Root Piles Research Articles

Research on the failure mechanisms and preventive actions of tunnels under butterfly load in loess regions

The results of surrounding rock pressure measurements from numerous loess tunnels exhibit a butterfly distribution pattern, subsequently referred to as ’butterfly load’. This pattern significantly deviates from the load distribution and magnitude—referred to as ’specification load’—calculated by current Chinese tunnel design specifications. By means of model experimental and numerical simulation, the mechanical behavior, failure cause, failure mechanism and preventive actions of tunnel structure under different load distribution are studied. The experimental results indicate that the ultimate bearing capacity of the tunnel structure under the butterfly load is significantly lower than that under the specification load, with the ultimate bearing capacity decreasing as load unevenness increases. The butterfly load increases the unevenness of the axial force distribution throughout the annular structure and also causes the structure to produce reverse moments in the vault and arch shoulder. Similarly, the morphological characteristics of the cracks show that the butterfly load most significantly influences the tunnel vault and arch shoulder. Under the influence of the butterfly load, the lining structure is prone to developing cracks at the arch shoulder and arch foot positions, with tensile cracks located at the vault position. Aiming at the failure mechanism of tunnel structure under butterfly load, it is suggested to implement advanced small conduit grouting at the tunnel arch to suppress the dislocation of the formation and to lay root piles at the arch foot to improve the bearing capacity of the tunnel bottom. The prevention effect of this technology in different loess regions is sandy loess > general loess > clay loess.

Study on the pullout bearing characteristics of bio-inspired root piles in coral sand foundation under different root buried depths

Root foundation is a new type of bio-inspired foundation, which has a broad application prospect in the development and construction of China's South China Sea area due to its good bearing characteristics such as pullout bearing capacity. The effect of root buried depth on the uplift bearing characteristics of root piles is analyzed through the model test of uplift bearing of root piles with different root buried depths in coral sand foundation. The results show that increasing the root buried depth can improve the ability of the root pile to control the uplift displacement, and there exists an optimal buried depth or range of buried depths that can effectively improve the pile foundation's uplift bearing capacity and control the ultimate displacement. The attenuation of axial force at the root is in the form of a step; with the increase of root buried depth, the maximum value of lateral friction resistance develops from the lower part of the pile body without root to the upper part of the pile body without root. The range of the bearing ratio of the root section of the pile with root buried depth of 260mm, 340mm and 420mm is 12.33%-15.68%, 9.98%-17.82% and 7.61%-21.65%, respectively; the smaller the root buried depth is, the higher is the ratio of the bearing ratio of the root section at the beginning of loading, and the bigger the root buried depth is, the bigger is the ratio of the root's final bearing ratio. The change of soil pressure around the pile increases and then decreases, and the densest point of soil compacting moves upward with the increase of root buried depth. The research results provide scientific basis for the design of root pile buried depth and root arrangement in actual projects.

Study on the pullout bearing characteristics of root piles in coral sand foundations under different water content states

Offshore wind energy will become one of the important sources of energy in the future, and root piles with good pullout bearing characteristics can be used for the foundation of offshore wind power generation facilities. Through the model test, the pullout bearing characteristics of root piles with different water content states in coral sand foundation are comparatively analyzed, and the pullout bearing deformation law and pile body force transmission characteristic law of root piles is revealed. The results show that when the coral sand foundation is saturated and the water level rises and falls, compared with the dry sand foundation, the pullout bearing capacity of the root pile decreases, and the ability to control the displacement is sharply weakened. The axial force curve at the root is in the shape of a step; after the foundation is affected by water, the maximum value of lateral friction resistance first appears at the bottom root, and the lateral friction resistance at the lower root is always greater than that at the upper root; the water level rise and fall leads to a decrease in the lateral friction resistance in the area of the pile without a root, and the lateral friction resistance in the area of the root is increased. Under saturated and water level rise and fall condition, the total bearing ratio of the root pile section increases, and the bearing ratio of the lower root pile section is always larger than that of the upper root. After loading, the soil in the upper part of the pile is compacted, and the soil in the lower part is loosened; the maximum value of soil pressure in the compacted area of the dry sand condition gradually moves downward, and the maximum value of soil pressure in the saturated and water level rise and fall condition is always located in the lowermost part of the compacted area; the pore water pressure in the saturated and water level rise and fall condition is basically equal with the hydrostatic pressure, and there is no obvious super pore water pressure generated. The research results provide scientific basis for the application of root piles in coral sand foundation.

Investigating the influence of root layer quantity on the pullout bearing characteristics of root piles in coral sand foundations

Root piles with good load-bearing characteristics can be used as foundations for offshore wind power facilities. By conducting model tests on root piles with varying numbers of root layers in coral sand foundations and studying the theoretical pullout ultimate bearing capacity, the influence of the number of root layers on the pullout bearing characteristics of root piles is analyzed. Subsequently, a formula for calculating the pullout ultimate bearing capacity of root piles is derived, drawing on the Vesic foundation bearing capacity theory and the Rankine soil pressure theory. The results indicate that increasing the number of root layers enhances the ultimate bearing capacity of root piles, enabling them to better withstand pullout forces and control displacement. The axial force curve of the roots exhibits a step-like pattern. Additionally, the maximum lateral mechanical resistance of the root piles in 3 and 5 layers gradually shifts downwards with increased loading. While the total bearing capacity of the root section increases, the rate of increase diminishes over time. Model test data validates the proposed theoretical formula for calculating bearing capacity, affirming its reliability. These research findings furnish a theoretical groundwork for the practical application of root piles in engineering.

Investigation into pullout characteristics of root piles within coral sand foundations at varied relative compaction

Root piles exhibit favorable bearing characteristics, rendering them suitable for the foundation of offshore wind power facilities to withstand uplift loads. This study compared the uplift load bearing advantages of root piles to those of equal diameter piles in coral sand foundations. Additionally, it analyzed the effects of relative foundation compaction on the uplift load bearing characteristics of root piles. The tests involved comparing root piles and equal diameter piles in coral sand foundations with relative foundation compaction levels of 0.5, 0.6, 0.7, and 0.8. The results indicate that root piles possess a 62.5% higher pullout capacity than equal diameter piles, with only a 10.7% increase in uplift displacement. As relative compaction increases, the pullout ultimate capacity of root piles initially rises rapidly and then levels off, while the corresponding uplift displacement experiences a sharp decline followed by a relatively stable state. With the increase of relative compaction, the location of the maximum attenuation of axial force and the maximum lateral friction resistance in the root region started to move deeper only under larger loads. Both the implementation of root piles and an increase in relative compaction greatly enhance the pile foundation’s ability to resist pullout forces and control deformation.

Mechanically stabilized wall (MSW) with geogrids as complement of partially executed anchored wall

This article presents the case study of Wall C11 of Lot 1 in the ringroads of Caraguatatuba and São Sebastião job site, whose original design consists of an anchored retaining wall with a concrete face supported on root piles. At the time of its interruption, only a part of the job had been executed. The alternative solution, mainly aimed at speeding up the completion of the job, was the execution of reinforced backfill with geogrids behind the concrete wall, eliminating the need for the remaining anchors. The presence of the bedrock top very close to the face of the retaining wall at some points could compromise the anchorage length required for the geogrids. In the construction phase, tests were carried out to verify the elements already executed, specially, the anchors. During the execution of the reinforced soil and after its completion, the instrumentation followed the displacements in the concrete wall.

Một phương pháp mới để cứu chữa sự cố hố đào sâu - một trường hợp thực tế

This case of study happened in Hue City, where a deep hole was required for a pumping station. The soil at that location is very soft. As the temporary shoring method for the excavation was sheet piles walls and braced beams were not enough, the failure was occurred during the excavation work, showed by the displacement and inclination of the sheet piles walls and deformation of braced beam. To solve the situation, based on the ideal of shear key principle, sheet piles and root piles panels installed perpendicular to hole’s wall to increase the shear strength of the surrounding soil. The work was completed successfully with the assistance of an intensive monitoring program. Through this paper, the authors want to shear with the colleagues an experience for excavation work in practice.

Study on safety assessment methods of gravity anchors based on a simplified mechanical model

Gravity anchor blocks are a common type of ground anchor used for suspension bridges, whose bearing depends on its large body and gravity. As there is no stratum requirement, the safety of the anchor block is important for bridge stability. This study summarizes all available estimation indexes, calculating methods, and evaluation criteria for gravity anchor block safety for the Ruili bank of the Banjin Dam grand suspension bridge. The anti-overturning, anti-sliding, base stress, and deformation safeties were comprehensively evaluated using methods including the suggested specification method (SM), simplified mechanical method (SMM), and finite element method (FEM), the results of which were compared and analyzed. The reasons for errors and improved formulas and working conditions were presented. The main conclusions were as follows. 1) The methods for calculating different evaluation indexes according to specifications lack consistency. Moreover, FEM requires that designers have good computer skills and has low feasibility in practice. 2) The SMM for gravity anchor block safety estimation as described in this study, whose indexes cover systematic and overall, the computational formula is simple and speedy, with relatively conservative results and good practicability. 3) SM, SMM, and FEM were all used for the safety estimation of gravity anchor blocks in Ruili bank. The anti-overturning and anti-sliding stability coefficients must all meet the specification requirements of 2.0. SM cannot be used to estimate the base tensile stress under limited conditions. SMM denoted tensile stress at 2.5 times the main cable design force, compared to 3.2 P for FEM. Deformation calculation methods are not given by SM, but can be suggested by SMM based on the elastic mechanics. The horizontal displacements under design load conditions were 122 mm (SMM), 108 mm (FEM), and 44 mm (composite foundation treated by root piles), with a safety standard of <80 mm. The vertical displacements were 338 mm (SMM), 110 mm (FEM), and 123 mm (composite foundation treated by root piles), with a safety standard of <160 mm. These findings proved the feasibility of SMM for the safety design of gravity anchor blocks in cases lacking regional experience.

Numerical simulation and field tests on vertical load bearing behaviour of bored root piles

Bored root piles are a new type of pile in which the load-bearing capacity can be significantly improved by jacking prefabricated roots into the soil through reserved holes. In this study, numerical simulations and computations are incorporated with field tests to fully investigate the complex load-bearing behaviour of bored root piles under vertical loading. The established numerical model is validated using a measured load-displacement curve obtained from self-balanced field tests. Using the validated numerical model, the load-transfer mechanism is investigated in terms of the load-displacement curve, axial force, shaft friction, and load-share ratios of different components. Results show that the roots share a significant proportion of the load, whereas the load-share ratios of the different root layers indicate temporal and sequence effects. Parametric studies are performed to investigate the effects of different root configurations on the vertical load-bearing behaviour of bored root piles. The results show that root layer spacing is the most important factor affecting the load-bearing capacity. Three failure modes of bored root piles are formulated in terms of root layer spacing. The results of this study can facilitate the design and application of bored root piles.

An extended BNWF model for root piles incorporating soil–root interactions under lateral loading

Root piles constitute a new type of pile in which a series of prefabricated reinforced concrete roots are inserted into the surrounding soil to improve the bearing capacity of the pile foundation. To investigate the nonlinear behaviour of root piles under lateral loading, an extended beam on nonlinear Winkler foundation (BNWF) model incorporating the soil–root interactions is proposed in this paper. The proposed model is aimed at simultaneously describing the additional bearing capacity provided by roots and the nonlinear soil–root interactions for laterally loaded root piles. Lateral loading tests are conducted on two full–scale root piles to verify the applicability and reliability of the proposed model. It is demonstrated that the proposed model can describe the lateral behaviour of root piles with a reasonable calculation accuracy. Further parametric analyses are conducted to investigate the lateral bearing behaviour of the root piles. It is found that the root pile increases the lateral bearing capacity of the piles owing to the roots. Increasing the number or length of roots and locating them in shallow soil layers can improve the lateral bearing capacity of the root piles. The obtained results are useful for the design and application of root piles.

Study of the Failure Mechanism of Mortar Rubble Using Digital Image Correlation, Acoustic Emission and Scanning Electron Microscopy

This paper describes an extensive experimental study of the compressive failure of different types of aggregates and the influence of aggregate type on the interfacial properties of mortars. Interfacial debonding was the main failure mode of mortar rubbles. The interlocking strength of aggregate and mortar strongly affected the compressive strength of materials. When basalt was used as the aggregate, I-II composite failure of the deflection crack occurred as well as interfacial debonding. The highest instantaneous AE energy of the granite mortar rubble was 1349 mV·ms, which was 4.1 times greater than that of the basalt mortar rubble (326 mV·ms). Acoustic emissions of mortar rubble were strongest in the 150–220 kHz range and gave an early warning of the damage load at high frequencies (160–320 kHz). The C-S-H gel formed by the hydration reaction adhered to the aggregate pores and exhibited a “root pile” effect that improved the bonding performance of the interfacial zone. The interfacial porosity of the basalt, granite and limestone mortar rubble was 21.29%, 18.70% and 30.0%, respectively. The limestone interface has a large porosity, the fractal cones was small (1.19), and there was an obvious sidewall effect, but the interfacial strength was weak. The pore structure had a significant effect on the interfacial bond strength. This multi-faceted analysis truly reflected the state and evolution of the damage of mortar rubbles, and the results were very effective for determining the mechanical mode of damage of mortar rubbles.

Contribution for a root pile installation control approach using a digital odometer

A root pile is a form of injection pile (cast-in-place with pressure, with very distinct construction aspects from the known micropile type). During the mortar shaft development, these piles are inserted using distinct injection pressures of up to 500 kPa. Static load tests are typically used to control root piles, which can be an expensive and time-consuming testing procedure. Static load tests were performed on eight monitored piles with diameters of 350 and 410 mm to investigate root pile performance control during pile installation. This research presents a refined and developed alternative methodology for confirming root pile performance using a digital odometer attached to the drill rig’s rotatory head. The methodology consists of monitoring variables obtained during pile installation related to pile bearing capacity. Moreover, empirical equations with simple and relevant applications to estimate root pile bearing capacity during installation are proposed. The developed equations produced results consistent with the values obtained from static load testing on the test piles. Therefore, the results suggest that the proposed methodology is a viable alternative for root pile performance control.

Performance control of root piles using a digital odometer

Root piles are injected and installed during mortar shaft construction, using pressures of up to 500 kPa. The executive control is typically done through static load testing, an expensive and time-consuming method. Static load tests on eight controlled piles (diameters between 310 mm and 410 mm) were performed, aiming at evaluating pile ultimate load. This study suggests an innovative, non-destructive approach to validate root pile field performance, using a digital speedometer connected to the drilling rig’s rotating head. The proposed method monitors variables related to bearing capacity during pile installation and proposes an empirical equation to estimate the ultimate bearing capacity of root piles. For the assessed piles, the predictions obtained with the proposed equations agreed fairly well with results from static load tests, proving it as a feasible and helpful option for the executive control of root piles, especially when load tests are not available.

Numerical analysis and field load testing of a suspension bridge with a root pile anchorage

The Qiupu River Bridge is the first suspension bridge with the application of a novel anchorage of root pile foundation. The static behaviors of the bridge were studied in this paper. Before the bridge was opened to traffic, the statically loaded field tests of the bridge have been carried out to check its load-carrying capacity. Then, a finite element (FE) model of the bridge was developed and calibrated to the measured initial equilibrium configuration. The efficiency of the FE model has been verified by comparing the calculated results of the model to the measured ones under the live loads, which indicates that calibration of the FE model to the initial equilibrium configuration is crucial to the construction of a baseline FE model. Based on the verified FE model, the influences of anchorage displacements on the bridge loading behavior were studied. Furthermore, a simplified analytical method was proposed to analyze the mechanism of structural responses under the anchorage horizontal displacements. The comparison of analytical results with FE results revealed a good agreement. It is demonstrated that the horizontal displacement of the anchorage of suspension bridges affects the structural responses more obviously than the vertical displacement, while the pylon is sensitive to the anchorage displacement. The deflection of the bridge deck results from the pylon displacement, which is caused by the unbalanced main cable force between the main span and side span. It is reasonable to take the internal force of pylons as the controlling condition of anchorage displacements. Finally, based on the analytical method, the parametric studies reveal that increasing the sag-to-span ratio and side-to-center span ratio can reduce the displacement and internal force of pylons caused by the anchorage displacement.

Use of the dynamic load test to obtain the pile capacity – the Brazilian experience

The dynamic load test is currently an important and usual tool for design, control, and quality assurance of deep foundations. The objective of this paper is to compare the expected geotechnical load capacity through empirical and semi-empirical Brazilian methods with the ultimate pile load obtained from the interpretation of Dynamic Load Tests (DLT; PDA). The stress-settlement curve was constructed from CAPWAP analysis with blows of different drop heights of increasing energy – test procedure proposed by Aoki (1989). Continuous flight augering (CFA) Franki and Root piles were evaluated in this study. These piles were tested in different cities in Brazil. Additionally, DLT results were compared with static load tests, and a good correlation was found with these field tests. The article aims to provide comparative background to guide foundation designers, as well as those who routinely develop these projects in Brazil.

A modified hyperbolicity-based load transfer model for nonlinear settlement analysis of root piles in multilayered soils

A modified hyperbolicity-based load transfer model for nonlinear settlement analysis of root piles in multilayered soils

Study the Losses in Sugar Beet Roots After Harvesting and Reducing it by using Different Storage Methods, Covering and Spraying Treatments during Storage Periods

A research experiment was conducted after sugar beet harvesting season of 2018/2019 to study the losses in sugar beet roots and reducing it by using different storage methods (shadow and sun light), covering (without, rice straw, sugar beet foliages and net) and spraying treatments (without, tap water and Mepiquat chloride at 0.5, 1.0 and 1.5 cm/L) during storage periods (one, two, three and four weeks) under environmental conditions of Dakahlia Governorate, Egypt. The experiment was carried out in factorial experiment in randomized complete blocks design with three replicates. The highest values of root fresh weight/plant, root length and diameter and infestation percentage and lowest root weight loss percentage were recorded when stored under shading. The highest values of root fresh weight/plant, root length and diameter and lowest root weight loss percentages were recorded when covering root piles with sugar beet foliages, followed by covering with rice straw. The highest values of root fresh weight/plant, root length and diameter and lowest root weight loss percentages and infestation percentages of sugar beet roots were recorded when spraying piles of sugar beet roots with Mepiquat chloride at 1.0 cm/L. It can be concluded that stored sugar beet roots after harvesting directly in piles under shading and covering with beet foliages and spraying piles with Mepiquat chloride at 1.0 cm/L to reduce losses in sugar beet roots after harvesting and during storage and achieve high apparent characters of roots under the environmental conditions of Dakahlia Governorate, Egypt

Comparative Study on the Influence of Pile Length and Diameter on Bearing Capacity and Efficiency of Root Piles, Straight-Shaft Piles and Pedestal Piles

In order to popularize the application of root pile (RP), which is a miniaturized application after the simplified construction procedure of root foundation and a promising new type of noncircular section pile, and to verify the bearing capacity and efficiency advantages of RP over traditional straight-shaft pile and pedestal pile (SP and PP, respectively), on the basis of validation of the numerical model, numerical simulation of the three pile types with different lengths and diameters and root sizes was carried out under vertical (including uplift and compressive) and lateral static loading. The results show that both the vertical and lateral bearing capacities and efficiency of RP are higher than those of SP and PP, and the larger the pile length and diameter are, the greater the advantages are. Specifically, compared with the bearing capacity of SP, vertical capacity could be improved greatly only by RP instead of PP when the pile length is no less than 15 m, and not the enlarged base of PP but the roots arranged on the upside of RP could improve lateral capacity. Besides, increasing pile length could not improve lateral capacity when the pile length is no less than 10 m, and both the vertical and lateral bearing capacities could be improved by increasing pile diameter. Two efficiency comparison methods were put forward, and their application condition, advantage and disadvantage were analysed to compare the efficiency of pile length and diameter in improving capacity, according to which the economic way to improve vertical and lateral capacities is to increase pile length and diameter, respectively. Enlarged base of PP and roots of RP could be played more fully when the displacement is large enough; therefore, the difference of capacity and efficiency of the three pile types with failure criterion of small displacement (10 mm lateral and 30 mm vertical displacement) is smaller than that with large displacement (30 mm lateral and 60 mm vertical displacement). Furthermore, in order to facilitate the application of RP, it is suggested that roots length should not exceed 0.5 times of pile diameter, and the height and width should not greater than the spacing between stirrup bars or main steel bar, respectively. In addition, roots annulus spacing of 0.4 ~ 0.5 m is appropriate, and the spacing of root layers should not be less than 2.4 m. On the whole, it is of great significance to popularize root piles in practice.

Experimental Investigation of the Uplift and Lateral Bearing Capacity of Root Piles

The root pile, a new type of pile foundation with a variable cross-section, is introduced. The mechanism of root piles under uplift and lateral loading and the construction process are presented and discussed. The bearing capacity of root piles under uplift loading, lateral loading, and combined uplift and lateral loading was compared with that of conventional straight piles through field loading tests, which were performed on two straight piles and two root piles. The results indicate that the uplift and lateral bearing capacity of root piles is significantly greater than that of straight piles; the combined uplift and lateral loading has almost no influence on the uplift bearing capacity of single piles, but would weaken their lateral bearing capacity; the roots are engaged gradually from the top to the bottom under uplift loading; the resistance of the roots increased substantially because of the interaction between the soil and roots; however, the side friction of the pile shaft above the roots decreased because of this interaction. Increasing the burial depth of the roots could improve the performance of the roots for building applications.

Study on Influence of Shield Side-piercing Construction on Pile Foundation of Nearby High-speed Railway Bridge

Taking the pile foundation of Suzhou Metro Line 3 passing through viaduct as the engineering background, the driving process of shield tunnel is simulated by finite element software. Through the analysis of surface settlement, bridge pier deformation and bridge pile foundation deformation in shield construction, the following conclusions are drawn: The foundation reinforcement “scheme of root pile + sleeve valve pipe grouting” has an obvious effect on reducing the ground settlement, the deformation of bridge piers and the deformation of pile foundation structure. The deformation and stress of the pile foundation of the bridge and the horizontal displacement of the pier above it are related to its distance from the tunnel, and the nearer it is, the greater its influence is. Whether the foundation is strengthened or not has little influence on the change law of axial force of bridge pile foundation, and the maximum axial force appears at the inflection point of horizontal displacement. Due to the excavation of shield tunnel, the bending moment of pile foundation changes in three sections along the depth direction, and the change of bending moment in the middle section is opposite to that on both sides.