The defendants, Empire Paving, Inc. (Empire), and its bonding company, American Insurance Company, doing business as Fireman’s Fund Insurance Company (Fireman’s Fund), appeal from the judgment of the trial court awarding damages to the named plaintiff, New England Rock Services, Inc. (Rock Services), under a contract between the parties. The principal issue on appeal is whether the trial court improperly concluded that an agreement made by the parties on December 9, 1995, modified an earlier contract executed by them on October 26, 1995.We affirm the judgment of the trial court.

   The following facts are relevant to the disposition of this appeal. On October 26, 1995, Empire entered into a contract with Rock Services under which Rock Services would provide drilling and blasting services as a subcontractor on the Niles Hill Road sewer project on which Empire was the general contractor and the city of New London was the owner. Pursuant to the contract, Rock Services agreed to drill and blast a certain amount of rock encountered on the sewer project. In return, Rock Services was to be paid an agreed upon price of $29 per cubic yard with an estimated amount of 5000 cubic yards, or on a time and materials basis, whichever was less.

   On October 31, 1995, Rock Services commenced work on the project. From the beginning, Rock Services experienced a number of problems with the project. The primary obstacle was the presence of a heavy concentration of water on the site. The water problem hindered Rock Services’ ability to complete its work as anticipated. The trial court found that it was the custom and practice in the industry for the general contractor to control the water on the site and that, on this particular job, Empire failed to control the water on the site properly. In an effort to mitigate the water problem, Rock Services attempted to ‘‘load behind the drill,’’ a process that allows a blaster to load the drilled hole with a charge immediately after the hole is drilled, before water has the opportunity to seep into the hole. The city fire marshall, however, refused to allow Rock Services to employ this method of drilling. Thereafter, in order to complete its work, Rock Services was compelled to use the more costly and time consuming method of casing the blasting hole, a process that requires the blaster to drive a plastic casing down into the drilled hole to prevent seepage.

   In late November, 1995, Rock Services advised Empire that it would be unable to complete the work as anticipated because of the conditions at the site and requested that Empire agree to amend the contract to allow Rock Services to complete the project on a time and materials basis. On December 8, 1995, Empire signed a purchase order that modified the original agreement. The modification required Empire to pay for the blasting work on a time and materials basis for the remainder of the project. Rock Services, thereafter, completed its work on the project.

   Upon completion of the work, Empire refused to pay Rock Services for the remaining balance due on the time and materials agreement in the amount of $58,686.63, and Rock Services instituted this action. The trial court concluded that the later purchase order was a valid and enforceable modification of the earlier contract. The trial court found that the parties intended the purchase order to modify the earlier agreement and that Empire’s assent to the modification was not made under duress but, rather, was a calculated business decision. After finding Empire’s withholding of the amount due to Rock Services wrongful, the trial court awarded Rock Services damages in the amount of $58,686.63, plus interest and costs. This appeal followed.

   On appeal, Empire claims that the trial court improperly found that the later purchase order was a valid and enforceable modification of the earlier contract. Specifically, Empire claims that the later agreement lacked the requisite consideration to be a valid and enforceable modification of the earlier contract. We disagree.

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   In concluding that the modification was valid and enforceable, the trial court determined that the later agreement was supported by sufficient consideration. * * *

‘‘The doctrine of consideration is fundamental in the law of contracts, the general rule being that in the absence of consideration an executory promise is unenforceable.’’ [Citation.] While mutual promises may be sufficient consideration to bind parties to a modification; [citations] a promise to do that which one is already bound by his contract to do is not sufficient consideration to support an additional promise by the other party to the contract. [Citations.]’’

‘‘A modification of an agreement must be supported by valid consideration and requires a party to do, or promise to do, something further than, or different from, that which he is already bound to do. [Citations.] It is an accepted principle of law in this state that when a party agrees to perform an obligation for another to whom that obligation is already owed, although for lesser remuneration, the second agreement does not constitute a valid, binding contract. [Citations.] The basis of the rule is generally made to rest upon the proposition that in such a situation he who promises the additional [work] receives nothing more than that to which he is already entitled and he to whom the promise is made gives nothing that he was not already under legal obligation to give. [Citations.]’’

   Our Supreme Court in [citation], however, articulated an exception to the preexisting duty rule: ‘‘‘[W]here a contract must be performed under burdensome conditions not anticipated, and not within the contemplation of the parties at the time when the contract was made, and the promisee measures up to the right standard of honesty and fair dealing, and agrees, in view of the changed conditions, to pay what is then reasonable, just, and fair, such new contract is not without consideration within the meaning of that term, either in law or in equity.’’’ * * * ‘‘‘What unforeseen difficulties and burdens will make a party’s refusal to go forward with his contract equitable, so as to take the case out of the general rule and bring it within the exception, must depend upon the facts of each particular case. They must be substantial, unforeseen, and not within the contemplation of the parties when the contract was made. They need not be such as would legally justify the party in his refusal to perform his contract, unless promised extra pay, or to justify a court of equity in relieving him from the contract; for they are sufficient if they are of such a character as to render the party’s demand for extra pay manifestly fair, so as to rebut all inference that he is seeking to be relieved from an unsatisfactory contract, or to take advantage of the necessities of the opposite party to coerce from him a promise for further compensation. Inadequacy of the contract price which is the result of an error of judgment, and not of some excusable mistake of fact, is not sufficient.’’’ [Citation.] * * * Empire argues strenuously that the water conditions on the site cannot qualify as a new circumstance that was not anticipated at the time the original contract was signed. * * *

   Empire’s argument, however, is misplaced. Rock Services does not argue that it was unaware of the water conditions on the site but, rather, that Empire’s failure to control or remove the water on the site constituted the new or changed circumstance. Rock Services argues that Empire’s duty to control or remove the water on the job site arose in accordance with the custom and practice in the industry and, therefore, Empire’s failure to control or remove the water on the site constituted a new circumstance that Rock Services did not anticipate at the time the original contract was signed. * * *

   The judgment is affirmed.