employer liability for contractor

Where the obstruction or defect caused or created in the street is purely collateral to the work contracted to be done, and is entirely the result of the wrongful acts of the contractor or his workmen, the rule is that the employer is not liable; but where the obstruction or defect which occasioned the injury results directly from the acts which the contractor agreed and was authorized to do, the person who employs the contractor and authorizes him to do those acts is equally liable to the injured party.