Frank Rausche

Static and Dynamic Models for CAPWAP Signal Matching

Signal matching is the preferred analysis method for Dynamic Load Test (DLT) evaluations. It is applicable to DLT records of driven piles, auger-cast piles, drilled shafts, and even on dynamic penetrometers. Although signal matching is considered standard best-practice, required by many code specifications and therefore routinely used on thousands of deep foundation projects worldwide, and of significant importance to the deep foundation industry, many features of the CAPWAP® signal matching model and procedure are not well known. CAPWAPs signal matching is possible because of the availability of redundant measurements of load and movement, and it is necessary to determine the unknown boundary conditions. The goal of CAPWAP is the determination of dynamic and static soil resistance parameters of the generally accepted Smith-type pile-soil interface model. However, the classic Smith model cannot explain some of the phenomena that occur during the impact event. For reliable signal matching results, therefore, several modifications of the original Smith model were made. While some modifications fundamentally do not affect the signal match, other more substantial changes are of considerable importance to the reliable determination of the all-important static load bearing capacity result. Before discussing the CAPWAP procedure and its automatic analysis tools, this paper describes the more unusual CAPWAP pile and soil model parameters and their effects on the final results. Measurement and analysis results from actual projects demonstrate the various features of the program and aspects of the models. The paper includes a summary of recommended limits for model parameters, match qualities, and calculation procedures and a few suggestions for additional research.

Analysis of Post-Installation Dynamic Load Test Data for Capacity Evaluation of Deep Foundations

Construction of deep foundations causes changes to the original geotechnical condition of the surrounding soils and rock. The nature, extent, and effects of these alterations, and subsequent time-dependent modifications, on the long-term load bearing capacity of the foundation elements are mainly functions of the characteristics of the supporting geo-materials and type of installation method. Post-installation Dynamic Load Testing (DLT) is commonly used for capacity evaluation of driven and drilled deep foundation elements and is performed under impacts of the pile driving hammer or a special large drop weight loading device. Use of a large drop weight loading device is usually appropriate for cast-in-place shafts and driven piles in high-setup soils to activate the full bearing capacity. The DLT procedure consists of applying a specific number of blows (typically 2 to 10 blows) and measuring pile/shaft strain and motion time-records, and the resulting displacement. This paper presents discussions on the analysis of post-installation DLT data for capacity evaluation of driven and drilled deep foundations. Technical aspects related to the proper test record selection for analysis considering pile/shaft set per blow, changes of capacity from blow to blow with subsequent impacts, energy levels, and total penetration during the test are delineated. Analytical procedure and engineering evaluation of results are discussed with recommendations for practical use. Test records from actual case histories are utilized to demonstrate data characteristics and illustrate proposed numerical methods and recommended analytical procedures for rational evaluation of DLT results for engineering use.

Defect Analysis for CSL Testing

Cross-hole Sonic Logging (CSL) has become a common method to evaluate the integrity of drilled shafts. However, the interpretation of test measurements by this method requires some judgment and experience. Many previous proposals rely solely on the arrival time, or wave speed. A proposed method which takes into account both arrival time and signal strength is presented. If defects are detected, then in some cases a tomography analysis may be helpful to further quantify the result. The situations where tomography is useful will be discussed. Recommendations are given for the type and amount of data required for a tomography analysis. This discussion is illustrated with a case history of a test shaft with purpose built defects to demonstrate the advantages of these evaluation methods.

The Dynamic Components of End Bearing in Highly Cohesive Clay during Pile Driving Process

This paper introduces the study on the dynamic component of foundation end bearing in high plastic clays. The total toe resistance during pile driving composes both a static and a dynamic component. It is generally assumed that under the quasi-static condition (when the velocity becomes zero), the quasi-static resistance equals to the total resistance. However, experimental results show that in high plastic clay, the quasi-static resistance can be significantly larger than the static resistance. Directly using the quasi-static resistance as the total pile static resistance could cause overestimation of the bearing capacity. The origin of this phenomenon is analyzed and attributed to the visco-elastic effects and excessive pore pressure generated during pile driving process. To quantify these effects, analyses were conducted by simplified model and 3-D Finite Element Method. In the 3-D finite element method, the interaction between pile-soil system is introduced through frictional contact boundary. Soil is described by a visco-elastic model and a model that can describe analogously pore water built-up. The results are analyzed to determine the relationship between the computed dynamic toe resistance variations and the extent of effective viscosity. Subsequently this determines the relative contributions of static and dynamic components to the total toe resistance. Data collected in an instrumented driven pile with toe resistance measurement will also be analyzed to investigate this phenomenon. A tentative criterion is proposed to account for the dynamic components of end bearing in highly cohesive clay.

Monitoring Quality Assurance for Deep Foundations

Constructing deep foundations is both an art and a science and their as-built acceptance should be based on evidence of quality confirmed by testing. Early prevention of problems is the most effective means to avoid costly and time consuming construction delays. Recent developments in equipment and methods aid in real-time monitoring of various pile installations. For driven piles, monitoring of diesel hammers has been done for years using the Saximeter. The measured kinetic energy of any hammer type can now be transmitted by telemetry to the Saximeter unit and stored electronically in an installation log for downloading or computer printout. Traditionally, the Pile Driving Analyzer (PDA) is used by an experienced engineer who collects and interprets measurements on the construction site, and later issues a report after returning to the office. The engineers travel and availability often dictates the testing schedule and impacts the construction activity. Utilizing wireless cell phones, dynamic pile testing can now be done remotely at the contractors convenience with substantial cost savings and even more substantial time savings. In the office, the PDA engineer receives and simultaneously views the measured data in real-time, immediately analyzes the data, and summarizes the monitoring results, often issuing the test report within hours of the test. Thus, the foundation installation and quality assurance testing proceed without interruption or delay. Devices are available for auger cast-in-place pile installations to guide the contractor in real-time to installing a good pile with documented quality so that it can be accepted without doubts or time delays. For drilled shafts, the most common quality assurance tool is cross hole sonic logging (CSL). New 3D tomography analysis of CSL data holds further promise to evaluate the quality of drilled shafts.

Pile Damage Prevention and Assessment Using Dynamic Monitoring and the Beta Method

Driven piles are subjected to high stresses during installation. It is, therefore, important not to exceed acceptable stresses along the pile shaft and at the toe to prevent damage. Dynamic monitoring has been used for decades to evaluate not only the installation stresses, but also to check test piles for signs of structural damage. The Beta Method (β-Method) for evaluation of the location and extent of a potential damage was developed more than 30 years ago and has proven effective as a quality control (QC) and quality assurance (QA) tool. As an aid in the process of pile rejection or acceptance, the β-method also offers a rating scale that translates the automatically determined β-number into a helpful pile integrity assessment tool. The reliability of this algorithm has been proven by numerous extracted piles. However, one limitation of the β-method concerned detection of damage near the pile toe where high toe resistance effects and/or stress wave reflections reduce the effectiveness of the traditional β-method. In the past, therefore, near-toe damage was determined by the testing engineer, not only by visual inspection of the dynamic monitoring data, but also by reviewing the pile toe compressive stresses throughout the monitored driving history and the strength and stiffness of the soil response from the pile toe. This approach has now been automated and subjected to tests on existing data. After a review of the existing methods of pile stress and damage calculations, the paper presents the new method, illustrating its effectiveness by examples from measurements on both concrete and steel piles.

Improved Methods for Rapid Load Tests of Deep Foundations

Rapid Load Tests have been promoted as an alternative for Static Pile Load Tests since the late 1980s. Rapid Load Tests create a relatively long duration force pulse in comparison with Dynamic Load Tests. However, a reliable prediction of pile static capacity with this test based on the so-called Unloading Point Method (UPM) has been subject to debate. Estimates of static capacity by UPM have overestimated static load test results when the applied force pulse produced too little axial movement. To resolve these issues, a drop mass Rapid Load Test called the Hybridnamic Test (HT) was developed with a ram mass up to 700 kN to reliably estimate the pile static ultimate bearing capacity up to a 35 MN. This paper discusses using a multi-cycle Hybridnamic Test in soil of mostly sand. A new interpretation method called Fully Mobilized UPM, with a required minimum net penetration per test cycle, was applied to this case study and comparison of estimated soil resistance was made between conventional UPM and Fully Mobilized UPM, and with signal matching of the dynamic force pulse data using CAPWAP. Finally a practical method to prepare a Static Load-Displacement Curve based on multi-cycle HT results is proposed.

Soil Resistance Predictions From Pile Dynamics

The present work extends the application of the force and acceleration records to the calculation of the distribution of soil resistance along the pile. It also shows how the records are used to predict the magnitude of dynamic resistance that the soil applies to the pile, an important factor in choosing efficient hammer characteristics. A method for obtaining a more accurate simplified prediction of total static bearing capacity is also presented. It should be emphasized that the aforementioned predictions are all made from measurements at the pile top only. The work is correlated by presentation of results for 24 pile tests which include construction static load tests as well as specially instrumented load test piles. The application of these results can have considerable impact on foundation costs. Static load tests are very costly and time consuming. In Ohio a single test on a service pile using tension reaction piles (also service piles) typically costs

Alternate Verification Methods for Augercast Piles

3,000 to

Dynamic Determination of Pile Capacity

5,000. This static test provides much useful information about the particular pile which was tested. However, due to variability of soil properties the information may be of less value for other piles in the structure. This is reflected in the large factors of safety commonly used for piles. The proposed dynamic measurements methods can be applied to a substantial number of service piles at less than the cost of a single static load test. The dynamic analysis herein differs from the general dynamics problem in which either the boundary force or acceleration record is given as input and the other record calculated as output. In the present dynamic analysis, both force and acceleration are shown and thus one of the two records can be viewed as redundant information. The second record is, therefore, used in the present analysis to give information on pile resistance effects; e.g., in the absence of soil resistance, the acceleration at the pile top completely determines the force at the top from Newtons and Hookes laws. The presence of resistance along the pile and at the pile tip affects the force at the top in a precise and predictable manner and makes it possible to compute t e magnitude and location of resistance forces along the pile. A simple soil resistance model is used, which consists of an elastoplastic shear resistance and a linear viscous damper. The dynamic analysis will be reviewed herein to present the basic ideas behind the work. The detailed mathematical expressions are given in Refs. 3 and 9.