Sand-bag and steel sheet pile combined cofferdam for hard formation and construction method of cofferdam

The invention discloses a sand-bag and steel sheet pile combined cofferdam for hard formation and a construction method of the cofferdam. The sand-bag and steel sheet pile combined cofferdam comprises an inner cofferdam body and an outer cofferdam body which are disposed on the hard formation. Clay is disposed in the clearance between the inner cofferdam body and the outer cofferdam body, sand is then filled, and a steel sheet pile is then driven to penetrate through the sand and clay and into the hard formation deeply. The clay is disposed at the bottom of the clearance between the inner cofferdam body and the outer cofferdam body, the sand is disposed above the clay, and the steel sheet pile penetrates through the sand and the clay into the hard formation deeply to form a water stopping core wall. Due to the fact that the clay is high in viscosity, poor in water permeability and the like, when the steel sheet pile penetrates through the clay, the clay is adhered to the bottom of steel sheet pile and penetra