Article: Only a few towns in western Libya—such as Bani Walid, Sebha and Sirte—remained Gaddafist strongholds. Retreating to Sirte after Tripoli's fall, Gaddafi announced his willingness to negotiate for a handover to a transitional government, a suggestion rejected by the NTC. Surrounding himself with bodyguards, he continually moved residences to escape NTC shelling, devoting his days to prayer and reading the Qur'an. On 20 October, Gaddafi broke out of Sirte's District 2 in a joint civilian-military convoy, hoping to take refuge in the Jarref Valley. At around 8.30am, NATO bombers attacked, destroying at least 14 vehicles and killing at least 53. The convoy scattered, and Gaddafi and those closest to him fled to a nearby villa, which was shelled by rebel militia from Misrata. Fleeing to a construction site, Gaddafi and his inner cohort hid inside drainage pipes while his bodyguards battled the rebels; in the conflict, Gaddafi suffered head injuries from a grenade blast while defence minister Abu-Bakr Yunis Jabr was killed.

Question: After fleeing to a construction site, how was Gaddafi injured?
grenade blast