Pile Tests Research Articles

Evaluating the effect of post-grouting on long bored pile based on ultra-weak fiber Bragg grating array

Fiber optic sensing has been widely acknowledged as a dependable tool to monitor pile performance. However, its applicability is restricted by the limited multiplexing capacity of fiber Bragg gratings (FBG) or the high equipment cost and poor real-time performance of distributed fiber optic sensing systems. In this paper, a novel sensing cable and system were developed based on ultra-weak FBG array for strain measurement of large-scale engineering structures. Through the laboratory calibration and field test of a post-grouting pile, the good strain measurement performance of the sensing cable and system were confirmed. Based on the distributed strain data obtained by the UWFBG array, the strengthening mechanism of the pile was well explained. The UWFBG array and sensing system could realize distributed, high-precision, and real-time strain measurement with a spatial resolution of 1 m within 1.5 km, which has great potentials in the monitoring of geo-environment and engineering structures.

Dynamic compressive and splitting tensile characteristics of rubber-modified non-autoclaved concrete pipe piles

Non-autoclaved concrete pipe piles are gaining attention as an environmentally friendly alternative to autoclaved concrete pipe piles. The purpose of this study was to investigate the changes in the impact resistance of a non-autoclaved concrete pipe pile with the addition of rubber. To this end, various volume fractions of rubber particles were used to replace the fine sand in the non-autoclaved pipe pile concrete (0%, 5%, 10%, 15% and 20%). The dynamic and static mechanical responses of rubber-modified non-autoclaved pipe pile concrete were explored through quasi-static and dynamic compression and splitting tensile tests. The results showed that (1) The tensile-compression ratio of the rubber-modified non-autoclaved pipe pile concrete increased with the rubber content, and the brittleness of non-autoclaved concrete was improved by adding rubber. (2) When subjected to impact load, rubber particles could absorb some of the impact energy, reduce the impact load on the aggregate, and prevented the aggregate from being crushed. (3) The impact toughness index and energy consumption ratio of non-autoclaved pipe pile concrete increased with the increase of rubber content. When the rubber content was 10%, the impact toughness index and energy consumption ratio of non-autoclaved pipe pile concrete were 52.2% and 51.2% higher than those of non-autoclaved pipe pile concrete without rubber particles, respectively, and its impact resistance was the best. The impact resistance of rubber-modified non-autoclaved pipe pile concrete was better than that of traditional autoclaved pipe pile concrete, and met the mechanical performance requirements of prestressed high-strength concrete pipe pile. It could be used as a new prestressed high-strength concrete pile material, which helped to reduce energy consumption in the production process of piles. The study provided parameter basis for the subsequent drop hammer impact test and numerical simulation of pipe piles.

Thermo-mechanical characteristics of laterally loaded energy piles under multiple heating-cooling cycles in mountain area

In this paper, model tests of energy piles used in mountain area under asymmetry filed condition with and without an external lateral load under multiple heating-cooling cycles were conducted. The asymmetry of the soil will produce asymmetric temperature field and stress field in mountain area. The variation of pile top displacement, horizontal soil pressure, and pile bending moment due to cyclic temperature load were measured and analyzed. The results showed that the pile top horizontal displacement with an external lateral load decreased when the energy pile was heated and increased when it was cooled. However, the variation trends of pile top horizontal displacement with and without an external lateral load showed a significant difference. When the pile subjected to an external load was heated, the soil pressure at the upper part behind the pile would increase and that at the lower part would decrease, and the change was opposite during cooling. The bending moment would be changed by the thermal load, and it increased during heating with an external lateral load, while the variation of bending moment was not obvious during cooling.

Field Test Study on the Bearing Capacity of Extra-Long PHC Pipe Piles under Dynamic and Static Loads

Pretensioned prestressed high strength concrete (PHC) pipe piles are widely used in various engineering foundations, which have the advantages of high single pile bearing capacity, strong adaptability to geological conditions and high degree of construction mechanization. In order to study the vertical compressive bearing performance and settlement characteristics of ultra-long PHC pipe piles, high strain dynamic detections and static load tests were carried out on four PHC piles with a diameter of 0.9 m on site. It can be seen from the field test that the bearing capacity of the prefabricated pipe piles was time-dependent. By the end of the dynamic test, the bearing capacity of each test pile increased by 27% to 66%. The static load test also verified the rationality of the value of the restitution coefficient. Therefore, the final bearing capacity of the pile foundation can be predicted by using the high strain initial driving results and the restitution coefficient, which can reduce the repeated driving process, effectively save the cost and improve the engineering efficiency. Under 2.1 times the design load, the change range of the pile concrete modulus is from 37.5 GPa to 52 GPa, the change range of the pile side friction resistance is from 0 kPa to 97 kPa and the change range of the pile end to pile bottom load ratio is from 0% to 7.54%. During the test, the shaft friction and end bearing of the lower part of the piles were not fully mobilized. The shaft friction resistance, the end resistance and the movement behavior of the pile top and the end of the piles can provide parameter references for the subsequent design and construction of the piles.

Three-Stage Analysis Method for Calculating the Settlement of Large-Diameter Extralong Piles

In terms of safety, large-diameter extralong piles require highly accurate settlement prediction because of their high load-bearing capacity. According to the bearing mechanism of the large-diameter extralong pile, in this paper, a novel and scientifically accurate three-stage analysis method is proposed for predicting pile settlement based on Randolph’s study of pile forces in homogeneous soils, in view of the limitation that the accuracy of the conventional settlement calculation method is only applicable to the pile in the end-bearing frictional bearing condition. The three-stage analysis method fully considers the deformation characteristics of the pile in three different bearing states: pure friction bearing stage, end-bearing friction bearing stage, and damage stage. The critical load of TP4-1 in this paper is 9620.89 kN, and the ultimate compressive bearing capacity is 19279.09 kN, as calculated using the three-stage analysis method, and thus the three bearing states are clearly defined. The settlement under all levels of load obtained using this method can be very close to the real-case settlement, and the maximum error of the TP4-1 pile is only 0.52 mm, as observed by numerical simulation, static load test of TP4-1, and experiments on the other three test piles.

Impacts of reference strain rate and strain level dependency on rate effects in fine-grained soils

Viscous rate effects in fine-grained soils may occur in many engineering applications (e.g. dynamically penetrated anchors, sampling, rapid load testing of piles or high-speed soil characterisation), where high strain rates are induced during shear. Often it is difficult to correct for this enhancement of soil strength as fundamental understanding and quantification of rate effects is an area where further research is required. This is at least in part due to limitations of soil characterisation equipment and instrumentation. With the aim of tackling some of these issues a high-speed triaxial testing system was developed incorporating lubricated end platens, which was used to test kaolin at strain rates from 0·1 to 180 000%/h. This allowed observation of behaviour from fully drained, partially drained and undrained conditions, through to viscous effects. Results were used to investigate how rate effects are influenced by strain and consolidation level and how rate normalisation parameters are affected. This was then used to improve a commonly adopted rate characterisation approach such that it performed better across a wider range of strain rates and can be used for more consistent evaluation of rate effects between studies.

Behavior of Piles Reinforced by Geosynthetics Under Lateral Load

This paper introduces the results of eleven lateral pile loading testsperformed on concrete piles reinforced with different materials such as FRP bars,geosynthetics geogrids, and composite of two materials to check their efficiency incarrying the lateral load. The lateral loading pile test was applied on three groupsconsisting of ten reinforced concrete pile specimens, and control concrete pilespecimen reinforced by steel bars. All samples have the same dimensions (150 mmdiameter x 1050 mm length). This research assumed that the pile was placed in avery soft clay soil and rested on a crushed stone layer, so the frictional effect of thesoil was neglected. A comparison has been carried out between experimental resultsfor all samples. The experimental results illustrated that the lateral loads carried bypiles were increased up to 25.3% by using FRP bars, biaxial geogrid, and uniaxialgeogrid. Moreover, a non-linear finite element analysis was verified by Abaqusstandard software and achieved a great rapprochement with the experimental results.Finally, a comparison was carried out between the reinforcement cost for allsamples, which showed that using these composite piles decreased its cost up to59%.

Field testing study on jacked pile penetration characteristics in laminated clay based on FBG sensing technology

In order to analyze the change of static pressure pile characteristics in silt and silty clay with the penetration depth, two PHC pipe piles were penetrated in the field in the actual project, and the research was based on the full section test technology and FBG test technology. The results show that the pile pressure increases with the increase of the pile penetration depth; The change of pile tip resistance is affected by the soil layer hardness; When the pile tip is in the transition stage, the pile tip resistance increases significantly; In the process of pile penetration, the hardness and brittleness of the soil layer will be affected by the pile penetration, and the different soil layers will have different effects on the pile tip resistance of the two piles; When the pile goes deep into the soil, the pile side resistance will change slowly, but it will usually increase. This fluctuation is related to the difference of the soft and hard layers of the soil layer, and will also be subject to performance constraints; The pile end resistance ratio in hard soil will be smaller than that in soft soil, which will further affect the load sharing ratio of the test pile. The test results can provide support for practical engineering technology.

Evaluation of Methods Based on CPTu Testing for Prediction of the Bearing Capacity of CFA Piles

Analysis of pile bearing capacity is an important task in the investigation of soil-structure interaction. The paper is dedicated to the prediction methods for the pile bearing capacity calculation based on the cone penetration test (CPTu) results, namely UniCone method, Laboratoire Central des Ponts et Chaussées method (LCPC), and the method involved in the Eurocode 7—2. A set of CFA piles was tested to obtain reference bearing capacity. The ability of the prediction methods to determine the bearing capacity of the pile was investigated. In each evaluation criteria using statistical tools, the methods were ranked based on their performance. The results of the study indicate that the UniCone method is most applicable for the given conditions. The EC 7—2 method showed the largest variability of results, and we do not recommend its application without a deeper analysis. The applicability of any presented method cannot be considered final or universal. It is advisable to use more modern and updated methods which have been developed from a larger database of pile tests. The development of these methods should continue by expanding the database of tested piles together with the application of more advanced rock environment testing procedures.

Effect of Base Grouting on the Bearing Capacity of Bored Piles

Post-grouting is a widely used technology for ground improvement and the strengthening of bored pile foundations. In situ experiments have shown that post-grouting is effective in improving the bearing capacity of bored piles. However, due to the cost and complexity of the in situ tests, the total number of test piles is limited. Therefore, it is not surprising that uncertainty is always associated with the collected data. This may significantly affect the conclusion of analyses, and most current studies have neglected this effect. In this paper, statistical analyses were carried out in order to investigate the effect of uncertainties in the data and the impact of post-grouting on the bearing behaviors of bored piles. The results show that post-grouting can significantly improve the bearing behavior of piles. This study is expected to provide technical guidance for post-grouting in foundation engineering.

Earth pressure distribution on laterally loaded offshore monopiles

Unlike conventional pile foundations, a monopile is a large diameter open-ended steel tube driven into the seabed. Accurate quantification of the earth pressure distribution along a monopile due to lateral loads plays a pivotal role in the design of offshore wind turbines and remains a problem of great interest. This paper presents an attempt to address the problem by means of three-dimensional (3D) finite element modeling, with focus on the effect of pile slenderness on earth pressure distribution and the associated soil deformation mechanisms. The study shows that as pile diameter increases, the deformation mode of the pile will change from flexural to rotational deflection while the deformation pattern of the surrounding soil varies from being the wedge-type to a combination of the wedge-type and the rotation-type. The magnitude of either normal contact stress or shear stress at a pile section decreases with increasing pile diameter, but the distribution pattern of the normalized contact stress (either normal or shear stress) is nearly the same. The implications of these findings for engineering practice are two-fold: (a) Use of the p-y relationship established from field tests on piles of reduced scales in the design of large-diameter piles may still result in a potential risk of overestimating the lateral bearing capacity if the test pile diameter is not large; and (b) a unified factor independent of pile diameter can be introduced to average the contact stress across the pile section in the development of simplified methods.

Cumulative cyclic response of offshore monopile in sands

Satisfactory performance of offshore wind turbines is governed by the cyclic response of its foundation under wind and wave. Most methods of evaluating the lateral cumulative deformation of monopile due to cyclic loading are based on small-diameter pile tests installed in dry sand. This paper investigates the cumulative deformation of large-diameter monopiles installed in saturated sand. A numerical model was developed employing FLAC3D and was validated by comparing its predictions with the results of triaxial tests and centrifuge tests. The validated numerical model was used to evaluate and compare the cyclic responses of monopiles between saturated case and no pore pressure case. To evaluate the cumulative deformation for offshore monopile due to cyclic loading, the numerical model was refined to account for the effects of number and amplitude and frequency of cyclic loading, soil permeability coefficient, soil relative density and pile diameter. It was found that the larger subsidence and the less capacity of soil around the pile is caused by accumulation and dissipation of transient excess pore pressure, thus the monopile in saturated sand would experience the larger cumulative deformation. Correspondingly, based on this parametric study, an analytical model was developed to calculate the pile-top cumulative displacement of offshore large-diameter monopile under lateral cyclic loading considering the effects of pile diameter. This analytical model was validated against the results of centrifuge tests and model pile tests in saturated sands.

Distribution and dissipation laws of excess pore water pressure based on pile-soil interface during pile-sinking in saturated clay

PHC pipe pile cooperated with jacked method is widely used in recent years. The excess pore water pressure of pile-soil interface caused during jacked PHC pipe driving would produce important effect on jacked pile construction and bearing characteristics. Two full scale jacked-in piles are penetrated into the clayey soil and studied by monitoring the law of generation and dissipation of excess pore water pressure. The law of excess pore water pressure with penetration depth and time is also analyzed. From this test results, excess pore water pressure of the pile-soil interface increases to the peak and then begins to decrease, but it remains large. The excess pore water pressure of S1 test pile pile-soil interface decreases from 201.4 kPa to 86.3 kPa, and decreases from 374.1 kPa to 114.3 kPa for S2 test pile. The excess pore water pressure of pile-soil interface rises first at the initial stage of consolidation, and it dissipates when the hydraulic gradient between pile-soil interface and soil around the pile disappears. The dissipation degree of excess pore water pressure reaches from 75% to 85%. The proposed solution provides certain engineering significance for jacked pile construction and bearing capacity design in the clayey foundation.

An Application of Spherical Cavity Expansion Theory in Soft Rock Pile-Base Resistance

The failure mode should be taken into account in the analytical solution for pile base resistance. In order to determine the pile’s failure pattern in soft rock, the graphical model test method is used. That the compression zone below the pile base in soft rock is compatible with the spherical cavity expansion is shown. An improved calculation model for the ultimate base resistance may be developed by considering the force balance of the rigid cone under the pile-end in the ultimate stress state and the limit pressure required to expand a spherical cavity. To account for the influence of socket length, the limiting pressure for the spherical cavity expansion in infinite space is adjusted to that in semi-infinite space. The ultimate resistance of a base is computed by factoring in the length effect. The normalization of the base resistance-displacement relation is provided based on the findings of pressure chamber experiments on pile load-settlement, which are independent of socket length and overburden pressure. This semi-empirical technique is verified via the use of pressure chamber experiments and field testing of piles in soft rock.

Effects of soil unloading and grouting on the vertical bearing mechanism for compressive piles

The present study conducts a series of model cast-in-place piles tests to analyze the vertical bearing characteristics and the grout returned diffusion mechanism along the pile shaft of tip post-grouting piles with different grouting and pile shaft soil unloading conditions. The model test results show that the pile tip post-grouting technology can improve the weak difference in the bearing characteristics and axial force distribution of the pile foundation caused by pile shaft soil unloading. The pile shaft resistance can be improved by the returned grout in the pile shaft. The pile shaft resistance will mainly bear the pile top load when the pile top load and the grouting volume are relatively small, and the pile tip resistance will mainly bear the pile top load with the increases of the pile top load and the grouting volume. The grouting return height increases with the increase in pile shaft soil unloading degree, and the analyzed values of the grouting returned height based on the pile bearing characteristic results are close to the measured values. The grouting diffusion mechanism of the cement on the pile tip and pile shaft soil during the pile tip post-grouting process is revealed.

Soil Interaction of H-shaped Steel-RC Stepped Pile of Integral Abutment Bridge: Experimental Evaluation

The reinforced concrete piles (RC) in integral abutment bridges (IABs) are challenging to meet the longitudinal deformation of bridges with significant lateral stiffness. The H-shaped steel (HS) piles are costly and prone to corrosion. This paper proposes a new form of pile comprised of HS and RC piles in series to meet the longitudinal deformation demand of IABs. Tests were conducted for one HS pile under reciprocating low-cycle pseudo-static test and two HS-RC stepped piles with different bending stiffness ratios of HS/RC (0.25 and 0.5) using the HS-RC pile test model,. The influence of the stiffness ratio on the mechanical behavior of stepped pile-soil was studied. The mechanical behavior of the stepped pile was also compared with single HS and RC piles. and the applicability of the m and p-y curve methods to the calculation of horizontal displacement of stepped pile is discussed. The results show that the elastic deformation range of the HS pile is 2 mm to 25 mm, and its horizontal deformation capacity and bearing capacity are excellent. As the stiffness ratio increases, the stepped pile-soil system's yield displacement and yield load increased. The stiffness ratio has no significant effect on the failure mode of the piles. The hysteretic ring of the stepped pile shows a squeezing shape at the initial loading stage, but becomes a full spindle in the later loading stage. The stepped pile shows an excellent energy consumption effect and a horizontal deformation ability, indicating it is suitable for IABs. The m method has better accuracy only up to 2 mm displacement, while the p-y curve method still has higher accuracy within 15 mm.

Прогноз несущей способности свай в билинейной постановке по результатам испытаний свай

The article analyzes the possibility of using the analytical method for forecasting the bearing capacity of piles in a bilinear formulation for prefabricated piles. The pile bearing capacity forecasting method in a bilinear formulation is an alternative pile bearing capacity forecasting technique that allows obtaining a load-settlement graph for a single pile, which makes it possible to forecast the pile bearing capacity in an analytical formulation. There were carried out 12 full-scale tests of bored piles and 19 factory-made piles for static indentation load. For each test, there was performed a numerical simulation of pile testing for a static indentation load and an analytical calculation of the bearing capacity of the pile in a bilinear formulation. The results of the analytical calculation are compared with the results of field tests of piles and the results of numerical simulation. Based on a comparison of calculated and experimental data, conclusions have been made about the possibility of using an analytical calculation of the bearing capacity of piles in a bilinear formulation.

Investigations of piles by bidirectional static loading test in Astana soils

The article presents the results of vertical static load tests of diametral and deep bored piles specially conducted at the construction site of EXPO-2017 in Astana, Kazakhstan. The methodology for determining the bearing capacity of the pile is summarized. Bored piles with a length of 31.5 m and a diameter of 1000 mm were tested. Vertical static load tests were conducted for the following load configurations: Bidirectional Static Loading Test (according to ASTM D8169), Static Compression Loading Test according to ASTM D1143-07 and Static Loading Test according to GOST 5686-12. The method presented in the paper can serve as practical guidelines to assess the capacities of bored piles installed in the field. This geotechnical investigation is important for understanding the soil-structures interaction on difficult soil ground conditions related to the construction sites.

Study on the thermal response of spiral energy piles based on field test

Field tests of spiral energy piles under the combined effect of temperature and loading are relatively few. Based on the field test, the heat transfer efficiency, pile strain, axial force and shaft friction of two spiral energy piles were studied. The major findings of the experimental studies were: First, when the double spiral energy pile was heated, the temperature distribution was more uniform; the total heat transfer and the heat transfer rate were higher than those of the single spiral energy pile. Second, the pile strain distribution was such that smaller values were noticed at both pile ends while larger values were in the middle part of the pile. The additional tensile stresses of the two piles generated during cooling reached 4.06 MPa and 4.75 MPa, which exceeded the tensile strength of concrete. Finally, during heating, the shaft friction was negative in the middle and upper pile and positive in the middle and lower pile. The single spiral energy pile showed two neutral points. The downward load generated by the single spiral energy pile was about 885 kN higher than that generated by the double one. The aforementioned changes should be focused on in the actual project.

The horizontal load test on pressure-injected piles in damped subsiding Soils

In connection with the needs of the present, the question of the optimal design of building foundations, taking into account the regional characteristics of loess-like silty clay soils, common in Mongolia, becomes relevant. In recent years, in the Orkhon-Selenginsky region and especially in the Darkhan region, industrial and civil buildings and engineering structures are being actively built up and planned to regulate the problems of overpopulation and smoke in the city of Ulaanbaatar, Mongolia. The regularity of the process of changing the stress-strain state of loess-like subsidence soils of the base due to technogenic moistening is subject to the general theory of subsidence soil mechanics, but each region of the world has its own distinctive feature depending on territorial and climatic conditions. Settling clayey soils of Mongolia in their natural state have a relatively high content of water-soluble and slightly soluble carbonate and other salt compounds and, due to their cemented and crystallized structural bonds, they have relatively high mechanical properties, but the mechanical properties, due to the weakening of structural bonds during soaking, sharply decrease to a value of c =7.0…10.2 kPa, φ=16….÷20°, E=3.5…4.5 MPa and subsidence begins depending on the type of soil at pressure Р=0,15 MPA and W= ( 6.7…8.6)% or W_sat, while the relative subsidence index is ε_sl>0.01. At pressure Р=0,2MPa and W=(6.1…7.6)%, subsidence begins, and at pressure Р=0.25MPa and W=(5.2…6.9)% drawdown begins. These moisture values are calculated respectively at the given pressures as the initial settling moisture W_sl. This article discusses the results of field tests of bored piles for a horizontal load, taking into account the moistening of the subsidence soil of the foundation of buildings and structures.