shear strength of soil - dsu.ac.kr
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Shear Strength of Soil
Chapter 12
동신대학교 토목공학과 지반공학연구실
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Chapter 12 Geotechnical Engineering
1. Mohr-Coulomb Failure Criterion
Mohr-Coulomb 파괴이론
c = 점착력(cohesion)
= 내부마찰각
= 파괴면에서의 수직응력
f = 전단강도
A, B, C 위치에 따른 응력상태
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Chapter 12 Geotechnical Engineering
1.1 Failure caused by Shear
전단파괴면의 경사
최대주응력 = 1
최소주응력 = 3
Mohr 원과 파괴포락선
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Chapter 12 Geotechnical Engineering
1.1 Example for Friction angle
압밀배수 삼축압축시험(CD test)이 정규압밀점토에 대해 실시되었다. 실험결과에서 내부마찰각( )을 구하여라. 그리고, 파괴면이 최대주응력면과 이루는 각()을 구하여라.
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Chapter 12 Geotechnical Engineering
1.2 Direct Shear Test Equipment
변형률제어 직접전단시험 장비
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Chapter 12 Geotechnical Engineering
1.3 Direct Shear Test
직접전단시험
응력제어(stress-controlled) 시험
변형률제어(strain-controlled) 시험
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Chapter 12 Geotechnical Engineering
1.4 Direct Shear Test for Dry Sand
전단변위-전단응력:
전단변위-시료높이:
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Chapter 12 Geotechnical Engineering
1.5 Shear Strength Parameter (dry sand)
건조한 모래의 전단강도 정수 결정
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Chapter 12 Geotechnical Engineering
1.6 Direct Shear Test for Saturated Soil
배수 직접전단시험
과압밀점토, 과잉간극수압 소산
시험시간 2-5일 소요
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Chapter 12 Geotechnical Engineering
1.7 Example for Direct Shear Test
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Chapter 12 Geotechnical Engineering
1.7 전단강도 Example
어떤 흙에 대해서 직접 전단시험을 한 결과 수직응력이 10kg/cm2일 때 전단저항이 5kg/cm2이었고, 또 수직응력이 20kg/cm2 일 때에는 전단저항이 8kg/cm2 이었다. 이 흙의 점착력은?
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Chapter 12 Geotechnical Engineering
1.8 Interface of a foundation and soil
경계면 마찰각 결정
𝑐𝑎′ = 부착력(adhesion)
𝛿′ = 흙과 기초재료 사이의 유효마찰각
𝜏𝑓 = 𝑐𝑎′ + 𝜎′ tan 𝛿′
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Chapter 12 Geotechnical Engineering
2. Triaxial Shear Test
삼축압축시험
고무막(rubber membrane)
유체압(confined pressure)
축차응력(deviator stress)
압밀배수시험(CD test)
압밀비배수시험(CU test)
비압밀비배수시험(UU test)
응력제어방식
변형률제어방식
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Chapter 12 Geotechnical Engineering
2.1 Consolidated Drained Test
압밀배수시험 (CD test)
과잉간극수압 소산
압밀되는 시간소요
전응력=유효응력
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Chapter 12 Geotechnical Engineering
2.2 Consolidated Drained Test
압밀배수시험 (CD test)
과입밀점토
정규압밀점토
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Chapter 12 Geotechnical Engineering
2.3 Consolidated Undrained Test
압밀비배수시험 (CU test)
전응력 유효응력
축차응력 = 간극수압
• Skempton 간극수압계수
𝐴 = 𝐴𝑓 =∆𝑢𝑑 𝑓
∆𝜎𝑑 𝑓
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Chapter 12 Geotechnical Engineering
2.4 Unconsolidated Undrained Test
비압밀비배수시험 (UU test)
구속압력(3)을 가하는 동안 배수를 허용하지 않는다.
축차응력(d)에 의해 전단파괴
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Chapter 12 Geotechnical Engineering
2.5 Example (CD test)
정규압밀 점토를 대상으로 압밀배수시험을 실시하여 결과는 다음과 같다.
𝜎3 = 276 kN/m2
Δ𝜎𝑑 𝑓 = 276 kN/m2
1. 내부마찰각(𝝓′)과 파괴면이 최대주응력면과 이루는 각(𝜽)을 결정하라.
2. 파괴면의 수직응력(𝝈′)과 전단응력(𝝉𝒇)을 구하라.
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Chapter 12 Geotechnical Engineering
2.5 Example (CD test) Solution
• 𝜏𝑓 = 𝜎′ tan𝜙′ (𝑐′ = 0 이므로)
• sin𝜙′ =𝜎1′−𝜎3
′
𝜎1′+𝜎3
′ =282.9−110.4
282.9+110.4= 0.438
• 𝜃 = 45 +𝜙′
2= 45° +
26°
2= 58°
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Chapter 12 Geotechnical Engineering
3. Unconfined compression Test
일축압축시험
구속압력(3)은 “0”
축하중(1) = 최대주응력에 의해 전단파괴
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Chapter 12 Geotechnical Engineering
4. Sensitivity and Thixotropy of Clay
점토의 예민비 (1~8)
불교란(undisturbed)상태
교란(remolded)상태
quick clay = 10 ~ 80
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Chapter 12 Geotechnical Engineering
4.1 Sensitivity and Thixotropy of Clay
점토의 틱소트로피
흙의 교란된 후,
함수비 변화 없음
강도를 회복
틱소트로피 강도 비
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Chapter 12 Geotechnical Engineering
5. 응력 경로 (Stress path)
삼축압축시험의 결과로 여러 점을 연결
시료가 받는 연속적인 응력 상태
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Chapter 12 Geotechnical Engineering
5. 응력 경로 (Stress path)
Mohr 원의 정점을 잇는 선(점 D)
수정파괴포락선(modified failure envelope), 𝑲𝒇선
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Chapter 12 Geotechnical Engineering
1. 수평토압: 정지토압, 주동토압
Lateral Earth Pressure: At-Rest, Active Rankine Theory
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Chapter 12 Geotechnical Engineering
2. 수평토압: 수동토압
Lateral Earth Pressure: Passive Rankine Theory
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Chapter 12 Geotechnical Engineering
5. Example (lateral earth pressure)
다음 옹벽에 대해 Rankine의 주동상태에서 벽체의 단위폭당 작용하는 토압과 그 작용점의 위치를 결정하라.
𝑃𝑎 =1
2𝐾𝑎𝛾𝐻
2
𝐾𝑎 =1−sin 𝜙′
1+sin 𝜙′
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Chapter 12 Geotechnical Engineering
5. Example (earth pressure)
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Chapter 12 Geotechnical Engineering
6. Example (shallow foundation)