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Abstract: Small H-beams such as the No.14–20 I-steel can be inserted into soil-cement retaining walls to form small H-beam soil-cement compound walls, functioning both as a retaining wall and a cutoff wall for braced structure excavations. Being different from the mixed soil-cement wall (SMW), the interaction between soil-cement and small H-steel is very good. We have carried out a series of bending experiments on small H-beams in soil-cement model compound beams to study the mechanism of interactions. The results show that the interaction between H-beams and soil-cement is very good, whether the H-beam is single or double. Joint forms of double H-beams at one end have little effect on both the contribution coefficient and on ultimate deflection before crack-ing. 52365
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But after cracking, the joint forms greatly affect the contribution coefficient. We conclude that the rigid joint girder for double H-beams is a better choice in practice. Key words: small H-steel; soil-cement compound beam; rigidity contribution coefficient; ultimate deflections; soil type; experi-mental study 1 Introduction The soil-cement mixed wall (SMW) technique was developed in Japan during the 1980s and has been successfully used in thousands of projects. The SMW technique is basically a soil improvement technology using mechanical means to mix in-situ soil with ce-ment slurry or other hardening reagents to form a soil-cement wall consisting of overlapping soil-ce-ment columns. The design of soil-cement walls for excavation support should include the design of rein-forcement members, usually wide-flanged H-beams, in order to resist bending moments and shear stress along the longitudinal direction of the wall. As well, it should also include the design of the soil-cement between reinforcement members to resist and redis-tribute the horizontal stress over neighboring rein-forcement members[1]. As a cutoff wall, pore size in soil-cement should be small enough and the cracking should not cross the wall. The SMW technique is usually used for deep excavation where bending mo-ments are large enough and the deflection of the wall is also large[2]. Therefore, the design concept for SMW walls is to use H-beams in order to resist all bending moments. The interaction of H-beams and soil-cement is not important, but it cannot be ignored. When the SMW is used for shallow excavation (less than 10 m in excavation depth), the H-steel used should be small because the bending moment is also small. The soil-cement wall is usually composed of two or three ranks of soil-cement piles, thus the thickness of the wall is large and the deflection of the wall small. In this situation, the interaction cannot be ignored either. Therefore, an improved SMW wall to consider this interaction was developed in China for shallow excavation (excavation depths usually less than 10 m). That is, the overlapping two or three rows of soil-cement columns are first formed in-situ, then small H-steel are inserted as reinforcement, forming a compound wall of soil-cement columns and small H-steel (for our purpose, H-steel refers to H-beam). In short, we called this improved SMW wall a com-pound wall or beam[3]. Due to the small stiffness of this H-beam, the interaction between soil-cement and the small H-beam is strong and the effect of both H-steel and soil-cement is of vital importance for the deflection and bending capacity of the compound wall[4]. We have studied the interaction of small H-steel and soil-cement in a compound model beam. stiffness contribution coefficient α of the H-beam does not change during the entire deformation proc-ess. We take α as 1.0 for the H-beam. Curves obtained from our experiments show the following features. For a single compound H-beam, the coefficient of the sand-cement compound beam is approximately twice that of the clay-cement com-pound beam because Young’s modulus of sand-ce-ment is much higher (see Table 2). Before cracking of the double compound H-beam, the joint forms of H-beams at one end have only a small effect on the soil-cement stiffness contribution coefficient. This indicates that the working mechanism of a small H-beam in a small rigid reinforced soil-cement wall may be quite different from that of a concrete bracing wall composed of double rows of cast-in-place piles. The coefficient of double compound H-beam soil-cement with strutting cannot be calculated by the framework theory of two-rank concrete piles. Further research is necessary. 3.2 Stiffness contribution coefficient before crack-ing Before cracking, compound beams are basically elastic structures. Soil-cement stiffness contribution coefficients change little, whether the compound beam is a single or a double H-beam. The adhesive stress between soil-cement and H-steel does not de-crease and the soil-cement and H-beam work together, even under tiny cracks. Therefore, as an elastic struc-ture, the stiffness can be calculated by the composite elastic beam. Table 3 shows the stiffness of the com-pound beam, calculated by elasticity theory, and a comparison with our measurements. Table 3 Bending stiffness and relative stiffness contribution coefficients before cracking Beam No. Measured stiffness (kN/m2) Calculated stiffness (kN/m2) α for H-steel β for soil-cement0HC0 345 53 – – 1H00 11.16 11.47 – – 1HCB 360 655 1.0 0.464 1HCM 202 131 – – 1HSB 1309 1408 1.0 0.916 2HCN 676 1103 1.0 0.84 2HCH 674 1103 1.0 0.844 2HCR 667 1103 1.0 0.824 Note: Measured stiffness obtained through back analysis of load–deflec-tion curve. Modulus of soil-cement E, not E50 is used. The H-beams of Table 3, are made by manual welding of steel plates. Its bending stiffness is 11.47 kN/m2 which is consistent with the measured stiffness of 11.16 kN/m2. Young’s modulus of soil-cement is obtained by back analysis of clay-cement beam stiff-ness and is 942 MPa. This modulus is quite different from the E50=112 MPa measured from the clay-ce-ment sample. Furthermore, the stiffness measured from a single H-beam located at the middle clay-ce-ment beam is larger than calculated, showing that the modulus of elasticity of soil-cement is much larger than E50.