The Pentagon is putting increased emphasis on using superhardened silos as the long-term way to ensure survivability of the giant, 10-warhead MX intercontinental ballistic missile, according to administration sources.

The ability to build new silos of cement and steel that could withstand a nearby nuclear blast could also reduce the need for building a costly, hardened, mobile transporter to carry the small, single-warhead ICBM, nicknamed Midgetman, now under development, sources said.

The Air Force plans to spend $219.7 million this fiscal year on superhardening, with testing on a full-size silo scheduled. Another $59.6 million is going into research for the hard mobile transporter for Midgetman.

At a sharply lower funding level of $9.6 million, paper studies are to continue on deep underground basing for ICBMs.

A Defense Department working group this summer looked into "appropriate alternative strategic missions that could be located deep underground," according to recently released congressional testimony. One finding, according to Air Force officials, was that use of deep basing would be more likely for communications, command and control facilities than for missiles.

Up to now, the plan has been to put the MX in existing Minuteman silos beginning in late 1986, with the possibility that they might be hardened in the future. That approach, which critics say would leave the MX vulnerable to a Soviet attack, has been questioned on Capitol Hill and has made it more difficult for the administration to win additional production funds for the controversial missile.

However, at a closed session of a House Appropriations subcommittee hearing four months ago, Thomas E. Cooper, assistant secretary of the Air Force for research, development and logistics, said that if future hardness tests were successful and enough money made available by Congress, "we could deploy an MX missile in a superhardened silo in late 1988."

Such a silo, Cooper said, could be anywhere from 25 to 50 times stronger than current Minuteman silos and permit an estimated 80 percent of the missiles to survive a nearby nuclear warhead explosion. Air Force witnesses have told Congress in the past that only 10 percent of the U.S. ICBM force would survive without superhardening.

A move to superhardening could help solve both the military and political problems now facing the embattled MX, according to congressional and administration sources.Hardened silos could be built at the same Air Force base in Wyoming where the MX now is scheduled to be deployed, eliminating the need for more land or new command and control systems.

Cooper said present Minuteman silos, in which the new MX missiles are to be placed beginning in December 1986, could withstand "about" 2,000 pounds of pressure per square inch (psi) from a nuclear blast. Recent tests on a one-fifth scale silo using the new cement and steel construction techniques "indicate we should be able to realize hardnesses up to 100,000 psi," Cooper said.

Key tests on a large-size silo in the coming year would not only determine whether the new approach to a cement and steel silo could survive a near direct hit, but also whether the missile could then be launched up through the resultant debris. Plans for actually putting MX missiles in superhardened silos will not be undertaken, Cooper said, until next year's test results are in.