Ulrich Engineers, Inc.

       civil engineers

                 specializing in geotechnics

cialists underground
 

Living with Deep Foundation Defects

By

Edward J. Ulrich, Jr.¹

Ulrich Engineers, Inc., 2901 Wilcrest, Suite 200, Houston, Texas 77042.

PH: 713.780.7645 FAX: 713.706.3686 EMAIL: edulrich@ulrichengineers.com

CELL:  281.635.5040

    Defect details in bored pile and drilled pier foundations are presented for multiple installation methods to a depth of 70 ft (22 m) in projects along the Texas Gulf Coast. Over 2000 bored pile and drilled pier foundations covering four projects are reviewed.     For nearly five decades drilled pier soldier pile walls have been an economic alternate to the reinforced concrete slurry wall which has enjoyed wide spread popularity across the North American Continent; hence the bored pile and drilled pier foundations used to form a basement wall on a project are numerous and are each exposed to the bottom basement level as construction proceeds. The experiences of design, preparation of construction documents and construction geotechnical engineering along with exposure allow a unique assessment of the design and construction compatibility of the completed drilled piers and bored piles. One project includes the observed limitations of low strain nondestructive testing to assess the presence of soldier pile defects for 24 in. (50 cm) diameter bored piles.

    Bored pile and drilled pier foundation defects are complex and do not lend themselves to easy assessment by non destructive testing. Accordingly, it is concluded that defects in bored pile and drilled pier foundation construction are inherent and that control of the construction means and methods is the only rational means available to build deep foundations with manageable defects. Hidden defect factors or reduced structural factors are needed for the design of deep foundations which will limit concrete carrying capacity below column design values because of the construction limitations, but the hidden defect factor cannot be a replacement for the control of construction means and methods by rational construction geotechnical engineering.

    Defects are inherent in the installation of deep foundations (Davisson, et al., 1984). Construction means and methods is the most salient variable in the development of successful deep foundations (Terzaghi, 1958; Ulrich, 1989-2007). Since a simple effective method to conclusively evaluate deep foundations for defects is still under development (MacNab et.al.,2000), control of the construction means and methods with relevant construction documents and construction geotechnical engineering still remains the most vital element of building successful deep foundations with defects that are manageable (Ulrich, 2007).     The defect details in bored pile and drilled pier foundations are presented for multiple installation methods to a depth of 70 ft (22 m) in projects along the Texas Gulf Coast. For nearly half a century drilled pier soldier pile walls have been an economic alternate to the reinforced concrete slurry wall; hence the drilled piers used to form a basement wall on a project are numerous and are each exposed to the bottom basement level as construction proceeds. The experiences of design, preparation of construction documents and construction geotechnical engineering along with exposure allows a unique assessment of the design and construction compatibility of the completed drilled piers and bored piles.

    Deep foundation defects are complex and do not lend themselves to easy assessment by non destructive testing. Accordingly, defects in deep foundation construction are inherent. Structural design should include a hidden defect factor or reduced resistance factors to limit concrete carrying capacity below column design values because of the construction limitations and the inadequacies of conclusive assessment of the as-installed deep foundations, but the hidden defect factor is not a replacement to the control of construction means and methods by rational construction geotechnical engineering.    The projects summarized are excavation wall systems installed to depths ranging from 20 to 70 ft with drilled pier soldier piles, bored piles and auger cast piles (ACIP) that formed the excavation wall systems, similar to the section shown in Figure 1 and illustrated in Figure 2. Over 2000 bored piles and drilled piers are reviewed in four projects. Subsurface conditions are of stiff to very stiff clay with sand inclusions and layers below the groundwater level. The depth to groundwater may range from 20 to 30 ft below the ground surface.

    Since the implantation of the modern American Concrete Institute (ACI) standards for the construction of drilled pier foundations in 1989, preparation of the project documents by the underground specialist, along with Geotechnical Construction Engineering, the author has been involved with bored pile and drilled pier projects that have included successes that would not have been possible using past local methods of design-build operation. Defects are inherent in deep foundations, especially drilled piers and bored piles, and successful deep foundations are those in which the defects have been reduced to manageable levels.

                                                        Figure 1. Section View of Excavation Wall System with Tieback Bracing – Texas Commerce Tower (Ulrich, 1989).

Figure 2. Cantilever Drilled Pier Excavation Wall System, 20 ft (6m) deep, 42 in. (100 cm) Diameter Piers - The Tallest Building in the Texas Medical Center (Ulrich, 2007)

    Texas Commerce Tower boasted Houston’s deepest basement excavation to 64 ft (20 m) with four parking levels when constructed in the early 1980’s (Ulrich, 1989, 1991). The basement excavation was restrained by 328 drilled pier soldier piles, 24 in. (60 cm) dia., to lengths of 50 (15m) and 70 ft (22) below street level. The soldier piles are termed cast-in-place-piles for diameters less than 30 in. (76cm) according to ACI definitions. Another 100 drilled piers at 42 in. (100 cm) diameter formed the interior basement wall for the Level 4 Basement.

Design. The soldier piles served double duty as excavation sheeting and the permanent basement wall. The closely spaced drilled pier soldier pile basement wall was Houston’s equivalent to a concrete slurry wall wherein the temporary and permanent walls are combined to enjoy cost savings that could not be achieved by installing a separate wall for construction conditions only. The wall system had many years of local experience since Houston Area contractors introduced the use of closely spaced drilled pier soldier piles as an economical alternative to H-Pile and lagging temporary wall systems for fifteen years prior to this project using diameters ranging from 18(46 cm) to 36 in (92cm). The soldier piles were designed to have clear spacing of 14 in. (36 cm) to facilitate the installation of four tiers of 12 in. (32 cm) diameter hollow stem augered tiebacks, Figure 1. The piers were cleaned of soils to the pier centerline and a concrete face wall was gunited to the piers as the excavation proceeded downward. The drilled pier soldier pile diameters ranged from 24 (60 cm) to 42 in. (100 cm). The 24 in. (60 cm) diam. soldier piles contained asymmetrical reinforcing totaling 1.3% to 2.0% of the soldier pile sectional diameter for pier lengths ranging from 46 ft to 70 ft. Concrete maximum coarse aggregate was 1.5 in. (4 cm). Concrete slump ranged from 3 (8 cm) to 5 in. (14 cm). Bar spacing was close, often less between 1.5 (4 cm) to 3 in (8 cm).

Soldier Pile Construction. Pier installation followed classical auger methods using temporary casing and concreting from the surface as the casing is withdrawn. The pier excavation proceeded by augering dry until wet sand was encountered and the wet hole method was engaged to facilitate casing installation in water bearing sand. When an adequate seal in the clay below 65 ft (20 m) could be obtained, the casing was cleaned out and the pile was advanced to the design penetration. In the event that an adequate casing seal could not be obtained at the desire penetration, the pile was advance to penetrations deeper than the design requirement. After the cased soldier pile was cleaned out, the reinforcing was set and the pile was concreted as the casing was withdrawn. Temporary casings reached 70 ft (22m).

    Project document preparation, construction, and Installation methods were consistent with local practice at the time of construction, circa1980. The geotechnical engineer did not participate in the preparation of project documents or provide geotechnical construction engineering. Modern changes to the ACI standards on drilled pier installation had not been formulated.

    Over 50% of the drilled pier soldier piles had extensive defects from the cap beam to the first tieback level, about 15 ft below street level, Figures 3 - 6. Although the defects were less frequent below the first tieback level, the frequency was severe. The results of Sonic Echo Testing (ADSC, 1998) of over 200 drilled pier soldier piles were found to be reliable in only 20% of the attempts when the soldier piles were uncovered as the basement excavation proceeded. Multiple tests were performed on several piles.

                                                        Figure 3. Defective Drilled Pier Soldier Piles with the mud intrusions and contaminated concrete cleaned out by shovel and low impact air spade.

                                                                                                                Figure 4. Soldier Piles Necked with Mud after Cleanout

Figure 5. Soldier Pile with Discontinuous Concrete at Reaction Plate from Large Coarse Aggregate and Low Slump

                                                                                                                Figure 6. Soldier Piles with Segregated Concrete and Defects Which Have Been Cleaned Out.

Bentonite Mud. Drilled Piers supported a one level basement wall and interior columns with diameters ranging from 36 in. (90 cm) to 48 in (122 cm) and lengths to 100 ft (33m). Project documents were prepared by the Architect before the modern American Concrete Institute standards were expanded to include pier installation by the Slurry Displacement Method. Piers were designed for side friction only and installation used Bentonite drilling mud. Bar spacing frequently was 2 to 3 times the maximum coarse aggregate size. The results of construction were disappointing with 20 percent of the piers classified as unacceptable, Figures 7 and 8. The concrete could not displace the slurry adequately when the slurry weight exceeded 100 pcf (1600 kg/m³), Figure 8.

Figure 7. Pier cleaned of mud intrusions and defective concrete using low impact power spade. Reinforcing spacing too tight.

Figure 8. Concrete Could Not Displace Mud Slurry Greater Than 100 Pcf.

Polymer Slurry. Nearly 400 drilled pier soldier piles, 30 in. (76 cm) diameter, were installed for an excavation in Rice University using the Slurry Displacement Method and a specification prepared by the design geotechnical engineer in compliance with the modern American Concrete Institute foundation standards. Polymer Drilling Fluid was used to form the drilling fluid. A slime ranging in thickness from ¼ to 1 in. separated the pier concrete from the formation, Figure 9. This experience once occurred in connection with a previous project (Ulrich & Ehlers, 1993). The slime had an undrained shear strength equivalent to very soft clay and could reduce the pier side friction by 100% if the entire pier was encapsulated. A cogent reason for the formation of the slime was not found.

Figure 9. Slime Encapsulated Several Piers Installed Using Polymer Drilling Fluid.

Air Voids. Discontinuities without soil or mud-in fills, Figure 10, have been found in a few piers installed by the slurry displacement method (Ulrich, 1989, 1993 & 2007). The voids often penetrate to ½ the pier diameter and may cover 1/3 to ½ of the circumference. The defects are attributed to air entrapped during the concreting process.

Figure 10. Discontinuities without mud in-fills have been found in some piers installed by the slurry displacement method.

Auger Cast Piles were installed for a rotary Car Dumper Excavation at a power plant in East Texas (Ulrich, 2003). The piles were 30 in. (76 cm) diameter by 80 ft (24 m) long. Four levels of tieback anchors were used to hold back the excavation wall. Tieback Levels 1 thru 3 are shown in Figure 11. Dense sand inclusions and layers were penetrated between the first and second level tiers. The installation specification and construction engineering followed Deep Foundation Institute requirements.

Figure 11. Irregular Pile Sections with Discontinuities between the First and Second Tieback Tier Levels Where Dense Sand Penetrated.

Bored pile and drilled pier foundation defects are complex and do not lend themselves to easy assessment by non destructive testing, especially if non-destructive testing is the sole means of evaluation of a dripped pile or pier defect. . The load carrying capacity of a bored pile or drilled pier is the sum of two components: the structural integrity of the foundation element and the relationship of the foundation element to the supporting earth mass, geotechnical capacity. Defects in deep foundations are inherent even if construction means and methods are flawless. Control of the construction means and methods is the only rational means available to build deep foundations with manageable defects.

Control of construction means and methods begins with the development of rational project documents by the foundation specialist along with construction geotechnical engineering to provide compliance with the project documents, assess insitu conditions related to design, and guide the needed design modifications to cope with actual site conditions. Successful deep foundation projects with a unique foundation design approaches using the method described above should be expected Figure 2 (Ulrich, 1993, 2007).

Hidden defect factors applied to column design or structural design factors reduced from those engaged for column design are needed for the design of deep foundations (Davisson, et. Al, 1984) to limit concrete carrying capacity below that of column design values because the limitations of modern construction are still more severe than in column construction. Defects are inherent to deep foundation construction and only though the rational control of construction means and methods can the defects be reduced to manageable magnitudes. Although deep foundation defects are inherent, the hidden defect factor cannot be a replacement for project documents governed by the design geotechnical engineer and the control of construction means and methods by rational construction geotechnical engineering.

Non-destructive testing is still developing and the presentation of conclusive correlations of tested and exposed foundations is encouraged to develop a strong industry confidence.

The Modern ACI standards and reports published by ACI Committees 336 and 543 prior to 2002 serve as appropriate guides in the successful design and construction of drilled piers and bored piles.

REFERENCES

Davisson, M.T., Manuel, F.S., and Armstrong, R. M. [1984]. Allowable S Stresses in Piles, FHWA-RD-83-059, and Report No: HRD-10/2-84 (1010) EW, Dept. of Transportation, FHA; Springfield, Va.:179 pp.

Macnab, A., et, al. [2000]. "Nondestructive Evaluation Of Drilled Shafts," Report of a Task Force Sponsored by the Geo-Institute Deep Foundations Committee, Journal of the Geotechnical and Geoenvironmental Engineering, Vol. 126, No. 1, January, ASCE, pp 92-95.

Terzaghi, K. (1958). Consultants, Clients, and Contractors, Journal of the Boston Society of Civil Engineers, Vol. 45, No. 1, pp 1-41.

Ulrich, E. J. (1989). "Tieback Supported Cuts in Overconsolidated Soils," Journal of Geotechnical Engineering, ASCE, Vol. 115, No. 4.

Ulrich, E. J. (1991). "Subgrade Reaction in Mat Foundation Design," Concrete International, American Concrete Institute, Vol. 13, No. 4, April, pp 41-50.

Ulrich, E. J. (1993). "Tallest Building in the Texas Medical Center-St. Luke's Medical Tower," SP -152, Design and Performance of Mat foundations state of the art review, American Concrete Institute, Farmington Hills, MI , pp 1-267.

Ulrich, E. J. (1994), Session Chairman and Editor. SP -152, Design and Performance of Mat foundations state of the art review, American Concrete Institute, Farmington Hills, MI , pp 1-267.

Ulrich, E. J. (2000). Arrest of a Texas Landslide, Concrete International, ACI, Farmington Hills, MI.

Ulrich, E. J. (2003). Abyss in East Texas, Concrete International, ACI, Farmington Hills.

Ulrich, E. J. (2007). The Tallest Building in the Texas Medical Center: Memorial Hermann Medical Tower, Geo-Denver: New Peaks in Geotechnics, Proceedings of the Geo-Denver 2007 Congress, ASCE, Denver, February.