
221 S.E.2d 512 (1976)
27 N.C. App. 725
W. E. GARRISON GRADING COMPANY
v.
PIRACCI CONSTRUCTION CO., INC.
No. 7514SC490.
Court of Appeals of North Carolina.
December 17, 1975.
Certiorari Denied February 3, 1976.
*514 Nye, Mitchell & Bugg by Charles B. Nye, Durham, for plaintiff.
*515 Dees, Johnson, Tart, Giles & Tedder by J. Sam Johnson, Jr., Greensboro, for defendant.
Certiorari Denied by Supreme Court February 3, 1976.
BROCK, Chief Judge.
To quote from defendant's brief, "All of the error which the Defendant assigns relates to this fundamental question: What was the contract between the parties, and did the Plaintiff prove liability under that contract?" In particular defendant contends that the presiding judge erred by not limiting its liability to excavation work performed by plaintiff which was (1) authorized by written change order and (2) measured by the cross-section method in accordance with the express terms of the 5 January contract. The essence of defendant's argument is that the 5 January contract exclusively governs its liability to plaintiff.
The presiding judge found that the written order procedure of the 5 January contract was modified by the subsequent oral agreement between plaintiff and defendant concerning the unit prices of borrow and mucking excavation. This finding is supported by competent evidence. Furthermore, it is apparent that the so-called "extra" borrow and mucking work was performed at the request and under the supervision of defendant's engineer. "The provisions of a written contract may be modified or waived by a subsequent parol agreement, or by conduct which naturally and justly leads the other party to believe the provisions of the contract are modified or waived. . .. This principle has been sustained even where the instrument provides for any modification of the contract to be in writing. (citations omitted)" Graham and Son, Inc. v. Board of Education, 25 N.C.App. 163, 212 S.E.2d 542 (1975). Applying this principle to the facts in this case, we hold that the presiding judge properly found defendant liable as a result of its oral agreement with plaintiff and its subsequent conduct.
Even if deemed liable for borrow and mucking operations, defendant argues that the quantity of excavation for which plaintiff is entitled to compensation should be based solely on cross-section calculations as prescribed by the 6 January contract. Again this argument is a casualty of the court's findings of fact:
"28. Upon the completion of work, the parties attempted to arrive at agreed quantities and in this connection jointly approved a final survey or cross-section of the completed works. [The parties could not agree that the final cross-section was an adequate measure or basis to determine the amount or quantity of work performed by comparing it with the original cross-section made before work was commenced, and the Court finds as a fact that the final cross-section of the completed work could not and would not be a proper basis of measurement for the work actually performed by the plaintiff in part because of:
"a. The final approved cross-section made after the completion of all work would not reflect the shrinkage in the fill areas after compaction.
"b. A comparison between the original and final cross-section in the fill areas would not correctly reflect the amount of top soil replaced in the fill areas and would not reflect accurately the mucking or top soil operations. Neither would a comparison of the cross-sections have any bearing on the materials in the fill areas which had to be removed more than once as borrow operations. Neither would the two cross-sections reflect or measure the amount of work required by the constant restaking of the project and changing of grades made by the defendant during the period that the construction work was being performed.
"Therefore, based upon all the testimony and evidence, the Court finds as a fact that the measurement of the actual quantities of material moved by the plaintiff must be based on some other method of measurement other than by use of a comparison of the original and final cross-sections *516 of the plaintiff's work because of the foregoing and because there was no interim engineering work for evaluating quantities.]
"[29. That the plaintiff offered uncontested testimony and evidence that the only reasonable and prudent way to determine the quantities or volume of material moved by the plaintiff on the construction project was, in the last analysis, based on load counts which the plaintiff kept on a daily basis during the construction of the project; that the plaintiff's evidence and testimony reflects the fact that all of its billings from start to finish concerning general and borrow excavation were based on load counts and the evidence tends to show, and the Court finds as a fact, that from the inception of the contract until its completion, the majority of its billings, if not all billings, were based on load count due to lack of engineering on behalf of the defendant; that the Court further finds it a fact that measurements made by load count are reasonably correct, are accepted in the trade as being reasonably certain and are a proper basis for determining the quantities of work performed by the plaintiff on the subject project because of the lack of interim engineering performed by the defendant and because of the plaintiff's being forced into an unbalanced project because of the defendant's failure to stake out the project properly on numerous occasions.]"
In summary, due to changes in the stakeouts as the work progressed and defendant's failure to provide the requisite engineering, it was not possible to accurately measure the amount of excavation by the cross-section method. "It is a salutary rule of law that one who prevents the performance of a condition, or makes it impossible by his own act, will not be permitted to take advantage of the nonperformance. (citations omitted)" Mullen v. Sawyer, 277 N.C. 623, 178 S.E.2d 425 (1971).
Defendant assigns error to the relief granted to plaintiff on the grounds that it is based upon quantum meruit. The presiding judge's conclusions of law clearly indicate that plaintiff's relief is derived not from quantum meruit principles but from "its contracts and agreements with the defendant under the conditions of the contracts and agreements or under the conditions of the contracts and agreements as waived or under the conditions as ratified and affirmed by the defendant. . . ." This assignment of error is overruled.
In the pleading stage of the trial defendant answered with a counterclaim to the effect that the cross-sections taken after the grading work had been completed indicate that plaintiff was overpaid. During trial defendant's attorney made the following statement to the trial judge:
"Your Honor, as I indicated before the luncheon recess, Mr. Rupp has asked to speak, or maybe before he begins his testimony with reference to the counterclaim. We have discussed it during lunch and I have told him that in our written contract, specifically paragraph 9.7.5 the statement is made `The making of final payment shall constitute a waiver of all claims by the owner except those arising from . . .' various matters that are not at issue here, and that in view of our position in this case that final payment has been made, that it would be both inconsistent and right that we not put in evidence of our counterclaim, and so that is the decision of the defendant consistent with its view of the contract, and therefore I would not introduce any evidence through Mr. Rupp of the counterclaim."
Defendant assigns error to the court's finding that defendant voluntarily agreed to the dismissal of its counterclaim. If we should determine that the trial judge erroneously found that defendant voluntarily dismissed its counterclaim, it would afford defendant no relief. At best, the difference between defendant's statement to the trial court and a voluntary dismissal of the counterclaim is technical. Most outstanding, *517 however, are the pretrial stipulations of the questions to be resolved by the trial court. Each of these questions was resolved upon the evidence offered at trial. None of them is affected by the existence or non-existence of defendant's counterclaim. We fail to see prejudicial error, if error exists, in the recitation in the judgment that defendant had voluntarily stipulated that its counterclaim be dismissed. Defendant had the burden of proof upon its counterclaim, but by its own election it offered no evidence to substantiate its claim. Under such circumstances we fail to see how defendant can suggest that its counterclaim still exists.
Affirmed.
VAUGHN and MARTIN, JJ., concur.
