construction decisions were not discretionary functions. Since the district court found that negligent construction and design were both proximate causes of the canal breaks, we may affirm the district court’s finding of liability on the part of the Bureau for the breaks.
Ill
The United States next argues that provisions in its Repayment Contract with Kennewick foreclose recovery for damages caused by the canal breaks. The government may of course assert a contractual defense in a tort action under the FTCA without running afoul of the Tucker Act, 28 U.S.C. § 1346(a)(2), which vests in the Court of Claims exclusive jurisdiction over contract claims against the United States in excess of $10,000. See 28 U.S.C. § 1346(c); see also Woodbury v. United States, 313 F.2d 291, 296-97 (9th Cir.1963).
Federal law controls the interpretation of a contract entered pursuant to federal law when the United States is a party. United States v. Seckinger, 397 U.S. 203, 209-10, 90 S.Ct. 880, 884-85, 25 L.Ed.2d 224 (1970) (Seckinger). For guidance, we may look to general principles for interpreting contracts. Saavedra v. Donovan, 700 F.2d 496, 498 (9th Cir.), cert. denied, 464 U.S. 892, 104 S.Ct. 236, 78 L.Ed.2d 227 (1983). The interpretation of a contract presents a mixed question of law and fact subject to independent review. James B. Lansing Sound, Inc. v. National Union Fire Insurance Co., 801 F.2d 1560, 1564 (9th Cir.1986).
“A written contract must be read as a whole and every part interpreted with reference to the whole.” Shakey’s Inc. v. Covalt, 704 F.2d 426, 434 (9th Cir.1983). “Preference must be given to reasonable interpretations as opposed to those that are unreasonable, or that would make the contract illusory.” Id. “The fact that the parties dispute a contract’s meaning does not establish that the contract is ambiguous.” International Union of Bricklayers & Allied Craftsman Local No. 20 v. Martin Jaska, Inc., 752 F.2d 1401, 1406 (9th Cir.1985). A contract is ambiguous if reasonable people could find its terms susceptible to more than one interpretation. Castaneda v. Dura-Vent Corp., 648 F.2d 612, 619 (9th Cir.1981).
Article 32 in the Repayment Contract, entitled “Computation of Costs,” provides:
The cost, which makes up the various obligations to be paid by the District to the United States under this contract, shall embrace all expenditures of whatsoever kind in relation to the function for which the charge is made, including, but without limitation by reason of this enumeration, cost of surveys and investigation, labor, property, material and equipment, engineering, legal, superintendence, administration, overhead, general expenses, inspection, special services and expenditures made by the United States on account of damage claims of all kinds, whether or not involving the negligence of the officers, agents or employees of the United States.
Kennewick contends that Article 32 is definitional only; it exists to determine the costs of various obligations incurred by the parties under other provisions of the contract but creates no obligations itself. To support this reading, Kennewick points to Article 32’s title, “Computation of Costs,” and its allegedly plain language. Kenne-wick also identifies contractual obligations contained in other provisions of the contract, including obligations for the costs of inspection, maintenance, and construction. With regard to the construction costs, Kennewick argues that these costs were fixed in 1953 by Article 8(a). Article 8(b) allows adjustments of those obligations but, according to Kennewick, Article 8 was meant to terminate when the fixed charges were paid. Without such termination, argues Kennewick, the construction charge “would remain open forever.” Thus, Article 32 imposes no obligation on Kennewick to indemnify the United States for claims asserted after the fixing of construction costs arising out of negligent construction in which Kennewick played no role. The government argues, however, that Kenne-wick’s interpretation frustrates the statu