Civil Engineering personal statements work best when they show how you think about load, risk, materials, water, ground conditions, transport, sustainability and public value. It helps to use one or two concrete examples rather than trying to cover every branch of the subject.
The Imperial course begins with core technical areas including mechanics, structural mechanics, materials, fluid mechanics, geotechnics, and energy and environmental engineering. A strong statement can link your reading, projects or observations to those kinds of foundations without pretending to know degree-level detail already.
Make the example recognisably civil-engineering rather than generic STEM. For instance, connect a bridge, drainage route, slope, construction site or low-carbon material comparison to load paths, structural mechanics, soil mechanics, water resources or engineering geology, then explain what you calculated, tested or reconsidered.
Avoid a paragraph that only says you like maths, physics and problem solving. A strong approach is to write about what you tested, calculated, observed or changed your mind about, because civil engineering is full of trade-offs rather than neat slogans.