현장시험과 공동팽창이론을 통한 CGS 공법의 적용성 평가

Study on Applicability of CGS Method based on Field Experiments and Cavity Expansion Theory

Abstract

국내 서해안 지역은 대부분 퇴적된 연약점성토로 이루어져 있으며, 구조물을 시공하기 전에 현장에 적합한 지반개량 공법의 적용이 필요하다. 본 연구에서는 CGS(Compaction Grouting System) 공법의 적용성을 파악하기 위하여 매립토 및 연약점성토로 이루어진 인천국제공항시설 현장에 공법을 적용하였다. CGS 공법은 현장 성토부의 지반개량 효과를 얻기 위함뿐만 아니라 교대의 말뚝기초 역할을 위해 적용되었으며, 본 공법의 지반개량 효과를 파악하기 위하여 공법 적용 전 후 지반의 개량 효과를 콘 관입시험을 수행함으로써 비교 분석하였다. CGS 공법의 적용 결과 현장지반의 전반적인 강도 증대 효과를 얻을 수 있었으며, 이는 현장교대의 하중을 충분히 견딜 수 있는 것으로 파악되어 말뚝기초로의 역할과 지반개량공법으로써의 CGS 공법의 적용성을 검증하였다. 또한 공동팽창이론을 적용하여 상부 구근의 크기 예측을 수행하였으며, 시공 후 상부구근 육안 확인을 통하여 시공 후 구근예측에 있어 공동팽창이론의 적용 가능성을 평가하였다.

Grounds of the western coast of the Korean Peninsular are mostly composed of soft and cohesive soils, and it is necessary to carry out soil improvement before construction. The CGS (Compaction Grouting System) method has been commonly applied for the purpose of not only improving soft ground but also serving as the pile foundation of a bridge. In this paper, the CGS method was applied to the Incheon International Airport facility site, which consists of reclaimed landfill and soft clay soil, so as to evaluate the applicability of this soil improvement method to soft clay ground formations. Futhermore, results of construction were intensively studied along with a series of field experiments and theoretical consideration. The cone penetration tests were performed to assess the ground improvement effect of the CGS method. Consequently, the application of CGS method led to an increase in soil strength enough to be used as the pile foundation to support the bridge at the site. In addition, the size of the upper grout-bulb was estimated by adopting the cavity expansion theory and compared with that of actual grout bulb exhumed in the field. Therefore, it is proved that the cavity expansion theory can be utilized to predict and evaluate the improvement of soft ground.

Fig. 1. Conceptual diagram of CGS method

Fig. 2. Ideal schematic view of grout bulb growth for analytical solution

Fig. 3. Construction field plans and fence diagrams

Fig. 4. Cone penetration test result (cone resistance)

Fig. 5. Grout-bulb excavation and its size measurement

Fig. 6. Analytical solution of cavity expansion (Yu & Houlsby,1991)

Table 1. Comparison between CGS method and other methods

Table 2. Site investigation results (bridge-1 & 2, refer Fig. 3)

Table 3. Soil properties input in cavity expansion theory (Yu & Houlsby, 1991)

Table 4. Measured and calculated injection pressure with different dilation angle (3m)