Assessment of Equivalent Heights of Soil for the Lateral Earth Pressure Against Retaining Walls Due to Design Truck Load

표준트럭하중에 의해 옹벽에 작용하는 수평토압의 등가높이 산정

Abstract

Limit state design has been implemented in Korea since 2015; however, there exists no specification of lateral load determination on retaining wall due to the Korean standard traffic load on retaining wall's backfill surface. The lateral load from traffic depends on lane number, standard truck's axle loads and locations, loading distance from the inner wall. The concept of equivalent height of soil accounting for traffic loadings is typically used for design of retaining walls to quantify the traffic loads transmitted to the inner wall faces. Due to the different characteristics of the standard design trucks between Korea and US (AASHTO), the direct use of the guidelines from AASHTO LRFD leads to incorrect estimation of traffic load effects on retaining walls. This paper presents the results of evaluation of equivalent height of soil to reflect the Korean standard truck, based on the findings from analytical solutions using Bounessq's theory and numerical assessment using 2D finite element method. Consequently, it was found that the equivalent heights of soil from the Korean standard truck load were lower for lower retaining wall height.

교통하중으로 인해 교통시설 하부구조인 옹벽에 전달되는 수평토압은 도로의 차선 수, 차량하중 및 옹벽으로부터 이격거리 등에 영향을 받는다. 차량하중에 의해 유발되는 토압은 등가상재하중높이로 표현하며, 표준트럭의 축하중 크기와 위치에 따라 달라진다. 한계상태설계법은 2015년부터 국내 도로교 설계에 적용되어 왔으나, 우리나라 실정을 고려한 토압하중계수(등가상재하중높이)가 제시되어 있지 않아 설계에 적용하는데 어려움이 있다. 따라서, 본 연구에서는 국내 표준트럭의 축하중 크기 및 위치를 반영한 등가상재하중높이를 산정하였다. 탄성체 지반에 대하여 Boussinesq 이론을 적용시켜 계산한 등가상재하중높이와 2차원 수치해석 산정치를 비교하였다. 그리고 수치해석 상의 한계와 옹벽의 장기안정성을 고려하여 AASHTO 기준치와 차별화된 등가상재하중높이를 제안하였다. 우리나라 교통하중으로부터 도출된 등가상재하중높이는 AASHTO에서 제안하는 등가상재하중높이보다 옹벽의 높이가 낮을 경우 다소 낮게 평가되었으며 옹벽의 높이가 높을 경우 높게 평가되었다.

Fig. 1. Information of truck’s axles loads and location and trucks’ formations: (a) the standard Korean design truck loads (adopted from MOLIT, 2016), (b) Case 1 - trucks parallel and adjacent to the wall with a distance 0 m, (c) Case 2 - trucks parallel to the wall with a distance of 0.3 m, (d) Case 3 - trucks perpendicular to the wall (plan view)

Fig. 2. Point load(P) on surface in cartesian coordinates

Fig. 3. Earth pressure induced by treuck load and lane loads acting: (a) parallel to the wall and (b) perpendicular to the wall

Fig. 4. Calculation of total lateral pressure against retaining wall and its elevation

Fig. 5. Typical model section (Case 1; height of wall: 6.0 m)

Fig. 6. Typical results of numerical analysis for unloading and loading traffic load (Case 2); (a) H=1.5 m, (b) H=3.0 m, (c) H=6.0 m

Fig. 7. Distribution of lateral earth pressure acting on the wall at initial and final loading stages with different wall heights; (a) Case 1, (b) Case 2, and (c) Case 3

Fig. 8. Variation of lateral earth pressures with lane loads and truck loads between the initial and the final loading stages with different wall heights: (a) Case 1, (b) Case 2, (c) Case 3

Fig. 9. Magnitudes of total earth pressures acting on the wall with lane and truck loads applied: (a) Case 1, (b) Case 2, and (c) Case 3

Fig. 10. Heights (e_h) on which the total lateral earth pressures measured from the bottom of the wall: (a) Case 1, (b) Case 2, and (c) Case 3

Table 1. Properties of materials used in the modeling

Table 2. Properties of pavement used in the modeling

Table 3. Summary of numerical analysis

Table 4. Equivalent height of soils (h_eq) on retaining walls (unit: meter)