GEI Consultants Inc., a geotechnical, environmental, water resources, and ecological science and engineering firms, is marking its 40th anniversary.

Established in 1970 as Geotechnical Engineers Inc., the firm was founded by five partners. Originally located in Winchester, GEI is now headquartered in Woburn and employs 400 staff in 20 offices nationwide.

"Throughout GEI’s 40-year history, despite changes in markets and the economy, the one thing we could always count on was the loyalty of our clients," said Francis D. Leathers, president of GEI. "We’re proud of our relationships, many of which date back to our beginning, and we value the client support that has allowed us to remain successful through the decades."

As the firm enters the century’s second decade, GEI is expanding its Midwestern presence by opening offices in Illinois, Wisconsin and Michigan.