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　1.72×10−5　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.　©　2024　Biodiversity　Research　Center　Academia　Sinica.　All　rights　reserved.