A modular building system (10, 900) for building a hardscape structure, the system comprising: a plurality of preformed hardscape structure sections including: a first hardscape structure section including a first section face and at least one first section groove (204, 910) defined in the first section face; a second hardscape structure section having a second section face and at least one second section groove (304, 912) defined in the second section face, the second hardscape structure section being disposed adjacent the first hardscape structure section such that the second section face is adjacent the first section face; a connecting member (400, 920, 7602, 7702) having a first engagement portion engaged in one of the at least one first section groove and a second engagement portion engaged in one of the at least one second section groove to thereby secure together the first and second hardscape structure sections (200, 300); and a compression member (950, 7604, 7704) configured to be disposed against the first and second section faces (202, 302), in alignment with the connecting member (400, 920, 7602, 7702), the compression member (950, 7604, 7704) being connectable to the connecting member (400, 920, 7602, 7702) such that the compression member (950, 7604, 7704) abuts the first and second section faces (302, 202) and frictionally engages the first and second section faces (302, 202) to prevent sliding longitudinal movement of the connecting member (400, 920, 7602, 7702) along the corresponding first and second section grooves (204, 910; 304, 912) in which the connecting member (400, 920, 7602, 7702) is received; wherein the compression member includes a compression plate (952, 1354, 3004, 7610, 7710) which is continuous and which overlaps the first and second section faces (202, 302). a plurality of preformed hardscape structure sections including: a first hardscape structure section including a first section face and at least one first section groove (204, 910) defined in the first section face; a second hardscape structure section having a second section face and at least one second section groove (304, 912) defined in the second section face, the second hardscape structure section being disposed adjacent the first hardscape structure section such that the second section face is adjacent the first section face; a first hardscape structure section including a first section face and at least one first section groove (204, 910) defined in the first section face; a second hardscape structure section having a second section face and at least one second section groove (304, 912) defined in the second section face, the second hardscape structure section being disposed adjacent the first hardscape structure section such that the second section face is adjacent the first section face; a connecting member (400, 920, 7602, 7702) having a first engagement portion engaged in one of the at least one first section groove and a second engagement portion engaged in one of the at least one second section groove to thereby secure together the first and second hardscape structure sections (200, 300); and a compression member (950, 7604, 7704) configured to be disposed against the first and second section faces (202, 302), in alignment with the connecting member (400, 920, 7602, 7702), the compression member (950, 7604, 7704) being connectable to the connecting member (400, 920, 7602, 7702) such that the compression member (950, 7604, 7704) abuts the first and second section faces (302, 202) and frictionally engages the first and second section faces (302, 202) to prevent sliding longitudinal movement of the connecting member (400, 920, 7602, 7702) along the corresponding first and second section grooves (204, 910; 304, 912) in which the connecting member (400, 920, 7602, 7702) is received; wherein the compression member includes a compression plate (952, 1354, 3004, 7610, 7710) which is continuous and which overlaps the first and second section faces (202, 302).