You should define your customers properly first.
Suggest you prepare 7-10 customer personas to define your customers unbiasedly first.
If my guess is correct, you have a mixture of Govt and private projects. Social media marketing may help you to connect people but not securing your business - you may et through bids or tenders.
For Govt projects - if you can connect to officers/ decision-makers check with them how to go about to secure a project and establish a regular communication/ relationship so that you can be updated with new projects available.
Another common approach is to join an engineering/ architect association as a member or website subscription. You may also advertise in those association websites or publications. You should consider participating in exhibition events where you can present your business and connect/ partner with people in the common industry.
If you know of some large construction companies that consistently have a lot of projects, you should consider trying to work closely with them so that they can offer you can regular projects.
Partner with salespersons may be an option if s/he can bring in a lot of good projects and have long list of rich leads.