Fawad S. Niazi

Cone Penetration Test Based Direct Methods for Evaluating Static Axial Capacity of Single Piles

The direct cone penetration test (CPT) based pile design methods use the measured penetrometer readings by scaling relationships or algorithms in a single-step process to enable the assessment of pile capacity components of shaft and base resistance (fp and qb, respectively) for evaluation of full-size pilings. This paper presents a state-of-the-art review of published works that focus on direct CPT evaluation of static axial pile capacity. The review is presented in a chronological order to explicate the evolution over the past six decades of an in situ test based solution for this soil-structure interaction problem. The objective of this study is an attempt to assemble maximum published methods proposed as a result of past investigations in one resource to afford researchers and practitioners with convenient access to the respective design equations and charts. In addition to an all-inclusive summary table and the design charts, a compilation of significant findings and discussions thereof are presented. Furthermore, potential future research directions are indicated, with special emphasis on the optimal use of the modern multi-channel hybrid geophysical-geotechnical seismic CPT to evaluate the complete axial pile load–displacement response.

Evaluating Axial Elastic Pile Response from Cone Penetration Tests (The 2009 Michael W. O'Neill Lecture)

Abstract Axial pile performance can be rationally evaluated within an elastic continuum framework using field results from seismic piezocone tests (SCPTu). Adopting a versatile Randolph-type elastic pile model, the approach can be applied to either traditional top down loading using an anchored reaction beam or the newer Osterberg cell that simultaneously pushes the base and shaft in opposite directions. The axial load distribution within the shaft is also evaluated. For site-specific data at a given site, the SCPTu is an optimal means for collection of subsurface information because it combines penetrometer readings and downhole geophysics in one sounding. The results obtained are at opposite ends of the stress-strain-strength curves, specifically the peak strength for capacity interpretations and the small-strain stiffness (Emax) for evaluating the initial deformations. Axial pile capacity can be analyzed using both direct and indirect CPT methods. As such, the approach allows for a complete representation of the axial load-displacement-capacity curve for pile response. Case studies are presented for deep foundations situated in stiff clays at two national geotechnical test sites located in Houston and College Station, Texas, using top down loading, as well as a third case study of a drilled shaft in clay till loaded by O-cell in Alberta.

Drilled Shaft Response in Piedmont Residuum Using Elastic Continuum Analysis and Seismic Piezocone Tests

The axial load-displacement-capacity response of drilled shaft and bored pile foundations can be evaluated during the analysis and design stage using an elastic continuum framework and results from in-situ testing methods, in particular, the seismic piezocone test (SCPTu). The SCPTu is an optimal and economical means for collection of geotechnical data because the same sounding provides up to five separate measurements with depth: cone tip resistance (qt), sleeve friction (fs), porewater pressure (u2), time rate of consolidation (t50), and shear wave velocity (Vs). Moreover, the SCPTu provides information on soil behavior at both ends of the stress-strain-strength curves, namely the peak strength and geostatic stress state for evaluating axial pile capacity and the small-strain stiffness (Gmax = ρt•Vs2) for the initial soil-pile deformations. Using a Randolph-type analytical elastic pile model, the approach can handle either traditional top down compression loading by dead-weight or reaction beam systems, or the more recent Osterberg cell that juxtaposes base and side resistances in opposite directions. A case study involving O-cell tests on a drilled shaft in Piedmont residuum and partially-weathered rock in Atlanta are presented.

Field Stiffness Reduction Evaluations from Back-Analysis of Pile Load Tests

Starting with the fundamental small-strain shear modulus (Gmax), the mechanical nonlinear stress-strain-strength behavior of soil manifests itself in the form of stiffness reduction under loading. Back analyses of 105 pile load tests combined with a knowledge of soil parameters at the site and pile geometries provide a framework for evaluation of operational stiffnesses at different levels of loading. This paper explains the framework for obtaining the operational stiffnesses from pile load tests and presents stiffness reduction curves (G/Gmax) in terms of logarithm of pseudo-strain (w/d), where w = pile displacement and d = pile diameter. The result affords an approach to evaluating the nonlinear load-displacement response of piles without resort to evaluating the pile capacity. The framework utilizes the Randolph elastic pile model to generate stiffness reduction curves from top-down axial compression load tests, tension tests, and O-cell configurations.

Discussion: Cai et al. (2011). Evaluation of Pile Bearing Capacity from Piezocone Penetration Test Data in Soft Jiangsu Quaternary Clay Deposits

This discussion article provides observations on an earlier published article by Cai et al. (2011), which presented piezocone penetration test (CPTu) based pile capacity evaluations. These observations include: (1) neglect to the most reliable reading of CPTu (i.e., tip resistance (qt)) in shaft capacity calculations; (2) no information provided concerning their definition of influence zone and qt averaging technique in their base capacity evaluations; (3) influence of installation method not investigated for the different categories of piles in their database; (4) no information/discussion included regarding their selected criteria in defining the pile capacity; and (5) inconsistencies in the methodology adopted for their comparative and reliability studies. Also included in this discussion article are suggestions and references to certain recent studies that may possibly be considered for future research on the topic.

Enhanced UniCone Expressions for Axial Pile Capacity Evaluation from Piezocone Tests

Using a large database derived from 153 full scale load tests on axially loaded pile foundations, a more refined direct cone penetration test (CPT) method of pile capacity analysis is developed from the 1997 UniCone approach proposed by Eslami and Fellenius (1997). A larger database permits separation of pile type and installation method: drilled, augered, jacked, and driven. Also, tension tests are separated from compression load tests. It is found that the original 5-part zonal soil behavior classification by piezocone test (CPTu) can be subdivided into finer categories of soil type. Furthermore, the pile side coefficient (Cse) linking effective cone resistance (qE = qt - u2) to pile unit shaft resistance (fp = Cse qE) can be related as a continuous function, rather than just five discrete values. The continuous function for Cse is provided via use of the CPT material index (Ic). A refined definition of the pile toe resistance (qb = Cte qE-toe) is made where Cte is also expressed as a continuous function of Ic. A case study is presented to demonstrate the improved assessments of the axial pile capacity via the proposed modifications.

An Update on Pile-CPTu Direct Correlations

The axial pile capacity can be estimated from the cone penetration test (CPT) using either indirect methods or direct methods. The indirect methods employ a dual-phase methodology of estimating the geoparameters for evaluating the shaft and base resistance (fp and qb, respectively) of the pile capacity. The direct methods use the penetrometer readings to directly obtain fp and qb. These correlation efforts from the results of pile and penetrometer testing have resulted in a large variety of design methodologies. The newer multi-channel piezocone test (CPTu) device allows for derivation of more elaborate correlations. To update the pile design formulations, a quick review of the CPTu-based methods is presented, followed by the results of a comprehensive study, where the most updated and the largest pile-CPTu database were used. Simplified correlations, based on cone soil classification index (Ic), are presented for fp and qb for different pile types.

A Review of the Design Formulations for Static Axial Response of Deep Foundations from CPT Data (DFI 2013 Student Paper Competition Runner-Up)

Abstract Axial capacity analysis of deep foundations has been a topic of great interest in the soil-structure interaction problems. Soil behavior is governed by a series of complex stress-strain changes that occur during pile installation and subsequent loading. Owing to the difficulties and uncertainties on the basis of the soil strength-deformation characteristics, one of the most frequently followed design practice is to refer to the formulae correlating directly the pile axial capacity components of unit base resistance (qb) and unit shaft resistance (fp) to the data collected from cone penetration test (CPT). The elementary basis for such formulations has been the idea of considering cone penetrometer as a minipile foundation. This has resulted in plethora of correlative relationships in the past over 60 years. Such correlations, although empirical, have been worked out on the basis of load test results from both instrumented and un-instrumented full scale piles and are able to accommodate many important variables. A quick review of the evolution process and development of such design formulations is presented. An existing method is refined and modified to bring more convenience in extended applications. Few recommendations are proposed for future research directions, where the latest version of CPT i.e., seismic piezocone test (SCPTu) can be used to advance from capacity singularity to the complete axial pile load – displacement (Q – w) response.