Any pipe can be filled with flowable fill. All abandoned pipes should be filled. Saves a lot of time when you find a 'strange 'pipe in an excavation. Pull the hose from the upstream end to 10 feet from downstream end. Place a plug in downstream end with a 1 inch pipe on the top of the plug past the end of the plug. Start neat cement grout pumping. Once grout is coming out of the 'overflow' pipe, cap it. Proceed to retrack hose as you pump. watch volume being pumped and extract hose at a rate to always have 25 feet of hose buried in grout. You can use a CCTV camera to wtch if you are concerned about your calculations for volumes.