Study on the Style Design and Anchoring Mechanism of Enlarged Head Anchors

To resolve insufficient traditional bolt supports due to the complexity of geological conditions, the optimal design of an expanded head bolt was investigated by using theoretical calculations and experiments. The results show that the drawing capacity of an expanded head bolt is affected by the bearing capacity of front and rear ends, side bearing capacity, and side friction resistance. For a circular anchor bolt, stepped anchor bolt, and semi-ellipsoidal anchor bolt, with an increase in the front section’s radius, the lateral friction resistance of the inner anchor section is gradually shared by the bearing force of the front end of the inner anchor section; the bearing effect of the front end of the inner anchor section is enhanced; and the pulling performance of the anchor bolt is enhanced. Therefore, the pulling force of the circular anchor bolt is at the maximum, followed by the stepped anchor bolt, and the semi-ellipsoidal bolt is at the minimum. The increase in the rear section can provide greater lateral friction resistance and end-bearing force. Compared with cylindrical enlarged head anchors, the circular, stepped, and semi-elliptic enlarged head anchors have a smaller front section but a larger rear section, and the reduction in the front section’s bearing capacity is less than the increase in the side bearing capacity and rear-end bearing capacity; thus, the cylindrical bolt has the lowest pulling force. Compared with the front radius, the back radius has more influence on the drawing ability of the enlarged head anchor. The longer the inner anchorage section, the larger the distribution range in the compression zone that is formed in the soil body and the smaller the range in the tension zone that is formed in the rear. The increase in the length of the inner anchorage section is conducive to improving the reinforcement effect of the soil in front of the anchorage section in the bolt. Therefore, this parameter plays an important role in the redistribution of the soil in front of the force. The ultimate pull-out force of a circular table-shaped tensile bolt is the highest, followed by the stepped bolt, and the semi-elliptic bolt comes in third, with the cylindrical bolt exhibiting the lowest pull-out force; the circular table-shaped enlarged head anchor constitutes the best style design.