Apparent Coefficient of Friction, f* to be Used in the Design of Reinforced Earth Structures. Technical Bulletin: MSE - 6

Transcription

1 The Reinforced Earth Company 8614 Westwood Center Drive Suite 1100 Vienna, Virginia Telephone: (703) Telefax: (703) Apparent Coefficient of Friction, f* to be Used in the Design of Reinforced Earth Structures Technical Bulletin: MSE - 6 October 1995 Atlanta Boston Dallas/Ft. Worth Irvine, CA Lafayette, IN Orlando Seattle Vienna, VA

2 TABLE OF CONTENTS Page No. I. Introduction... 1 II. f* Used in the Design of Reinforced Earth Structures... 1 III. Discussion of the Influence of Water on the f* Value Used in Design IV. Influence of the Use of Uniform Fine Sands for Wall Backfill on the Apparent Coefficient of Friction, f* LIST OF FIGURES LIST OF TABLES

3 LIST OF FIGURES Figure 1: Values of Apparent Coefficient of Friction (f*) From Pullout Tests Figure 2: Apparent Coefficient of Friction, f o * in Relationship to the Coefficient of Uniformity, C u Figure 3: Gradation Curves and Determination of the Coefficient of Uniformity, C u, for Pullout Test Soils Figure 4: Apparent Coefficient of Friction, f*, at Optimum Moisture and in a Saturated Condition Figure 5: Apparent Coefficient of Friction, f*, in Uniform Fine Sand LIST OF TABLES Table 1: Direct shear Properties of Soils at Optimum Moisture and in a Saturated Condition

4 I. Introduction The topic of sliding shear resistance between galvanized steel reinforcing strips and soil has been the subject of numerous research studies in several countries. These studies have produced abundant data that, on first examination, are difficult to explain but will, after more detailed scrutiny, generally yield to the usual concepts of the shear strength properties of granular materials and sliding friction between materials. Several types of tests have been used to measure the apparent coefficient of friction, f*, between the soil and reinforcing material. These include: 1. Direct shear (sliding shear) tests between soil and reinforcing material. 2. Reinforcing strip pullout tests from a pullout test apparatus. 3. Reinforcing strip pullout tests from Reinforced Earth walls - model scale, prototype, and full scale structures. Of all the testing procedures used, the direct pullout test from a pullout test apparatus in the laboratory is the one most available to practicing engineers for the evaluation of design parameters. Other testing procedures require more specialized equipment, and generally involve higher cost which may not be justified by either the size of the project or the economic gain that may result from more refined data. Therefore, from a designer's standpoint, it is important to base the apparent coefficient of friction, f* on the abundant data that is available in the literature. II. f* Used in the Design of Reinforced Earth Structures All parameters assumed for the design of Reinforced Earth structures are conservatively taken as minimum values by enveloping the abundant data available in the literature. The available data, summarized in Figure 1 (the same figure referenced in the commentary of the 1994 AASHTO Standard Specifications for Highway Bridges Section C5.8.5) presents a summary of pullout test results for ribbed reinforcing strips in a variety of backfill materials ranging from silty sands to coarse gravels. Note that the f* envelope extending from a maximum value of 2.0 at the ground surface, to a minimum value of tan at a depth of 6 meters envelopes the data. As discussed in Section of the 1994 AASHTO standard specifications for highway bridges, a value of 2.0 may be used in the design of Reinforced Earth structures. 1

5 In the actual design of a Reinforced Earth wall, the value of the apparent coefficient of friction at the ground surface, f o *, is based on one of the following: 1. The backfill source for the project is known prior to design, therefore a value is selected that is appropriate for the backfill material to be used. 2. The backfill source is unknown, however, backfills used on previous projects have been consistent, or, the specified backfill gradation is restrictive in that it insures a certain type of backfill material. In either case, a reasonably representative value of f o * can be confidently selected. 3. Backfill sources local to the project are limited, such as uniform sands in Florida, such that an appropriate value of f o * can be assigned for the region. 4. The backfill source in unknown and the specified gradation is not very restrictive. A value of 1.5 is used in the design, and then checked once the gradation of the backfill is known. In addition, it is important to note that a minimum factor of safety of 1.5 with respect to reinforcement adherence is provided by design to cover any uncertainties associated with the value selected for f o * and for the resulting soil-reinforcement bond safety of the structure. Selection of f o * will depend on the coefficient of uniformity, C u, of the soil and on the angularity of the material (in the case of gravel). The coefficient of uniformity, C u, is equal to the size of the sieve opening through which 60 percent of the material passes, divided by the size of the sieve opening through which 10 percent of the material passes (D 60 /D 10 ). Generally if more than 10 percent of the material passes the #200 sieve, the #200 sieve opening (75µm) is used for the denominator. Figure 2 presents a summary of f o * values from project specific pullout tests. Note that the equation f o * = log C u (referenced in the literature) is well below all of the data points. Using this equation, the f o * value can be conservatively computed for use in design. For simplicity, three specific values of f o * are generally, but not exclusively, used in a design, depending on soil type: f o * = 1.2 f o * = 1.2 is to be used for projects constructed with backfills having a coefficient of uniformity Cu < 2, i.e., materials such as uniform fine sands, uniform medium sands, and uniform rounded 2

6 coarse materials such as pea gravel, unless substantiated otherwise by pullout tests in a representative backfill. f o * = 1.5 f o * = 1.5 is to be used for most projects constructed with backfills having a coefficient of uniformity 2 Cu < 7 (i.e., poorly graded sands, poorly graded river run gravels, and sand containing up to 25% passing the #200 sieve). Note, AASHTO specifications restrict fines to not more than 15% for all reinforced soil structures. f o * = 2.0 f o * = 2.0 is to be used for projects constructed with well graded backfills having a coefficient of uniformity Cu 7, or when using coarse well-graded angular materials, such as crusher run materials or poorly graded angular gravels. III. Discussion of the Influence of Water on the f* Valve Used in Design Hundreds of Reinforced Earth structures have become fully saturated in service or remain permanently inundated throughout the service life of the structure. There is not a significant reduction in the friction developed between the reinforcements and the saturated soil for backfills meeting the strict gradation requirements for Mechanically Stabilized Earth (MSE) backfill. The excellent performance of saturated MSE structures is due to the following: a. The soils specified for construction are granular and are of high shear strength, generally > 36, yet 34 shear strength is typically used in design. b. The soils are compacted during construction to relatively high densities while at or slightly dry of optimum moisture content. c. The soils are subjected to low strain levels in service and are normally stressed in plane strain. d. There is no mechanism for excess pore pressures to develop because of the use of reasonably free draining backfills. 3

7 The following AASHTO requirements assure that the frictional characteristics of the soil, and, therefore, the apparent coefficient of friction between the soil and reinforcements, will remain essentially unaffected by saturation. a. 0-15% passing the number 200 sieve (restricts the percentage of silt and clay particles in the soil and thus ensures a material that is relatively free draining). b. A minimum shear strength of 34 determined by direct shear tests on a sample of material compacted to 95% of standard proctor density at optimum moisture content. c. Plasticity index less than or equal to six (restricts the percentage of plastic clay particles). Soils meeting these restrictive requirements, in addition to the required particle durability testing, will not be susceptible to a significant reduction in shear strength when becoming fully saturated. Of course, the same holds true for the apparent coefficient of friction, f*, between the strip and soil, since this parameter is directly related to the shear strength of the soil. In 1982, Terre Armee Internationale researched the effects of saturated conditions on the shear strength of soils and on the apparent coefficient of friction, f*. One of the soils studied meets all of the requirements specified by AASHTO for MSE structures. A second soil meets all of the AASHTO requirements except that the plasticity index is high and the material is borderline with respect to the percent passing the #200 sieve (75 µm). The results of these tests are presented herein as follows: a. Table 1 presents index properties and direct shear results for each soil at both optimum moisture content and in a saturated, undrained condition. b. Figure 3 presents, for each soil, a gradation curve and the determination of the coefficient of uniformity, C u, and the corresponding f o * value (from Figure 2) to be used in design. c. Figure 4 presents a plot of the apparent coefficient of friction determined by pullout tests at various heights of overburden for both optimum moisture and saturated conditions. In addition, the f* envelope that would be used for design, based on the material's gradation, is also shown. When a soil sample is sheared under poorly drained conditions, the resulting shear strength is generally less than that obtained under well drained conditions. However, for soils meeting 4

8 AASHTO MSE backfill specifications, the decrease in shear strength is not significant. Although the apparent coefficient of friction decreases due to saturation, the reduced values remain well above the envelope used for design. In addition, in the design of structures that become submerged, the forces to be resisted generally include the effects of a minimum 3 foot drawdown (differential head). In the reinforcing strip resistance calculation, reduced overburden on the reinforcing strips due to soil buoyancy is also included. Therefore, the influence of water in a saturated structure is already addressed in the design. There is no adjustment necessary of the f o * value used in the design. IV. Influence of the use of uniform fine sands for wall backfill on the apparent coefficient of friction, f* Available research demonstrates that the equation f o * = Log C u is conservatively representative of the f o * value that should be used for design when using uniform materials for backfill. Florida DOT launched an extensive research program to study the use of uniform fine sands in the construction of MSE walls. Uniform beach sand with a fine to medium grain size is the predominant backfill material available in Florida to construct MSE walls. One portion of the research program consisted of reinforcement pullout testing in beach sand to study the apparent coefficient of friction, f*, for ribbed reinforcing strips. Figure 5, enclosed is from a paper written by two Florida DOT engineers on the results of this research. The paper is not published, however, it is available from Florida DOT or The Reinforced Earth Company. Note that all of the pullout test results can be conservatively enveloped by a line used for design of Reinforced Earth structures in Florida. The design envelope originating from f o * = 1.5 at the ground surface, decreasing to tan 30 at a 20 foot depth is a conservative envelope despite the backfill material having a coefficient of uniformity just less than 2.0. In fact, f* values between 2.0 and 2.5 were determined near the top of the structure in the pullout tests. This testing demonstrates that the equation f o * = log C u is conservative, even in the case of very uniform materials with a coefficient of uniformity less than 2.0. The results of similar studies on materials with a coefficient of uniformity less than 2 are shown in Figure 2. Clearly, the f o * values are greater than log C u in every case. C:\wp51\pla\techbull\techbul6.doc 5

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14 Table 1: Direct shear Properties of Soils at Optimum Moisture and in a Saturated Condition. Soil 1 Soil 2 Percent Passing #200 Sieve (i.e., silt 6% 15% and clay) Percent Finer than 15 µm 4% 12% (i.e., clay) Plasticity Index, PI Non-Plastic 14 * Direct Shear Test on Soil at Optimum Moisture Content **, Phi Angle Shear Test on Soil Saturated and Consolidated under 4000 psf * PI greater than AASHTO limit of 6 ** All shear tests performed under fast shearing conditions (1mm/min) Soil 1: Soil 2: Clean poorly graded sand Sand with some clay