반복 단순전단 시험기

반복 단순전단 시험기 Electromechanical Dynamic Cyclic Simple Shear

 

영신컨설턴트 (02) 529 8803 ystcha@naver.com 2022 3

 

해상풍력 말뚝에서 바람 파도 반복하중에 의한 말뚝 주변지반의 강도는 감소되어 반복단순전단시험으로 반복강도 저하를 측정할 수 있습니다.

반복하중에 의한 주변지반 강도 감소 cyclic degradation

지반강성 감소 Reduction in strength and stiffness

변형속도에 의한 감소 Strain rate effects (clays)

간극수압에 의한 감소 Accumulated pore pressure

변형에 의한 감소 Accumulated deformations

감쇄 Damping

 

 

 

진동하중에 의한 변형증가

 

뚝주변지반의 강도 감소는 말뚝심도에 따라 다름

 

 

말뚝심도에 대한 P-Y curve, 반복하중 수

 

 

반복단순전단시험에 의한 P-Y curve

 

 

반복 단순 전단력과 비배수강도 반복하중 수

 

 

반복 단순전단 시험 응력

Boundary stresses in the direct simple shear test during (a) consolidation (Ko conditions), and (b) specimen shearing.

 

 

 

반복단순 전단 시험기

 

GDS Electromechanical Dynamic Cyclic Simple Shear Device (EMDCSS)

 

 

시험 예 1Hz 일정체적 조건 반복단순시험 Example Test Results

Typical results from a dynamic simple shear tests are shown below.

The test on a remoulded clay sample was performed at 1Hz while maintaining constant volume condition using active height control.

 

반복단순전단 일정체적조건에서 간극수압  (y)  상승 ,  진동수  (x) 20Hz 에서 파괴

 

반복단순전단 일정체적조건에서 전단변형 (y) 상승, 진동수 (x)

 

반복단순전단 일정체적조건에서 전단응력 (y) 전단변형 (x) 연직응력 감소

 

반복단순전단 일정체적조건에서 전단응력 (y) 연직응력 (vertical stress) 감소 (x)

 

말뚝주변지반의 강성에 따른 일정 연직강성 제어 Testing in Constant Normal Stiffness

 

Testing in Constant Normal Stiffness (CNS) conditions have been widely used to better represent the effects of soil contraction/dilatation on the normal stress at the interface between the soil and the piles.

 

g. 7 Static simple shear results showing the difference between the constant normal load and constant normal stiffness conditions   일정 연직하중 조건(적색)과 일정연직강성조건 (검정) 연직응력 y 연직변형 x

 

Fig. 8 Results from the dynamic CNS simple shear tests with varying constant normal stiffness and horizontal strain amplitude.   일정 연직하중 조건(적색)과 일정연직강성조건 (검정) 연직응력 y 전단변형 x

 

 

일정연직강성조건 1245kPa/mm /0.25mm (검정), 일정연직강성조건 625kPa/mm /0.25mm (파란색) 일정연직강성조건 625kPa/mm /0.5mm (적색) 전단응력 y 연직응력 x

 

 

일정연직강성조건 1245kPa/mm /0.25mm (검정), 일정연직강성조건 625kPa/mm /0.25mm (파란색) 일정연직강성조건 625kPa/mm /0.5mm (적색) 전단응력 y 전단변형 x

 

일정연직강성조건 1245kPa/mm /0.25mm (검정), 일정연직강성조건 625kPa/mm /0.25mm (파란색) 일정연직강성조건 625kPa/mm /0.5mm (적색) 연직응력 y 진동수 x

 

일정연직강성조건 1245kPa/mm /0.25mm (검정), 일정연직강성조건 625kPa/mm /0.25mm (파란색) 일정연직강성조건 625kPa/mm /0.5mm (적색) 연직응력 y 연직변형 x

Fig. 8 Results from the dynamic CNS simple shear tests with varying constant normal stiffness and horizontal strain amplitude

 

 

3D and plan view of a soil element, showing shear stresses τX and τY applied along two axes (in red and blue). Note the direction of the resultant stress, τXY, can be varied by changing the relative magnitudes of τX and τY

 

파도와 바람의 반복하중으로 해상말뚝의 주변지반의 강성이 작아지고 이런 조건 constant normal stiffness condition (CNS) (e.g. with a spring)를 반복단순전단시험에 적용할 수 있다.

 

LeHane and White (2004) showed that, during loading, contraction of the shear band caused a reduction in the normal stress acting on a pile. In contrast, dilation of the shear band caused an increase in the normal stress.

 

This behavior can be recreated in laboratory interface shear tests by imposing a constant normal stiffness condition (CNS) (e.g. with a spring) on the sample. is hypothesized that one of the major mechanisms responsible for a reduction in axial capacity of offshore piles under cyclic loading is contraction of the soil along interface between the pile and soil (termed the shear band).

 

Contraction along the interface reduces the normal stress acting on the pile, which will cause the shear strength of the soil surrounding the pile to decrease because soil strength is frictional in nature (i.e. stress dependent).

 

 

Constant Normal Stiffness condition for pile-soil interface. (Lehane & White 2004)

Δt is the change in interface thickness due to dilation or contraction.

The initial normal load acting on the pile “σh” will increase or decrease due to dilation or contraction by Δσh

 

Δσh= 4Δt/D = Δt

Δσ : Shear modulus of the soil around the pile

D : Pile shaft diameter

4Δt/D : Stiffness of soil represented as spring stiffness

 

Variation in the lateral stress during static tension tests. (Lehane & White 2004)

 

shows how the normal stress (referred to as lateral stress in the graphs) increases with increasing pile displacement until reaching a maximum value.

 

 

연직응력 제어 constant normal stiffness

According to Boulon (1988a, 1989) and Wernick (1978a, 1978b), such results can be explained considering the existence of a very thin zone at the contact between structure and sand, defined as “interface,” where an intense localization of shear strains occurs. 

A particular feature of the interface during the shearing displacement is the tendency to dilate (or to contract), while the surrounding soil can be regarded as an elastic confinement medium (“confined dilatancy”).

 

constant normal stiffness (CNS) K = Δσn (normal stress) / ΔU (vertical displacement)

 

Typical trend of K = Δσn / Δu in a CNS test (imposed value of K 1000 kPa/mm).