Effect of Penetration Rate on Cone Penetration Resistance in Saturated Clayey Soils

In this paper, the effects of penetration rate on cone resistance in saturated clayey soils are investigated. Shear strength rate effects in clayey soils are related to two physical processes: the increase of shear strength with increasing rate of loading and the increase of shear strength as the process transitions from undrained to drained. Special focus is placed on this second effect. Cone penetration tests were performed at various penetration rates both in the field and in a calibration chamber, and the resulting data were analyzed. The field cone penetration tests were performed at two test sites with fairly homogeneous clayey silt and silty clay layers located below the groundwater table. Additionally, tests with both cone and flat-tip penetrometers in sand-clay mixtures were performed in a calibration chamber to investigate the change in drainage conditions from undrained to partially drained and from partially drained to fully drained. A series of flexible-wall permeameter tests were conducted in the laboratory for various clayey sand mixtures prepared at various mixing ratios in order to obtain values of the coefficient of consolidation, which is required to estimate the penetration rates below which penetration is drained and above which penetration is undrained. A correlation between cone resistance and drainage conditions was established based on the results of the calibration chamber and field penetration tests.     

Correcting Liquefaction Resistance for Aged Sands Using Measured to Estimated Velocity Ratio

Factors for correcting liquefaction resistance for aged sands using ratios of measured to estimated shear-wave velocity (MEVR) are derived in this paper. Estimated values of shear-wave velocity (VS) are computed for 91 penetration resistance-VS data pairs using previously published relationships. Linear regression is performed on values of MEVR and corresponding average age. Age of the sand layer is taken as the time between VS measurements and initial deposition or last critical disturbance. It is found that MEVR increases by a factor of about 0.08 per log cycle of time, and time equals about 6?years on average when MEVR equals 1 for the recommended penetration resistance-VS relationships. The resulting regression equation is combined with the strength gain equation reported by Hayati et al. 2008 in “Proc., Geotechnical Earthquake Engineering and Soil Dynamics IV,” to produce a MEVR versus deposit resistance correction relationship. This new corrective relationship is applied to create liquefaction resistance curves based on VS, standard penetration test blow count, and cone tip resistance for sands of various ages (or MEVRs). Because age of natural soil deposits is usually difficult to accurately determine, MEVR appears to be a promising alternative.     

Liquefaction Resistance of Clean and Nonplastic Silty Sands Based on Cone Penetration Resistance

Liquefaction of granular soil deposits is one of the major causes of loss resulting from earthquakes. The accuracy in the assessment of the likelihood of liquefaction at a site affects the safety and economy of the design. In this paper, curves of cyclic resistance ratio (CRR) versus cone penetration test (CPT) stress-normalized cone resistance qc1 are developed from a combination of analysis and laboratory testing. The approach consists of two steps: (1) determination of the CRR as a function of relative density from cyclic triaxial tests performed on samples isotropically consolidated to 100 kPa; and (2) estimation of the stress-normalized cone resistance qc1 for the relative densities at which the soil liquefaction tests were performed. A well-tested penetration resistance analysis based on cavity expansion analysis was used to calculate qc1 for the various soil densities. A set of 64 cyclic triaxial tests were performed on specimens of Ottawa sand with nonplastic silt content in the range of 0–15% by weight, and relative densities from loose to dense for each gradation, to establish the relationship of the CRR to the soil state and fines content. The resulting (CRR)7.5-qc1 relationship for clean sand is consistent with widely accepted empirical relationships. The (CRR)7.5-qc1 relationships for the silty sands depend on the relative effect of silt content on the CRR and qc1. It is shown that the cone resistance increases at a higher rate with increasing silt content than does liquefaction resistance, shifting the (CRR)7.5-qc1 curves to the right. The (CRR)7.5-qc1 curves proposed for both clean and silty sands are consistent with field observations.     

Penetrometer-Based Assessment of Spudcan Penetration Resistance

Estimation of spudcan penetration resistance is an important design step to guarantee the stability and functionality of offshore mobile jack-up units. Dependence on in situ penetrometer test data to evaluate the stratigraphy and resulting spudcan capacity profile has been increased. However, this becomes difficult in intermediate soil types in which the degree of consolidation during penetration falls between the extremes of fully drained or fully undrained. In this study, a penetrometer-based methodology utilizing results from cone and T-bar penetration tests is developed. Three main steps are involved, comprising estimation of the relative penetration resistance of spudcan and cone or T-bar penetrometer under fully drained and fully undrained conditions, and then quantifying the effect of the different normalized penetration rates for spudcan and penetrometer. Values of the various correlation parameters for the proposed model are evaluated. The validity and accuracy of the proposed methodology are evaluated through case studies from centrifuge tests in clay and a field example of spudcan installation in interbedded carbonate silts and sands. The comparisons confirm the potential of the proposed methodology for interpretation of penetrometer tests and application to the prediction of foundation performance.     

Updated Liquefaction Resistance Correction Factors for Aged Sands

Data from over 30 sites in 5 countries are analyzed to develop updated factors for correcting liquefaction resistance for aged sand deposits. Results of cyclic laboratory tests on relatively undisturbed and reconstituted specimens suggest an increase in the correction factors of 0.12 per log cycle of time and an average reference age of 2 days for the reconstitute specimens. Laboratory and field test results combined with cyclic resistance ratio (CRR) charts suggest an increase in the correction factors of 0.13 per log cycle of time and an average reference age of 23 years. A reference age of 23 years seems appropriate for the commonly used CRR charts derived from field liquefaction and no liquefaction case history data. Because age of natural deposits is often difficult to accurately determine, a relationship between measured to estimated shear-wave velocity ratio (MEVR) and liquefaction resistance correction factor is also derived directly from the compiled data. This new MEVR-liquefaction resistance correction factor relationship is not as sensitive to MEVR as in the relationship derived indirectly in a previous paper.     

CPT-DMT Correlations

Although the cone penetration test (CPT) and flat-plat dilatometer test (DMT) have been used for over 30 years, relatively little has been published regarding comprehensive correlations between the two in situ tests. This paper presents preliminary correlations between the main parameters of the CPT and DMT. The key to the proposed correlations is the recognition that the main DMT parameters are normalized and hence, should be correlated with normalized CPT parameters. The suggested correlations are developed and evaluated using published records and existing links to various other parameters as well as comparison profiles. The suggested correlations may guide future more detailed correlations between these two in situ tests.     

Analysis of Factors Influencing Soil Classification Using Normalized Piezocone Tip Resistance and Pore Pressure Parameters

This paper discusses the development of a framework for classifying soil using normalized piezocone test (CPTU) data from the corrected tip resistance (qt) and penetration pore-water pressure at the shoulder (u2). Parametric studies for normalized cone tip resistance (Q = qcnet/σv0′) and normalized excess pressures (Δu2/σv0′) as a function of overconsolidation ratio (OCR = σvy′/σv0′) during undrained penetration are combined with piezocone data from clay sites, as well as results from relatively uniform thick deposits of sands, silts, and varietal clays from around the globe. The study focuses on separating the influence of yield stress ratio from that of partial consolidation on normalized CPTU parameters, which both tend to increase Q and decrease the pore pressure parameter (Bq = Δu2/qcnet). The resulting recommended classification chart is significantly different from existing charts, and implies that assessment of data in Q–Δu2/σv0′ space is superior to Q–Bq space when evaluating piezocone data for a range of soil types. Still, there are zones of overlap for silty soils and heavily overconsolidated clays, thus requiring that supplementary information to Q and Δu2/σv0′ be obtained in unfamiliar geologies, including variable rate penetration tests, dissipation tests, CPT friction ratio, or soil sampling.     

Resistance Factors for Use in Shallow Foundation LRFD

In shallow foundation design, the key improvements offered by LRFD over the traditional working stress design (WSD) are the ability to provide a more consistent level of reliability between different designs and the possibility of accounting for load and resistance uncertainties separately. In the development of LRFD, a framework for the objective, logical assessment of resistance factors is needed. Additionally, in order for LRFD to fulfill its promise for designs with more consistent reliability, the methods used to execute a design must be consistent with the methods assumed in the development of the LRFD factors. In this paper, a methodology for the estimation of soil parameters for use in design equations is proposed that should allow for more statistical consistency in design inputs than is possible in traditional methods. Resistance factors for ultimate bearing capacity are computed using reliability analysis for shallow foundations both in sand and in clay, with input parameters obtained from both the cone penetration test and the standard penetration test, and for both ASCE-7 2000 and AASHTO 1998 load factors. Resistance factor values are dependent upon the values of load factors used. Thus, a method to adjust the resistance factors to account for code-specified load factors is also presented.     

Correlation of Elastic Constants with Penetration Resistance in Sandy Soils

In this note, we investigate some existing correlations between soil modulii and penetration resistance using a direct approach. We make use of settlement data from case histories for both shallow and deep foundations over sandy soils. Knowledge of foundation dimensions, loads, and measured settlements permits us, using elasticity theory, to calculate the theoretical soil modulus. We can then directly compare the calculated modulus with measured penetration resistance. A total of 276 case histories are considered. The resulting data set is used to assess the efficacy of two well-known correlations between soil modulii and penetration resistance. We find that both correlations provide reasonably good representations of the data, but have a conservative bias in the sense that the predicted modulus values are smaller than the best-fit results. From an engineering standpoint, both correlations represent reasonable solutions and our results should lend comfort to future users of either correlation.     

Resistance Factors for Use in Load and Resistance Factor Design of Driven Pipe Piles in Sands

Load and resistance factor design of foundations is done in the offshore industry and is now being done in bridge projects in the United States. Common methods used to establish resistance factors include calibration to assumed factors of safety and reliability analysis using field load test databases. Reliability analyses are the preferred tools for this work but the needed probabilistic information regarding design method uncertainty is difficult to obtain. Furthermore, field load test databases, while relatively attractive for assessing design uncertainty, are not able to discriminate between uncertainties caused by soil variability, test methods, and model design relationships. In contrast to previous efforts, this paper illustrates an approach to uncertainty assessment that seeks to isolate the various sources of uncertainty. Using this approach, reliability analysis is used to develop resistance factors for the design of driven pipe piles in sand. The resistance factor results are used to highlight some of the differences between design methods that are exposed by the proposed uncertainty assessment technique. A brief design example is also given that illustrates the use of the resistance factors.     

Effect of Penetration Rate on Penetrometer Resistance in Clay

This paper discusses the effects of penetration rate on the penetration resistance in soft clay for various shaped penetrometers (cone, T-bar, ball, and plate) and for T-bars with different aspect ratios. Constant rate (“normal”) and variable rate (“twitch”) penetration tests, where the penetration rate was successively halved over eight steps with the penetrometer advanced by one or two diameters in each step, were undertaken in the beam centrifuge at the University of Western Australia. The tests were conducted on samples reconstituted from clay collected from the Burswood site in Western Australia. The twitch tests showed higher penetration resistance than the corresponding normal tests after the penetration rate had been reduced by a factor of 16 due to cumulative effects of partial consolidation. The penetration rate at which the resistance started to increase due to partial consolidation was used to estimate the consolidation coefficient, cv, of the reconstituted clay. The interpreted cv values were similar to values estimated from other consolidation data, both in the centrifuge and from laboratory Rowe cell and constant rate of strain consolidation tests. In addition, results from in situ twitch tests at the Burswood site were examined to evaluate viscous effects on the penetration resistance.     

Estimating Liquefaction-Induced Lateral Displacements Using the Standard Penetration Test or Cone Penetration Test

A semiempirical approach to estimate liquefaction-induced lateral displacements using standard penetration test (SPT) or cone penetration test (CPT) data is presented. The approach combines available SPT- and CPT-based methods to evaluate liquefaction potential with laboratory test results for clean sands to estimate the potential maximum cyclic shear strains for saturated sandy soils under seismic loading. A lateral displacement index is then introduced, which is obtained by integrating the maximum cyclic shear strains with depth. Empirical correlations from case history data are proposed between actual lateral displacement, the lateral displacement index, and geometric parameters characterizing ground geometry for gently sloping ground without a free face, level ground with a free face, and gently sloping ground with a free face. The proposed approach can be applied to obtain preliminary estimates of the magnitude of lateral displacements associated with a liquefaction-induced lateral spread.     

Database Assessment of CPT-Based Design Methods for Axial Capacity of Driven Piles in Siliceous Sands

Numerous cone penetration test (CPT)-based methods exist for calculation of the axial pile capacity in sands, but no clear guidance is presently available to assist designers in the selection of the most appropriate method. To assist in this regard, this paper examines the predictive performance of a range of pile design methods against a newly compiled database of static load tests on driven piles in siliceous sands with adjacent CPT profiles. Seven driven pile design methods are considered, including the conventional American Petroleum Institute (API) approach, simplified CPT alpha methods, and four new CPT-based methods, which are now presented in the commentary of the 22nd edition of the API recommendations. Mean and standard deviation database statistics for the design methods are presented for the entire 77 pile database, as well as for smaller subset databases separated by pile material (steel and concrete), end condition (open versus closed), and direction of loading (tension versus compression). Certain methods are seen to exhibit bias toward length, relative density, cone tip resistance, and pile end condition. Other methods do not exhibit any apparent bias (even though their formulations differ significantly) due to the limited size of the database subsets and the large number of factors known to influence pile capacity in sand. The database statistics for the best performing methods are substantially better than those for the API approach and the simplified alpha methods. Improved predictive reliability will emerge with an extension of the database and the inclusion of additional important controlling factors affecting capacity.     

Undrained Anisotropic Monotonic Behavior of Sand from In Situ Tests

A procedure for estimating the undrained stress-strain behavior of sand from drained self-boring pressuremeter and seismic piezocone penetration tests is proposed in this paper. The procedure offers an inexpensive alternative to laboratory testing and avoids the uncertainty of the empirical methods based on index measurements such as the Standard Penetration Test blow count and the tip resistance in a Piezocone Penetration Test (CPTU). To check its validity, the proposed procedure was used to infer the undrained triaxial stress-strain curves and the results were compared with laboratory triaxial tests on undisturbed samples. The undrained limit equilibrium stability of a dike was also assessed using the inferred stress-strain behavior to illustrate the usefulness of the procedure. The result of the stability analysis was found to be in qualitative agreement with the observed performance of the dike during a recent field experiment attempting to trigger static liquefaction.     

High Overburden Stress Effects in Liquefaction Analyses

A reevaluation is presented of two factors that can strongly affect the estimation of liquefaction resistance for clean sands under high effective overburden stresses (σv′): the relation used to normalize penetration resistances to a σv′ of 1 atm (i.e., CN), and the adjustment factor for the effects of σv′ on cyclic resistance ratio (i.e., Kσ). These two factors have been investigated in a number of ways and several relations exist for each of them. An improved CN relation is developed based on cone penetration theory and validation against calibration chamber test data for both cone penetration and standard penetration tests. A relative state parameter index (ξR) is shown to provide a consistent theoretical framework for interrelating the penetration and cyclic loading resistances. It is subsequently shown that the CN and Kσ relations are interrelated through the sand properties and relative density (DR) in ways that have compensating effects on the predicted cyclic resistance. The derived relations provide an improved representation of the effects of high σv′ levels, and reduce the conservatism that results when some established relations are extended to σv′ levels higher than they were calibrated for.     

Estimation of Load Capacity of Pipe Piles in Sand Based on Cone Penetration Test Results

Pipe piles can be classified as either closed- or open-ended piles. In the present paper, the load capacity of both closed- and open-ended piles is related to cone penetration resistance qc through an experimental program using calibration chamber model pile load tests and field pile load tests. A total of 36 calibration chamber pile load tests and two full-scale field pile load tests were analyzed. All the test piles were instrumented for separate measurement of each component of pile load capacity. Based on the test results, the normalized base resistance qb/qc was obtained as a function of the relative density DR for closed-ended piles, and of both the relative density DR and the incremental filling ratio (IFR) for open-ended piles. A relationship between the IFR and the relative density DR is proposed as a function of the pile diameter and driving depth. The relationship between IFR and DR allows the estimation of IFR and thus of the pile load capacity of open-ended piles at the design stage, before pile driving operations.     

Spatial Distribution of Excess Pore-Water Pressure due to Piezocone Penetration in Overconsolidated Clay

This paper presents the results of an analysis of the spatial distribution of the excess pore-water pressure induced by piezocone penetration into overconsolidated clays. From the experimental results obtained for moderately and heavily overconsolidated clays, it was observed that the excess pore-water pressure increases monotonically from the piezocone surface to the outer boundary of the shear zone and then decreases logarithmically, approaching zero at the outer boundary of the plastic zone. It was also found that the size of the shear zone decreases from approximately 2.2 to 1.5 times the cone radius with increasing overconsolidation ratio (OCR), whereas the plastic radius is about 11 times the piezocone radius, regardless of the OCR. The expressions developed in this study based on the modified Cam clay model and the cylindrical cavity expansion theory, which take into consideration the effects of the strain rate and stress anisotropy, provide a good prediction of the initial pore-water pressure at the piezocone location. The method of predicting the spatial distribution of excess pore-water pressure proposed in this study is based on a linearly increasing Δushear in the shear zone and a logarithmically decreasing Δuoct, and was verified by comparing the pore-water pressure measured in overconsolidated specimens in the calibration chamber.     

Standard Penetration Test-Based Probabilistic and Deterministic Assessment of Seismic Soil Liquefaction Potential

This paper presents new correlations for assessment of the likelihood of initiation (or “triggering”) of soil liquefaction. These new correlations eliminate several sources of bias intrinsic to previous, similar correlations, and provide greatly reduced overall uncertainty and variance. Key elements in the development of these new correlations are (1) accumulation of a significantly expanded database of field performance case histories; (2) use of improved knowledge and understanding of factors affecting interpretation of standard penetration test data; (3) incorporation of improved understanding of factors affecting site-specific earthquake ground motions (including directivity effects, site-specific response, etc.); (4) use of improved methods for assessment of in situ cyclic shear stress ratio; (5) screening of field data case histories on a quality/uncertainty basis; and (6) use of high-order probabilistic tools (Bayesian updating). The resulting relationships not only provide greatly reduced uncertainty, they also help to resolve a number of corollary issues that have long been difficult and controversial including: (1) magnitude-correlated duration weighting factors, (2) adjustments for fines content, and (3) corrections for overburden stress.     

Simplified Cone Penetration Test-based Method for Evaluating Liquefaction Resistance of Soils

This paper presents a new simplified method for assessing the liquefaction resistance of soils based on the cone penetration test (CPT). A relatively large database consisting of CPT measurements and field liquefaction performance observations of historical earthquakes is analyzed. This database is first used to train an artificial neural network for predicting the occurrence and nonoccurrence of liquefaction based on soil and seismic load parameters. The successfully trained and tested neural network is then used to generate a set of artificial data points that collectively define the liquefaction boundary surface, the limit state function. An empirical equation is further obtained by regression analysis to approximate the unknown limit state function. The empirical equation developed represents a deterministic method for assessing liquefaction resistance using the CPT. Based on this newly developed deterministic method, probabilistic analyses of the cases in the database are conducted using the Bayesian mapping function approach. The results of the probabilistic analyses, expressed as a mapping function, provide a simple means for probability-based evaluation of the liquefaction potential. The newly developed simplified method compares favorably to a widely used existing method.     

Influence of Penetration Rate on Penetrometer Resistance

Penetration resistance in fine-grained soils varies with the rate of penetration. Considering undrained behavior as a reference, as the rate of penetration is reduced, soil resistance increases because of the effects of partial consolidation and soil strengthening immediately ahead of the probe. Many penetration tests have been performed under different rates of penetration to identify the range of drainage characteristics of the soils used, correlating these conditions with laboratory interpretations and in situ tests. A backbone curve relates the variation of the normalized point resistance with the normalized rate of penetration. This work presents an analytical approach to the backbone curve equation used to fit test data. In addition, this paper presents a set of centrifuge tests with variable penetration rates performed with a soil classified as silty tailings, which has different geotechnical behavior from most of the soils used by previous researchers.