Nonlinear Response of Piled Gravity Base Foundations Subjected to Combined Loading

서지훈,김영호,구정민,추연욱 대한토목학회 2019 KSCE JOURNAL OF CIVIL ENGINEERING Vol.23 No.5

This study investigates the soil-structure interaction of a piled Gravity Base Foundation (piled GBF) supporting an offshore wind turbine tower. The piled GBF is a gravity base foundation supported by five piles with one center and four outer piles. A series of three-dimensional finite element analyses were performed to investigate p-y characteristics in the piled GBF. The results were validated against centrifuge test data prior to undertaking a detailed parametric study, exploring the relevant range of parameters in terms of foundation type, vertical load due to the gravity base self-weight, loading height, undrained shear strength, and the pile bearing condition. Overall, the center pile of the piled GBF showed the highest ultimate soil reaction force, while the leading pile indicated the lowest value. Due to the rotation of mat and self-weight of GBF, significant shear failure was mobilized near the leading pile, which reduced the ultimate soil reaction force. This was also confirmed through the comparison studies with group pile (no mat), piled raft (low self-weight of mat) and rock-socketed piled GBF (no rotation of mat).

Analysis of Piled Piers Considering Riverbed Scouring

Jeong, Sang-Seom,Suh, Jung-Ju,Won, Jin-Oh 한국지반공학회 2002 지반 : 한국지반공학회지 Vol.18 No.3

본 연구에서는 지반말뚝, 말뚝-말뚝캡, 그리고 말뚝-유체간의 상호작용 해석을 수행하여 교각세굴을 고려한 말뚝기초의 거동을 해석하였다. 지반-말뚝의 상호작용은 비선형 하중전이곡선(p-y, t-z, 그리고 q-z 곡선)을, 말뚝-말뚝캡의 상호작용에서는 군말뚝의 배열과 말뚝-말뚝캡 사이의 구속조건을 고려하였다. 말뚝유체의 상호작용은 세굴에 의한 지반의 강성 저하를 고려하여 지반-말뚝의 상호작용에 포함하여 해석하였다. 그 결과 세굴심이 깊어질수록 말뚝에 발생하는 최대 휨모멘트의 값이 증가함을 알 수 있었으며, 이를 바탕으로 세굴에 따른 군말뚝의 안정성 평가에서는 지반-말뚝 및 말뚝-말뚝캡의 상호작용을 고려한 해석을 수행하는 것이 바람직함을 알 수 있었다. This paper describes a simplified numerical procedure for analyzing the response of bridge pier foundations due to riverbed scouring. A computationally efficient algorithm to analyze the behavior of a pile group is proposed by considering soil-pile, pile-cap, and pile-fluid interactions. The complex phenomenon of the pile-soil interaction is modeled by discrete nonlinear soil springs (p-y, t-z and q-z curves). The pile-cap interaction is considered by geometric configuration of the piles in a group and connectivity conditions between piles and the cap. The pile-fluid interaction is incorporated into the procedure by reducing the stiffness of the soil-pile reactions as a result of nonlinearity and degradation of the soil stiffness with river bridge scouring. Through the numerical study, it is shown that the maximum bending moment increases with increasing scour depth. Thus it is desirable to check the stability elf pile groups based on soil-pile and pile-cap interactions by considering scouring depth in the riverbed.

Regression versus Artificial Neural Networks: Predicting Pile Setup from Empirical Data

Bashar Tarawneh,Rana Imam 대한토목학회 2014 KSCE JOURNAL OF CIVIL ENGINEERING Vol.18 No.4

Piles have been used as a deep foundation for both inland and offshore structures. After installation, pile capacity may increasewith time. This time dependent capacity increase is known as setup, and was first mentioned in the literature in 1900 by Wendel. When accounted for accurately during the design stages, the integration of pile setup can lead to more cost-effective pile design as itwill reduce pile length, pile section, and size of driving equipment. In this paper, Both Multiple Linear Regression (MLR) andArtificial Neural Networks (ANN) models were developed for predicting pile setup for three pile types (pipe, concrete, and H-pile)using 169 dynamic load tests obtained from the published literature and the authors' files. In addition, the paper discusses the choiceof variables that were examined to obtain the optimum model. Furthermore, the paper compares the predictions obtained by thedeveloped MLR and the ANN models with those given by four traditional empirical formulae. It is concluded that the ANN modeloutperforms both the MLR model and the examined empirical formulae in predicting the measured pile setup. Finally, static load testdata was used to further verify the developed models. It’s noted that the optimal ANN model overestimated pile capacity by 17% to21% for the H-piles, and underestimated pile capacity by 12% to 17% for the pipe piles.

연직하중을 받는 소규모 무리말뚝의 거동

이영남,이승현,박영호 한국지반공학회 2001 한국지반공학회논문집 Vol.17 No.6

무리말뚝의 하중지지능력에서 말뚝캡이 부담하는 비율을 파악하고 무리말뚝을 구성하는 개개 말뚝의 하중전이 특성을 알아보고자 재하시험을 수행하였다. 직경 92.5mm의 강관말뚝 24본(4개씩 6열)을 지표 아래 3m 깊이까지 근입 시컥 풍화암 상단에 말뚝선단이 위치하도록 하였다. 최대하중 320t을 지표면에 접촉되어 있는 $1.5\times2.3m$ 크기의 말뚝캡에 재하하였다. 최대 시험하중인 320t에서는 말뚝캡이 전체하중의 약 22%에 해당하는 하중을 분담하였다. 무리 말뚝 재하시험시 말뚝의 평균 극한지지력은 16.4t이며, 이 값은 말뚝캡을 타설하기 전에 무리말뚝의 모서리에 위치한 말뚝에 대하여 수행한 단말쪽 재하시험으로부터 구한 극한지지력보다 상당히 크게 나타났다. 변위장중첩 효과로 인하여 무리말뚝 중앙에 위치한 말뚝은 작은 하중이 작용하여도 외곽부의 말뚝과 같은 침하량을 기록한 것으로 나타났다. 무리말뚝에서는 주면마찰력의 분담율이 전 하중단계에서 약 60%로 일정하나 단말뚝 시험에서는 하중이 증가함에 따라 주면마찰력의 분담율이 82%에서 65%로 감소하였다. Pile load tests were carried out to investigate the contribution of the pile cap to the carrying capacity of a pile group and load transfer characteristics of piles in the group. A group of 24 piles$(4 \times6 array)$</TEx> of 92.5mm diameter steel pipe were installed to the depth of 3m fron the ground surface, the top of weathered rock. A maximum load of 320ton was applied to the pile cap, $1.5\times2.3m$, in contact with the ground surface. At the maximum load of 320ton, the pile cap has carried 22% of the total load. Average ultimate capacity of pile in the pile group was estimated to be 16.4ton, substantially higher than that of single pile, installed at the corner and tested before pile cap construction. For the same magnitude of settlement, the pile in the center carried less load than the pile at the perimeter due to strain superposition effect. Piles in the group showed almost constant contribution(approx. 60%) of side friction to the total capacity for all of the loading stages, while that of single pile decreased from 82% to 65%.

Settlement analysis of pile cap with normal and under-reamed piles

Madisetti Pavan Kumar,P. Markandeya Raju,G. Vincent Jasmine,Mantini Aditya 사단법인 한국계산역학회 2020 Computers and Concrete, An International Journal Vol.25 No.6

The use of pile foundations has become more popular in recent years, as the combined action of the pile cap and the piles can increase the bearing capacity, reduce settlement, and the piles can be arranged so as to reduce differential deflection in the pile cap. Piles are relatively long, slender members that transmit foundation loads through soil strata of low bearing capacity to deeper soil or rock strata having a high bearing capacity. In this study analysis of pile cap with considering different parameters like depth of the pile cap, width and breadth of the pile cap, type of piles and different types of soil which affect the behaviour of pile cap foundation is carried out by using Finite Element Software ANSYS. For understanding the settlement behaviour of pile cap foundation, parametric studies have been carried out in four types of clay by varying pile cap dimensions with two types of piles namely normal and under-reamed piles for different group of piles. Furthermore, the analysis results of settlement and stress values for the pile cap with normal and under-reamed piles are compared. From the study it can be concluded that settlement values of pile cap with under-reamed pile are less than the settlements of pile cap with normal pile. It means that the ultimate load bearing capacity of pile cap with under-reamed piles are greater than the pile cap with normal piles.

A Simplified Calculation Method for Stress Concentration Ratio of Composite Foundation with Rigid Piles

Linchang Miao,Fei Wang,Weihua Lv 대한토목학회 2018 KSCE Journal of Civil Engineering Vol.22 No.9

Piles in a composite foundation share loads with the surrounding soil instead of carrying all loads alone as a pile foundation. Stress concentration ratio, defined as the ratio of the stress on the top of piles to the stress on the surrounding soil, is essential for the design of piles in the composite foundation. An equal strain condition is widely accepted and adopted in the existing methods of calculating the stress concentration ratio. In other words, the piles should have identical relative displacement with the surrounding soil in an equal strain condition. This assumption may work for flexible and semi-rigid piles; however, it may not be suitable for rigid piles considering the significant stiffness difference of pile and soil. The relative displacement of rigid piles and surrounding soil can significantly change the stress distribution on piles and soils due to the pile-soil interaction (i.e., pile side friction). Therefore, the pile-soil interaction should be considered in the design of the rigid piles in the composite foundation. In this study, a simplified theoretical solution of the stress concentration ratio was derived based on the deformation coordination between rigid pile and soil to consider the pile-soil relative displacement and thus the pile-soil interaction. The pile side friction was assumed as linearly increased with the pile-soil relative displacement and then the stress concentration ratio was derived based on the force and deformation equilibrium in the composite foundation. A neutral point (i.e., location with zero pile-soil relative displacement) was included in the derivation to take the negative skin friction along the pile into account. Lab-scale and full-scale field tests were conducted to verify the effectiveness of the proposed method. Comparison of measured stress concentration ratios with calculated ones using the proposed method demonstrates the effectiveness of the proposed method.

Parametric study of laterally loaded pile groups using simplified F.E. models

Chore, H.S.,Ingle, R.K.,Sawant, V.A. Techno-Press 2012 Coupled systems mechanics Vol.1 No.1

The problem of laterally loaded piles is particularly a complex soil-structure interaction problem. The flexural stresses developed due to the combined action of axial load and bending moment must be evaluated in a realistic and rational manner for safe and economical design of pile foundation. The paper reports the finite element analysis of pile groups. For this purpose simplified models along the lines similar to that suggested by Desai et al. (1981) are used for idealizing various elements of the foundation system. The pile is idealized one dimensional beam element, pile cap as two dimensional plate element and the soil as independent closely spaced linearly elastic springs. The analysis takes into consideration the effect of interaction between pile cap and soil underlying it. The pile group is considered to have been embedded in cohesive soil. The parametric study is carried out to examine the effect of pile spacing, pile diameter, number of piles and arrangement of pile on the responses of pile group. The responses considered include the displacement at top of pile group and bending moment in piles. The results obtained using the simplified approach of the F.E. analysis are further compared with the results of the complete 3-D F.E. analysis published earlier and fair agreement is observed in the either result.

Centrifuge modelling of pile-soil interaction in liquefiable slopes

Haigh, Stuart K.,Gopal Madabhushi, S.P. Techno-Press 2011 Geomechanics & engineering Vol.3 No.1

Piles passing through sloping liquefiable deposits are prone to lateral loading if these deposits liquefy and flow during earthquakes. These lateral loads caused by the relative soil-pile movement will induce bending in the piles and may result in failure of the piles or excessive pile-head displacement. Whilst the weak nature of the flowing liquefied soil would suggest that only small loads would be exerted on the piles, it is known from case histories that piles do fail owing to the influence of laterally spreading soils. It will be shown, based on dynamic centrifuge test data, that dilatant behaviour of soil close to the pile is the major cause of these considerable transient lateral loads which are transferred to the pile. This paper reports the results of geotechnical centrifuge tests in which models of gently sloping liquefiable sand with pile foundations passing through them were subjected to earthquake excitation. The soil close to the pile was instrumented with pore-pressure transducers and contact stress cells in order to monitor the interaction between soil and pile and to track the soil stress state both upslope and downslope of the pile. The presence of instrumentation measuring pore-pressure and lateral stress close to the pile in the research described in this paper gives the opportunity to better study the soil stress state close to the pile and to compare the loads measured as being applied to the piles by the laterally spreading soils with those suggested by the JRA design code. This test data shows that lateral stresses much greater than one might expect from calculations based on the residual strength of liquefied soil may be applied to piles in flowing liquefied slopes owing to the dilative behaviour of the liquefied soil. It is shown at least for the particular geometry studied that the current JRA design code can be un-conservative by a factor of three for these dilation-affected transient lateral loads.

Analyzing load response and load sharing behavior of piled rafts installed with driven piles in sands

Park, D.,Park, D.,Lee, J. Elsevier 2016 Computers and geotechnics Vol.78 No.-

In this study, the load carrying behavior of piled rafts embedded in sand were investigated using the 3-D finite element analysis. Focus was given on piled rafts installed with driven piles. Various foundation and soil conditions were considered in the analyses. To simulate driven piles, the soil influence zone around driven pile was set where the relative density and lateral stress were increased to reflect the changes in soil condition and stress state due to pile driving. For both bored- and driven-pile cases, the values of load sharing ratio showed non-linear variation with settlement. The load sharing ratios for driven piles were higher than for bored piles within a certain settlement range. With further increasing settlement, however, the values of load sharing ratio for driven piles became similar to those of bored piles with limited variation. This indicates that load-sharing model can be applied for both cases to estimate the load capacity of piled rafts.

수평력을 받는 무리말뚝의 하중분담특성

안병철(Ahn Byungchul),오세욱(Oh Sewook) 한국지반환경공학회 2010 한국지반환경공학회논문집 Vol.11 No.3

본 연구에서는 풍화토 지반에 설치된 수평하중을 받는 H-pile 무리말뚝의 하중분담특성 및 상호작용계수(p-multiplier)를 분석하였다. 말뚝의 배열(2×3, 3×3), 말뚝의 간격(2D , 4D , 6D), 그리고 지반밀도(상대밀도 : 40%, 80%)를 고려한 수평재하 실험을 실시한 결과 다음과 같은 결과를 얻을 수 있었다. 무리말뚝내 각각의 말뚝에 작용된 평균수평하중은 말뚝의 수가 감소할수록 평균 수평저항력이 증가하는 것으로 나타났다. 말뚝간격과 지반밀도에 따라 단말뚝과 무리말뚝의 p-y 곡선을 분석한 결과 말뚝간격이 2D에서 6D로 증가함에 따라 무리말뚝의 상호작용계수값이 느슨한 지반의 경우 0.85~0.94(앞열말뚝), 0.57~0.79(중간말뚝), 0.60~0.71(후열말뚝), 조밀한 지반의 경우 0.76~0.82(앞열말뚝), 0.58~0.73(중간말뚝), 0.53~0.70(후열말뚝)의 값을 각각 나타냈다. 이와 같이 단말뚝과 무리말뚝간 상호작용계수는 말뚝간격이 증가할수록 그 값이 증가함을 알 수 있었으며, 또한 앞열의 무리말뚝이 중간 및 후열말뚝들보다 보다 큰 상호작용계수 값을 나타냈다. This paper analyzed the characteristics of p-multiplier and the load distribution of H-pile group installed in weathered soil under horizontal loading. The results of this study conducted in pile arrangement (2×3, 3×3), the pile center to center spacing (2D , 4D , 6D), and soil density (relative density: 40%, 80%) were drawn as follows. As to the average horizontal loading applied to each pile in pile groups, the fewer number of piles was, the larger average horizontal resistance became. As the result of analysis on p-y curves of single piles and pile groups according to the pile distance and the soil density, as the pile spacing was increased from 2D to 6D , the interaction coefficients of pile group showed 0.85~0.94 (piles in the front row), 0.57~0.79 (piles in the middle row), and 0.60~0.71 (piles in the rear row) in the loose ground and showed 0.76~0.82 (piles in the front row), 0.58~0.73 (piles in the middle row), and 0.53~0.70 (piles in the rear row) in the dense ground. As above, the wider pile distance was, the larger interaction coefficient value was shown among piles. In addition, piles in the front row showed bigger interaction coefficients than that of piles in the middle and back row.