FASE DE EXCAVACION DE CIMENTACIONES Y APERTURA DE ZANJAS

S C O P E A N D S U M M A R Y

Salinity and pH are two common measures used to characterize pore fl uids of all geo materials. Pore fl uids are commonly extracted in the laboratory when working with fi ne - grained and peat materials. The samples do not need to be intact, but the chemi-cal composition must be preserved during transport. When working with free - draining materials such as sands, it will be necessary to extract the pore fl uids while in the fi eld.

T Y P I C A L M AT E R I A L S

B A C K G R O U N D The chemical composition of the pore fl uid is important to the behavior of fi ne - grained soils, the durability of structures in contact with soils, the evaluation of contaminant transport, and the use of ground water. The salinity and pH of the pore fl uid have a This chapter presents background and procedures for the measurement of both the pH and the salinity of the pore fl uid in soils. The measurements are appli-cable to the pore fl uid of all types of soils but the procedures in this chapter are restricted to fi ne-grained material and peat. Salinity and pH are commonly used to characterize the pore fl uid. There are several methods available to make such measurements. The chapter provides rapid, simple methods that are approximate, yet suffi ciently adequate for most geo-engineering applications.

signifi cant impact on the mechanical properties of fi ne - grained soils. Both infl uence the thickness of the double layer around clay particles and consequently the interaction between these particles. In general, the fi ner particles are more sensitive to pore fl uid chemistry than larger particles. The conditions of the pore fl uid during deposition (e.g., a marine environment) will control the initial fabric or arrangement of particles.

Changes in the pore fl uid chemistry subsequent to deposition can have a signifi -cant infl uence on the mechanical behavior. One classic example is the high degree of strength sensitivity observed when salt is leached from clay deposits. Pore fl uid chemis-try can enhance the rate of corrosion of materials in contact with soil as well as systems used to extract groundwater. Measurements of pore fl uid chemistry are used exten-sively to evaluate the extent of contamination into deposits, infi ltration of salt water into aquifers, and monitoring the security of landfi ll liners.

Although pH and salinity measurements are applied in very different ways in engi-neering practice, both parameters contribute to soil characterization and provide infor-mation about the chemical properties of the pore fl uid. The procedures used to make the measurements are very similar. Both measurements require processing the sample to extract pore fl uid in the laboratory and provide an introduction to important analytic techniques. Figure 6.1 shows the typical equipment used to extract pore fl uid from a soil specimen.

The measurement of pH and salinity have been selected for this chapter because they are common in the geotechnical arena, provide useful information for engineering practice, and illustrate methods used to test pore fl uid chemistry. Therefore, these two measurements are grouped together in this one chapter. Evaluation of pH and salinity are only two of a host of chemical assessment measurements. Many of these measure-ments are made using similar procedures and ion - specifi c probes. Others are measured by performing analytical chemical assays on the extracted pore fl uid.

Figure 6.1 Equipment used to extract pore fl uid from a soil specimen.

pH pH is a dimensionless representation of the concentration of free hydrogen ions in an aqueous solution. It is calculated as the negative of the logarithm (base ten) of the concentration of hydrogen ions. In this context, concentration is the number of moles

(of hydrogen atoms) per liter of water. pH commonly ranges in value from 0 for a very strong acid to 14 for a very strong base, although these are not absolute limits. For each whole - number decrease in pH there is a 10 - fold increase in the number of hydrogen ions in the solution. Even pure water contains electronically charged free ions due to the dissociation of water molecules. One liter of pure water contains 10 ^{- 7} moles (equal to grams for hydrogen because the atomic weight is 1) of positively charged hydrogen ions (H⫹) and an equal number of negatively charged hydroxyl ions (OH ⫺ ). This liq-uid is neutral with a pH equal to 7 and has a balanced ionic charge. It is important to note that water equilibrated with air at standard conditions does not have a pH of 7. The carbon dioxide in the air will dissolve (very quickly) in the water, turning the water into a weak acid having a pH of about 5.5. Since pH is represented using a logarithmic scale, this reaction creates more than a 30 - fold increase in the hydrogen ions.

When working with soils, pH is measured either by contacting the surface of the soil with a pH sensitive device or by submersion of the device in the pore fl uid.

The surface measurement can result in lower pH values because the soil particles in contact with the probe have surface charge and can bias the measurement. The surface contact measurement is most common when conducting corrosion analysis of facilities installed in the ground because the method more closely mimics the fi eld situation. Pore fl uid measurements generally require addition of fl uid to the sample to yield enough particle - free fl uid for the measurement. Measurements are made on each sample using a distilled water solution and a dilute salt solution of 0.01 M calcium chloride (CaCl ₂ ).

The addition of a salt solution is used to reduce the effect of natural salts in the soil on the measurement.

Equipment is available to measure pH using either a hydrogen specifi c probe or disposable indicating paper. pH paper is made with a variety of color - indicating organic compounds. These dyes are designed to provide a specifi c color in the presence of a known hydrogen concentration. The paper is available in different ranges and sensitivi-ties. In general, papers are limited to pH increments of 0.1.

The ion - specifi c probe provides a more accurate measurement over a wider range of values but must be maintained properly and calibrated routinely. pH probes are used in combination with a very high inductance voltmeter and generally sold as a unit called a pH meter. The probe is an electrode consisting of a very thin glass bulb that is plated to be sensitive to hydrogen ions. The inside of the bulb is fi lled with a reference solu-tion, typically calomel. An electrical contact is submerged in the reference solution. An electrical potential (differential voltage) is created across the glass, which is a function of the hydrogen ion concentration. A second electrical contact is required to measure the voltage across the glass bulb. This contact is submerged in a salt solution inside the probe that is in contact with the fl uid outside the probe. The salt solution connection to the outside liquid is achieved through a hole in the glass fi lled with a permeable mem-brane. The salt solution slowly leaks from the probe. The probe must be stored in a salt solution and the reference solution needs to be replaced on a regular basis. Always refer to the manufacturer ’ s instructions for proper maintenance.

Three ASTM standards are available to measure pH for geotechnical purposes:

D4972 pH of Soils, D2976 pH of Peat Materials, and G51 Measuring pH of Soil for Use in Corrosion Testing. The method used in G51 is a surface contact measurement and normally performed in the fi eld. The procedures contained in D4972 and D2976 are essentially the same method of measurement with a few noncritical differences in the discussions. This chapter makes use of D4972 Method A (pH Meter) to measure the pH.

Figure 6.2 shows the equipment necessary to measure pH using the pH meter.

Important considerations for the measurement of pH include:

Frequent calibration verifi cation of the pH probe is critical to consistent meas-urements. Buffer solutions are available in a variety of pH values, with 4, 7, and 10 being the most common. Erratic readings or incorrect readings in the buffer solutions require immediate action. Most meter systems provide the ability to

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Figure 6.2 Equipment used to measure the pH of pore fl uid:

centrifuges tubes with soil and pore fl uid separated (left); probe positioned in a 10 mL beaker (center); pH meter (right).

recalibrate based on buffer solutions. While this is a great feature, it still requires proper stocking of the buffer solutions.

Proper care of the pH probe is absolutely necessary. The reference salt solu-tion needs to be replenished on a regular basis and the probe should be cleaned routinely and always stored in the salt solution. There are many products on the market to help with proper maintenance of the probe.

While it would seem that distilled water, with a pH in the middle of the scale, would provide a convenient calibration point, this is far from the truth. Distilled water has low ionic strength, which results in very high electrical conductivity and makes the task of measuring pH extremely diffi cult.

The test methods require addition of distilled water to the soil in order to provide enough fl uid for submersion of the pH probe. Diluting the pore fl uid will change the pH. The magnitude of the error increases as the soil pH deviates from 7 and as the amount of dilution increases. If more accurate values are required, then the measurements should be corrected for the dilution.

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Salinity Salinity is the amount of salt that is dissolved in the pore fl uid. This is sometimes

referred to as the Total Dissolved Solids (TDS). Salinity is expressed in terms of grams per liter. Seawater is generally at a salinity of about 35 grams of salt per liter of seawater (g/L) but it varies somewhat with geographic location and season. Sea salt has a com-plex chemical composition. It is mostly sodium chloride (about 85 percent), with lesser amounts of sulfate, magnesium, calcium and potassium in decreasing concentrations.

When soils are deposited in a marine environment, the salt remains in the intersti-tial pore fl uid. Geologic processes can alter the salinity of the pore fl uid after deposition.

Reduction of the void ratio by compression expels salt with the pore fl uid and generally does not change the salinity. Evaporation will increase the salinity, as evidenced in salt fl ats and by the clays in the Great Salt Lake region of Utah, where the pore fl uid salinity can be up to 300 g/L.

Leaching is a common process that leads to a reduction in the salinity. Artesian pressure below sedimentary clay layers produces upward fl ow of fresh water, which displaces the saline pore fl uid. Diffusion of salt through the network of pore space to create a balance in concentration can also be important in sedimentary deposits. Diffu-sion can either increase or decrease local salt concentrations.

The presence of salt in the pore space has various repercussions. Salt can be important for project - level concerns, which are beyond the scope of this text. In such

situations, measurement of salinity may be required simply to evaluate the in situ conditions. More to the point for measuring the properties of soils is the fact that salin-ity infl uences mechanical behavior, the interpretation of some test measurements, and can damage equipment.

Salinity in the pore fl uid causes an error in many common index measurements.

The magnitude of this error increases with increasing salinity. This is the case for water content, void ratio, dry mass, specifi c gravity, and particle size analysis using sedi-mentation. All of these test results can be explicitly corrected provided the salinity is known. Appendix C provides a more detailed discussion relative to corrections as well as equations for the phase relationships that account for salinity.

While not a matter for data correction, the infl uence of salinity on mechanical behavior is also important for understanding the measurements. Salinity infl uences everything from the liquid limit to the undrained strength of cohesive soils. Having a measurement of the salinity provides the information to properly compare measure-ments on different samples and consider changes that may occur if salinity changes over time. Salinity, similar to pH, alters the electrolyte concentration in the pore fl uid. The ions alter the thickness of the double layer and ultimately the physical interaction of the particles. Much like pH, salinity changes must be substantial to be important. Soil behavior changes would be expected as salinity changes by a factor of 2. Leached or quick behavior requires salinity in the single digits, but not all soils with low levels of salinity will be leached or quick.

There are at least three methods in common use to measure the salinity of fl uid.

They each make use of very different principles of measurement and require different levels of effort and equipment. All three methods require separation of the pore fl uid from the soil.

The most straightforward approach is to make gravimetric measurements. This method requires extraction of a known volume of pore fl uid, oven - drying the fl uid to remove the water, and then determining the mass of the evaporate. This method meas-ures the TDS and makes no distinction between fi ne soil particles and salts that were in solution. A high - precision analytic balance is required to obtain reasonable resolution.

The method works fi ne for high salt solutions but quickly loses precision as the salinity decreases.

The second method takes advantage of the fact that the angle of refraction changes with salt content. One such method is described in ASTM D4542 Pore Water Extraction and Determination of the Soluble Salt Content of Soils by Refractometer. The refraction method makes use of a readily available and fairly inexpensive tool, but has a limited salinity range and poor resolution for low concentrations.

The third method is based on measurement of electrical conductivity. The electrical conductivity of distilled water is extremely low because distilled water does not contain free ions, which are the mobile atoms or molecules capable of transporting electrons.

The addition of ions to the water provides charge carriers and increases the electrical conductivity. For a given ion (or salt), the change in conductivity is proportional to the change in concentration. The method has the distinctive advantage that equipment is available to measure conductivity over many orders of magnitude. This advantage provides comparable precision (represented as the coeffi cient of variation) for all ranges of interest. The electrical conductivity method is used in this text. Figure 6.3 shows the equipment necessary to measure the electrical conductivity of the extracted pore fl uid.

The method suffers from the fact that it is indirect. All free ions in solution will contribute to the electrical conductivity, but each with its own specifi c contribution.

Each ion has a different equivalent ionic conductance and molecular weight. The meas-ured conductivity will be the sum of the contribution of each ion. This complication is handled by establishing a calibration curve for the salt of interest. Figure 6.4 presents a typical calibration curve for sea salt that has been normalized to the conductivity at 12.5 g/L. Normalizing the curve provides a generic relationship that can be used with different devices and different reference salts.

Figure 6.3 Equipment used to measure the electrical con-ductivity of pore fl uid: electrical conductivity probe connected to a handheld meter.

Important considerations for the measurement of salinity include:

The centrifuge is used to accelerate the rate at which particles will fall out of suspension. The smaller clay particles have the most potential to stay in solution and are also the particles with the most surface charge. These small particles will function as ions and alter the measurement. The supernatant should be clear when making the measurements.

Cleaning the probe is essential for reproducible results. A small drop of dis-tilled water or saline water from a prior measurement will immediately change the concentration of the small volume of the supernatant. The probe must be meticulously washed and dried in between each measurement.

Purity of distilled water is important to the measurement since contamination in the water will increase the conductivity. The error is more important as the

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0.001 0.01 0.1 1 10 100

0.1 1 10 100 1,000

Sea Salt Concentration, SS (g/L) Normalized Electrical Conductivity (C/C12.5g/L)

10,000

Figure 6.4 Relationship between normalized electrical conductivity and sea salt con-centration.

T Y P I C A L VA L U E S

Soil Type pH Salinity (g/L)

Boston Blue Clay* — 8 to 31

Mexico City Clay** 7.2 to 9.1 1 to 6

*Personal database.

**After Fernández, 1994.

Table 6.1 Typical values of pH and salinity of selected soils.

Equipment Requirements pH

1. Potentiometer equipped with glass-calomel electrode system. Follow the manufacturer’s instructions for proper operation of the pH meter.

A silver/silver chloride electrode system or similar is also acceptable.

Reagents

1. Water: distilled, deionized water

2. Acid potassium phthalate buffer solution (0.05 M) or commercial pH 4.0 buffer solution

3. Phosphate buffer solution (0.025 M) or commercial pH 7.0 buffer solution

4. Commercial pH probe storage solution

5. Calcium chloride stock solution (CaCl₂*2H₂O) (1.0 M). Dissolve 147 g of CaCl₂*2H₂O in water and increase the volume to one liter 6. Calcium chloride solution (0.01 M). Take 10 mL of stock 1.0 M

solu-tion (item 5) and add water to increase the volume to 1 L to create a 0.01 M solution.

Salinity

1. Electrical conductivity meter using an alternating current (1000 Hz works well) Wheatstone bridge design. Many commercial units are available.

salinity of the pore fl uid decreases. The fact that the measurement is made on a dilution makes this consideration more important.

The calibration curve expresses the grams of soluble salt per liter, SS, with sea-water salt as the reference salt. When the dissolved salts contributing to the electrical conductivity differ markedly from seawater composition, the absolute value of the grams of salt per liter may be inaccurate. However, even small rela-tive changes in salt concentration are accurately measured as long as the salt composition at the location under investigation does not change markedly. The plot of C/C _o versus SS gives a unique curve whether seawater or reagent - grade sodium chloride is used as the salt.

The test should not be performed on dried material. Drying will transport salts to the boundaries of the specimen, making it diffi cult to obtain representative samples. This is especially true of oven - dried samples.

Typical values of pH and salinity of selected soils are presented in Table 6.1 .

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Calibrate the pH meter at a pH of 4 and 7 using the buffer solutions. Adjustment of the pH meter should follow the manufacturer ’ s directions. At the completion of calibration, the probe should read the proper pH value in each buffer. The probe should be repaired or replaced if the readings are not repeatable in the buffer solutions.

The measurement system must be calibrated to obtain the relationship between salt con-centration and electrical conductivity. The relationship will depend on the geometry of the probe (area of electrodes and distance between plates characterized by the cell constant), the particular salt, and the conductivity meter. The conductivity meter

The measurement system must be calibrated to obtain the relationship between salt con-centration and electrical conductivity. The relationship will depend on the geometry of the probe (area of electrodes and distance between plates characterized by the cell constant), the particular salt, and the conductivity meter. The conductivity meter