Continuous Flight Auger Piles Research Articles

The Influence of Granite Size and Workability on the Compressive Strength of Continuous Flight Auger Pile Concrete

Concrete achieving 30N/mm² in Continuous Flight Auger (CFA) concreting is a challenge due to coarse aggregate size and the required workability of the concrete for easy pumping. This study investigated the impact of workability and coarse aggregates on the compressive strength of Continuous Flight Auger concreting. Aggregates sizes of ¾ and ⅜ inches were used for the experiments and the slump value for the mix ratios were set in range 145 to 190mm. Three mix ratios; 1:2:4, 1:1.5:3 and 1:1:2 was adopted and mix composition was done by volume method based on what is expected on construction site. Twelve (12) cubes (150x150mm) of each type of coarse aggregate with and without superplasticizers (strength enhancement, and retarder) for the various mixes were cast for 7, 21, 28 and 58 days to determine their compressive strengths. Granite sizes ¾ and ⅜ with mix ratio 1:1:2 was found to attain the targeted strength without admixtures, whereas the ¾ inches aggregates for all the mixes with the two admixtures exceeded the targeted strength with at least 13 percent. Hence, concrete of Grade C30 is achievable for CFA pile concrete without using admixture.

Effect of soil-pile contact parameters on pile bearing capacity value

The soil and foundation elements are fundamental to the behavior of a structure. When the foundation soil has a low capacity, it is necessary to transmit the loads to a deeper and more resistant layer, using deep foundations. A pile load test is a field measurement of the bearing capacity (vertical or horizontal) of a foundation element. In addition to a physical model of a load test, the use of computational tools can bring additional understanding. In order to analyze the responses obtained by numerical models compared to field test results, tools that use Finite Element Methods (FEM) and that have a series of constitutive models become a great alternative. This paper aims to study the behavior of Continuous Flight Auger (CFA) piles, aiming to understand the soil-pile interaction regarding the effect of soil-pile contact parameters on pile bearing capacity value. In this way, experimental data from three load tests were compared with numerical simulations using the Mohr-Coulomb constitutive model. Also, a parametric study was completed based on the collected soil data from the literature of the region that most affected the settlement behavior of the load pile. It was concluded that there is a direct and significant influence of the friction coefficient on the pile bearing capacity. In addition, by incorporating more real soil data and parameters representing its actual formation, the results of the pile load test simulation can be taken as an alternative to estimate the ultimate capacity of a vertically loaded pile in the absence of the static load test, which has been widely observed in engineering practice.

Investigation of laboratory and field tests of piles installed by displacement technology

When compared to conventional pile systems, the Drilled Displacement System (DDS) and Continuous Flight Auger (CFA) piles offer advantages in terms of improved load-carrying capacity, quicker installation, and less spoil generation. In this article, experiments on miniature piles built utilizing the Drilled Displacement System (DDS) and Continuous Flight Auger (CFA) technologies in a soil-filled test tank are described. For the study, model piles with dimensions of 20 mm in diameter and 300 mm in length were used, scaled at 1/20. The model piles are subjected to static loading, and throughout each phase of the loading process, the relevant displacements are measured. For both DDS and CFA piles, the analysis's findings on load-settlement curves and estimates of ultimate bearing capacity were compared. According to the study, the DDS piles outperformed the CFA piles in terms of load-carrying capacity. Additionally, full-scale field tests with static loads were performed on DDS-drilled piles with dimensions of 400 mm in diameter and 6 m in length. The field test's load-settlement response exhibits good agreement with the model testing. Overall, the study's findings offer insightful information about the behavior and performance of DDS piles that may be used to improve the design and installation of these piles in various soil types.

Comparison of bearing capacities of model drilled micro piles using DDS and CFA technologies

Drilled piles installed using CFA (Continuous Flight Auger) and DDS (Drilled displacement system) technologies are relatively new construction products in the market of Kazakhstan for the last 15 years, but already today the technology has a significant practical value in the construction of the cities of Astana, Almaty and other regions. This article presents model tests of bored piles installed using Drilled Displacement System (DDS) and Continuous Flight Auger (CFA) technologies on a volumetric bench. For testing we used 1:20 scale, piles diameter was 20 mm, length was 300 mm. Drilling was performed using augers that were prepared in advance through a 3D printer. The load on the model piles was applied in steps of 39 N up to the ultimate load of 391 N. Based on the results of the study, the «settlement-load» plots of the DDS and CFA model piles were obtained, as well as a comparison of the bearing capacity of these piles by static test method. Based on the study, it was found that the DDS piles exhibit good bearing capacity performance compared to the CFA piles. Overall, the results of the study provide valuable information on the performance behavior of DDS and CFA piles, which can be used to optimize their design and installation in different soil types.

Comparative Investigation of Axial Bearing Performance and Mechanism of Continuous Flight Auger Pile in Weathered Granitic Soils

Axial bearing performance and mechanism of continuous flight auger (CFA) pile in weathered granitic soils, i.e., a widespread special soil in South China, were investigated by field test in this study. Load–settlement responses of four CFA piles were examined, and evolutions of shaft/base resistances were captured by ultra-weak fiber Bragg gratings (UWFBG) with a reflectivity ≤−40 dB. Performances of CFA piles were compared with those of a slurry displacement (SD) pile at the same site, thirteen pretensioned spun high-strength concrete (PHC) piles in the literature and empirical data in design code. Test results show that the ultimate bearing capacity of the CFA pile is highest among different pile types, and typically is twice that of the SD pile. Again, CFA pile produces the highest shaft resistances at 140 kPa and 153 kPa in two weathered granitic soils, while the base resistance of 3080 kPa is between those of the SD pile and the PHC pile. By field excavation, the superior mechanism of the CFA pile is suggested to avoid the formation of in-between bentonite layers and prevent preferential baseflow along fissures, both of which can weaken the soil–pile interface. Overall, this study provides fundamental data through UWFBG and explanations based on field observations which underpin the need for developing a design code specified for CFA piles in South China.

Semi-empirical method for the bearing capacity of continuous flight auger piles based on installation energy

The prediction of load capacity and the control of the execution of the Continuous Flight Auger (CFA) piles are often exercised with components of empiricism and intuition. This fact is often added to the uncertainties arising from the formation of the soils and the limitations of preliminary studies that support the project design. In this context, aiming to aid the executive control of CFA, a semi-empirical method is proposed based on the soil type, geometric dimensions of the piles, and the installation energy obtained during the pile excavation. The method makes it possible to determine the CFA pile load capacity during the execution process of each pile of pilling. As a consequence of the proposed method, the settlement of each pile can be controlled through the quantification of the energy required or the work carried out to excavate each pile through a specific software installed in the machinery monitoring system that increases the safety and reliability of the piling.

The 2022 Pacheco Silva lecture: The influence of residual loads on pile foundation behavior

Residual loads can affect the load transfer and the settlement-induced in-service loadings, although they do not alter the bearing capacity. When residual loads are present and not measured or evaluated, the settlement estimate is greater than predicted if these loads are known. Residual loads can be measured when the pile instrumentation is nullified before pile installation, in the case of displacement piles, or before the first loading in non-displacement piles, such as bored cast-in-place piles, continuous flight auger piles, and micro-piles. In the case of underpinning foundation and piled raft, when the loading transfer is shared by the original and new foundation, or by the piles and the raft, it is essential to know the stiffness of each foundation element to estimate the load partition. If residual loads are present, pile stiffness is greater than when not considered in the design. The paper revisits this theme of practical relevance. A historical review of the most relevant research involving pile residual load measurements, pile loading tests including the interpretation of residual loads locked at a pile toe, and a new procedure for residual loads prediction are provided. A comparison is made of the experimental residual loads observed in some of the instrumented cases and the values estimated with the suggested procedure. The development of residual loads at the pile toe as a function of the toe resistance to total capacity ratio is very similar to the variation of the soil density as a function of soil moisture content in soil compaction.

Seasonal variation of the shaft resistance of CFA piles in unsaturated soil: a case study

Excessive settlements or failure of piles observed in some Brazilian unsaturated soil sites indicates that the shaft resistance can vary with the seasonal variability of top soil water content, especially in the case of foundations of transmission towers or installed at the periphery of buildings, situations in which the soil is more susceptible to suction fluctuation due to the higher exposure to rainfall action. The objective of the research described in this paper was to provide field evidence with the purpose of alerting practising engineers and to help researchers to better understand the contribution of matric suction on pile capacity. For this aim, the seasonal variability of the shaft resistance of Continuous Flight Auger (CFA) piles in a typical Brazilian unsaturated tropical soil was evaluated by performing axial tensile load tests on eight instrumented CFA piles in different periods of the year. Soil suction was monitored throughout the period of tests, and the results indicate that the shaft resistance of the pile upper part varies considerably during the year and tends to decrease with decreasing soil suction. This work provides valuable insights into the application of unsaturated soil mechanics, which is usually disconnected to classical geotechnical practice.

Three-dimensional numerical analysis of the generalized group effect in monitored continuous flight auger pile groups

The interaction mechanisms related to the group effect between piles and between pile groups significantly influence the soil-structure interaction process. This interaction causes the superposition of stresses and, in general, makes the pile group settlement different from the settlement of an isolated pile. The objective of the present paper is to evaluate the soilstructure interaction mechanisms of buildings with foundations of monitored continuous flight auger piles (CFA) in a stratified soil mass, with the presence of an intermediate soft soil layer. Hence, it is particularly analyzed the group effect between piles of a group and the group effect between all pile groups from a foundation of a study case instrumented by means of numerical modeling, considering the effect of the soft soil layer. The results show the significant group effect on displacements, showing the increase in settlement due to the overlapping of the tension bulbs of the piles and neighboring pile groups.

An Energy-Based Big Data Framework to Estimate the Young’s Moduli of the Soils Drilled during the Execution of Continuous Flight Auger Piles

Understanding the soil mass and how it behaves is determinant to the quality and reliability of a foundation design. Normally, such behavior is predicted based on laboratory and in situ tests. In the big data era, instead of executing more tests, engineers should understand how to take advantage of ordinary execution procedures to obtain the parameters of interest. Sensors are key components in engineering big data frameworks, as they provide a large number of valuable measured data. In particular, the building process (excavation and concreting) of continuous flight auger piles (CFAPs) can be fully monitored by collecting data from sensors in the drilling machine. This makes this type of pile an ideal candidate to utilize a big data methodology to indirectly obtain some constitutive parameters of the soil being drilled. Thus, in the present paper, the drilling process of CFAPs is modeled by a new physical model which predicts the energy spending during the execution of this type of pile. This new model relies on other fundamental properties of the soils drilled, such as unit weight, cohesion and internal friction angle. In order to show the applicability of the big data methodological framework hereby developed, a case study was conducted. A work site in Brasília-DF, Brazil, was studied and the execution of three CFAPs was monitored. Soil surveys were carried out to identify the soil strata in the site and, therefore, to validate the estimates of Young’s moduli provided by the new formulas. The 95% confidence intervals of Young’s moduli obtained for silty clay, clayey silt and silt were, in MPa, [14.56, 19.11], [12.26, 16.88] and [19.65, 26.11], respectively. These intervals are consistent with literature reports for the following materials: stiff to very stiff clays with low-medium plasticity, medium silts with slight plasticity, and stiff to very stiff silts with low plasticity, respectively. These were the types of materials observed during the site surveys; therefore, the results obtained are consistent with literature reports as well as with field surveys. This new framework may be useful to provide real-time estimates of the drilled soil’s parameters, as well as updating CFAPs designs during their execution. This way, sustainable designs can be achieved, where substrata materials are better characterized, avoiding over-designed structures.

A novel short pile foundation system bonded to highly cemented layers for settlement control

While design methods of deep foundations are mainly developed for homogenous soil deposits, the presence of highly cemented layers could lead to underestimation of resistance and overestimation of settlement of pile foundations. This study presents a novel approach using competent caliche layers bonded to the top and bottom of a continuous flight auger (CFA) pile as a new composite foundation system named caliche stiffened pile (CSP). The key objective is to optimize the required pile length in a cost-effective approach without ameliorating soil properties. Settlements of the CSP foundation for a high-rise building were monitored and full-scale tests were conducted to measure piles’ capacity. Finite element back analyses were performed to avoid adverse effect of sample disturbance in settlement calculations. A back calculation of a test fill embankment was performed to determine soil stiffness parameters by simulating an unscheduled imposed load to the structure. Impacts of the CSP on controlling the settlement of pile foundation and optimizing the required pile length are investigated using finite element analysis and a parametric study. The proposed CSP foundation can reduce the CFA pile settlement significantly in the presence of caliche layers with thickness equal or greater than a pile diameter at CFA pile head and toe, where the CSP is located.

Model and field tests of drilled displacement system piles

Drilled Displacement System (DDS) and Continuous Flight Auger (CFA) piles are two popular techniques used for building pile foundations that offer advantages over traditional pile systems, including improved load-carrying capacity, reduced installation time, and less spoil generation. This article presents laboratory-scaled model tests conducted on model piles installed using the Drilled Displacement System (DDS) and Continuous Flight Auger (CFA) technologies on a test tank setup filled with soil. Model piles considering a scaling factor of 1/20 with a diameter of 20 mm and a length of 300 mm were adopted for the study. Static loading is applied to the model piles and the corresponding displacements are measured during each loading phase. The results of the analysis were compared for load-settlement curves and ultimate bearing capacity estimates for both DDS and CFA piles. Based on the study, the DDS piles were observed to perform well in terms of load-carrying capacity compared to the CFA piles. Further, full-scale field tests under static load conditions were carried out on DDS-drilled piles of diameter 400 mm and length 6 m. The load-settlement response from the field test shows good agreement with the model tests. Overall, the results of the study provide valuable insights into the behavior and performance of DDS piles that can be used to optimize their design and installation in different soil types.

Estimating Shear Strength Properties of the Surrounding Soils Based on the Execution Energies of Piles

Historically, empirical relations are the basis of everyday foundation design. These relations, however, rely on specific datasets, which may not represent the true conditions observed in the field. Even in situ tests rely on empirical correlation formulas, which link observed phenomena to soil properties. These correlations should be updated according to the specific design conditions. Big data (BD) workflows enable the use of massive data available to update the correlations and to provide more accurate predictions of the parameters studied. Thus, in this paper, a BD approach is used to study the relation between the drilling process of continuous flight auger piles and the shear strength properties (SSPs) of the surrounding soils. Soil surveys were carried out to identify the soil strata in the site and to validate the estimates of the SSPs. The results show that indirect measurements are in accordance with typical undrained shear strength and friction angles of the materials considered.

Execution energy of continuous flight auger piles as an assessment tool to evaluate the mechanical response of the soil mass

Allying technology, theory and engineering practice is one of the main challenges of modern foundation engineering. Current foundation and retaining walls designs may lead to oversizing or undersizing due to the spatial variability of the stratigraphic profile. As the design procedure of piled foundations involves defining the setting depth of the tip, sometimes the piles may not behave as expected, as the soil mass may change significantly even in a small region. In this paper, a study was conducted on the geoenergetic behavior of the soil mass and of the piles of a construction site located in Brasília, DF, Brazil. Previous studies showed that the energy needed to drill a continuous flight auger pile (CFAP) can be related to the strength of the drilled strata. Therefore, a methodological framework was built to further discuss how the geomechanical behavior of CFAPs foundations could be assessed by analyzing the energy needed to drill such piles. Statistical methods, in special the Bootstrap method, were used to assess the possible influences of construction procedures on pile behavior. It was studied how the execution energies of piles vary with initial depth and relative position in the terrain, as well as the influences that they suffer because of the order of execution within a same foundation group and due to the proximity to surrounding retaining walls. The proposed methodology can be used to improve the energy control procedure and performance evaluation of CFAPs, allowing a complementary reliability assessment for the foundation and retaining walls designs and implementations.

Thermal design of energy piles for a hotel building in subtropical climate: a case study in São Paulo, Brazil

The use of shallow geothermal energy through energy piles for the air-conditioning of buildings is increasing worldwide. This type of renewable energy technology is still not utilized in Brazil, where the hot dominating weather regions and the air cooling demand predominate. In this case of unbalanced heat transfer to the ground, the efficiency of the system may decrease with time due to the excessive heat injection into the soil. In order to investigate the possibility of an efficient application of this technology in São Paulo city, a balanced use of the ground for a ground-source heat pump (GSHP) system utilizing energy piles is evaluated in the present paper. Energy foundations were designed to meet the balanced heating and cooling loads (air conditioning and water heating) of a hypothetical business hotel building located in a site at the campus of the University of São Paulo, where thermal response tests (TRTs) were conducted on different types of energy pile. The number of energy piles required to supply the building thermal loads were estimated using the pile heat exchanger modelling software PILESIM 2.1 and compared with an analytical model prediction. The evaluations were done for three different types of pile tested at the site chosen for this study: micropiles, steel pipe, and continuous flight auger (CFA) piles. The results indicate that the ground heat extraction should be considered for the use of GSHP systems with energy piles in air cooling-dominated scenarios similar to the case studied here.

Assessment of Brazilian onshore wind turbines foundations

Abstract Onshore wind turbine foundations are subjected to large overturning moments. The wind action imposes cyclic and dynamic loading conditions which occur in extreme and service scenarios. Deep foundations, when used, transfer this large overturning moment through a pile group which combines the axial and lateral resistance of all piles. The paper explored the key aspects of onshore wind turbine foundations in Brazil. The main reason to explore this subject is that several authors consider onshore wind turbine foundations a well-understood topic; however, limited data from actual situations have been published, especially in developing countries where wind energy projects have only recently started. Thus, a survey of Brazilian energy companies and foundation designers was conducted, and the first Brazilian database of wind turbine foundations was created. This database contains data from more than three thousand Brazilian wind turbine foundations. The key aspects, types and dimensions of these foundations were summarized. Worldwide, concrete gravity foundations are the most used foundation type for onshore wind turbines. In Brazil, 43.3% of wind turbines have shallow foundations, essentially concrete gravity, and 56.7% have deep foundations, mostly continuous flight auger piles. This first stage of the research identified that Brazilian wind turbine foundations are significantly different from the ones of other countries. Approximately 70% of Brazilian wind turbine deep foundations used continuous concrete flight auger piles, most of them embedded in sandy soils. Concrete and steel are the main materials used in wind turbine structures.

VERTICALITY OF BOREHOLES

The study contains the problem of the borehole shape of during drilling. Usage of a drilling bit and tubular or screw rods is traditional and most common drilling methods. The requirement of "verticality" for boreholes in the drilling process is necessary to comply with design solutions and design schemes. At the same time, the actual implementation of this important quality is often very difficult. An analysis of the regulatory documentation is carried out to ensure the requirement during conducting geotechnical investigations, and during installing pile foundations made in the ground: bored piles and continuous flight auger piles (CFA). Data about the researchers in the field of assessing of bending of the pile axis are presented for CFA piles. The article presents the results of field studies of the borehole verticality at two investigation sites. At the first site, a borehole was drilled with a depth of 9.0 m, at the second site, ten boreholes were drilled with a depth of 25.0 m. The spatial position of the boreholes was determined by an inclinometer. The results of the measured verticality deviations of the boreholes are presented on the spatial graph for clear visibility. A conclusion is made about the influence of the borehole shape on its vertical projection during conducting of geotechnical investigations. A series of comparative numerical calculations was performed using the Midas GTS NX program to assess the effect of the verticality deviations of the boreholes during installing bored piles on their bearing capacity. A single pile action in the ground was simulated for the pressing and pulling loads. It is concluded that the shape of the borehole influences on the single pile action in the ground.

Laboratory Model Study on the Pile-Forming Mechanisms and Bearing Deformation Characteristics of CFA Piles

To study the pile-forming mechanism and bearing deformation characteristics of continuous flight auger (CFA) piles, a series of procedures, including helical drilling, pulling up/grouting, and inserting cage/pile forming, were simulated in clay-sand double-layer foundations by a homemade model drilling machine system in laboratory model tests. The effects of two different pile-forming methods on the load transfer and bearing characteristics of the piles were investigated by performing a model test comparison of CFA piles and bored piles. The experimental results show that there exist a soil improvement effect around the pile and a diameter expansion effect during the drilling process and grouting process for the CFA pile, which can effectively improve the lateral friction resistance of the pile. Compared with the bored pile, the pile diameter in the middle of the CFA pile increased by 19%, and the total lateral friction resistance of the CFA pile increased by 9.1% at a high load (1500 N). The comparative results of the model tests show that the bearing capacity of a single CFA pile increased by 50.0% and that the total settlement decreased by 40.5%. The results of the in situ test piles show that the load-settlement curves of the two pile types are similar under low-medium loads and that the lateral friction resistance of the CFA pile under high loads is better developed, which is relatively consistent with the model test results.

FIELD STUDY ON SOFT SOIL IMPROVEMENT USING CONTINUOUS FLIGHT AUGER (CFA) PILES

The continuous flight auger (CFA) piling technique is commonly used where the presence of groundwater or potentially collapsible material reduces the cost-effectiveness of bored piling techniques. Whilst generally considered a soil replacement technique, the process of pumping concrete under pressure during the pile installation can result in a significant improvement of the surrounding soil, particularly in low strength soil conditions. In this study, a case study is analyzed. An oil storage tank was intended to be constructed on soft clay in southern Iraq. The tank has a diameter of 46 m. Cone penetration test, drilling boreholes, and field tests have been conducted in the tank site. A decision was made to improve the mechanical properties of soft clay of the site by CFA piles before construction of the tank. The depth of CFA pile constructed is 16 m, with a diameter of 0.5 m and spacing between piles is 4.2 m. Three types of in-situ tests have been conducted: Conventional plate load test on the soil before and after improvement; large plate load test for soil after improvement; and pile load test on CFA pile. It was concluded that construction of CFA piles shows an increase in the bearing capacity by about 110% for the soil directly around piles and 70% for soil within 2 m from the center of piles. Large plate load test result for the pile with surrounding soil revealed that the ultimate bearing capacity increases to 500 kN/m2. So that the overall loading area showed an increase in the bearing capacity of about 350% when CFA piles are constructed.

Sustainability-Oriented Multi-Criteria Analysis of Different Continuous Flight Auger Piles

With increasing construction activity and concrete consumption globally, the economic, environmental, and social impacts of human activities continue to increase rapidly. Therefore, it is imperative to assess the choice and construction of each structure and structural component from a sustainability-based perspective. In this study, such a multi-criteria decision-making approach using the MIVES method is applied to the choice of grouped continuous flight auger (CFA) piles. Different alternatives of CFA piles are studied: length (10 and 20 m), reinforcement (steel cage reinforcement and structural fibers), and aggregates (natural crushed aggregates and recycled aggregate concrete sourced from stationary and mobile recycling plants), based on experimentally verified mix designs. All alternatives were analyzed considering economic, environmental, and social requirements, using a decision-making tree with eight criteria and eleven indicators, with weights assigned by an expert panel. The results of the analysis showed a clear advantage in terms of all three sustainability requirements for CFA piles with steel fibers and recycled aggregate concrete, with all solutions with steel cage reinforcement having significantly lower values of the sustainability index. Such results demonstrate the need for implementing innovative solutions even in structural members such as CFA piles that are often considered in insufficient detail.