A $66 million lawsuit has been filed charging that a law firm and its major client, Shell Oil Co., conspired to cover up years of pollution at the Rocky Mountain Arsenal. Travelers Insurance Co., which insured Shell's arsenal operations from 1952 to 1975, charged that the oil giant failed for decades to tell underwriters about environmental destruction at the chemical production site north of Stapleton International Airport. The complex lawsuit, filed Nov. 1 in California, is unusual in its accusation that the Denver law firm Holme Roberts conspired with a client. The Army, which leased arsenal space to Shell, sued the oil giant in December 1983 to force it to join the $2 billion cleanup of the 27-square-mile facility. Shell hired Holme Roberts as its attorneys in the case. One month later, Travelers agreed to pay 55 percent _ $16 million _ of Shell's costs of defending itself against that suit. Travelers is asking for return of the $16 million plus $50 million in punitive damages. Travelers said Shell ``knowingly and intentionally released pollutants into the environment since commencement of its operations at the arsenal in 1952.'' It also charged that Holme Roberts learned of the extent of Shell's pollution at the arsenal while representing Shell. A spokesman for Holme Roberts said the lawsuit was ``totally without foundation,'' and a Shell corporate spokeswoman said the accusations were ``totally without merit.'' In a separate case, Shell is suing Travelers and 250 other insurance companies to force them to share the entire cost of the cleanup bill. That trial is in its second year in San Mateo County (Calif.) Superior Court. Travelers charged in its lawsuit that Shell and Holme Roberts are using Travelers-financed evidence from the Army case against Travelers itself in the California insurance trial.