B767-300 CAPT REPORTS TXWY EXCURSION AT INTERSECTION OF TXWY R AND N AT LFPG.

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TAXI CLRNC WAS TO 'TAXI TO RWY 8L; HOLDING POINT S1 VIA TXWY R AND S1.' WE COPIED AND READ BACK THE CLRNC. DELAYED START CHKLIST; RWY CHANGE CHKLIST ITEMS; AND BEFORE TKOF CHKLIST; DOWN TO RWY ITEMS; WERE COMPLETED. TAXI DIAGRAMS OF THE ARPT WERE OUT AND IN USE BY THE FO; RELIEF PLT; AND MYSELF. WE HAD TAXIED ON THIS RTE 3 DAYS PRIOR ON A PREVIOUS LEG OUT OF LFPG. APCHING RT1; GND CALLED US AND INFORMED US TO GIVE WAY TO A NBOUND A320 COMING FROM OUR R ON TXWY N. THERE WERE ACTUALLY 2 A320'S BUT ONLY ONE WAS A FACTOR AND WE GAVE WAY TO IT. AT THIS POINT; WE JUST PASSED RT1; AND I FELT THAT WE HAD MISSED OUR TURN POINT. AS WE APCHED THE INTXNS OF TXWYS R; N; AND C; WE REQUESTED A L TURN ON TXWY N IN ORDER TO GET TO HOLDING POINT S1. GND APPROVED THE L TURN. IT WAS A TIGHT TURN AND I THOUGHT I HAD GIVEN MYSELF A WIDE ENOUGH BERTH TO NEGOTIATE THE TURN; BUT I DID NOT AND THE L LNDG GEAR DEPARTED THE PAVEMENT INTO THE GRASS. AT FIRST I THOUGHT IT WAS A WEAK SECTION OF TARMAC; BUT AFTER WE REQUESTED ASSISTANCE FROM GND; ARPT FIRE PERSONNEL CAME OUT AND INFORMED US THAT THE WHEELS WERE IN THE GRASS. IT WAS DETERMINED THAT THE ACFT WOULD HAVE TO BE OFFLOADED OF PAX AND CARGO BEFORE THE ACFT COULD BE MOVED. AIRSTAIRS AND BUSES WERE BROUGHT OUT TO DISEMBARK THE PAX AND TAKE THEM BACK TO THE TERMINAL; AND CARGO WAS OFFLOADED. THERE WERE NO INJURIES AND NO DAMAGE TO THE ACFT ACCORDING TO THE COMPANY MECHS. THERE ARE NUMEROUS FACTORS CONTRIBUTING TO THE INCIDENT; MOST DEALING WITH THE COMMERCIAL ARPT DIAGRAM AND ARPT MARKINGS. THE FIRST IS THE WAY THE TXWYS ARE MARKED ON THE COMMERCIAL DIAGRAM. THERE IS NO DIRECT LINK BTWN TXWYS R AND S1 AS THE DIAGRAM IS PRINTED. THERE IS AN ARROW ON THE DIAGRAM FOR THE PREFERRED FLOW FOR EASTERLY DEPS ON RWY 8L; BUT IT TAKES ONE FROM TXWY R TO TXWY U TO UC1 AND THEN MAKES A L TURN ONTO WHAT THE DIAGRAM DEPICTS AS S1; THE '1' IN 'S1' SOMEWHAT COVERED BY THE PREFERRED ROUTING ARROW. TXWY SIGNAGE IS ALSO LACKING IN THAT AREA. THE ONLY SIGN THAT WE SAW IN THE AREA WAS FOR TXWY C. HOWEVER IF ONE LOOKED FURTHER UP TXWY C; THERE WERE PAINTED MARKINGS FOR TXWY C AND S1. THE S1 MARKING DOES NOT COINCIDE WITH THE MARKINGS ON THE COMMERCIAL DIAGRAM. ARPT PERSONNEL COMMENTED THAT THERE WAS NO TAXI LINE MARKING FOR A L TURN FROM TXWY R TO TXWY N. THERE IS NOT. HOWEVER; IF ONE LOOKS AT THE PREFERRED TAXI RTE; THERE IS ALSO NO TAXI LINE FOR A L TURN FROM TXWY U TO UC1 FOR WBOUND TFC. THERE IS ONE FOR EBOUND TFC AT THAT INTXN. ANOTHER FACTOR WAS FAMILIARITY WITH THE ARPT. THOUGH I HAD TAXIED THAT WAY ON OTHER OCCASIONS; AND WE HAD DONE SO ON THE PREVIOUS LEG OUT OF LFPG; WE DID NOT DISCUSS IT THOROUGHLY AS WE DID THE PREVIOUS TRIP.