Toe Resistance Research Articles

Effects of piggyback twin tunnelling on a pile group: 3D centrifuge tests and numerical modelling

The effects of tunnel construction on existing single piles have been extensively investigated, but the influence of twin tunnel advancement on an existing pile group is seldom reported in the literature. In this study, a series of three-dimensional (3D) centrifuge model tests and numerical simulations using an advanced hypoplastic soil model were carried out to investigate the response of an existing 2 × 2 pile group to piggyback (i.e. vertically aligned) twin tunnelling at various depths. Soil parameters for 3D numerical back-analyses were obtained from stress-path triaxial tests. In each twin tunnelling simulation in a centrifuge, the first tunnel was advanced three-dimensionally by controlling volume loss in-flight near the mid-depth of the pile shaft, before the second tunnel was constructed either next to the toe of the pile group (test ST), below but to one side of the pile group (test SB) or directly beneath the pile toe (test SU). The piggyback twin tunnelling in test ST resulted in the largest transverse tilting (of 0·2%) but the smallest settlement of the pile group under a working load. This is because the second tunnelling caused significant non-uniform change in vertical effective stress underneath the four piles in the group. On the contrary, the tunnelling directly beneath the pile group (i.e. test SU) caused the smallest tilting but the largest settlement of the pile group (4·6% of pile diameter) and substantial mobilisation of shaft resistance. This is attributable to the most significant and uniform loss of toe resistance of each pile in the group resulting from stress relief from the second tunnelling. Two distinct load transfer mechanisms can be identified in the pile group, namely downward load transfer in test ST and upward load transfer in tests SB and SU.

Instrumented Mechanically Stabilized Earth Wall Reinforced with Polyester Straps

A wall of mechanically stabilized earth (MSE) with large concrete panel facing, reinforced with polyester straps, is an emerging technology in the United States. To enable the design of this wall system through AASHTO's methodology, the system's classification of extensibility must be established. Therefore, a section of such a wall was instrumented as part of a newly constructed interchange on I-95 at Christiana, Delaware. The section was one of several similar walls that allow access to bridges composing the interchange. Monitoring included tensile force distribution at selected straps and force at the connection of a few straps to the large concrete panels. The recorded data were from April 2012, the beginning of construction of the instrumented wall, to June 2014, about 20 months after construction ended. When this reinforced wall system was classified as extensible, the maximum load in the straps and connections was about one-fourth the predicted value from using the AASHTO method, and the lengths of the reinforcing straps were adequate to enable the wall to sustain the required loads. The indicated conservatism of the measured versus the predicted maximum load in the reinforcements was attributed to significant underestimation of the reinforced soil strength in AASHTO's design and, potentially, to a large toe resistance. The data suggest that AASHTO's design, which is based on extensible reinforcement for this family of MSE walls, is adequate.

New Method and Application of Negative Friction Resistance Monitoring for PHC Pipe Pile

The theoretical calculation of PHC pipe pile negative friction resistance is difficult to obtain valuable results due to many complex factors involved, such as soil properties, consolidation conditions and actual working environment, so the field monitoring is the only feasible method. The pile negative friction resistance can be got by pre-buried sensors inside the pile to monitor the changes of stress and strain of shaft. However, for PHC pipe pile, the sensors can not be embedded and protected effectively in the process of pile manufacturing, which makes the negative friction resistance of PHC pipe pile become a very thorny problem. Based on the improved field monitoring method and data analysis of sliding micrometer technology, a new method was introduced into the PHC pipe pile negative skin friction resistance testing in this paper. We can got not only the PHC pipe pile negative friction resistance, distribution range and neutral point position with different load or time, but also the strain distribution, axial force distribution, skin friction resistance and toe resistance distribution of pile, etc. The practical engineering application shows that the new method is very effective to monitor PHC pipe pile negative friction resistance and very beneficial to further research on formation mechanism and theoretical calculation of negative friction resistance of PHC pipe pile.

Numerical Evaluation of the Behavior of GRS Walls with Segmental Block Facing under Working Stress Conditions

AbstractThis study numerically evaluated the combined effects of different controlling factors (i.e., wall height, stresses induced during backfill compaction, reinforcement stiffness, toe conditions, and facing stiffness) on the behavior of geosynthetic-reinforced soil (GRS) walls under working stress conditions. The results were compared with values predicted by different currently used methods. It was shown that toe resistance has a major effect on the prediction capability of those design methods. Parametric analyses have shown that the amount of tension in the reinforcement varies with restraint at the base of the block facing. For free-base conditions with a constant value for reinforcement stiffness, the tension in the reinforcements was the same irrespective of the facing stiffness and the wall height. For the fixed-base conditions, the amount of tension in the reinforcements and horizontal toe load varied as a function of the facing stiffness. Comparing individual measurements of reinforcement te...

매개변수연구를 통한 진동타입말뚝 해석기법 평가

진동해머에 의해 시공되는 말뚝의 해석기법을 개발하고 매개변수연구를 수행하여 개발된 해석기법의 신뢰성을 평가해보았다. 편심모멘트와 진동수에 따른 매개변수해석을 통해 구한 가속도를 비교분석해 본 결과 최대가속도의 크기는 대략적으로 편심모멘트에 비례하였으며 진동수의 제곱에 비례하였다. 또한 변위진폭은 편심모멘트에 비례하는 결과를 나타내었고 진동수에는 거의 영향을 받지 않았는데 이러한 경향은 공운전시의 거동특성과 유사하다. 매개변수해석을 통해 구한 동적 하중전이곡선을 비교해 볼 때 편심모멘트와 진동수의 크기에 관계없이 최대 동적단위선단저항력의 크기는 동일하였으며 최대 동적단위주면마찰력의 크기는 편심모멘트에 영향을 받으나 진동수와는 무관하였다. 매개변수해석결과를 종합적으로 비교분석해 볼 때 개발된 해석기법은 합리적인 해석결과를 보인다고 볼 수 있다. Technique for analyzing a pile installed by vibrohammer was developed and parametric studies were executed in order to evaluate reliability of the developed technique. Comparing the accelerations obtained from parametric studies of varying eccentric moment and frequency, it can be seen that magnitude of maximum acceleration was proportional to the eccentric moment and square of frequency. It can also be seen that amplitude of displacement was roughly proportional to the eccentric moment but has nothing to do with the frequency. It can be said that all of the analysis results reflect characteristics of behavior of a pile in case of free vibration. Comparing the dynamic load transfer curves, maximum dynamic unit toe resistance was constant regardless of the eccentric moment and the frequency and it can be seen that dynamic unit skin friction was affected by the eccentric moment not by frequency. Comparing all of the analysis results, it can be said that the developed technique is reliable.

EFFECT OF STIFFNESS OF THIN BEARING LAYER ON TOE BEARING MECHANISM OF STEEL PIPE PILE WITH A CONCRETE BULB

It has been known that the sufficiently stiff layer can be considered as a load-bearing layer of piles, even if its thickness is thin. The authors have studied the toe bearing resistance of pile on a thin load- bearing layer, in order to establish the design method about pile on a thin load-bearing layer. In this paper, the effect of stiffness of load-bearing layer on characteristics of toe resistance of steel pipe pile on a thin load-bearing layer is discussed through a series of numerical analyses. First, a field loading test of steel pipe piles with a concrete bulb on a thin load-bearing layer is reproduced through the numerical analysis, in which a soil-water coupled with an elast-plastic finite element method is applied, to confirm the availability of the numerical analysis proposed. Second, a series of numerical analyses are carried out in which the deformation modulus of the load-bearing layer and the thickness of load-bearing layer are chosen as a variable parameter. The results of the numerical analyses show that the stiffness of load-bearing layer affects the resistance when the yielding of load-bearing layer occurs.

Experimental Research on Statnamic Load Test of Long Rock Socket Bored Pile

Through the Statnamic load test on working bored pile, the displacement of pile top and the distributing of axis force along the bored pile were brought forward and two methods were used for interpretation the test result. The bearing capacity, axial force, shaft friction and pile toe resistance of pile were all illustrated. The result show that the Statnamic load test method can check the pile capacity quite easily, it is quite difficult to test the ultimate pile capacity.

Closure to “Pile Setup in Cohesive Soil. I: Experimental Investigation” by Kam W. Ng, Matthew Roling, Sherif S. AbdelSalam, Muhannad T. Suleiman, and Sri Sritharan

Closure to “Pile Setup in Cohesive Soil. I: Experimental Investigation” by Kam W. Ng, Matthew Roling, Sherif S. AbdelSalam, Muhannad T. Suleiman, and Sri Sritharan

진동타입기의 사하중이 말뚝의 하중전이 특성에 미치는 영향

Technique for analyzing pile installed by vibratory pile driver was developed and results of analysis obtained from variation of bias weight were studied. It can be seen from load transfer curve for dynamic skin friction that load transfer curve shift to downward as bias weight increases. Shape of load transfer curve for dynamic skin friction becomes closer to shape of coil as the bias weight decreases. Magnitudes of toe resistances were not affected by the bias weight. Shape of load transfer curve for dynamic toe resistance shows the similar tendency as the load transfer curve for skin friction exhibits. Vertical displacement increases as the bias weight increases and the shape of vertical displacement with time shows more distinct shape of wave.

Evaluation of the effects of facing stiffness and toe resistance on the behavior of GRS walls

This article presents physical and numerical model studies in order to assess the effects of facing stiffness and toe resistance on the behavior of GRS walls. Experimental tests were performed in a facility at the Geotechnical Laboratory of COPPE/UFRJ, using block and wrapped facing types. Tension along the reinforcements and the lateral and vertical movements of the walls were monitored. The results show that for the condition of no toe resistance facing stiffness may not significantly affect the mobilized tension in the reinforcements. Nevertheless, when the base of the face is restricted, varieties of facing stiffness may play an effective role in the magnitude of the tension in the reinforcements. The magnitude of reinforcement tension is not solely associated with facing stiffness, but also to the mobilized friction at the interface of the base of the facing column and foundation soil. Lateral displacements observed in the wrapped-faced wall were much greater than those verified in the block-faced wall, and this may be related to local movements near the face of the walls. Numerical modeling shows that the summation of the maximum tension in the reinforcements and toe load in fixed base condition remained almost constant (less than 5% of variation) and is greater than the value for a free base condition, irrespective of the magnitude of facing stiffness. This difference in the determined values for walls with a free and fixed facing base may be attributed to the lateral movement of the walls, which is more restricted in a fixed base condition.

Testing Study on Bearing Behavior of Belled Large-Diameter PHC Pipe Pile by NAKS (Nakabori Kakutei System) Construction Method

Through field static load test and pile shaft axial force test, field testing study is conducted on bearing behavior of NAKS-construction-method belled large-diameter PHC pipe pile; besides, the test result is compared with that of hammering-method PHC pipe pile and bored pile with same pile length and diameter under the same site condition. The result shows that the ultimate bearing capacity of NAKS-construction-method belled large-diameter PHC pipe pile is slightly inferior to hammering-method PHC pipe pile but obviously superior to that of bored pile. Compared with traditional hammering-method pipe pile, the pile side resistance of NAKS-construction-method belled pipe pile is smaller; however, the higher toe resistance will give better play to bearing capacity of bearing stratum of pile toe; moreover, it is found that under the action of ultimate load and failure load, the maximum settlement and final settlement of NAKS-construction-method belled pipe pile tip are obviously lower than that of other test piles, which is conductive to lessening foundation settlement of upper structure.

Discussion of “Load tests on full-scale bored pile groups”

Discussion of “Load tests on full-scale bored pile groups”

Pile Bearing Properties Analysis Based on Energy Method

Energy transfer of pile body in foundation is considered by the energy balance equation of pile unit. Bilinear model is used to calculate the pile side load transfer, and trilinear model to calculate the toe resistance load transfer. According to the deformation coordinating relations, pile load-settlement curve analysis method is obtained. Comparing the in-situ and calculation data, the results show that they are basically consistent. The conclusions are pile bearing properties can be analyzed reasonably by this method, and the calculation results can provide a reference to engineering practice.

Impact of Toe Resistance in Reinforced Masonry Block Walls: Design Dilemma

Reinforced masonry block retaining walls are comprised of a narrow column of stacked blocks at their exposed end. This column is placed on a nonstructural leveling pad to facilitate the placement of facing units. Theoretically, this column can generate very large toe resistance to sliding. A recent publication indicates that an accepted design methodology implicitly counts on this resistance in assessing the reinforcement load. Although not calculated in this design, it unconditionally considers that over 60% of the resultant horizontal force in a 12-m-high wall is carried by the toe, which is made up of 0.3-m-deep blocks. This paper elucidates this issue by explicitly identifying the magnitude of toe resistance and critically reviews whether such high resistance is universally suitable for design. It shows that high toe resistance may not be feasible for most foundation soils. The high impact of toe resistance on the reinforcement force poses a design dilemma as to the reliability of this resistance, even if attainable. Practically, the leveling pad is not intended to serve as a critical structural element and thus should not be relied on for maintaining the toe resistance in long-term design. Economically, ignoring the toe resistance has little impact on the overall cost.

杭先端面と羽根面の支持力機構に基づいた回転貫入杭の鉛直支持力算定式の構築

The authors conducted vertical loading tests on a model screw pile in a calibration chamber where the different mechanism of toe resistance mobilizing at the closed end and helical screw was in focus. Layered sand deposits were prepared in two different densities concerning relative position of the pile toe and the thin layer around it. A new formula to evaluate the toe resistance of a screw pile is proposed based on the test results. The formula shows fairly good applicability to estimate the toe resistance of full-scale piles by taking different soil modulus at the closed end and helical screw.

Applicability of the SPT-based methods for estimating toe bearing capacity of driven PHC piles in the thick deltaic deposits

Standard Penetration Test (SPT)-based design methods for pile foundations have been extensively used in Korean practice. However, their applicability for local application has seldom been examined, particularly for the thick deposits, such as in the Nakdong River deltaic area. This paper examines the applicability of three common SPT-based methods to deep sandy deposits in the delta. Routine SPT data, in which the test was completed for each of the N = 50 blows at less than 30 cm penetration in the last two increments of 15 cm, were adopted. A special SPT was conducted to examine the general relationship between penetration and blow counts in dense sands. Based on the special SPT and another well-documented case study, a simple linear extrapolation was developed to estimate data equivalent to N > 50. PDA (Pile Driving Analyzer) tests were also carried out to evaluate energy efficiency of the donut hammer used for the routine SPT. Energy correction factor (CE) was determined as 0.9. Using the corrected N-values, the estimated SPT-based toe resistances were compared with data obtained from PDA tests, field load tests on piles, and a CPT (Cone Penetration Test)-based method. Results indicate that the SPT-based methods generally manifest a rather low reliability. The recommended Meyerhof’s method is deemed applicable for the preliminary design.

동재하시험결과 분석을 통한 송도지역 590MPa급 고강도 강관말뚝의 항타관입성 및 지지력 특성 분석

강관말뚝의 경쟁력 향상을 위해 개발되어 현장적용되고 있는 590MPa급 고강도 강관말뚝의 항타관입성과 지지력 특성을 분석하기 위하여 인천 송도지역에 총 18본의 다양한 지름/두께 및 강도별로 강관말뚝을 시공하고 이에 대한 재하시험을 수행하였다. 항타관입성은 일반강도와 고강도간의 타격횟수, 관입깊이, 건전도 등을 확인하여 비교하였으며, 파일 지지력은 동재하시험을 수행하고 대표 말뚝에 대해서는 정재하시험을 수행하여 동재하시험의 적절성을 평가하였다. 또한 동재하시험결과를 GRLWeap 해석결과와 비교하고 입력변수에 대한 역해석을 수행하여 향후 고강도 강관말뚝을 송도지역에 시공할 때 보다 적절한 항타장비 선정 및 지지력 예측이 가능하도록 표준관입시험의 N값을 이용한 지반반력계수에 대한 보정식을 제안하였다. Domestic usage of high strength steel for pile has been limited to steel with yield strength (YS) of 490 MPa. However, design and construction cases abroad show beneficial usage of steel pipe with YS ranging in 500~700 MPa. In this study, YS 590 MPa steel pipe has been tested for driven pile foundation in Songdo area. Pile dynamic analysis (PDA) was carried out for 18 piles of which 16 piles have been reviewed for comparison of the PDA test results with those of GRLWeap analysis using SPT N value. Back analysis of PDA analysis was also carried out to narrow the deviation of standard SPT N value used in GRLWeap analysis. A regression equation is suggested for the shaft and toe resistance according to SPT N values for future GRLWeap analysis that can be used in the designing stage at Songdo area.

Influence of toe restraint on reinforced soil segmental walls

A verified fast Lagrangian analysis of continua (FLAC) numerical model is used to investigate the influence of horizontal toe stiffness on the performance of reinforced soil segmental retaining walls under working stress (operational) conditions. Results of full-scale shear testing of the interface between the bottom of a typical modular block and concrete or crushed stone levelling pads are used to back-calculate toe stiffness values. The results of numerical simulations demonstrate that toe resistance at the base of a reinforced soil segmental retaining wall can generate a significant portion of the resistance to horizontal earth loads in these systems. This partially explains why reinforcement loads under working stress conditions are typically overestimated using current limit equilibrium-based design methods. Other parameters investigated are wall height, interface shear stiffness between blocks, wall facing batter, reinforcement stiffness, and reinforcement spacing. Computed reinforcement loads are compared with predicted loads using the empirical-based K-stiffness method. The K-stiffness method predictions are shown to better capture the qualitative trends in numerical results and be quantitatively more accurate compared with the AASHTO simplified method.

Vertical bearing behavior of cast-in-place diaphragm wall in Shanghai

When diaphragm wall is used as the permanent vertical bearing structure, design standard of the bored pile adopted has to induce the risk or iste. The vertical load transfer mechanism and bearing capacity of the diaphragm wall are examined by a field testing program at the site in Shanghai soft clays. Test results indicate that the diaphragm wall almost behaves as a rigid body under the vertical load. It induces that the skin friction and the toe resistance of the wall develop simultaneously. The skin friction resistance carries the large portion of the vertical load, and the toe resistance of the wall provides about 9.2% of vertical bearing load. Toe-grouting technique is found to achieve a remarkable increase in skin friction and toe resistance. The toe resistance of the grouted wall provides about 17.5% of vertical load.

Required Unfactored Strength of Geosynthetic in Reinforced Earth Structures

Current reinforced earth structure designs arbitrarily distinguish between reinforced walls and slopes, that is, the batter of walls is 20° or less while in slopes it is larger than 20°. This has led to disjointed design methodologies where walls employ a lateral earth pressure approach and slopes utilize limit equilibrium analyses. The earth pressure approach used is either simplified (e.g., ignoring facing effects), approximated (e.g., considering facing effects only partially), or purely empirical. It results in selection of a geosynthetic with a long-term strength that is potentially overly conservative or, by virtue of ignoring statics, potentially unconservative. The limit equilibrium approach used in slopes deals explicitly with global equilibrium only; it is ambiguous about the load in individual layers. Presented is a simple limit equilibrium methodology to determine the unfactored global geosynthetic strength required to ensure sufficient internal stability in reinforced earth structures. This approach allows for seamless integration of the design methodologies for reinforced earth walls and slopes. The methodology that is developed accounts for the sliding resistance of the facing. The results are displayed in the form of dimensionless stability charts. Given the slope angle, the design frictional strength of the soil, and the toe resistance, the required global unfactored strength of the reinforcement can be determined using these charts. The global strength is then distributed among individual layers using three different assumed distribution functions. It is observed that, generally, the assumed distribution functions have secondary effects on the trace of the critical slip surface. The impact of the distribution function on the required global strength of reinforcement is minor and exists only when there is no toe resistance, when the slope tends to be vertical, or when the soil has low strength. Conversely, the impact of the distribution function on the maximum unfactored load in individual layers, a value which is typically used to select the geosynthetics, can result in doubling its required long-term strength.