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Geotechnical Engineering Circular (GEC) No. 8
Design And Construction Of Continuous Flight Auger Piles
Final
April 2007

Chapter 6: Recommended Design Method

The purpose of this chapter is to present a step-by-step generalized method for the selection and design of CFA piles.

The process consists of the following design steps:

  • Step 1: Initial Design Considerations
  • Step 2: Comparison and Selection of Deep Foundation Alternatives
  • Step 3: Selection of Pile Length and Assessment of Pile Performance under Specified Loads:
    • Calculation of Pile Length
    • Verification of Capacity and Performance for Axial and Lateral Loads
    • Verification of Pile Group Capacity and Group Settlement Calculations
    • Pile Structural Design
    • Miscellaneous Considerations
  • Step 4: Review of Constructability
  • Step 5: Preparation of Plans and Construction Specifications, Set QC/QA and Load Testing Requirements

The remainder of this chapter presents an outline of each of the design steps listed above and presents preliminary discussions of the most salient aspects of the design.

6.1 Step 1: Initial Design Considerations

The initial design considerations include a review of structure-specific and site-specific conditions for the project that are necessary for any foundation design. In this chapter, attention is focused specifically on those items that establish or preclude suitability of CFA piles for the project. Step 1 is subdivided into several components as described below.

6.1.1 General Structural Foundation Requirements

The first step in the entire process is to determine the general structural requirements for the foundation. Some important considerations include the following:

  • Project type: new bridge, replacement bridge, bridge widening, retaining wall, noise wall, sign or light standard. CFA piles may be considered for any of the above.
  • Construction sequencing: phased construction or all at once. Neither condition either precludes or favors CFA piles.
  • General structure layout and approach grades.
  • Surficial site characteristics. A stable working platform is required for CFA pile construction.
  • Special design events such as seismic, scour, vessel impact, etc. These factors should be considered in planning the site investigation and can have a significant effect on the selection of CFA piles.
  • Possible modifications to the structure that may be desirable for the site under consideration.
  • Approximate foundation loads and limitations on deformation.
6.1.2 Site Geology and Subsurface Conditions

A comprehensive review of this component of Step 1 is the subject of other FHWA publications (e.g., "Subsurface Investigations," document FHWA-HI-97-021, and National Highway Institute [NHI] Workshop on Soils and Foundations, document FHWA-NHI-66-083, and GEC No. 5: "Evaluation of Soil and Rock Properties, document FHWA-IF-02-034 authored by Sabatini et al. [2002]) and will not be repeated here in detail. In general, the components of the site characterization include a review of the site geology and foundation experience in the area, followed by a carefully planned and executed subsurface exploration program. In general, the consideration of CFA piles does not require specialized investigation techniques differing from those used for driven pile foundations. Important considerations for CFA piles include the general site stratigraphy and soil classification, the depth and characteristics of the most likely bearing formation, groundwater conditions, variability, and the presence and extent of unusual geologic features such as solution cavities, boulders, lenses, or layers of hard rock.

The use of cone penetration testing (CPT) is generally considered to be particularly well suited for design procedures used for CFA piles, but especially for drilled displacement (DD) piles. CPT soundings provide a continuous record of a strength measurement that correlates well with CFA and DD pile performance, CPT soundings can generally be performed in soils where CFA or DD piles are to be considered. It is also a very cost-effective tool compared to conventional borings for sounding a large area. Where conditions are such that CFA piles may be considered as a viable foundation alternates, the use of CPT soundings is recommended and encouraged as a part of the exploration program.

6.2 Step 2: Comparison and Selection of Deep Foundation Alternatives

The information from Step 1 must be evaluated and a foundation system selected. Alternatives to deep foundations may be considered, including shallow foundation systems and the potential use of ground improvement techniques to allow the use of shallow foundations. Where deep foundations are required, alternatives include driven piles, drilled shafts, micropiles, and CFA piles including both conventional CFA piles and DD piles. The selection of the optimum deep foundation system for a given project includes consideration of multiple factors and requires experience and judgment on the part of the designer. Table 6.1 outlines many of the considerations involved in the foundation selection process with respect to CFA piles.

Table 6.1: Design Consideration for Foundation Selection of CFA and DD Piles
Stratigraphic Favorable Ground Conditions? Type of Profile GWT Location Other Factors
Predominantly Clays: favor CFAVery soft surface may be unfavorable due to poor working platformIf below existing grade, not much of a factor in clays.Depth to competent material > 30 m (100 ft): not favorable
Sands: favor DDUniform or similar strata: favorableGWT depth > 3m (10 ft): favorableBoulders, rock layers or lenses: not favorable
Highly variable drilling resistance: not favorableArtesian conditions: not favorableGood working platform is especially important for DD because DD rigs are usually heavier than CFA
Cemented soils, weak rock: favor CFAHighly variable drilling resistance: not favorableArtesian conditions: not favorableBoulders, rock layers or lenses: not favorable
Rock: not favorable for CFA or DD------
Structural Loading Conditions Other Conditions
Approximate maximum ultimate lateral loads per vertical pile (kip) vs. recommended diameter (in.)
  • Low headroom requirements: may favor CFA piles
  • Close proximity to existing structures: not favorable to CFA or DD due to potential ground movements during construction
  • Noise and vibration considerations: may favor CFA or DD piles vs. driven piles
  • Potential obstructions below grade: not favorable to CFA or DD piles
soft or loose soil:
  • 12 kip - 18" dia.
  • 20 kip - 24" dia.
  • 30 kip - 30" dia.
  • 45 kip - 36" dia.
Stiff or dense soil:
  • 20 kip - 18" dia.
  • 35 kip - 24" dia.
  • 50 kip - 30" dia.
  • 70 kip - 36" dia.
Approximate maximum ultimate axial compressive loads per pile (kip) vs. recommended diameter
  • 400 kip - 18" dia.
  • 700 kip - 24" dia.
  • 1,000 kip - 30" dia.
  • 1,500 kip - 36" dia.
Special Considerations Seismic Scour Contaminated Spoils Over Water
Lateral subsurface soil movements from seismic events produce lateral load conditions not favorable to CFA or DD Deep scour may result in high moments at depth: not favorable to CFA or DD Contaminated ground conditions: may favor DD due to avoidance of spoils Work over water: not favorable to CFA or DD piles
Economic Factors Qualified Local Subcontractors Available? Relative Numbers of Piles Project Size
Necessary for CFA or DDLarge numbers of piles: favorable to CFA or DDSmall project with few piles and many moves: may not favor CFA or DD

The subsequent steps are presented for CFA piles and DD piles. Alternative deep foundation types are described in existing FHWA design manuals for drilled shaft foundations (e.g., FHWA Report No. IF-99-025 by O'Neill and Reese [1999]), for driven pile foundations (e.g., Publication for NHI Course FHWA-NHI-132021 by Hannigan et al. [2006]), and for micropile foundations (e.g., Publication NHI-05-039 by Sabatini et al. [2005]).

6.3 Step 3: Select pile length and calculate performance under specified loads

6.3.1 Limit States for Design

The design method presented is based on an Allowable Stress Design (ASD) for geotechnical conditions, in which a factor of safety is applied to ultimate limit state conditions to obtain allowable resistance values for design. Resistance factors for Load Resistance Factored Design (LRFD) have not been calibrated for geotechnical aspects of CFA pile design. Structural design of the pile is in accordance with LRFD (AASHTO, 2004) as for other reinforced concrete structural elements.

In general, there are three limit state conditions that must be satisfied for design of CFA and other deep foundations:

  1. Geotechnical Ultimate Limit State (GULS). The pile should have a load resistance that is greater than the expected loads (service loads) by an adequate margin to provide a required level of safety (safety factor). For axial compressive loads, the GULS is defined as the load resistance at a displacement equal to 5% of the pile diameter in an axial static load test, as shown in Figure 6.1. The Davisson criterion, commonly used for driven piles, and shown for reference in Figure 6.1, will sometimes underestimate the ultimate resistance and is not appropriate for CFA piles. For lateral loads, the GULS may be defined as a push-over failure of the foundation or alternately as some deflection limit at which collapse of the structure above the foundation may occur. Uplift loading conditions and group behavior for axial, rotation, or lateral are additional geotechnical ultimate limit states that may control in some cases. The GULS for preliminary design is determined by calculations, and these calculations may be refined based on site-specific load testing. Note that the GULS is often referenced using the words "capacity" or "failure", which are a poor choice of words, because no collapse or condition of plunging may exist at the GULS and the pile may have a capacity to support additional loads beyond the GULS. The state of deformation associated with the GULS is not to be confused with deformations at service load levels. The GULS provides a definition of foundation resistance.
  2. Service Limit State (SLS). The pile should undergo deformations at service load levels that are within the tolerable limits appropriate for the structure. The actual definition of the service limits should be determined by a rational assessment of the sensitivity of the structure to deformations. Short-term deformations for transient loadings are a function of the mobilization of pile resistance as indicated in Figure 6.1. However, long-term settlements under structural dead loads are a function of group settlements and should be computed accordingly (as described in Chapter 5). For bridge structures, the serviceability requirements for deformations are in accordance with AASHTO (2002) Section 4.4.7.2.5. Settlements should generally be such that angular distortions between adjacent foundations do not exceed 0.008 in simple spans and 0.004 in continuous spans. It should be noted that only post-construction settlements affect serviceability of the bridge structure. Tolerable movement criteria for lateral displacement should be developed considering the effects of combined lateral and vertical displacements on the structure. AASHTO (2002) Section 4.4.7.2.5 requires that horizontal movements be limited to 25 mm (1 in.) where combined horizontal and vertical movements are possible, and be limited to 38 mm (1.5 in.) when vertical movements are small relative to horizontal movements.
  3. Structural Ultimate Limit State (SULS). The pile must have sufficient structural capacity when the pile is subjected to combined axial and flexural loads such that structural yielding of the pile is avoided. The SULS provides a second definition of foundation strength.

Figure 6.1: GULS and Short-Term SLS for Axial Load on a Single CFA Pile

Illustration showing the geotechnical ultimate limit state (GULS) and the short-term loading (SLS) for axial load on a single CFA pile.

Most engineers using ASD methods are familiar with the concept of a factor of safety, which divides the resistance at the GULS to determine an allowable load per pile. The service (unfactored design) loads are compared against the allowable loads per pile to evaluate strength and provide the margin of safety against the limit-state condition (often referred to as "failure").

For design of CFA piles using ASD, the resistance computed at the GULS is compared with service loads such that:

(Equation 6.1)

Σ Qi ≤ R / SF

Where:

Qi=value of service (unfactored) load of type i
R=computed resistance at GULS
SF=factor of safety

CFA piles should generally be designed having a factor of safety of at least 2.5. Lower factors of safety are warranted where site-specific load tests are performed and QA/QC systems are used to provide verification that the performance of production piles is reasonably consistent with that of the test pile(s). A factor of safety of 2.0 may be used for design for axial loads provided that the following conditions are met:

  1. At least one conventional static load test (per ASTM Standard D1143-81) is performed at the site, to a load exceeding the computed ultimate by 50% or to a load producing displacement equal to 5% of the pile diameter, whichever comes first. Multiple load tests are required where the site extends for a great distance of roadway structure or if, in the judgment of the project geotechnical engineer, the site spans across significant geologic or stratigraphic differences in site conditions. Dynamic or rapid load tests, as described in Chapter 7, may be substituted for some of the conventional static load tests and their results can be correlated to static test or local conditions.
  2. Automated monitoring systems are used on production piles to verify that production piles are constructed in a similar way and to achieve similar performance in the test pile(s).
  3. The site geology, stratigraphy, and soil properties are not highly variable. Engineering judgment must apply in this instance, and higher factors of safety for design are warranted for conditions with unusual variability in soil properties or geologic conditions. It is acceptable to use a higher factor of safety for some portion of the computed resistance. For example, base resistance or the resistance within some deeper strong layer may be considered as more variable than other portions of the profile that contribute to resistance and therefore, an engineer may apply judgment to assign a higher factor of safety to this portion of the resistance.
  4. The site conditions do not pose difficult construction conditions for CFA piles.

Note that load testing and monitoring as described above should be required on all transportation projects constructed using CFA or DD piles. Exceptions may include soundwalls, sign foundations, or similar structures for which the design is not controlled by axial resistance.

It is also important to note that the foundation design engineer should consider the factors of safety cited above as minimum values, and should use a higher factor of safety in any circumstance where there exists concerns for site variability, lack of redundancy, or potentially difficult construction conditions. An especially critical structure such as a lifeline structure for hurricane evacuation or seismic safety concerns may also warrant a higher safety factor. The values suggested above are for typical conditions and routine projects.

6.3.2 Design Procedures

To complete a design, it is necessary to perform static analyses and estimate pile lengths and pile diameters necessary to provide the required compression, lateral, and uplift load capacity. For most routine projects, it is anticipated that the ultimate limit state (capacity) conditions usually control the design of individual CFA piles rather than serviceability (deflection) requirements, therefore, the most efficient approach for design is to check first ultimate limit state conditions and then serviceability. Note that some instances of serviceability may control, such as with large pile groups over a deeper compressible stratum.

In general, smaller diameter and greater length piles tend to be more economical than larger diameter, shorter length piles with similar axial resistance. However, pile diameter is often controlled by lateral shear and bending moment considerations. Lateral load considerations almost never control the length of CFA piles because the soil resistance mobilized to resist lateral loads tends to be within the shallow strata extending to a depth no more than about 10-pile diameters. If significant lateral loads (per pile) are anticipated, the design is accomplished by first performing an initial analysis of lateral loading to define the required diameter and also the depth to the point where calculated bending moments are negligible. This lateral check is followed by static analysis of axial load capacity to define the pile length requirements.

Note that a range of diameters and lengths may be considered for groups of piles, as it is feasible to consider foundations with larger numbers of smaller capacity piles vs. fewer numbers of larger capacity piles. At this stage in the design, it is appropriate to consider a range of pile capacities for possible different group configurations. The procedures described in Chapter 5 are used to perform the analyses associated with determining axial and lateral resistance of various design alternatives.

The recommended procedure for performing a foundation design of CFA or DD piles is outlined below. This outline is developed for the design of a foundation for a structure such as a bridge. A similar procedure for sign foundations or wall components may differ in specifics of some components, but will follow a similar general outline.

  1. Develop Idealized Profile. Using the borings and geologic descriptions from the site investigation program, group the borings into zones for foundation design according to similarities in the soil profile and properties, and establish idealized geotechnical design profiles for each representative zone at the site. The differing zones should adequately cover the range of conditions at the site, and the designers may develop multiple profiles for each zone to evaluate the possible range of geotechnical (and groundwater) conditions within each identified zone. It may also be necessary to consider several cases of scour associated with different loading conditions. The layers and characterization of soils should be consistent with the methods for estimating axial and lateral load transfer as outlined in Chapter 5 of this document.
  2. Develop Geotechnical Design Parameters. For each stratum defined in the idealized profiles, evaluate the geotechnical strength and stiffness parameters to establish design parameters for each layer. These may include:
    1. Soil strength parameters such as undrained shear strength and drained friction angle or other measurements, such as SPT-N60 values and cone tip resistance. These strength parameters will be used either directly or through correlations to estimate unit side-shear and end-base resistances and to develop p-y curves for lateral loading.
    2. Soil stiffness or modulus and other parameters related to deformation characteristics for use in developing p-y curves for lateral loading and performing settlement analyses of pile groups.
    3. Other soil properties that may be needed for design, such as unit weights and index tests for classification.

    Note that the actual values used for design are typically based on judgment and experience along with an understanding of the site geology and potential variability (Sabatini et al., 2002).

  3. Obtain the Loadings for the Foundation. The design loadings will likely include several cases of both axial and lateral loads. Many different load combinations exist of dead loads, traffic loads, wind loads, etc. Some cases may be combined with scour and some may include extreme event loadings. If downdrag or uplift due to expansive soils is to be considered, these should be noted at this time for inclusion where appropriate into subsequent analysis and design steps. Analyses of the load combinations will reveal the maximum axial and lateral loads imposed to the pile and represent the critical design cases.
  4. Safety Factor(s) for Design. The safety factors cited in the previous section are suggested for general use in design for strength, and differ depending on the level of site-specific testing and quality control. Large values may be applied in cases where unusual variability in subsurface conditions exist, difficult construction conditions are likely, or if other considerations for the structure dictate. For example, where base resistance contributes a large portion of the axial resistance and the properties of the bearing stratum are quite variable, it may be appropriate to use a significantly greater safety factor on base resistance than side-shear even where a load test is performed.
  5. Select a Trial Design Pile Group to Establish Individual Pile Loads. The geometry and layout of the pile group, along with the number of piles used to support the foundation loadings will determine individual loads per pile. At this point in the process, experience and/or some preliminary estimates should suggest some reasonable values of nominal axial and lateral resistance for single piles so that an efficient layout can be developed. Engineers are encouraged to evaluate numerous alternatives in the preliminary design stage.
  6. Select a Trial Design for Individual CFA or Drilled Displacement Piles.
    1. Design for Lateral Loading. Lateral analyses may only be needed if lateral loads are significant. As a guide, lateral shear forces on vertical piles that are less than about 9 to 22 kN (2 to 5 kips) per pile for 460 to 915 mm (18 to 36-in.) diameter piles, would probably not justify lateral analysis at this point in the design process and the designer could skip the lateral analyses and move on to design for axial load. If battered piles are used to resist lateral forces these guidelines apply to the resulting forces transverse to the pile axis and the longitudinal component of the force on the battered pile is considered in design for axial loading.

      Select a diameter that is sufficient to provide the necessary nominal lateral load resistance and service load requirements for deflection. Note that when lateral loads are significant, the final design of CFA piles for lateral loading is typically controlled by structural design considerations and the necessary flexural strength and reinforcement. At this point in the design process, it is prudent to consider lateral loading because it may control the pile diameter (but usually not the length). Note that high bending stresses combined with unsupported length due to scour or liquefaction may preclude the use of CFA piles altogether.

      A preliminary lateral analysis using a computer code as described in Chapter 5 is warranted at this step in the process. The relative significance of lateral load magnitude is certainly dependent upon soil conditions, and weak surficial soils tend to result in more significant bending moments for a given lateral shear force. Batter piles may also be considered for large lateral forces if downdrag or constructability considerations do not preclude their use, and groups of piles including batter piles are best analyzed using 3-D group analysis programs such as GROUP (Ensoft, 2006) or FB-Pier (BSI, 2003).

      The lateral load analysis of a trial pile design should proceed as follows:

      1. Select a trial pile diameter and length (although the design is generally not sensitive to length for CFA piles having length/diameter ratios of 20 or more). Select a trial longitudinal reinforcement. A longitudinal reinforcement with a cross-sectional area of around 1% of the pile cross-sectional area is typically a good initial value to consider. Construct a computer model with p-y curves for the load conditions likely to be most critical for lateral load considerations.
      2. With the pile modeled as a linear elastic beam, evaluate foundation strength conditions by computing the foundation response of the pile due to service loads multiplied by a factor of at least 2.5 to ensure that the pile embedment into the soil has adequate reserve capacity. Service load deflections or structural strength requirements generally control design, but this "push-over" type analysis is performed to ensure that adequate reserve strength exists with respect to the soil resistance. At the same time, service load and factored load cases can be computed to provide design information that will be used in subsequent steps. Note that there may be several different load combinations to be evaluated, although there is usually a clearly dominant lateral loading case. This check is to ensure that push-over conditions do not control the design. As mentioned earlier, normally service deflections or structural limit state will control lateral load design. The strength check ensures that the available soil resistance exceeds the structural capacity of the pile in flexure and thus the foundation should have ductility. Note that for analysis of a single pile that will be representative of a group of piles, it is appropriate to include a p-multiplier (less than 1.0) which is equal to the average p-multiplier for the group. The p-multiplier concept is described in Chapter 5 and other references, and is incorporated into most computer programs for lateral analysis using p-y curves.
      3. Verify that the magnitude and depth of longitudinal reinforcement is adequate for the maximum bending stress computed with the computer program used in step ii. Details of structural design and adequacy are contained in Section 5.6.4. Check to see that the reinforcing design is constructible, as discussed in Section 2.2.2.5.
      4. With the pile modeled as a nonlinear reinforced concrete beam, evaluate foundation deflections by computing the foundation response of the pile, or pile cap (when pile groups are used), due to service loads. If the deflections are larger than the service load requirements, the pile diameter may need to be increased (go back to step 3.F.a.i) or the layout changed (go back to step 3.E). If the lateral loads are very high, it may be appropriate to consider batter piles, CFA piles reinforced with steel pipe, or alternative deep foundations.
      5. Note that if seismic loads are an important component of lateral load considerations, the possibility of subsurface ground movements must be considered, as briefly described in Chapter 5. Although the recommended procedure for design of CFA piles is with ASD using the AASHTO (2002), sections of the AASHTO (2006) design code address issues relating to subsurface ground movements, the effect on pile foundations, and ductility requirements for piles. Subsurface ground movements, which can occur at large depths, may subject CFA piles to significant bending stresses at locations well below the ground surface. Installation of conventional reinforcement in CFA piles may be problematic in such conditions. The use of structural steel pipe or H sections for reinforcement in CFA piles, or selection of alternative deep foundation systems should be considered.
    2. Design for Axial Loading. After the diameter is selected, determine the length of pile required to provide the necessary axial resistance. Analyses of ultimate axial resistance are performed using the methods outlined in Chapter 5. This step may be a trial and error process. Many engineers may prefer to automate the computations using a spreadsheet or other computer solution that incorporates the methods outlined in Chapter 5. These methods produce profiles with depth of the nominal and allowable axial resistance for each of the idealized profiles, and loading conditions, and foundation location established for the project. The allowable axial resistance (ultimate axial resistance divided by the factor of safety) is compared to the service loads to ensure that the design meets strength requirements.
    3. At this point it is also prudent to consider constructability, and cost effectiveness of the pile length determined. If the design is problematic from either standpoint, go back to step 3.E and consider an alternative pile layout or alternate deep foundation types.
  7. Pile Group Capacity. The pile group capacities may be evaluated by hand calculations for simple load conditions and soil layering, as is detailed in Section 5.5.2. However, for pile groups with complex 3-D load conditions or soil layering, the pile group capacity may require analyses using computer codes such as FB-Pier, GROUP, or similar computer-based pile group analysis methods. These tools can be effectively used to optimize foundation layout and load distribution to the piles.
  8. Pile Group Settlements. For groups of piles subject to sustained permanent loads, long-term settlements in excess of the short-term deformations associated with individual pile load-deflection response (i.e., due to the deeper influence of the pile group) will present a service load condition that should be considered. The group settlement may be evaluated by hand calculations for simple load conditions and soil layering, as is detailed in Section 5.5.3. However, for pile groups with complex 3-D load conditions or soil layering, the pile group capacity may also require computer-based pile group analysis methods. It is noted that the advantages of DD piles in achieving high capacity at shallow depth may be offset by settlement considerations if relatively compressible strata exist at depth. Where downdrag loading is relevant, this should be assessed as outlined in Chapter 5 and design modifications maybe be required. If necessary, longer piles may be required to accommodate settlement or downdrag concerns and the design process may require returning to step 3.E.
  9. Pile Group Lateral Behavior. The group behavior was taken into account using p-y multipliers during the preliminary analyses of single piles (see step F.a.ii). This may be sufficient for most pile groups, and a sensitivity analysis may be considered to determine primarily the effect of the pile-head fixicity. However, for pile groups with complex soil-pile interaction or cap designs, the pile group lateral capacity may also require computer-based pile group analysis methods.
  10. Structural Design. After the design has been selected to satisfy considerations of geotechnical strength (GULS) and serviceability (SLS), the final structural design of the piles and pile cap must be completed. This step will involve the final lateral pile analysis or verification and the pile structural design, including reinforcement and grout or concrete material requirements. The structural design for CFA piles is very similar to that of drilled shaft foundations, except for slight differences in properties of grout compared to concrete and the tendency to use a rebar cage that does not extend the full length of the pile. The structural design for the pile is detailed in Section 5.6.4.

6.4 Step 4: Constructability Review

An evaluation of constructability is an integral part of the design process and constructability factors should have been considered already in making foundation type selection. A final review of constructability should be performed at this point, including a review of the checklist below for items for evaluating constructability of CFA piles and, if appropriate, DD piles. Note that there are always concerns of some type about constructability, and special concerns should be noted and identified on plan notes and special provisions. It is helpful to highlight these issues to potential bidders so that responsible bids can be prepared and special concerns addressed in the successful bidder's installation plan. A checklist of items to consider follows.

  1. Are pile length and diameters appropriate? Typical CFA pile diameters are typically 0.4 to 0.6 m (16 to 24 in.), and are rarely constructed in excess of 0.9 m (36 in.) in diameter. Typical DD pile diameters are 0.4 to 0.6 m (16 to 24 in.). Lengths of over 30 m (100 ft) are generally undesirable and not optimum.
  2. Can bearing strata be penetrated to depth indicated? Avoid designs that require penetration into hard materials to achieve capacity if the overburden soils may be subject to soil mining or if the bearing stratum is too hard to drill effectively.
  3. Is there a risk of soil mining due to loose water-bearing sands? Avoid CFA in such materials; DD piles may be more effective.
  4. Is there a potential effect on nearby structures? Drilling in close proximity to nearby structures can be risky due to potential subsidence.
  5. Is the rebar cage appropriate? Rebar cage length less than about 12 m (40 ft) is preferable.
  6. Is there a pile cutoff detail? Avoid pile cutoff more than a few feet below the working grade if possible. Deeper cutoff will require casting the pile to the surface and chipping down after the grout or concrete has set.
  7. Is construction sequence is feasible? Consider site access, existing structures, obstructions, pile cap footprint. Avoid installing CFA or DD piles over water. A stable working platform is necessary, especially for DD piles which require larger, heavier rigs than conventional CFA piles.
  8. Are low headroom conditions required? Low headroom working conditions are best if avoided. If it is necessary to use low headroom construction, use smaller piles (usually 0.45 m [18 in.] diameter or less) and smaller working loads per pile to avoid installation problems with small lightweight rigs.
  9. Is there a plan for resolving construction questions prior to production?

6.5 Step 5: Prepare Plans And Construction Specifications, Set QC/QA And Load Testing Requirements

At this step, general specifications should be reviewed, and project special provisions and plan notes should be developed if needed for incorporation into the project plans and specifications. Guide construction specifications are provided in Chapter 8. Field QC/QA requirements will vary depending on the project type, (e.g., soundwall foundations in favorable soil conditions may have minimal requirements, whereas bridge structure foundations will include extensive monitoring). Load testing requirements should be established, consistent with the design considerations and factor of safety.

6.6 Example Problem

6.6.1 Introduction

This example problem is intended to demonstrate the step-by-step design methodology for CFA piles described in the previous sections. In the interest of brevity, the design will focus on the foundation for a single column of one pier, for a single load case. An actual project would utilize the methodology for a variety of foundation locations, additional load cases and alternate subsurface profiles as may occur across the project site for the structure in question. The example problem is developed in English units only.

6.6.2 Step 1: Initial Design Considerations
6.6.2.1 General Structural Foundation Requirements

This project will consist of a series of new bridges to be constructed as a part of an interchange. The layout is such that relatively good access is available and a large number of foundations are required. The magnitude of the axial and lateral loads per each foundation is not unusually large. The working platform is relatively soft in some areas, but suitable for equipment; timber crane mats may be needed.

6.6.2.2 Site Geology and Subsurface Conditions

The site is in the coastal plains, at a location where relatively soft to medium strength alluvial sediments are present at shallow depths. This soil is predominantly cohesive, but with frequent silt or sand layers. Groundwater is typically present at depths ranging from 6 to 10 ft below existing grade, leaving the cohesive soils in the upper 5 ft, which exhibit a stiffer crust due to desiccation. The shallow sediments are underlain by older, more competent overconsolidated clays of Pleistocene age. The stiff clays extend to depths of engineering significance for this project. The general conditions for the entire site, as indicated by the site investigation, is shown in Figure 6.2. The relatively low strength and high compressibility of the shallow alluvial sediments require that deep foundation support be utilized.

Figure 6.2: Typical Conditions at the Project Site

Illustration showing typical conditions at the project site.

6.6.3 Step 2: Comparison and Selection of Deep Foundation Alternatives

A number of deep foundation alternatives may be considered for this site, including driven piles, drilled shafts, and CFA piles. Because the soils are predominantly clays and no strong bearing stratum is present, deep foundations will derive the majority of capacity from side-shear. This type of profile favors the use of CFA piles, although prestressed concrete piles may be a feasible alternative. Drilled shafts are also a viable alternative, but because of the shallow groundwater and frequent sand layers it is likely that dry hole construction may not be possible and slurry drilling would be required. CFA piles are likely to be very cost-effective in the conditions described in Step 1 and will thus be considered further. The cohesive soils do not favor the use of DD piles as their strength is not appreciably improved from the construction process nor does it benefit from the higher lateral stress attained.

Possible difficulties with the use of CFA piles for this site are:

  • If extremely soft organic layers were present within the alluvium, these could pose stability problems with fluid grout. However, no substantial thicknesses (more than a few feet thick) of organics were evident.
  • Large lateral loads could be detrimental to CFA piles due to the soft shallow stratum and the limited strength in flexure for these piles.
  • Downdrag could be problematic for CFA piles at the abutments, where fill will likely produce significant settlement. This is a consideration for any type of deep foundation, however, and does not preclude the use of CFA piles.
6.6.4 Step 3: Select Pile Length and Calculate Performance Under Specified Loads

For this step in the design process, one intermediate bent will be selected and a design developed for the columns at that bent. This bent is to be constructed as a two-column pier, with each column supported by a pile footing. The design will be completed using ASD methods.

Step-By-Step Design Procedure for Axial and Lateral Loads.

  • 6.6.4.A Develop idealized profile. Based on the borings at the intermediate bent location with consideration of nearby borings and engineering judgment, the profile shown in Figure 6.3 is developed for this location. Note that this is the same profile used for one of the examples in Chapter 5.
  • 6.6.4.B Develop geotechnical design parameters. Undrained shear strengths have been obtained based on unconsolidated, undrained triaxial tests, and a design strength profile is developed as shown below. Note that a pile footing is anticipated, with the base of the cap at 4 ft below grade. This elevation will be the top of the pile for analysis purposes.

    For settlement considerations of pile groups installed into the deeper stiff clay, this clay is heavily overconsolidated with a recompression index, Cr of 0.015. The void ratio averages approximately 0.6. Consolidation settlements of the shallow clay will be of interest to parts of the project, but will not directly affect the deep foundation calculations in this example.

  • 6.6.4.C Obtain the nominal loadings for the foundation. At this interchange, there is no waterway to cause scour and extreme event loadings are not significant. Axial loads are controlled by combined dead load and live load, and lateral loads are produced by wind. Although there are typically a number of load cases to be considered, this simplified problem will consider foundation loads as follows for each of the two columns:
    • Group Vertical service load (dead + live) = 500 kips
    • Dead Load / Live Load = 2.5
    • Horizontal service load (due to wind) = 50 kips
    • Overturning moments at the base of the column = 250 ft-kips service loads (transient)

    Post-construction axial settlements should not exceed 0.5 in. under service dead loads and lateral deflections at the pile cap should not exceed 0.25 in.

  • 6.6.4.D Establish Factor of Safety for Design. For this project, an extensive field load test program will be developed and used, so that a factor of safety of 2.0 may be used.
  • 6.6.4.E Select a trial design pile group to establish individual pile loads. Several different pile layouts may be used, but as a first trial consider a 5 pile group of 18-in. diameter CFA piles. With piles spaced at 3 diameters on center (i.e., 4.5 ft center-to- center), the layout of such a group is shown in Figure 6.4.

    Figure 6.3: Idealized Soil Profile for Design

    Graphical illustration showing idealized soil profile for design.

    The five-pile layout provides some redundancy in the foundation and distributes the resistance over a broad footprint so that the moment on the column is resisted by axial forces in the piles. A preliminary estimate of individual pile factored loads is as follows:

    Axial pile force due to axial column load:

    500 kip per column/5 piles = 100 kips/pile

    The axial pile force (Fpile) due to the overturning moment on column (Moverturn) can be computed from static equilibrium as follows: only Npiles = 4 piles contribute to moment as the center piles lies along the neutral axis. The distance normal to the neutral axis of the outer four piles (dpile) is used for calculation.

    Figure 6.4: Five-Pile Footing Layout

    Illustration showing five-pile footing layout.

    For static equilibrium about the center of the group, the applied overturning moment must equal the overturning moment resulting from pile axial forces as follows:

    Moverturn = Npiles × Fpile × dpile

    Load per pile due to moment = 250 kip-ft / [(4 piles) × (4.5 ft) (cos 45°)] = ±20 kips/pile

    Cap weight (assume a 2.5-ft thick cap):

    10 ft × 10 ft × 2.5 ft × 0.15 kip/ft3 / 4 piles = 8 kips/pile

    Total axial load per pile = Axial force due to column load + Load due to moment + Cap weight = 100 + 20 + 8 kips = 128 kips/pile

    Note that piles at the opposite side of the group are those with the minimum load, and are 40 kips/pile less than the maximum. In some cases, the pile cap may not need to be included in the load.

    For design, round off and use 130 kips/pile as the required allowable pile capacity

    For lateral loads due to wind, use lateral load per pile = 50 kips/5 piles = 10 kips/pile

  • 6.6.4.F Select a trial design for individual CFA pile. Because lateral loads may control pile diameter, check lateral loading first. Note that combined lateral and axial loading could be evaluated using pile group design software, such as FB-Pier (BSI, 2003) or GROUP (Ensoft, 2006). In many simple cases and in this simple example, an analysis can be performed on a single pile that is representative of the group behavior. For this example, a single pile is evaluated using LPILE (Ensoft, 2006) to demonstrate the calculation methods and design philosophy. It is obvious that the piles will need to extend for some length into the stiff Pleistocene clay stratum to generate axial resistance; however, the lateral response is unlikely to be affected by this length because the length to diameter ratio for an 18-in. diameter CFA pile will be large. A 40-ft long pile is used for lateral analysis, even though a longer pile will likely be required for axial loading considerations.
    1. Design for Lateral Loading

      LPILE is used to compute the response of a single linear elastic pile of 18 in. in diameter that extends into the stiff clay stratum for a short distance. Analyses are performed for a range of loads, including loads up to twice the service load of 10 kips per pile, in order to verify that a ductile lateral load vs. deflection response is obtained (e.g., the pile has greater capacity in excess of the design loads with increasing deflection). The top of the pile is assumed to be restrained against rotation; i.e., the pile has a full moment connection and is fixed to the cap by connecting the reinforcement into the cap. The cap will resist rotation because of the rotational stiffness from the axial resistance of the piles. To account for lateral group effects, an average P-multiplier for the pile group is used. P-multipliers range from 0.4 to 0.8 for piles within a group spaced at 3-diameters on center in clay soils (see Section 5.6.3 for a discussion of P-multipliers).

      First, a linear elastic analyses (concrete and steel strength are assumed to be linearly elastic with no cracking) with a p-multiplier at the mid-range of 0.6 is performed to verify that the lateral deflections are small and the 18-in. diameter pile has adequate lateral resistance. Subsequently, the model is revised to include nonlinear behavior of the reinforced concrete pile. As a first try, a rebar cage having longitudinal steel equal to at least 1% of the cross-sectional area (i.e., Ds = 2.54 in.2) is used. Six #7 bars provide a total of 3.6 in.2 area and are used for subsequent analyses. Grade 60 reinforcement of fy = 60 ksi and fc = 4,000 psi concrete or grout is used for this design. Note that repeated analyses with a range of values are very easy once the basic model is established.

      The results of these analyses are illustrated using Figures 6.5 through 6.8. The design engineer can quickly evaluate results graphically on the computer screen and revise the analyses as required without lengthy printouts. The figures are used in this example to illustrate the design process.

      The lateral load vs. deflection response shown in Figure 6.5 indicates that the piles in the group should support a lateral shear of 10 kips with a deflection of around 0.2 in. or slightly less at the pile top. The curve for the linear elastic pile case was performed to provide an initial evaluation of soil response without regards to the structural capacity of the pile. The nonlinear analyses should be expected to provide a more realistic estimate of actual pile response. Note that this does not include any potential contribution from the pile cap which is typically bearing against the soil; therefore, this analysis is likely conservative. The actual distribution of forces in the piles in the group may range from around 8 to 12 kips for a displacement of 0.2 in., depending on the row position, and p-multiplier used, as was discussed in Section 5.6.3. The consideration of the average condition is sufficient for this illustration problem.

      Figure 6.5: Computed Lateral Load vs. Deflection Response. NL = nonlinear pile behavior, L.E. = linear elastic beam

      Graphical illustration showing computed lateral load as a function of deflection response (in inches). Figure shows curves for Non-linear and linear responses.

      The resulting deflection profiles illustrate the length of pile over which the lateral soil resistance is mobilized. Deflection vs. depth for the mid range of the p-multipliers used (Pm = 0.6) is shown for a 10 kip lateral load (mid-range from 8 to 12 kips range) in Figure 6.6. This is considered representative of the average pile case. Bending moments in the pile vs. depth for that case are also shown in Figure 6.6. The maximum bending moment occurs at the top of the pile, at the connection to the pile cap. Both deflections and bending moments indicate that depths below 30 ft are relatively unaffected by lateral loads applied at the pile top.

      An evaluation of bending moments for the range of lateral loads from 8 to 12 kips corresponds to the range of maximum bending stresses in the piles of the group at a deflection of approximately 0.2 in. Maximum bending moments vs. deflection for the range of p-multiplier values used is illustrated in Figure 6.7. The maximum value for use in design (i.e., for a deflection of 0.2 in.) is around 700 in.-kips. Note that the stiffer pile (Pm = 0.8) has the highest moment at a given deflection, because this stiffer pile is supporting a larger share of the lateral load. In any event, it is appropriate to design all of the piles to encompass the maximum possible range of conditions. If the simplified method had been used with calculations performed only for the average p-multiplier of 0.6, the estimated deflection would have been almost the same to that obtained using the row-dependent p-multipliers. The maximum moment would have been calculated at 720 in.-kips, based on the 600 in.-kips for the average pile, with a 20% increase to account for variations in pile stiffness.

      Figure 6.6: Deflection and Bending Moments vs. Depth for Example Problem
      (lateral load = 10 kips)

      Two graphs illustrating distributions of deflection and bending moments as a function of depth for example problem in which the lateral load is equal to 10 kips).

      Figure 6.7: Maximum Bending Moments as a Function of Pile Top Deflection

      Graph illustrating maximum bending moments as a function of pile top deflection. Figure shows curves for non-linear response.

      Figure 6.8 shows the moment vs. curvature relationship computed with LPILE for an 18 in.-diameter pile reinforced with 6 #7 bars, and with 3 in. of cover. The range of 80 to 130 kips of axial force encompasses the anticipated range of axial pile loads and provides a structural check on combined axial and flexural responses. This figure suggests that the maximum bending moments computed in the 700 in.-kip range are well within the factored moment resistance of this pile for purposes of preliminary design. The ultimate moment resistance is in the 1,500 to 1,800 in.-kip range, as is evident by the large increase in curvature with little increase in bending moment (approximate curvature of 0.0003).

      At this point, the preliminary evaluation of an 18-in. diameter pile for lateral loads is completed. Before completing a final design considering lateral loads, the design for axial resistance should be performed. Note also that by evaluating a range of lateral loads at this preliminary design stage, it would be easy to re-evaluate lateral load response for pile groups with different numbers of piles or for different loads that may be representative of other foundation cases on the project.

    2. Design for Axial Loading

      The maximum service loads were determined to be 130 kips per pile. Using a factor of safety of 2.0, the ultimate pile resistance required for this design is 260 kips, corresponding to a tip embedment of 69 ft below grade. Computations of axial resistance with depth for an 18-in. diameter pile in this soil profile were made in Section B.1 of Appendix B. The results of these calculations are reproduced as Figure 6-9. Axial structural capacity was considered in combination with flexure in the previous step.

      Figure 6.8: Bending Moment vs. Curvature for a 18-in. diameter Pile with 6 #7's

      Graph illustrating bending moment as a function of curvature for an 18-inch diameter pile with 6 number 7 bars.

      Note that load testing of a pile with similar characteristics and soil conditions is required to utilize the safety factor of 2.0 selected for design. The load test must be designed to test the pile load resistance to at least 390 kips, i.e., 3 times the design load.

    3. Consider Constructability and Cost Effectiveness.

      The required embedment depth of 69 ft below grade is fairly deep, but well within the range for which CFA piles of this size can be constructed efficiently. It is possible to consider using 24-in. diameter piles of shorter length, for which a 4-pile group could be easily constructed. It is also feasible to increase the design group to a 6-pile arrangement of 18 in. piles to use shorter piles which may require a rig with less torque or crowd.

      Another constructability issue to consider is the rebar cage. The calculations for bending moments suggest that a cage needs to extend to a depth of 30 ft, which is within the range of embedment that is relatively easy to construct. Without significant tensile forces or seismic ground motions that could produce significant bending stresses at greater depth, there is no necessity to install a deeper cage. The six bar cage developed in this preliminary design step does not appear to pose a constructability problem.

      It may be noted that the great majority of the resistance of the proposed design comes from side-shear, with relatively little contribution from end-bearing resistance. Of most significance is the embedment into the stiff Pleistocene clay stratum, a point which should be made via the project documents so that the inspection team can specifically seek to verify the embedment into the Pleistocene. It generally should be possible to note a change in drilling resistance when this significantly stiffer stratum is encountered below the alluvium.

  • 6.6.4.G Pile Group Capacity. Limiting pile group capacity may be estimated using the block failure concept outlined in Section 5.5.2.1.2. This rarely controls but must be checked. A quick check can be made on the side-shear on the block in the stiff clay stratum only, without even considering the base resistance of the block. The width of the outside of the 5-pile group is 2 times the projected distance from the center pile to the center of the corner piles plus one pile diameter or:

    2 × (4.5 cos 45°) + 1.5 = 7.9 ft

    The group is embedded 69 - 29 = 40 ft into the stiff clay. Therefore, the surface area of the block resisting in side-shear is:

    4 × 7.9 × 40 = 1,264 ft2

    The average undrained shear strength within 29 and 69 ft below grade is 1.9 ksf. Therefore, the side-shear resistance on the block is:

    1,264 ft2 × 1.9 ksf = 2,402 kip.

    This resistance is substantially larger than the resistance of the 5 individual piles, indicating that block failure of the group does not control. Base resistance can be added if necessary, but this quick check is sufficient to verify the block failure does not control design.

  • 6.6.4.H Pile Group Settlements. Long-term settlements in clay soils resulting from dead load are a consideration, as these sustained loads may result in significant compression or consolidation settlement of the underlying soils. The step-by-step procedure outlined in Section 5.5.3.3 is followed for this calculation.
    1. The depth of the "equivalent footing" is approximately 2/3 of the depth within the stiff clay stratum. Because the pile extends to 69 ft below grade and the stiff clay starts at a depth of 29 ft below grade, the equivalent footing is located at:
    2. 2/3 × (69 - 29 ft) = 0.67 × 40 = 56 ft, or 27 ft into the stiff clay deposit or 56 ft below the ground surface. The side of the equivalent footing for a 5-pile group shown in Figure 6-4 is 2 × (4.5 cos 45°) + 1.5 = 7.9 ft. Because the DL/LL ratio is 2.5, the sustained dead load (SDL) is the total service load (TSL) times the ratio to the dead load of the total service load or:

      SDL = 500 × 2.5/(1+2.5) = 357 kips

      the cap weight is assumed to be = 10 × 10 × 2.5 × 0.15 = 37.5 kip

    Therefore, the total dead weight causing long-term settlement is 395 kip.

    The bearing pressure on the equivalent footing is thus 395 kip/7.92 = 6.33 ksf.

    1. The soil is divided into 6-ft thick layers below the equivalent footing. The first of three layers will have an effective vertical stress, po, at the center of the layer (or at depth of 56 + 3 = 59 ft) equal to:
      po =6soil zsoil,i) - γw zw
      Σ
      i

      where:

      γsoil=soil unit weight of layer i
      zsoil,i=soil layer thickness of layer i
      γw=unit weight of water
      zw=depth below the groundwater table
      =29 ft × 0.110 kcf + 30 ft × 0.120 kcf - 52 ft × 0.0624 kcf = 3.55 ksf

    Load spreading at a 1H:2V ratio at the center of the first layer produces a stress change, Δp, of 395k/(7.9 + 3)2 = 3.32 ksf and a final stress, pf = po + Δp = 3.32 + 3.55 = 6.87 ksf.

    The settlement, S, of this layer is:

    S1 = H [ ( Cr / ( 1 + eo ) ) log ( pf / po ) ]

    where:

    H=layer thickness = 6 ft = 72 in.
    Cr=recompression index = 0.015
    eo=void ratio = 0.6
    S1=72 [(0.015 / 1.6) log (6.87/3.55)] = 0.19 in.

    For other layers, the computation follows in Table 6.2. Note that the calculation is performed until a depth at which Δp/po < 10%

    Figure 6.9: Computed Axial Resistance vs. Depth

    Graphical illustration showing computed axial resistance as a function of depth.

    Table 6.2: Pile Group Settlement Computation
    Layer Number Depth Interval (ft) Thickness (in.) po (ksf) Δp (ksf) S (in.)
    156 - 62723.553.320.19
    262 - 68723.91.380.09
    368 - 74724.240.750.05
    474 - 80724.590.470.03
    Total:0.36

    Add to the consolidation settlement above the theoretical elastic shortening (Sel) of the piles acting as a column above the equivalent footing. These piles support a total load of 395 kips with an area equal to that of 5 piles; therefore, using an estimated elastic modulus for the piles of 3,000 ksi, it results

    Sel = (1/2) × (PL/AE) = 1/2 × [(395 kip × 56 ft × 12 in./ft) / (5 × 254 in.2 × 3,000 ksi)] = 0.04 in.

    Long-term settlements under dead load for this foundation are thereby estimated at no more than 0.36 + 0.04 = 0.40 in., or about 0.5 in. It should be noted that a portion of the dead load is applied onto the piles before the pier cap and girder bearing plates are finalized, and the portion of the settlement that occurs during this period will not affect the bridge structure. Therefore, if the long-term settlements due to total dead load appear to be too high, it would be prudent for the design engineer to re-evaluate the settlements to estimate the actual magnitude of the post-construction portion of the total settlement.

  • 6.6.4.I Pile Group Lateral Behavior. Lateral deflections have already been estimated in the previous step (6.6.4.F.a) at less than 0.2 in. for the design lateral loads using appropriate P-multipliers to account for group effects. A full 3-D computer model of the proposed foundation may be analyzed using GROUP (Ensoft, 2006) or FB-Pier (BSI, 2003) or similar software. The use of these sophisticated programs is not necessary for this simple problem but may be a convenience for users who are familiar and efficient in using such software.
  • 6.6.4.J Finalize Structural Design. The design satisfies geotechnical limit states and serviceability limit states. The final sub-step in designing the piles is to finalize the structural design of the pile and select reinforcement. Structural design follows the procedures outlined in Section 5.6.4.

    The depth required for the full cage section must first be determined. It was seen in Figure 6.6 that the pile clearly exhibited "long pile"behavior, and the depth to the counter-flexure point in the displacement profile (second point of zero displacement with depth) was approximately 25 ft. The cage may thus terminate at this depth because the piles are not bearing in weak rock or intermediate geotechnical material (IGM) sockets.

    Longitudinal reinforcement was chosen as the recommended minimum of 1%, as has been determined previously in computations of lateral resistance to provide adequate lateral structural resistance. Combined axial force and flexure was considered in the analysis presented in Figure 6.8.

    For the maximum computed shear force in any pile, check to determine if the concrete section has adequate shear capacity without shear reinforcement (Chapter 5, Section 5.6.4.2).

    With 6 #7's bars and 3-in. cover on the longitudinal reinforcing, the radius of the ring formed by the centroids of the longitudinal bars is (Eq. 5.59):

    rls = B/2 - dc - db/2 = 9 - 3 - 0.44 = 5.56 in.

    The area of the cross-section that is effective in resisting shear is (Eq. 5.59):

    Av = B [ B/2 + 0.5756 × rls ] = 219 in.2

    The concrete shear strength using the average axial force of 100 kips is (Eq. 5.61a):

    Vc = [ 1 + φ × 0.00019 ( P/Ag ) ] × ( fc )0.5

    = [ 1 + 0.85 × 0.00019 × 100,000 / ( π × 92 ) ] × ( 4,000 )0.5 = 68 psi

    Note that the maximum shear will likely occur on a front row pile with greater axial force.

    Note that ignoring the effect of axial force and computing Vc = ( fc )0.5, a value of 63 psi is obtained, which is slightly conservative but generally sufficient for design.

    The factored shear resistance provided by the concrete is thus:

    φ Vn = φ Vc Av = 0.85 × 68 × 219 / 1,000 (lb/kip) = 12.6 kips > 12 kips (max. shear)

    Thus, shear reinforcement is not required and the transverse reinforcement can consist of the minimum recommended of #3 ties at 12 in. spacing.

    A single center longitudinal bar will extend to the full length of the CFA pile. Because these piles are not subject to uplift forces, the longitudinal bar may be a # 9 bar (minimum size allowed). Note that this bar must extend through the full cage section to the top of the shaft, and would be allowed to contribute to the structural capacity of the top cage section if the design engineer elects to do so in the analyses.

6.6.5 Step 4: Constructability Review

Finally, a brief check on constructability issues is provided in a question and answer format:

  1. Are pile length and diameters appropriate? Yes, 18 in. diameter and less than 70 ft embedded length is within normal sizes.
  2. Can the bearing strata be penetrated to depth indicated? Avoid designs which require penetration into hard materials to achieve capacity if the overburden soils may be subject to soil mining or if the bearing stratum is too hard to drill effectively. Yes.
  3. Is there a risk of soil mining due to loose water-bearing sands? Yes, if soils are cohesive.
  4. Is there potential effect on nearby structures? No.
  5. Is the rebar cage appropriate? Yes.
  6. Is there a pile cutoff detail? Yes. Avoid pile cutoff more than a few feet below the working grade if possible. Deeper cutoff will require casting the pile to the surface and chipping down after the grout or concrete has set. These piles may be formed to the surface and cut down later. If the contractor chooses to dip the pile down while still fluid, there must be a temporary form provided to prevent cave-ins from contaminating the top of the pile.
  7. Is construction sequence feasible? Yes. Consider existing structures, obstructions, pile cap footprint. Avoid installing CFA or DD piles over water. A stable working platform is necessary, especially for DD piles, which require larger, heavier rigs than conventional CFA piles. The working pad may require stabilization; this is worthy of a note on the plans that the surface may be soft.
  8. Are low headroom conditions required? No.
  9. Is there a plan for resolving construction questions prior to production? Yes. Include pre-construction meeting discussion.
6.6.6 Step 5: Prepare Plans and Construction Specifications, Set Field QC/QA and Load Testing Requirements

These items follow the general work for developing plans and specifications for transportation projects involving deep foundation work. The following two chapters are focused on the aspects of inspection, testing, and specifications for CFA projects. Note that for this example, a load testing program is required because the design relied on the use of a lower factor of safety and because of the increased reliability of a design that must be validated by site-specific load test data.

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Updated: 04/07/2011
 

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