A method of constructing a foundation for overfilled arches or other bridges by means of a combination of a precast and a cast-in-place concrete foundation, comprising steps of: utilizing a precast concrete foundation unit having a first elongated upright wall member and a second elongated upright wall member spaced apart from the first elongated upright wall member to define a channel therebetween, characterized in that at least one upright support extending laterally across the channel and interconnecting the first elongated upright wall member and the second elongated upright wall member, wherein an inner side of the first elongated upright wall member includes a first lengthwise recess facing the channel and an inner side of the second upright wall member includes a second lengthwise recess facing the channel in opposed and aligned relationship with the first lengthwise recess, wherein the upright support includes a plurality of through openings; subsequent to casting of the precast concrete foundation unit, inserting a first plurality of elongated metal reinforcement members into the channel such that each elongated metal reinforcement member extends laterally between the first lengthwise recess and the second lengthwise recess with a first end of the elongated metal reinforcement member positioned in the first lengthwise recess and a second end of the elongated metal reinforcement member positioned in the second lengthwise recess; subsequent to casting of the precast concrete foundation unit, inserting a second plurality of elongated metal reinforcement members through the through openings such that each elongated metal reinforcement member of the second plurality extends generally parallel to the first and second elongated upright wall members (302,304); subsequent to casting of the precast concrete foundation unit, placing the precast concrete foundation unit at a desired use location of the construction site; delivering concrete into the open cell of the precast concrete foundation unit while the precast concrete foundation unit remains at the desired use location; and allowing the concrete to cure-in-place such that the first plurality of elongated metal reinforcement members and the second plurality of elongated reinforcement members become embedded in the cured-in-place concrete, wherein, prior to the delivering and allowing steps, each elongated metal reinforcement member of the first plurality is tied to at least one elongated metal reinforcement member of the second plurality to maintain a desired position of each elongated metal reinforcement member of the first plurality within the channel. utilizing a precast concrete foundation unit having a first elongated upright wall member and a second elongated upright wall member spaced apart from the first elongated upright wall member to define a channel therebetween, characterized in that at least one upright support extending laterally across the channel and interconnecting the first elongated upright wall member and the second elongated upright wall member, wherein an inner side of the first elongated upright wall member includes a first lengthwise recess facing the channel and an inner side of the second upright wall member includes a second lengthwise recess facing the channel in opposed and aligned relationship with the first lengthwise recess, wherein the upright support includes a plurality of through openings; subsequent to casting of the precast concrete foundation unit, inserting a first plurality of elongated metal reinforcement members into the channel such that each elongated metal reinforcement member extends laterally between the first lengthwise recess and the second lengthwise recess with a first end of the elongated metal reinforcement member positioned in the first lengthwise recess and a second end of the elongated metal reinforcement member positioned in the second lengthwise recess; subsequent to casting of the precast concrete foundation unit, inserting a second plurality of elongated metal reinforcement members through the through openings such that each elongated metal reinforcement member of the second plurality extends generally parallel to the first and second elongated upright wall members (302,304); subsequent to casting of the precast concrete foundation unit, placing the precast concrete foundation unit at a desired use location of the construction site; delivering concrete into the open cell of the precast concrete foundation unit while the precast concrete foundation unit remains at the desired use location; and allowing the concrete to cure-in-place such that the first plurality of elongated metal reinforcement members and the second plurality of elongated reinforcement members become embedded in the cured-in-place concrete, wherein, prior to the delivering and allowing steps, each elongated metal reinforcement member of the first plurality is tied to at least one elongated metal reinforcement member of the second plurality to maintain a desired position of each elongated metal reinforcement member of the first plurality within the channel.