Waste concrete powder (WCP) is proposed to replace part of the cement to seek environmentally friendly grouting materials for ground improvement in mine goaf. The optimal mixing proportion was selected based on the performance indexes of the water‐separation ratio, stone rate, viscosity, setting time, and compressive strength. X‐ray diffraction (XRD) and scanning electron microscopy (SEM) tests were also conducted to analyze mineralogical phases and investigate the microscopic mechanism. Test results show that the slurry prepared by the substitution rate of 70% and adding 0.05% water‐reducing agent meets well the requirements of ground grouting in mine goaf. The WCP produced by grinding mainly exerts microaggregate effect in the slurry due to low activity. A lot of pores on the surface of WCP were shown by SEM which can absorb the water in the slurry and increase the stone rate. The WCP application for ground grouting in mine goaf can not only recycle WC but also provide new grouting materials for goaf ground.

Combined with a strategic policy for the recycling of construction waste, this paper puts forward a programme for treating a goaf foundation with waste concrete powder, waste brick powder, and cement grout (BCP cement grout). The BCP cement grout is prepared by replacing part of the waste concrete powder with waste brick powder, and the changeable rule of the setting time, water separation rate, stone rate, and viscosity of the grout with different rates for replacement are studied. X-ray diffraction (XRD) and scanning electron microscopy (SEM) are carried out. The mineral composition is analyzed and the microscopic mechanism is studied. The results show that the ratio of BCP cement grout developed by the experiment is reasonable; the unit cost is less than 160 yuan/t, which is 40% lower than that of pure cement grout; and both a good economic effect and good environmental effect are obtained. It has the advantages of low water separation rate, high stone rate, and low viscosity. When the ratio of cement powder: waste concrete powder: waste brick powder is 3:5:2 or 3:6:1, the 28 d compressive strength of the stone body is more than 2 MPa, meeting the filling requirements for a goaf foundation. It is not only technically feasible but also economically reasonable to apply construction waste powder to fill the goaf foundation. Recycling and utilizing construction waste can also achieve the ecological restoration of mined-out areas, highlighting the ecological benefits.

Y-shaped piles, characterised by three concave arches, are adopted in practice as they exhibit advantages over conventional circular piles. To facilitate their practical design, eight model tests were conducted in dry sand on Y-shaped piles (Y₁ and Y₂) and circular cross-sectional piles (C₁ with the same section area A_p as pile Y₂ and C₂ with larger A_p but the same perimeter as Y₂). Under compressive or tensile load, the tests provided load–settlement (displacement) data and the distributions of resistance along the pile shaft and at the pile base. The tests revealed the expansion (compressive loading) and contraction (tensile loading) effects on the pile capacity for the Y-shaped sections. Pile Y₂ had a 70% larger ultimate capacity than pile C₁. It induced a shaft resistance over 100% larger than that of C₁ and 15% larger than that of C₂. The ratio of ultimate tensile capacity to ultimate capacity (about 0·15) of the short Y-shaped piles was much lower than that of in situ piles. A simple expression considering shape effects was established to estimate the ultimate capacity of a pile under compressive loading and the results compared well with the model tests. Compared with the circular piles, the Y-shaped piles showed increases of 15% and 30% in shaft and base resistance, respectively, for compressive loading.

Y-shaped piles are a new type of pile whose cross-section is like the letter Y: they are often used in ground improvement for road or train subgrades in the eastern coastal region of China. To investigate the bearing behaviour of Y-shaped piles in saturated sand, a series of model tests under compressive and horizontal load for Y-shaped piles, circular pile (the same cross-sectional area of Y-shaped pile), and circular pile (the same perimeter of Y-shaped pile) were carried out. Comparative analysis was conducted on bearing capacity, axial force and side resistance distribution, load sharing ratio, bending moment, and lateral soil pressure distribution along the embedded length. The results show that the bearing capacity of a Y-shaped pile does not increase in proportion, and the shaft resistance is weakened to some extent in saturated sand; Y-shaped pile can effectively improve the compressive bearing capacity for the same amount of concrete. The lateral bearing capacity of a Y-shaped pile has directionality, and hanging a circular section into a Y-shaped section may improve the horizontal bearing capacity for the same amount concrete, but cannot give full play to the advantage of the larger side area for horizontal bearing capacity in saturated sand.