In tunnel boring machine (TBM) tunnels

the pea gravel as a filling layer between the tunnel lining segments and surrounding rock is of significant importance for the load-bearing capacity and impermeability of the segments. Due to the poor flowability of cement slurry

it fails to adequately fill the backfill layer

resulting in defects such as voids behind the walls and inadequate grouting. Enzyme-induced calcium carbonate precipitation technology (EICP) has emerged as an environmentally friendly and efficient reinforcement method. The grouting material is liquid

exhibiting excellent fluidity and diffusivity

making it a promising solution for grouting in pea gravel backfill layers. To optimize the effectiveness of EICP grouting in pea gravel

an attempt was made to use standard sand and pea gravel as backfill aggregates. In order to quantitatively analyze the optimal mixing ratio

experiments were conducted with different ratios of pea gravel to sand (0.5

0.75

1.0

1.25

1.5) and varying grouting frequencies (9

12

15 times) in sand column solidification tests. Through unconfined compressive strength tests

permeability tests

determination of calcium carbonate content

ultrasonic velocity measurements

and scanning electron microscopy (SEM) microscopic analysis

the impact of different ratios of pea gravel to sand on the solidification effectiveness of EICP was analyzed from both macro and micro perspectives. The results indicate that the optimal ratio for EICP reinforcement of mixed pea gravel and sand is 1:1.5. After 15 grouting cycles

the uniaxial compressive strength of the specimens can reach up to 4.55 MPa

and the permeability coefficient is 1.72×10 m/s. Samples with a higher sand content exhibit a notable phenomenon where interparticle voids are readily filled and compacted by calcium carbonate crystals. This process results in a higher proportion of effective bonding among calcium carbonate crystals

consequently contributing to an elevated unconfined compressive strength of the stone body. The findings of this study can provide a theoretical basis for the engineering application of EICP technology in reinforcing TBM backfilled pea gravel.