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2026 Vol.42, Issue 4 Preview Page
Development of a Controlled-Velocity Free-Fall Penetrometer Using Electromagnetic Induction Braking and Evaluation of the Undrained Shear Strength of Clay

전자기 유도 제동 기반의 속도 제어형 자유 낙하 관입기 개발 및 점토의 비배수 전단강도 산정

Jiseok Oh1
,
Hyunwook Choo2*
오 지석1
,
추 현욱2*
1Member, Graduate Student, Dept. of Civil and Environmental Engineering, Hanyang Univ.
2Member, Associate Prof., Dept. of Civil and Environmental Engineering, Hanyang Univ.
1정회원, 한양대학교 건설환경공학과 석박통합과정
2정회원, 한양대학교 건설환경공학과 부교수
*Corresponding Author
31 August 2026. pp. 117-128
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Abstract

Rapid and reliable evaluation of undrained shear strength (Su) is essential for the stable design of marine and coastal infrastructure on soft clay deposits. Conventional laboratory strength tests can be time-consuming, and their results can be affected by sample disturbance. Although fall-cone tests and free-fall penetrometers have been proposed as alternatives, they do not allow independent control of penetration energy and impact velocity. To overcome these limitations, this study developed a velocity-controlled free-fall penetrometer based on electromagnetic induction braking. The proposed device consisted of a permanent-magnet penetrator, a conductive induction tube, detachable weights, and a laser displacement sensor. The device used eddy-current braking to reach a constant terminal velocity before soil penetration. Penetration tests using the developed equipment were conducted on kaolin clay prepared under various water content conditions, and the results were comparatively verified with those from triaxial compression and unconfined compression tests. The results confirmed that electromagnetic braking effectively limited the rapid increase in shear strain rate during soil penetration. Furthermore, it demonstrated high precision, maintaining an error rate of within 10% across all tested ranges, regardless of the water content or the magnitude of the clay’s shear strength, thereby significantly enhancing the reliability of Su estimation.

Keywords
Electromagnetic induction
Fall-cone test
Free-fall penetrometer
Undrained shear strength

해양 및 연안 연약 점토 지반의 안정적인 설계를 위해서는 비배수 전단강도(Su)의 신속하고 신뢰성 있는 평가가 필수적이다. 기존 실내 강도시험은 시험 시간이 길고 시료 교란의 영향을 받을 수 있으며, 그 대안으로 제시된 낙하 원추 시험과 자유 낙하 관입기는 관입 에너지와 충격 속도를 독립적으로 제어하기 어렵다는 한계가 있다. 본 연구에서는 이러한 한계를 개선하기 위해 전자기 유도 제동 원리를 적용한 속도 제어형 자유 낙하 관입기를 개발하였다. 제안된 장비는 영구자석 관입기, 유도관, 탈부착식 무게추, 레이저 변위 센서로 구성되며, 와전류 제동을 통해 지반 관입 전 일정한 종단 속도에 도달하도록 설계되었다. 다양한 함수비 조건의 카올린 점토에 대해 개발된 장비 관입체의 관입 시험을 수행하고 삼축압축시험 및 일축압축시험 결과와 비교 검증하였다. 분석 결과, 개발된 장비는 전자기 제동을 통해 지반 관입 시의 급격한 전단 변형률 속도 증가를 효과적으로 제어하였으며, 점토의 함수비 또는 전단강도의 크기와 관계없이 전 구간에서 오차율 10% 이내의 우수한 정밀도로 Su 산정 신뢰성을 향상시킬 수 있음을 확인하였다.

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Information
  • Publisher :The Korean Geotechnical Society
  • Publisher(Ko) :한국지반공학회
  • Journal Title :Journal of the Korean Geotechnical Society
  • Journal Title(Ko) :한국지반공학회 논문집
  • Volume : 42
  • No :4
  • Pages :117-128
  • Received Date : 2026-06-29
  • Revised Date : 2026-07-08
  • Accepted Date : 2026-07-12
  • DOI :https://doi.org/10.7843/kgs.2026.42.4.117
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