CAPT OF AIR CARRIER WDB ACFT DIVERTED OFF THE ATC TAXI CLRNC RTE ONTO A CLOSED TAXIWAY.

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OUR FLT WAS CLRED FROM THE DFW E AIR FREIGHT AREA TO RWY 17R VIA TAXIWAY '60' TO TAXIWAY 'M' THEN TAXIWAY '19' TO 17R. 17L WAS CLOSED DUE TO CONSTRUCTION; AND THE THRESHOLD OF 17R WAS DISPLACED. BEFORE PUSHBACK WE RECEIVED OUR ATC CLRNC EXCEPT FOR OUR TRANSPONDER CODE; WHICH WE WERE TOLD WOULD BE GIVEN TO US LATER. AS PER COMPANY PROC; I HAD THE ARPT DIAGRAM OUT FOLLOWING OUR TAXI PROGRESS. AFTER XING 17L I DIVERTED MY ATTN TO SETTING THE TRANSPONDER CODE JUST RECEIVED AND WRITING IT DOWN ON OUR FLT RELEASE. I FELT THE ACFT TURNING; SO I LOOKED UP AND FOUND THE CAPT HAD TURNED ON TAXIWAY 'L' TO PICK UP TAXIWAY '18.' I QUESTIONED HIM ABOUT COMING THIS RTE AND HE REPLIED HE ASSUMED TAXIWAY '18' WAS OPEN ON THIS SIDE OF RWY 17L. THERE WERE NO BARRIERS ON TAXIWAY 'L' OR ON TAXIWAY '18' BTWN 17R AND 'L' AS THERE WERE AT 'M' AND '18.' WE DISCUSSED THE DISPLACED THRESHOLD ON RWY 17R; BUT HE SAID THAT IT WAS FOR LNDG PURPOSES ONLY. AT THE END OF 17R ON '18' WE FOUND THE ARPT HAD CONSTRUCTED A TEMPORARY BLAST FENCE BTWN THE END OF 17R AND THE APCH LIGHTS. IT MUST HAVE BEEN PUT THERE TO PROTECT THE TRUCKS GOING TO AND FROM THE CONSTRUCTION ON 17L FROM THE JET BLAST OF ACFT TAKING OFF ON 17R. I TOLD THE CAPT THAT IT LOOKED VERY CLOSE TO ME; THAT WE MAY NOT BE ABLE TO TAXI ONTO RWY 17R WITHOUT HITTING THE FENCE OR LEAVING THE TAXIWAY ON THE L SIDE. I COULD SEE THAT THE L SIDE OF THE TAXIWAY NEAR THE EDGE WAS SOFT -- THERE WERE TIRE TRACKS FROM A TRUCK OR ACFT IN THE ASPHALT NEAR THE LIGHTS. THE TWR CLRED US FOR TKOF; THE CAPT AGREED IT LOOKED CLOSE; BUT HE THOUGHT HE COULD MAKE IT OK. HE TAXIED FORWARD ABOUT 15-20 FT AND ASKED HOW IT LOOKED ON MY SIDE NEAR THE BLAST FENCE. I TOLD HIM I WAS NOT COMFORTABLE WITH THE CLRNC BTWN OUR R ENG/WING AND THE TOP OF THE BLAST FENCE. WE WERE PAST THE HOLD LINE FOR 17R; SO WE INFORMED THE TWR WE MAY NOT BE ABLE TO CLR THE BLAST FENCE. WE ASKED IF THEY WOULD SEND ONE OF SEVERAL ARPT VEHICLES OVER TO WATCH OUR R ENG/WING AS WE COMPLETED OUR TURN ONTO THE RWY. 1 OF THE VEHICLES WITH A RADIO SAID HE WOULD WATCH OUR R SIDE AND PULLED HIS TRUCK NEXT TO THE BLAST FENCE. HE SAID WE WERE OK AND WATCHED AS WE COMPLETED THE TURN ONTO THE RWY. WE THANKED HIM FOR HIS HELP. AND PROCEEDED WITH THE TKOF WITH NO FURTHER PROBLEMS.