Load-carrying behavior of tranmission-tower connected foundations subjected to different load directions

AbstractConnected foundations comprise an effective option for improving the mechanical performance of transmission tower foundations. In this study, the load-carrying behavior of connected foundations for transmission tower structures was investigated focusing on the effect of the load direction based on the field experimental testing program. Improved performances of connected foundations were observed for load directions of both θ=0° and 45° considered in this study. The downward settlements at the compressive side for θ=45° were larger than those for θ=0°, while the upward displacements were similar. For both vertical and lateral displacements, the use of connected foundations was more effective for θ=45°, and the effectiveness became more pronounced as the connection-beam stiffness increased. However, the lateral load-carrying capacities for θ=0° and 45° were not significantly different for all connection-beam conditions. From the prototype-scaled model load tests, it was confirmed that the use of connected foundations for transmission tower structures is similarly effective for different load directions. Based on the test results, it was suggested that a unified design methodology is applicable for the stability analysis of transmission tower structures subjected to different load directions

Design Lessons from Load Tests on Open- and Closed-Ended Pipe Piles

Both the driving response and static bearing capacity of open-ended piles are affected by the soil plug that forms inside the pile during pile driving. In order to investigate the effect of the soil plug on the load capacity of pipe piles in general, field pile load tests were performed on instrumented open- and closed-ended piles driven into sand. For the open-ended pile, the soil plug length was continuously measured during pile driving, allowing calculation of an incremental filling ratio, IFR for the pile. The cumulative hammer blow count for the openended pile with final IFR of 77.5% was 16% lower than for the closed-ended pile. The limit unit shaft and base resistances of the openended pile were 51% and 32% lower than the corresponding values for the closed-ended pile. It was also observed, for the open-ended pile, that the unit soil plug resistance was only about 28% of the unit annulus resistance

Analysis of Plie Raft Interaction in Sand With Centrifuge Tests

In the conventional design for piled rafts, the load capacity of the raft is not in general taken into account and the load capacity of piles is only considered for the estimation of the total load carrying capacity of the piled rafts. As a consequence, piled rafts are often designed with excessively conservative safety margin, raising a need of further investigation of the load capacity mechanism of piled rafts. In this study, a series of centrifuge load tests using model group piles and piled rafts are conducted and used to compare the axial load carrying behaviors of group piles and piled rafts for different soil conditions. Instrumented model piles and rafts are manufactured and introduced into the centrifuge tests. Different density conditions of test sands were considered in the tests. From the test results, it is revealed that the load carrying capacity increase for piled rafts differ for different soil conditions. The load capacity of piled rafts is greater than those of the group piles by 13% for dense sand cases and by 22% for loose sand cases

Pile Design Based on Cone Penetration Test Results

The bearing capacity of piles consists of both base resistance and side resistance. The side resistance of piles is in most cases fully mobilized well before the maximum base resistance is reached. As the side resistance is mobilized early in the loading process, the determination of pile base resistance is a key element of pile design. Static cone penetration is well related to the pile loading process, since it is performed quasi-statically and resembles a scaled-down pile load test. In order to take advantage of the CPT for pile design, load-settlement curves of axially loaded piles bearing in sand were developed in terms of normalized base resistance (qb/qc) versus relative settlement (s/B). Although the limit state design concept for pile design has been used mostly with respect to either s/B = 5% or s/B = 10%, the normalized load-settlement curves obtained in this study allow determination of pile base resistance at any relative settlement level within the 0 – 20% range. The normalized base resistance for both non-displacement and displacement piles were addressed. In order to obtain the pile base load-settlement relationship, a 3-D non-linear elastic-plastic constitutive model was used in finite element analyses. The 3-D non-linear elastic-plastic constitutive model takes advantage of the intrinsic and state soil variables that can be uniquely determined for a given soil type and condition. A series of calibration chamber tests were modeled and analyzed using the finite element approach with the 3-D non-linear elastic-plastic stress-strain model. The predicted load-settlement curves showed good agreement with measured load-settlement curves. Calibration chamber size effects were also investigated for different relative densities and boundary conditions using the finite element analysis. The value of the normalized base resistance qb/qc was not a constant, varying as a function of the relative density, the confining stress, and the coefficient of lateral earth pressure at rest. The effect of relative density on the normalized base resistance qb/qc was most significant, while that of the confining stress at the pile base level was small. At higher relative densities, the value of qb/qc was smaller (qb/qc = 0.12 -0.13 for DR = 90%) than at lower relative densities (qb/qc = 0.19 - 0.2 for DR = 30%). The values of the normalized base resistance qb/qc for displacement piles are higher than those for nondisplacement piles, being typically in the 0.15 - 0.25 range for s/B = 5% and in the 0.22 - 0.35 range for s/B = 10%. The values of the normalized base resistance qb/qc for silty sands are in the 0.12 – 0.17 range, depending on the relative density and the confining stress at the pile base level. The confining stress is another important factor that influences the value of qb/qc for silty sands. For lower relative density, the value of qb/qc decreases as the pile length increases while that for higher relative density increases. For effective use of CPT-based pile design methods in practice, the method proposed in this study and some other existing methods reviewed in this study were coded in a FORTRAN DLL with a window-based interface. This program can be used in practice to estimate pile load capacity for a variety of..

Design of foundations bearing in sand based on CPT results

The bearing capacity of piles consists of both base resistance and side resistance. As the side resistance is mobilized early in the loading process, the determination of pile base resistance is a key element of pile design. Static cone penetration is well related to the pile loading process, since it is performed quasi-statically and resembles a scaled-down pile load test. In order to take advantage of the CPT for pile design, load-settlement curves of axially loaded piles bearing in sand were developed in terms of normalized base resistance (qb/qc) versus relative settlement (s/B). The normalized load-settlement curves obtained in this study allow determination of pile base resistance at any relative settlement level within the 0–20% range. Both non-displacement and displacement piles were addressed. In order to obtain the pile base load-settlement relationship, a 3-D non-linear elastic-plastic constitutive model was used in finite element analyses. A series of calibration chamber tests were modeled and analyzed using the finite element approach with the 3-D non-linear elastic-plastic stress-strain model. The predicted load-settlement curves showed good agreement with measured load-settlement curves. Calibration chamber size effects were also investigated for different relative densities and boundary conditions using the finite element analysis. The value of the normalized base resistance qb/qc was not a constant, varying as a function of the relative density, the confining stress, and the coefficient of lateral earth pressure at rest. The effect of relative density on the normalized base resistance qb/q c was most significant, while that of the confining stress at the pile base level was small. The values of the normalized base resistance q b/qc for silty sands were also addressed. Vertically loaded footings on sands were also modeled using the finite element method with the non-linear stress-strain model. The load-settlement responses obtained from these analyses were compared with those from an existing elasticity-based method. The application of cone penetration testing to footing design was investigated based on the results of the analyses

Load Tests on Pipe Piles for Development of CPT-Based Design Method

This research focused on the drivability and load-carrying capacity of both open and closed-ended steel pipe piles. Two pipe piles (one open-ended, the other closed-ended) were installed in a sandy soil to the same depth. The site was extensively characterized. SPT and CPT tests were performed both before and after pile installation. A variety of soil indices and shear strength parameters (such as the constant-volume friction angle) were measured in the laboratory. The piles were fully instrumented, permitting separate measurement of shaft and base capacity for the closed-ended pile and shaft, annulus and soil plug capacities for the open-ended pile. The results are presented in a variety of ways. In particular, values of pile resistance are presented normalized with respect to CPT cone resistance values both along the shaft and base of the piles for quick reference. The test results for the openended piles are quite unique. Two design methods are proposed for open-ended piles based on the field load test as well as on results found in the literature. In one method, pile resistances are referred to either the soil plug length or incremental filling ratio. In the other method, pile resistances are correlated to the CPT cone resistance. Comparisons of the proposed methods with the load test results and with methods currently in use are quite favorable. The present research suggests current pile design methods may be excessively conservative. It seems that cost savings from similar research, where complete measurement of all variables of interest both for the piles and for the soil deposit where the piles are installed are done, can be very substantial if the methods proposed here are validated further. It appears that such savings would be in the interest of DOT\u27s and the FHWA

Numerical investigation of the at-rest earth pressure coefficient of granular materials

© 2015, Springer-Verlag Berlin Heidelberg. The at-rest earth pressure coefficient, $$\hbox {K}_{0}$$K0, is one of the most fundamental values for evaluating in-situ soil stresses and designing foundation. Research has been expanded to investigate the correlation between $$\hbox {K}_{0}$$K0 and micro-scale characteristic of granular soils, beyond the macroscopic approach empirically correlated with internal friction angle. This study presents the evolution of $$\hbox {K}_{0}$$K0 values of irregularly shaped natural sand, spherical shaped smooth and rough surfaced glass beads along with the stress history, estimated by the discrete element method. The surface roughness and non-spherical particles were emulated by inter-particle friction coefficient and the clumped particles. Results exhibit that the $$\hbox {K}_{0}$$K0 during loading stage nonlinearly decreases with increasing values of friction coefficient and the assemblies with clumped particles present the lower values of $$\hbox {K}_{0}$$K0 than spherical particle assemblies of the same friction coefficient. The varying friction coefficient seems enough to capture the evolution of $$\hbox {K}_{0}$$K0 during loading, unloading and reloading cycles, while the natural sand inevitably requires the assembly with clumped particles to capture the experimentally observed $$\hbox {K}_{0}$$K0 evolutions.Link_to_subscribed_fulltex

Characterizing the hydraulic conductivity of soil based on the moving average of precipitation and groundwater level using a regional database

Hydraulic conductivity is an important geological and geotechnical characteristic, necessary for flow-related problems and underground construction. In many countries, databases for hydrology parameters and groundwater level (GWL) are well established, often not fully utilized for directly estimating hydraulic conductivity characteristics. In this study, a method for estimating hydraulic conductivity based on a regionally established database of hydrological, geological, and geotechnical parameters is proposed. For this purpose, 68 databases of hydrological, geological and geotechnical parameters in different regions in Korea were collected and adopted to develop a data-based estimation method of soil hydraulic conductivity. The time response of GWL to precipitation was considered as a key influence factor on the hydraulic conductivity of soil, as it directly affected the infiltration process of rainfalls into soil deposits. Moving average (MA) of precipitation was introduced, which gave the best correlation to GWL, to account for the effect of accumulated precedent precipitation. Case examples were selected and used to check the validity of the proposed method. HIGHLIGHTS Hydraulic conductivity of soil was estimated by hydrological and geological data.; The time response of GWL to precipitation was a key factor that can estimate the hydraulic conductivity of soil.; The concept of moving averge was adopted.