말뚝주변지반의 강성 (Constant Normal Stiffness)을 고려한 반복 단순전단 시험 cyclic simple shear

영신컨설턴트 (02) 529 8803 ystcha@naver.com 2021 4

 

해상풍력 말뚝기초의 반복 횡하중으로 말뚝주변 지반은 팽창과 수축을 반복하고 팽창 시 말뚝에 작용하는 연직응력은 상승하고 수축 시 연직응력은 감소한다. 말뚝 주변지반의 지반강성 (Constant Normal Stiffness) 연직응력에 입력하여 반복 단순전단시험을 할 수 있습니다.

해상풍력 발전

 

 

말뚝주변지반의 강성 (Constant Normal Stiffness)

 

Tests performed by Boulon and Foray (1986) indicated that the shear stress along piles is concentrated in a thin zone about ten times the grain diameter. This zone, referred to as the interface, is where the dilation and contraction occurs.

 

This interface is constrained by the soil surrounding it; it can be thought of as an infinite number of uncoupled springs along the interface.

 

These springs have an initial deflection that represents the earth pressure (normal stress) acting on the pile. If dilation occurs is the spring loaded, increasing thus the normal stress acting on the pile.

If contraction occurs the spring will be unloaded, decreasing the normal stress.

 

This is termed the constant normal stiffness (CNS) boundary condition.

 

말뚝 주변지반의 지반강성

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= 4 G Δt / D = Kn Δt

 

G : Shear modulus of the soil around the pile

 

D : Pile shaft diameter

 

Kn : Stiffness of soil represented as spring stiffness

 

 

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

 

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

 

 

 

말뚝 수평변형과 횡하중

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.

 

좌 단순전단 시험, 우 포화 가능한 2축 반복전단 시험기

 

 

반복단순전단시험은 전단 시 전단면에서 팽창과 수축이 발생하여 연직응력의 제어 (constant normal stiffness) 가 필요하여 반복 수평하중에 순응하는 연직응력이 필요하여 수평과 연직제어 2개의 acturator가 필요합니다.

 

3 (xyz) 축 제어 반복 단순 시험기

 

 

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 TESTING INCLUDED AS STANDARD IN EMDCSS

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. the firmware of GDS’ Electromechanical Dynamic Cyclic Simple Shear Device (EMDCSS) allows the apparatus to perform both static and dynamic simple shear tests in CNS conditions.

 

The upgrade allows the customer to choose the value of the constant normal stiffness required for the test in kPa/mm or kN/mm.

 

Static simple shear results showing the difference between the constant normal load and constant normal stiffness conditions.

3색 constant normal stiffness in kPa/mm 변동에 따른 반복전단시험

 

상 (좌) horizontal stress kPa y normal stress kPa x

(우) horizontal stress kPa y horizontal strain x

 

하 (좌) normal stress y cyclic number x ,(우) normal stress y axial displacement x

3색 constant normal stiffness in kPa/mm.

 

 

adaptive control 하중제어 시 시료의 응력에 순응하여 하중제어

 

삼축압축 시험과 단순 전단시험 시 시료의 강성을 무시한 변형제어와

 

시료 강성에 맞게 속도를 바꾸는 하중제어 합니다.

 

 

adaptive control 하중제어

New adaptive control method for GDS Dynamic TriaxialTesting System

 

 

ASTM D8296 – 19 반복 단순전단 시험

Standard Test Method for Consolidated Undrained Cyclic Direct Simple Shear Test under Constant Volume with Load Control or Displacement Control

ASTM D6528 - 17 

Standard Test Method for Consolidated Undrained Direct Simple Shear Testing of Fine Grain Soils

 

 

GDS ELECTRO-MECHANICAL DYNAMIC CYCLIC SIMPLE SHEAR EMDCSS 2축

 

adaptive control confirmware

GDS ELECTRO-MECHANICAL DYNAMIC CYCLIC SIMPLE SHEAR Product Code : EMDCSS

 

Displacement Range axial = +/- 25mm, shear = +/- 15mm: Accuracy = <0.1%

 

Load Range (kN) 5,10 , Operating Frequency (Hz) 0-5 Sample Sizes 50, 70, 100mm