Assist in designing underground spaces

Looking for an engineer ideally versed in designing small (500-1500 sq ft) spaces underground. Client request is for an underground area of size indicated to house equipment and provide short term or emergency shelter to remote workers. The equipment is electrical and facilities for equipment and personnel is prefabricated in present above grade facilities however the underground shelter has been requested to allow workers confirmed safety during especially bad weather, wildlife threats or during mandatory rest periods. We have numerous options available, however the client request is that the overfill be at least 15" and not to exceed 36" of topsoil. While we can truck in lighter topsoil to ease design requirements it is preferred that the design use an above average soil weight (Texas and Oklahoma) or stipulate a maximum safe load for the underground space roof area. We have seen examples utilizing many forms from Steel structures, fiberglass, earth bag walls, shipping containers, concrete block and even creosote soaked utility poles all proven in use for a decade or more. Our challenge is that many sites we are awarded are remote. Movement of large structures is almost impossible and use of heavy equipment is difficult and occasionally even lifting in small equipment is problematic. Instinct is that the ideal solution will be a combination of existing solutions. For instance a light steel frame placed in the excavated space then encased in a foam concrete or similar material perhaps with an earth bag lining and roof followed by covering the structure in 12"-18" of typical topsoil (16"-22" total overfill). Ideally the design would be a single 20x10 space, some supporting beams and cross beams are possible as the electrical equipment is modular. We earn the most if the space for the workers is larger. It needs of course to be safe however cost is a key factor and the space will essentially be hidden and found by workers using GPS with locked hatches as the terminal equipment is valued at $100,000-$150,000. We have specialist assigned to ensure habitability (air, facilities and ample living conditions) what we need is a simple fast and easy design that is cost-aware. Our bonus is dependent on speed of completion and bringing the structure online, number of workers that can use for occasional overnight stays (capacity) and of course cost and safety. We need a suggested make-up of the structure - primarily the load bearing roof and support that can carry the needed load of earth. We can already prepare the general site, drainage etc but need input on 1)- the options including rough cost and speed of construction given remote conditions, simplicity, SPEED - ideally a small space could be done in a couple days with limited equipment 2.) Any solution specific needs such as coatings, amount of ground water etc 3.) means to use all applicable technology such as foam concrete to speed construction or lower costs without sacrificing safety Steel sheet, I beams, utility poles (25-40' & 8"-10" in diameter) concrete, blocks, shipping containers and even fiberglass are all acceptable.