ESCALERAS DE MANO (DE MADERA O METAL)

10.1  Summarize the major techniques to measure matric suction and total suction. Summarize the  advantages and disadvantages for each technique including its practical measurement range. Matric suction is commonly measured with tensometers which directly measure negative pore water pressure, axis translation techniques which measure or control matric suction and cor- responding soil water content by separation of the pore air and water pressure, electrical and thermal conductivity sensors which indirectly measure matric suction through its relationship to electrical or thermal conductivity, and contact paper techniques which measures the equi- librium water content of filter papers in direct contact with unsaturated soil specimens under laboratory induced relative humidity and determines suction through correlation to the filter paper’s moisture content- suction characteristic curve (p. 417-419).

Total suction can be measured with humidity measurement techniques such as thermocouple psychrometers, chilled-mirror hygrometers. Humidity control techniques to measure total suction include isopiestic techniques using salt solutions, and two pressure techniques such as divided-flow humidity control. Total suction can also be measured with the non-contact filter paper method. Total suction techniques utilize Kelvin’s equation to convert humidity to suction (p. 419-420).

Tensiometers are usable over a practical range of approximately 0

₋

100 kP a and are useful for

directly measuring suction in the field under varying osmotic potentials since the tensiome- ter’s sensor tip is permeable to dissolved solutes. However, the sensor tip must be in direct contact with the pore water. Axis translation can measure soil suctions from approximately

0

₋

1, 500 kP a. This technique is useful for controlling the matric suction on a soil sample,

however, it must be performed with equipment not commonly practical for the field. Further- more the technique is primariliy applicable for coarse-grained soils since the HAE stone used in the techniques has an air-entry point of about 1 , 500 kP a, which limits the usefulness of this technique for fine soils. Further difficulties arise near residual or saturated conditions since the continuity of the pore air and water spaces is uncertain (p. 427). Electrical and thermal

conductivity sensors are useful from 0

₋

400 kP a   and are useful for automated electronic

applications in the laboratory and the field. However, the sensors may be highly responsive to changes in temperature or moisture content of the soil being measured and require fine calibration and care with respect to hysteresis of the sensor (p. 429-431). Thermocouple

laboratory and is limited to relative humidity readings of about 94%. Chilled-mirror hygrom- eters work quickly and are fairly simple to operate. They are accurate within 3% relative

humidity. Humidity control techniques are useful over a range of 4 , 000

₋

600, 000 kP a. These

techniques are indirect and not generally applicable for the field. Filter paper techniques are

useful for a total suction range of 1 , 000

₋

500, 000 kP a. This technique is very useful when

the filter paper is calibrated due to its wide range of total suction measurement. However, this technique is temperature dependent and requires a fair amount of time to perform ( 10 days).

10.2  If a sandy soil were encountered, what would be the appropriate technique(s) for laboratory  suction measurement? If a clayey soil were encountered, what would be the appropriate tech- nique(s) for laboratory suction measurement? 

A sandy soil would be best measured with a tensiometer, axis translation, or the contact filter paper method. For a clayey soil, non-contact filter paper and either humidity control or humidity measurement techniques would be the best suction measurements techniques to use.

10.3  Investigate the sensitivity of total suction to temperature by plotting Kelvin’s equation in terms  of suction versus relative humidity for three different temperatures. Summarize your findings   from this investigation.

Total suction is given in terms of relative humidity by equation 10.3 (p. 431). A specific

volume of water of  vwo = 0.001 m3/kg   and a molecular water mass of water vapor, ωvo =

18.016 kg/kmolare assumed. Temperatures of 250 K, 300 K, and 350 K were plotted and are shown in Figure S10.1. As shown on the graph, these values are fairly close together. Examination of equation 10.3 shows that total suction is directly proportional to temper- ature. However, total suction is directly proportional to the natural logarithm of relative humidity, indicating that total suction is much more dependent upon relative humidity than temperature.

10.4 A thermocouple psychrometer is used to measure the relative humidity of an unsaturated clayey  soil specimen. At equilibrium, the RH of the pore water vapor is 97.3% and the temperature  is 15  . What is the total suction of the soil in kPa? What is the likely water content for the  clay (give an approximate range)? 

The molecular mass of water vapor  ωv = 8.016 kg/kmol. Using equation 10.3,

Ψt =

 −

RT 

vwωv

ln(RH ) =

−

(285.15 K )(8.314J/mol

·

K )

(0.001m3_{/kg)(18.016 kg/kmol)} ln(0.973) = 3639.687 kP a

The total suction is about 3640 kPa with a likely water content for the clay between 30-50% gravimetric water content.

10.5  A Tempe cell test was conducted for a sample of unsaturated sandy silt. The data shown in  Table 10.7 was obtained (p. 260). Column 1 shows values of air pressure that were incremen- tally applied to the system. Column 2 shows the change in mass of the soil specimen as pore  water was expelled at each air pressure increment (e.g., when the pressure was increased from  15 kPa to 20 kPa, 1.15 g of water were expelled). After the system reached equilibrium for the 

Figure S10.1: Plot of Total Suction Related to Temperature vs. Relative Humidity

 final air pressure increment, the specimen was removed, weighed wet (23.71 g), oven-dried, and then weighed dry (23.00 g). Plot the matric suction characteristic curve for the soil. Problem data is shown in Figure S10.2, with data for the SWCC shown in Figure S10.3.

10.6 Concentrations of NaCl and KCl solutions shown in Table 10.8 (p. 461) were used to calibrate 

a batch of filter papers for total suction testing using the noncontact method. The filter paper 

water content  wfp (%) corresponding to each salt solution is shown. The average temperature 

during calibration was 25  . Plot the calibration curve for the filter paper in terms of total  suction versus filter paper water content.

Nacl (58.46   g/mol) and KCl(74.57   g/mol) concentrations were converted to molality and tables 10.3 and 10.4 (p. 437) were used to obtain values for the total suction of the filter paper.

10.7  An unsaturated soil specimen was tested for total suction using the calibrated filter paper from  Problem 10.6. The equilibrium water content of the filter paper after testing was measured as  12.0What is the approximate total suction of the soil? 

From the calibration curve Ψ = 353757e−0.3014w_fp

. With wf p = 12.0%, Ψ12.0  = 9505. The

total suction of the soil is approximately 9500 kPa.

Figure S10.2: Problem Data for 5

10.8 Evaluate the validity of the van’t Hoff approximation [eq. (1.16)] using Tables 10.3 and 10.4

(p. 461).

From Tables 10.3 and 10.4 using equation 1.16 with R = 8.314J/mol

_·

K  and uw0 = 0.000018 m3/mol

the following data in Table S10.6 is obtained, where bold values are computed with the van’t Hoff approximation.

The van’t Hoff approximation is valid as it produces an estimate for the osmotic suctions that approximates the overall trend of the data. However, the approximation only estimates the order of the osmotic suction.

For example, the osmotic suction for NaCl and KCl is approximately twice the van’t Hoff 

approximated value. This is most likely caused by the constant value of  uw0  used in the

equation, which in actuality is non-constant and different depending upon the concentration

of solution. A more accurate value for  uw0 (probably non-linear) will yield better results for

the osmotic suction estimate. Additionally, the van’t Hoff approximation does not account for the influence of short-range electrical fields or van der Waals fields on total suction.

Figure S10.3: SWCC

Figure S10.6: van’t Hoff Approximation Comparison for Total (Osmotic) Suction in NaCl and KCl Solutions as a function of Temperature

Chapter 11

Hydraulic Conductivity

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