풍화토 정착 인장형 앵커에서 주면전단거동분석을 위한 다중선형모델 적용 해석기법의 제안

Suggestion of Analytical Technique Applying Multi-Linear Models for Analysis of Skin Shear Behavior of Tension-Type Ground Anchors in Weathered Soil

Abstract

지반앵커의 정착장에 작용하는 정착응력 분포 특성은 매우 비선형적이며 공학적인 메카니즘이 비교적 복잡하기 때문에 다양한 지반조건 및 비선형적 주면전단거동을 구체적으로 모사하여 지반앵커를 설계하는데 어려움이 크다. 이런 한계로 인하여 현재 대부분의 관련 설계 기준서에는 편의상 정착장 전장에 걸쳐 일정한 주면전단응력분포를 가정하여 설계에 적용하고 있다. 따라서 본 연구에서는 인장형 앵커의 인발거동 특성을 분석하기 위하여 풍화토 지반조건을 대상으로 현장인발시험을 수행하였으며 이를 토대로 앵커 정착장의 주면전단거동을 정립하고, 정착장 거동특성을 비교적 간편하게 예측하기 위한 다중선형모델 및 이를 적용한 해석적 기법을 제안하였다. 현장시험결과와 해석적 결과가 상호 유사하게 나타남에 따라 본 연구에서 제시된 다중선형모델 및 이를 이용한 해석적 기법의 적용성 및 유효성을 확인할 수 있었다. 정착장 주면전단거동의 경우 최대인발하중 보다 작은 하중조건에서는 정착장 시작점에서 최대전단응력이 분포하게 되나 최대인발하중이 발생한 이후부터는 정착장 시작점에서 전단응력이 가장 작게 분포하고, 정착장 시작점으로부터 일정거리 이격된 지점에서 최대전단응력이 발생함을 확인하였다.

The characteristics of the skin shear stress distribution for the fixed length of the ground anchor are extremely nonlinear and the engineering mechanisms are complex relatively. So it is difficult to design the anchors simulating the actual behavior by considering various soil conditions and nonlinear behavior. Due to these limits, constant skin shear stress distributions for the whole fixed length of the ground anchor are usually assumed in the design for the sake of convenience. In this study, to assess the pull-out behavior of the tension-type ground anchors, the in-situ pull-out tests in weathered-soil conditions were carried out. Based on the test results, the skin shear behaviors for the fixed length of tension-type ground anchors were established and the multi-linear slip shear model predicting this behavior and an analytical technique applying this model were proposed. From the similarity between the results of the in-situ pull-out tests and those of the analytical technique, the applicability and availability of the multi-linear slip shear model and the proposed analytical technique were verified. The maximum shear stress was developed at the start point of the fixed length acting with the smaller load than the maximum pull-out load but the minimum shear stress was developed at the start point of the fixed length and the maximum shear stress was developed at the point apart from the start point of the fixed length after the maximum pull-out load.

Fig. 1. Schematic diagram tension-type ground anchors

Fig. 2. Typical multi-linear slip and shear stress relations of fixed length (Benmokrane et al., 1995)

Fig. 3. Equilibrium relationship of forces at each point of the fixed length

Fig. 4. Linear elastic behavior condition to grout-ground Interface

Fig. 5. Linear elastic-softening behavior condition to grout-ground Interface

Fig. 6. Linear elastic-softening-residual behavior condition to grout-ground Interface

Fig. 7. Cross-section condition of strand tendon and grout

Fig. 8. Pullout test of tension type ground anchors

Fig. 9. Compressive strength of grout samples by curing time

Fig. 10. Scheme of ground condition and measurement system installation

Fig. 11. Plotting of performance test results : FT-1(S) and FT-2(N)

Fig. 12. Plotting of elastic displacement and slip : FT-1(S) and FT-2(N)

Fig. 13. Flowchart for the implementation of the skin shear stress-slip model using pullout test results

Fig. 14. Slip-shear stress model of FT-1(S) and FT-2(N) anchors

Fig. 15. Comparison between the experimental data and the analytical approach of P-δ_s relationships : FT-1(S) & FT-2(N)

Fig. 16. Comparison between the experimental data and the analytical approach of axial load distribution : FT-1(S)

Fig. 17. Comparison between the experimental data and the analytical approach of skin shear stress distribution : FT-1(S)

Table 1. Nominal dimension and physical properties of strand tendon for pull-out test

Table 2. Dimension of test anchors for pull-out test

Table 3. Values for set up of multi-linear model with FT-1(S) and FT-2(N) test results