup with 25% initial water content

Figure 8: Remaining Water Contents in Aquariums versus Cups

From the slopes of the plot in Figure 8, the drainage rates in the cups are much faster than those in the aquariums. There could be several explanations for this to happen. First, the ratio between the mass of soil in the container and the surface area of the submerged transport layer is very different between the two configurations – 0.00625kg/cm2 for the cups versus 0.04133kg/cm2 for the aquariums. 1cm2 of the transport layer from the aquariums has to take care of 6.6 times more soil. Secondly, in order for water to evaporate, it has to work against gravity to go up to the tip extending into the atmosphere. The longer the path is, the slower the water will get to the tip. Based on the configurations of 30cm-high aquariums versus 10cm-high cups, the drainage of the cups should be much more effective.

Another observation from the plot in Figure 8 is that there are several adjacent points on the “Beach Sand” Aquarium curve and the Clayey Sand Aquarium curve that lie on horizontal lines. It seems that water was not drained out of the systems during that period of time. Indeed, water has always been drained, however, due to the 0.1kg-resolution of the scale, it did not capture any loss less than 100g. It took a couple of days for the cumulative loss to exceed 100g before it could be recorded.

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As expected, the drainage rate in the cup with clay is faster than that of the cup with “beach sand.” Under unsaturated conditions, finer soils are more conductive than coarse-grained soils. At zero suction, most water was drawn out of pores of coarse-grained soils. Under negative water pressure, water which is still held abundantly in pores of fine soils moves into areas with lower heads, resulting in conductive fine soils under unsaturated conditions. Once the water moves fast in the soil, it will be drained out fast too.

Like the small-scale experiment, cracks can also be seen along the length of the wicks of the ‘beach sand’ and clayey sand cups as the transport layer has been drained about 50% of the initial water contents within 7 days. However, cracks are much more visible in the clayey sand cup than those in the “beach sand” cup as seen in the following Figure 9. “Beach sand” could not take more water than the volume of its void, which did not change its initial volume; whereas clayey sand could take much more water than its void space. Once the water is drained out of clayey sand, it could lead to significant volume shrinkage, which results in serious cracking problems.

Figure 9: Cracks Are Visible in “Beach Sand” & Clayey Sand Cups

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3/ Distribution of Water Content at Different Depth Locations:

The soils in two aquariums were collected at 0cm, 5cm, 10cm, and 15cm and oven-dried to determine the water contents. Table 4 summarizes the distribution of water contents.

Table 4: Distribution of Water Content:

Cup Depth (cm) Water Content (%)

Clayey Sand

Aquarium

C1 0 35.86%C2 5 26.26%C3 10 25.54%C4 15 26.62%

“Beach Sand”

Aquarium

S1 0 26.94%S2 5 25.75%S3 10 25.27%S4 15 26.30%

The water contents along the wicks of two aquariums are quite homogenous, and these results are consistent with the average water contents of the end points of two soils as seen in Figure 8. Though both top thirds of two aquariums had been initially filled with water, only the top layer of the clayey sand had higher water content. Though the extra water was siphoned out of the tanks on the following day, clay by nature could hold more water than its void, resulting in the wetter top layer.

Conclusions and Suggestions:

The drainage results from cups are very promising – the water content could drop 50% within 6 weeks, which could be very ideal for any real systems to reduce the freeze-thaw effect. The drainage rate is not only dependent on the soil and the transport layer properties, but also is a function of initial water contents and time. The higher the initial unsaturated water content is, the more effective the system works. The longer the system is in place, the less the drainage rate will become. However, there is a great difference between the results of the two configurations: 266-ml cups versus 38-liter aquariums. Though there was not enough time to take more readings on the aquarium, it is obvious that the drainage rate of the cups by far beats that of the aquariums. Some of the reasons might be:

- The ratio between the soil in the container and the surface area of the submerged wick- Gravitational force on the water trying to flow up the wick

When system loss by not being able to seal the cups completely is taken into account, it could reduce the drainage rate by approximately 25%. Figure 10 and Table 4 will illustrate the more accurate effectiveness of the transport layer when system loss could be eliminated.

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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 426

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)Comparison between System Loss and Drainage by Transport Layer of 12% Water Content Cups

12% Control Cup12% Cups with WicksApproximation of Drainage by Wicks of 12% Cups

Figure 10: Approximation of Drainage from 12% Cups

Table 5: Approximation of Drainage from 12% Cups with Wicks:

Water Content (%)After 1 week

After 2 weeks

After 3 weeks

After 4 weeks

After 5 weeks

After 6 weeks

12% Sealed Control Cup 11.66 11.42 11.21 10.96 10.69 10.4

12% Cups with Wicks 10.53 9.48 8.68 7.74 6.83

6.25

Approximate Water Content 10.87 10.06 9.47 8.78 8.14 7.81

Though system loss was monitored on the 12% cups only, it could be said that there was loss on all other water content cups as long as they were sealed by aluminum foil and rubber bands. Therefore, the effectiveness of utilizing transport layer would not be as high as shown in Table 1. It is suggested that a control cup be set up for every single water content to see if system loss could also be positively proportional to initial water content just like drainage rate, and how it could affect the overall results.

The maximum drainage rate of liquid-filled cups was also measured. To be stable, every object tries to maintain an energy level as low as possible. Salt mixed in water is more disordered than water by itself; therefore brine is more stable than water. Compared to the lower energy state brine system, the water system needs to dissipate more energy to remain stable, resulting in more water being transformed into vapor and eventually faster drainage rate.

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During the period of 9 days to test drainage of the aquariums, the water contents dropped from 27% to 26%. It could be anticipated that the rate would get slower and slower as seen in the small-scale experiment, because drainage rate is inversely proportional to time. From the moisture characteristic curves in Figure 6, 26% for “beach sand” is equivalent to 20cm of water rising above the water table level, whereas 26% for clayey sand means that water rises 100cm above the water table level. For shallow water table levels, these capillary rise zones definitely cause serious problems to foundations under the effect of freeze-thaw. Instead of letting water working against gravity to be drained, if the transport layer is on an incline, it could improve the drainage rate. And if the ratio between the surface area of the submerged wick and the soil in the container could be maximized, we could expect more promising results.

The question is no longer whether the transport layer could draw water out of unsaturated soils, since it has been proved to be very effective through the small-scale experiment. Further research should focus on how to draw water quick enough to prevent water rise, but not so fast that it could cause cracks in foundations, which has been also seen in the small-scale experiment.

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