동적 단순전단 액상화 시험 liquefaction test.

동적 단순전단 액상화 시험 liquefaction test.

 

 

영신컨설턴트 (02) 529 8803 ystcha@naver.com 2020 2

 

 

 

액상화 시험

 

 

액상화시험은 실트 입자 유효경이 0.1mm보다 작고 균등계수가 5보다 작은 간극률이 44% 이상 느슨 모래포화 사질토 지반이 진동 시 체적감소로 과잉간극수압 발생하고 과잉간극수압이 누적되어 유효응력이 감소되어 이에 따라 전단강도가 감소해 지반이 액상으로 강도를 잃는 현상으로 사질토지반이 침강, 부상, 유동파괴, 지지력 상실 초래한다.

 

전단강도 공식 τ = (σ-u)tanφ + c에서

 

여기서, τ : 전단강도, σ : 유효응력, u : 간극수압, c : 점착력 이며

 

사질토는 점착력이 없으므로 c = 0 이고, u가 증가 시 σ와 같게 되면 τ=0

 

 

동적단순전단 시험기

 

 

 

액상화 진동시험 시 상대밀도 (relative density Dr)에 따라 액상화 저항 곡선을 만든다.

 

cyclic stress ratio (CSR) 와 number of cycles to liquefaction (NL)

 

 

 

동시 과잉간극수압 발생을 위해 일정 체적 단순 전단 시험

 

constant volume direct simple shear (CDSS)

 

direct simple shear (DSS) tests under monotonic loading (ASTM D6528-17).

 

The desirable boundary conditions for CDSS tests to represent loading in the field are constant vertical total stress, zero lateral strains, and zero axial strains (Boulanger 1990).

현장조건은 일정 연직응력, 횡방향 변형 제로, 연직변형 제로

 

To achieve these boundary conditions, there are three general kinds of CDSS tests used in practice (El Mohtar et al. 2018): undrained, constant vertical stress (CS) tests, constant volume (CV) tests using passive control (PC) to limit volumetric deformations, and CV tests using active control (AC) to limit volumetric deformations.

일정 체적시험 위해 수동 능동 연직변위 일정조건

 

일정 연직응력

CS tests are performed on saturated samples of soil and the drainage lines are closed during shearing to allow excess pore water pressures to generate. This test typically requires more effort than CV type tests due to the necessary steps of back-pressure saturating the sample to flush out air. It can also be time-consuming to allow the soil to consolidate, particularly if the sample contains a significant amount of fines. In these CS tests, the closed drainage valves enforce constant volume conditions during shearing.

 

 

액상화 시험 시 일정 체적조건 필요

 

 

일정 연직 체적 제어 방법

 

PC 수동방법 ( passive control, mechanical) 기계적

AC 수동방법 ( active control, feedback loop)

 

PC (passive control, mechanical) 방법은 기계적으로 고정하고

AC (active control, feedback loop)은 연직 변위계 (LVDT)나 vertical actuator의 피드백 제어

 

PC (passive control, mechanical) 방법

CV-CDSS tests using PC maintain constant volume with a physical locking mechanism that minimizes vertical deformations. In contrast, CV-CDSS tests using AC maintain constant volume via a feedback loop between a vertical LVDT and the vertical actuator to adjust the vertical load such that the vertical deformations are minimized.

 

Deformations measured at the position of the vertical actuator (i.e. outside of the locking mech-anism) may appear to be close to zero. However, if another LVDT is installed near the top of the soil sample, this LVDT will likely record much larger deformations than those measured at the level of the vertical actuator.

 

 

 

mparison of axial strain at two locations in the testing apparatus during the cyclic phase of a PC CV-CDSS (Dr = 19%, σ’v0 = 250 kPa).

기계적 고정식의 경우 actuator 위치는 일정하나 시료 부위는 침하 (int LVDT)

 

One of the likely reasons for the large ε at the level of the soil sample is that the components of the testing apparatus between the locking mechanism and the top of the soil sample are not stiff enough and/or have connections that add to the overall compliance of the apparatus. If the soil is contractive, then during cyclic loading the stress acting on the top platen will decrease and the force in the vertical components between the locking mechanism and the soil will relax, which could lead to an overall lengthening, thus causing ε in the sample (Dyvik and Suzuki 2018).

 

It is also possible for tests using PC to have stress paths that indicate irregular generation of equivalent excess pore water pressures, manifest-ing in irregular spacing between cycles in the stress path, prior to the initiation of liquefaction in the soil.

 

능동 일정 연직제어방법 AC (active control, feedback loop)

Though greatly reduced using AC compared to using PC to maintain constant volume, ε can still develop during cyclic loading in CV-CDSS tests.

능동 일정 연직제어방법 AC는 수동 일정 연직제어방법보다 일정하게 연직변형을 유지한다.

 

 

Comparison of axial strain at two locations in the testing apparatus during the cyclic phase of an AC CV-CDSS test (Dr = 67%, σ’v0 = 250 kPa).

 

 

참고문헌

Proceedings of the XVII ECSMGE-2019 Geotechnical Engineering foundation of the future