"以扶壁支撑" can be translated as "supported by buttresses" or "propped up with buttresses".
"Supported by buttresses" emphasizes the structural support provided by the buttresses.
"Propped up with buttresses" conveys a similar meaning but might imply a more temporary or external form of support, though in architectural contexts, both expressions are acceptable and commonly used.
Choose the one that best fits the context and tone of your sentence.