Historical Development of the Equations Used for the Design of CFA Piles in Granular Soils - Essay Example

Historical development of the equations used for the design of CFA piles in Granular Soils Continuous flight auger(CFA) can be considered to be present between driven piles and drift shaft, an idea that has a primary attribute towards different lateral stress changes during times of installation of the various types of piles. In most cases, especially during drilled shaft construction, soil stress seems to reduce or stay at the initial rate in the vicinity of excavation of a pile. When installing a driven pile, the process involved displaces the soil vertically, and hence, it raises the stress in the surrounding soil. According to Brown in the book Design and Construction of Continuous Flight Auger Piles Final, it is relatively important to note that the estimated static CFA capacity is achieved through methods that are developed for specific objects including drilled shafts and driven piles, mainly because the behavior of CFA piles’ load-settlement are same (2007, p.226). Several methods are specifically meant for CFA piles which take modification form that is made to the methods that were previously developed purposefully for drilled shafts, as well as driven piles (Brown, 2007, p.226). Additionally, Lord and Hayward in the book Shaft Friction of CFA Piles in Chalk, state that there is growth of End-Bearing Resistance and Side-Shear with Pile Displacement, which carries similar characteristics to other kinds of deep foundations. In this case, the overall axial compressive resistance (RT) of an identified CFA pile is achieved through the calculations of combining the side-shear resistance (RS) with the end-bearing resistance (RE). RT = RS + RB While calculating the total side-shear resistance requires a division of the pile length in to N pile sectors. Then side resistance of a specific pile segment “I” is achieved through multiplication of the surface area of the pile with the achieved unit side-shear resistance of the segment (Lord and Hayward, 2003, p.345). RS = ∑iN fs,i π Di Li However some methods presented use specifically an average unit side-shear for the pile length just instead of totaling individual pile segments. This is evident in the book Numerical modelling of soil-pile axial load transfer mechanisms in granular soils by Aguiar, Sofia, and Arézou Modaressi. For this case, the formula for the total side-shear resistance is: RS = fs-ave π D L where D is for the average diameter of the pile, and L represents the pile total embedment length. On the other hand, the formula for total end-bearing resistance is: RB=qp[πDB2] where qp stands for the unit end-bearing resistance, and DB stands for the diameter of the pile at the base (Aguiar et al., 2008, p.112). Brown in the book Design and Construction of Continuous Flight Auger Piles Final argues that the right estimation methods offered for static axial capacity of an individual CFA piles puts an assumption that the construction method of a conventional continuous flight auger will be introduced. In addition, quality assurance procedures and practices of construction consistent with the recognized herein are followed up to par in that excessive soil flighting and loosening of ground is avoided. It is confirmed that high-displacement auger cast piles use and also the use of amelioration leads to increase in pile capacity (Brown, 2007, p.345). The additional recommendations on design procedures are hereby organized broadly by the type of soil as either cohesive or non-cohesive in the subdivisions that follow thereafter. This information is given in the literature, Specification and design criteria for the construction of continuous flight auger piles in the Houston area: final report to the Texas Department of Transportation for project number 7-3921 written by Hassan and Michael. Notably, that soils that are silty require judgment on the side of the evaluation of the most reasonable approaches usable by the engineer. Generally, Soils should be categorized in relation to the anticipated characteristics under the load being put in to consideration that is as whether the soil is prone to undrainage or full drainage. Techniques of either cohesive or non-cohesive soils must be employed depending on this classification and hence further categorized by the availability of in-situ and or laboratory test data (Hassan and Michael, 1997, p.65). The recommended method or technique of End-Bearing and Side-Shear and End-Bearing through the use of undrained shear strength is the FHWA1999 method. It is mainly for drilled shafts in CFA piles especially in cohesive materials (Michael and Frank, 2002, p.343). For an identified pile segment, the formula for the ultimate unit-shear resistance (fs) becomes fs = α Su, in which Su represents the undrained shear strength of the soil at the location of the pile segment, while α stands for decrease factor that varies as α = 0.55 for Su / Pa ≤ 1.5, in which the standard atmospheric pressure is represented by Pa, for 1.5 < Su/Pa ≤ 2.5, α varies linearly from 0.55 to 0.45. This is argued out in the book by Michael and Frank called Deep foundations 2002 an international perspective on theory, design, construction, and performance: proceedings of the International Deep Foundations Congress 2002. Furthermore, Michael and Frank in the book Deep Foundations 2002 an international perspective on theory, design, construction, and performance: proceedings of the International Deep Foundations Congress 2002 state that neglect of the side-shear input on the capability of the bottom of the piles’ mono diameter length occurs only and only if the bottom pile is bearing on clay. And there is an existence of the soil’s potential to shrink away starting from the top most level of the pile on exposure to the atmosphere that is if the top layer is clayey. If there are suspicions that such conditions do exist, side-shear contribution resulting from the identified layer should be done away with in the greater of seasonal change depth or the top of 5 ft. or the depth of seasonal moisture change (Michael and Frank, 2002, p.202). According to Turner, John and Paul in GeoSupport 2004 drilled shafts, micropiling, deep mixing, remedial methods, and specialty foundation systems, there are several tests that are used to obtain certain geotechnical parameters used in the designing of equations. The in-situ tests, for instance, the Standard Penetration Test (SPT), the Cone Penetration Test (CPT, CPTU and SCPTU), the Flat Dilatometer Test (DMT), the Pressure Meter Test (PMT), and the Vane Shear Test (VST) have been exploited widely to resolve engineering parameters that are needed for geotechnical design. The in-situ tests mentioned at most cases are always combined with laboratory tests such as soil index tests, triaxial tests, unconfined compression tests, direct shear tests and consolidation tests. When we narrow down to CPT, the results from this shows that it has received great attention in the recent times from several departments of transportation. These tests can effectively take soil characteristics of large volumes of soils, thereby reducing the need to underdo very large numbers of time-consuming laboratory tests. In-situ is unlimited since it does not only save on time and cost but also does away with the concerns regarding sample disturbance resulting from soil sampling and storage (Turner et al., 2004, p.605). The commonly used in-situ testis The Standard Penetration Test (SPT) but however it suffers from dependency of SPT blow count N on the operator, it lacks accuracy and emphasis and also it lacks interpretation basing on theory. Despite the limitations, CPT offers fast and continuous soil profiling since it is not operator-dependent and in addition it has a strong theoretical basis for interpretation (Turner et al., 2004, p.615). Lord and Hayward who are the authors of the book called Shaft friction of CFA piles in chalk supports the fact that CPT test or method has been used increasingly by many departmental states of transport just because of its obvious advantages. This official trend highlights that saving costs can be reached at by practising geotechnical designs with the use of results obtained from advanced in-situ tests. For the proof of this, the transportation department of Indiana acquired CPT tool for improving the geotechnical service (Lord and Hayward, 2003, p.232). Hassan and Michael who wrote the Specification and design criteria for the construction of continuous flight auger piles in the Houston area: final report to the Texas Department of Transportation for project number 7-3921 says that remarkable advancements can be noticed in the interpretation of the CPT test but however there are still noticeable limitations involved in using such technique in determining geotechnical designs. This is mainly because most of empirical bond in between CPT findings and properties of the soil have been availed for textbook soils. In addition, some empirical relations basing on the CPT have developed with no consideration of certain important factors such as effect rate and proper estimation of undrained shear strength. Therefore it means that all the factors should be considered for a better performance and provides more accurate geotechnical design (Hassan and Michael, 1997, p.312). In Indiana cone factor evaluation was performed by the field cone penetration test and laboratory tests for clayey soils. Good enough, CPT effect rate was considered and the isotropic consolidated undrained compression test for shear strength was also used. This only provides the applicable area where the methods discussed were used. In the Indiana core factor evaluation, effectiveness of such methods is put in to practice and good results are obtained. As construction practices are concerned it is always recommended that safety measures are taken in to action to prevent unnecessary damages. According to Brown in the book Design and construction of continuous flight auger piles final, he says that, for health safety training to employees should be done. He further gives the safety measures to be put in to consideration. All employees, contractor and maintenance employees included who are involved with highly hazardous chemicals are supposed to be fully equipped with knowledge about the safety and health hazards of the substances they handle and process they work with for high protection of themselves, fellow employees and also the citizens of the nearby community (Brown, 2007, p.336). As a safety measure, safety information should be processed. Full and truthful written information concerning process chemicals, technology and equipment is very essential to an effective process of safety management program and to a process hazards analysis. The team that performs the process hazards analysis will take the compiled safety information as very vital and very important. Such measures will help in maintaining safety of employees, employers and the entire community that surrounds the firm. It is very advantageous to do so (Brown, 2007, p.337). Several methods may be applied to obtain the geotechnical parameters required in the design equations. This is always done during the historical development of the equations used for the design of CFA piles in granular soils. These tests are different, but some tests are not effective that is why such steps as the Flat Dilatometer Test (DMT), the Standard Penetration Test (SPT), the Cone Penetration Test (CPT), the Vane Shear Test (VST), the Pressure Meter Test (PMT), and are considered to be effective during the process. These tests are known as the in-situ tests. References Aguiar, Sofia, and Arézou Modaressi 2008, Numerical modelling of soil-pile axial load transfer mechanisms in granular soils. Brown, D. A. 2007, Design and construction of continuous flight auger piles final. Washington, D.C.: U.S. Department of Transportation, Federal Highway Administration. Hassan, K. M., and Michael W. Neill 1997, Specification and design criteria for the construction of continuous flight auger piles in the Houston area: final report to the Texas Department of Transportation for project number 7-3921. Houston, Texas: University of Houston, Center for Innovative Grouting Materials and Technology. Lord, J. A., and T. Hayward 2003, Shaft friction of CFA piles in chalk. London: CIRIA. Neill, Michael W., and Frank C. Townsend 2002, Deep foundations 2002 an international perspective on theory, design, construction, and performance : proceedings of the International Deep Foundations Congress 2002, Orlando, Florida: Geo-Institute, American Society of Civil Engineers. Turner, John P., and Paul W. Mayne 2004, GeoSupport 2004 drilled shafts, micropiling, deep mixing, remedial methods, and specialty foundation systems. Reston, Va.: ASCE Research Library. Read More