Summary of Earth Retaining Methods Utilizing Helical Anchors

Transcription

1 Summary of Earth Retaining Methods Utilizing Helical Anchors by Howard A. Perko, PE Consulting Engineer for Magnum Piering, Inc. March 4, 1999 (Revised November 11, 1999 and July 24, 2001) Abstract A summary of helical anchor use in retaining wall systems is presented. The summary includes engineering analysis of helical anchor capacity and an example retaining wall design. Previous research is discussed particularly regarding effects of inclination angle on pullout capacity, performance in soft clays, performance in sands, and stress-strain behavior. Introduction Tie-back, earth retaining walls are commonly restrained using grouted anchors. Another technology for retaining wall restraint is the use of helical anchors. A helical anchor consists of one or more helically shaped, galvanized steel blades attached to an elongate, central, galvanized steel shaft with square, tubular or round cross-section. A schematic diagram of an example helical anchor is shown in Fig. 1. The shaft is turned into the ground by application of torsion using a truck mounted auger or a torque head attached to a backhoe or front-end loader. Once the blades are advanced to the appropriate depth, they offer significant pull-out resistance. Tensile loads as high as 100 kips are achievable for particular anchors in certain soils. Most non-grouted manufactured helical anchors have a maximum ultimate pullout capacity of between 35 and 80 kips. Helical anchors offer numerous advantages over conventional grouted anchors. Installation of helical anchors progresses rapidly, and post tensioning can be immediately performed without waiting for grout to set. A helical anchor installed in an incorrect location can be easily removed and reinstalled. Likewise, helical anchors can be removed and salvaged if desired, such as in the case of a temporary bracing. Helical anchors can be installed in any weather and in limited access situations with commonly available equipment and smaller crews. Fig. 1 Example Helical Anchor -1-

2 Helical Anchor Capacity The pullout capacities of horizontally oriented helical anchors for retaining wall restraint can be determined the same way the uplift capacities of vertically installed helical piers are found (Ghaly and Clemence, 1998, A.B. Chance, 1993). There are three methods for predicting pullout capacity, namely cylindrical shear, individual bearing, and an empirical method based on installation torque. It has been shown that the empirical method yields more consistent results (Hoyt and Clemence, 1989). However, it is suggested that the cylindrical shear and individual bearing methods be used to determine minimum allowable blade areas and the installation torque method be used as a field verification of capacity. For retaining walls in critical areas where human life may be jeopardized by a failure, the capacity of helical anchors should be checked through post-tensioning. Cylindrical Shear Method In the cylindrical shear method, the entire volume of soil between the helical blades is assumed to be mobilized. Ultimate pullout capacity of a multi-blade anchor is a combination of shear along the cylinder of soil between the blades and bearing capacity of the top blade, given by (modified from Mitsch and Clemence, 1985, and Clemence, 1985) where R is average blade radius, R T is top blade radius, L is total spacing between all blades, F v is the vertical soil pressure at the helices, c is soil cohesion, N is angle of internal friction of the soil, and N c and N q are bearing capacity factors for general shear. (1) Individual Bearing Method The assumed failure mechanism in the individual bearing method consists of each helical blade displacing the overburden soil in a logarithmic spiral mode. Thus, the capacity of each blade can be estimated using the wellknown Terzaghi bearing capacity equation. Contributions of soil unit weight are ignored for uplift. Total ultimate pullout capacity is the sum over N blades, given by (modified from A.B. Chance, 1995) where A n is blade area. All other parameters have been defined previously. In both Eq. (1) and (2), it is important that capacity not increase with depth of embedment to impossible values. It is recommended that effective vertical soil pressure be limited to that imposed by soil about 15 times the average blade diameter above the helical blades in accordance with standard practice for deep bearing members (Bowles, 1988). Installation Torque Method Based on the empirical method, anchor pullout capacity is given by where K is the torque to capacity ratio and T is final installation torque. This method should only be applied when the depth to helix diameter ratio is at least 5. The value of K depends on the geometry of the helical anchor. For anchors with square shaft diameters less than 2 in, a value of 10 ft -1 is recommended by Hoyt and Clemence (1989). Manufacturer recommended K values should be used for other helical anchor geometries. Magnum Piering, Inc. recommends a value of 8 ft -1 for their 3-inch O.D. round-shaft helical (2) (3) -2-

3 anchors. A factor of safety of 3.0 is commonly used in bearing capacity calculations. However, when the foundation installation process includes an indirect measurement of soil strength at the foundation depth, a smaller factor of safety is permissible. A traditional example of this is pile driving where a much lower factor of safety is often allowed. The American Society of Civil Engineers (1996) explains that a factor of safety of 1.5 is acceptable for pile foundations. Since the instillation torque of helical anchors also provides an indication of soil strength at the depth of the helices, a lower factor of safety is permissible for allowable pullout capacity calculations. Typically a factor of safety of 2.0 is used in helical anchor design. Effect of Inclination Angle Gahly and Clemence (1998) showed theoretically and experimentally that the pullout capacity of helical anchors installed in sand at an angle is greater than that of vertical anchors. This difference was explained by the development of a larger zone of soil mobilization. However, in the case of retaining walls, it is anticipated that this effect is canceled out by the infringement of the larger zone of mobilized soil with the active soil wedge. Additional study is required. At present, it is recommended that the effect of inclination angle and increased strength be ignored in order to be conservative. Anchors in Soft Clays Laboratory experiments were conducted on model earth anchors in a cylindrical clay filled test cylinder by Rao and Prasad (1993), Rao, Prasad, and Shetty (1991), and Rao, Prasad, and Veeresh (1993). The blade spacing to diameter ratio was varied between approximately 1 and 5 for the model anchors. Effects of blade spacing on cylindrical shear pullout were analyzed. Experimental results indicated that at a blade spacing to diameter ratio of 1.5, the anchors exhibited individual bearing failure rather then cylindrical shear. The foregoing experiments were conducted on small laboratory model anchors. The maximum blade diameter was approximately 6 inches. Since cylindrical shear increases with R and plate bearing capacity with R 2, it is believed that the optimal blade spacing to blade diameter ratio increases for larger diameter helical anchors. Typically, helical anchors are manufactured with a blade spacing to blade diameter ratio of 3. Stress and Strain Behavior Ghaly and Hanna (1992), and Ghaly, Hanna, and Hanna (1991) tested miniature helical anchors with different geometries in a sand filled testing tank equipped with stress transducers. It was determined that blade geometry had a significant effect on the installation torque of the helical anchors, but had little effect on the pullout capacity. This result indicates that helical blade configuration and geometry must be taken into consideration when using the empirical method of determining pullout capacity based on installation torque. In the same study, it was determined that the zone of soil stress-strain influence surrounding the blades of a helical anchor experiencing 90% of its designed pullout capacity is limited by the ratio of depth to blade diameter and by the density of the surrounding sand. In particular, a transition between significant and minimal strain occurred at depth to blade diameter ratios of 7, 9, and 11 for loose, medium, and dense sand, respectively. It can be interpreted from these results that helical anchors should be extended to distances considerably beyond the anticipated active wedge of retained earth, such that these ratios are exceeded. In doing so, the zone of strain -3-

4 influence due to anchor pullout should not overlap the active wedge of retained earth. In cohesive soils, it is believed that the transition between significant and minimal strain occurs at smaller depth to blade diameter ratios as compared to that in sand. The transition between individual bearing limit state and cylindrical shear limit state at a depth to diameter ratio of 1.5 as discussed previously is an example of this phenomenon in clay. It is possible that helical anchors need only be extended a distance beyond the active zone of a retaining wall equal to 1.5 to 3 times the helical blade diameter. Until more testing is available, a distance of 5 times the helical blade diameter is suggested in clay soils. Fig. 2 Example Retaining Wall Design Example The usual procedure for design of a helical anchor retaining wall is as follows. I. Construct an earth pressure diagram II. Determine helical anchor spacing III. Compute helical anchor ultimate capacity IV. Determine required minimum anchor blade diameter(s) V. Specify minimum length of helical anchors VI. Check global stability VII. Design a retaining wall facing VIII. Select a suitable foundation for the wall facing IX. Specify surface and ground water drainage systems Most of these steps are similar to procedures used in all types of earth retaining wall design and can be found in a variety of texts on the subject such as Abramson, et al. (1995). In the following examples, certain methods specific to helical anchors are discussed in more detail. A section view of an example earth retaining wall design is shown in Fig. 2. In this example, the size and configuration of helical anchors is given. Their capacity needs to be determined. Helical earth anchors are spaced along the wall at the locations shown. The soil has a unit weight of 120 pcf, friction angle of 30 o, and no cohesion. This friction angle corresponds to N q equal to 23. The anchors have two 12-inch diameter blades spaced 2 feet apart and have a capacity:torque ratio of 8 ft -1. The anchors are oriented 15 degrees from horizontal and are approximately 6 feet below the ground surface. The anchors are installed to a distance of 9 feet beyond the theoretical active zone and to a minimum torque of 2,500 ft-lbs. The ultimate pullout capacity of one anchor by the cylindrical shear method is given by -4-

5 (4) where the vertical stress was assumed constant (5) over the entire length of the anchor for simplicity. By the individual bearing method, the pullout capacity of one anchor is (6) (7) In most cases, the cylindrical shear and individual bearing methods yield similar results. In this example, the strength in cylindrical shear was compromised by the shallow depth of the anchors and the absence of soil cohesion. The computed ultimate capacity of the helical anchor is always taken as the more conservative result of the two methods. Finally, if the helical anchors are installed to the recommended torque, the pullout capacity by the installation torque method is (8) (9) The capacity of helical anchors determined through the installation torque method has been shown to more consistently match field capacity tests in comparison with other methods. Reasons for this may include limited reliability of soil strength information, variations in soil strength within the ground, and non-isotropic states of stress. However, the results obtained using the installation torque method should not be far afield from the other more traditional foundation capacity methods. An engineer should weigh the results obtained from the cylindrical shear and individual bearing methods against the reliability of geotechnical information and the variability of soil consistency observed in exploratory borings and judge whether the capacity determined by the installation torque method is reasonable. Furthermore, the installation torque method is only valid if a deep mode of failure governs helical anchor capacity. In the foregoing example, the helical anchor blades are 9 diameters beyond the active zone. This distance is sufficient to insure a deep mode of failure for a medium dense sand (see previous section on stress and strain behavior). Furthermore, the capacity determined through torque correlations is near enough to that determined from cylindrical shear that it is reasonable. Thus, the allowable pullout capacity for each anchor using a factor of safety of 2.0 is (10) The capacity of the anchor is in a direction parallel with the anchor shaft. The angle of the anchor must be taken into account in the static force diagram. The allowable stress in the horizontal direction is (11) The downward component of this force must be taken into account in the design of a suitable foundation for the retaining wall facing. Helix piers provide a convenient foundation for helical anchor retaining walls. A remaining step in helical anchor design is to verify that the anchor itself is sufficient to withstand the calculated pullout capacity. Helical anchors are typically manufactured of high strength carbon steel having an ultimate tensile strength in the range of 35,000 to 70,000 psi. In this example, the helical anchor shaft must have a cross-sectional area of at least 0.07 to 0.14 in 2. Strength of the helical anchor section connectors should also be checked. Most helical anchor manufacturers provide -5-

6 information on the mechanical strength of their products. Friction along the shaft of a helical anchor also contributes to the pullout capacity (Gahly and Clemence, 1998). Deep, largediameter-shaft, helical anchors may develop a considerable portion of their strength from the shaft to soil interface. Since high strains result from turning during installation, residual shear strength parameters are appropriate. Wobbling during installation must also be taken into consideration, since it causes the soil to separate from along the anchor shaft. For short helical anchors, the adhesion and friction along the anchor shaft is anticipated to add only minimal additional strength. Therefore, these factors were ignored in the foregoing example. cohesion of 3,000 psf, and no friction (undrained rapid failure mode). Zero friction angle corresponds to N c equal to 6 and N cq equal to 1. The anchors are to be 6 and 12 feet below the ground surface. The minimum length of the anchors beyond the active zone was discussed in the previous section on stress and strain behavior. The results of the research by Ghaly and Hanna (1992) is summarized in Table 1. In the example, the site is characterized by clay soils. Therefore, the helical blades need to be a minimum of 5 feet beyond the active zone. Table 1. Minimum Length Beyond Active Zone Soil Condition Embedment Length Clay Loose Sand Medium Sand Dense Sand 5 ft 7 ft 9 ft 11 ft To determine the minimum helical blade requirements, an assumption is made about the blade areas, and the number of blades is computed by solving Eq. (2) for N. Fig. 3 Example Retaining Wall A section view of another example earth retaining wall design is shown in Fig. 3. In this example, the size and configuration of helical anchors is unknown and need to be determined. The minimum length of the anchors beyond the active zone also needs to be determined. From Peck s apparent pressure diagram, it has been determined that each anchor is subject to 30 kips force in the direction along the anchor shaft. The soil is a clay with a unit weight of 120 pcf, For the example, the number of 12-inch diameter blades required is where the vertical stress for the shallowest helical anchor was used to be conservative. In helical anchor retaining wall design, it is better practice to round-down the number of blades and implement a minimum installation (12) (13) -6-

7 torque requirement to verify capacity. This practice causes the helical anchors to be installed deeper with less blades rather than shallower with more blades. For the example retaining wall, a minimum of three (3) 12-inch diameter helical blades are required. The minimum installation torque criteria is typically specified on the plans in a statement such as, Helical anchors shall be installed to a minimum torque indicative of 30 kips allowable capacity. Manufacturer recommended capacity:torque ratio shall be used with a factor of safety of 2.0. The minimum required torque shall be met and maintained or exceeded for a distance of at least five feet. It is important in helical anchor design to specify that the required minimum torque be obtained and sustained for some distance. This ensures that the soil above the helical blades is at least as stiff/dense as the soils surrounding the helical blades. This distance should be at least five feet. The reinforced facing used in helical anchor retaining walls can be multi-layer or single layer reinforced shotcrete, precast panels, or any other structurally suitable system. Conclusions The use of helical anchors in earth retaining wall restraint is a viable alternative to grouted earth anchors that offers many advantages including ease of installation, immediate post tensioning, penetration through ground water and caving soils, removal, and reuse. There are three methods for determining anchor pullout capacity: cylindrical shear, individual bearing, and installation torque. When designing a helical anchor retaining wall, pullout capacity should be approximated using the most conservative result obtained from cylindrical shear and individual bearing methods. Installation torque should be used as a final field verification of helical anchor capacity. Effects of anchor inclination on pullout capacity are minimal. Helical anchors should be extended beyond the theoretical zone of active soil failure behind a retaining wall by certain distances that depend on soil type. Minimum required torque should be maintained for a distance of at least 5 feet. References Abramson, L.W., et al. (1995) Slope Stability and Stabilization Methods, Wiley Interscience, New York A.B. Chance, Co. (1993). Tension Anchor System for Tieback Applications. Manufacturer Technical Support Document, Centralia, MO. A.B. Chance, Co. (1995). Sample Calculations for Helical Pier Application. Manufacturer Technical Support Document, Centralia, MO. American Society of Civil Engineers (1996). Standard Guidelines for Design and Installation of Pile Foundations Publication No , ASCE Press, Reston, VA. Bowles, J.E. (1988) Foundation Analysis and Design, 4 th Edition, McGraw-Hill, Inc., New York Clemence, S.P. (1985). Uplift Behavior of Anchor Foundations in Soil. Proceedings of a Session Sponsored by the Geotechnical Eng. Div. of ASCE, Detroit, MI. Ghaly, A. and Hanna, A. (1992). Stresses and Strains Around Helical Screw Anchors in Sand. Soils and Foundations, Vol. 32, No. 4, pp Ghaly, A.M. and Clemence, S.P. (1998). Pullout Performance of Inclined Helical Screw Anchors in Sand. Journal of Geotechnical and Geoenvironmental Engineering, Vol. 124, No. 7, ASCE, Reston, VA, pp Hansen, J.B. (1970). A Revised and Extended Formula for Bearing Capacity. Danish Geotechnical Institute, Bulletin No. 28, Copenhagen. Hoyt, R.M. and Clemence, S.P. (1989). Uplift Capacity -7-

8 of Helical Anchors in Soil. Proceedings of the 12 th International Conference on Soil Mechanics and Foundation Engineering, Rio de Janeiro, Brazil. Mitsch, M.P., and Clemence, S.P. (1985). Uplift Behavior of Anchor Foundations in Soil. Journal of Geotechnical Engineering, ASCE, pp Rao, S.N., Prasad, Y.V.S.N., and Shetty, M.D. (1991). Behavior of Model Screw Piles in Cohesive Soils. Soils and Foundations, Vol. 31, No. 2, pp Rao, S.N., Prasad, Y.V.S.N., and Veeresh, C. (1993). Behavior of Embedded Model Screw Anchors in Soft Clays. Geotechnique, Vol. 43, No. 4, pp Rao, N.S. and Prasad, Y.V.S.N. (1993). Estimation of Uplift Capacity of Helical Anchors in Clays. Journal of Geotechnical Engineering, Vol. 119, No. 2, ASCE, pp