The University of Massachusetts at Amherst, already in the middle of a building boom, still needs an additional $1.8 billion in the next decade and beyond to keep aging buildings in shape, to add new facilities, and to maintain its infrastructure, according to a report. Four years ago, UMass-Amherst began a $790 million building effort. This year, five new residence halls opened, along with two renovated dining halls. The state, UMass officials point out, has covered only about 13 percent of the annual construction costs, forcing the university to rely on other money, such as student fees and bonds. The daunting price tag for future building costs highlights the gravity of the state's lack of investment in public higher education, say university officials and a key lawmaker.