Abstract: The gravel bolting tests has been used to verify the existence of pressure arch in bolted fractured rock,however,the pressure arch is difficult to understand and used due to the abstract concept.In order to study the formation mechanism of pressure arch and the functions of rockbolts in bolted fractured rock,the numerical simulation of gravel bolting tests by using the Particle Flow Code in 3 Dimensions(PFC3D)was achieved through developing the methods of batch generation and pretension for rockbolts simulated by particles.A series of gravel bolting tests were performed to visualize the pressure arch and reveal its formation mechanism and causes.The functions of rockbolts in gravel bolting tests were analyzed and verified.Base on the above,some suggestions were made for bolting in extremely fractured rock.The results show that:① A pressure arch can be formed in bolted gravel pack,which is an arched load bearing structure made of interlocked gravel particles.The load of the whole gravel pack is carried by the pressure arch,and the stability of pressure arch depend on the support from the gravel below the pressure arch.② Pressure arch is formed in the adaptive motion process of gravel particles under bolting.After the removal of bottom plate of gravel box,the gravity of upper gravel shifts to the periphery due to the downward movement of bottom gravel,forming arch legs in surrounding gravel.The gravel particles at the top of the arch interlock each other to form the vault,resulting in a formation of pressure arch based on the backing boards.③ The formation of a pressure arch needs to meet four conditions:a strong bottom base,a sufficient lateral constraint,a proper thickness of gravel pack and some balanced gravel particles below the arch.④ Lateral restraint,support and surface retention are exerted by rockbolts to gravel particles in gravel bolting tests.In the formation of a pressure arch,the rockbolts are pretensioned to generate lateral restraint on the gravel pack,so that the gravel particles have sufficient interlocking capacity for arching.After the formation,the rockbolts can maintain the stability of pressure arch by providing support and surface retention for gravel particles below the pressure arch.The pretention of rockbolts helps to increase the lateral restraint and supporting effect.⑤ The lateral restraint,supporting and surface retaining effects of rockbolts should be fully developed to build up a pressure arch in a large scale extremely fractured rock,including increasing pretension force,increasing the roughness of the rockbolt bodies,reducing the spacing between rockbolts,enlarging the size of plates and installing mesh,etc.