Examine the wall for cracks. Chisel out the edges of each crack with a hammer and chisel to create a smooth surface along both sides of the crack. Brush any debris out of the crack with a dustpan and brush. Prepare a batch of grout. Fill the crack with the grout using a grouting tool. Repeat for the opposite wall.
Measure the width of the wall using a tape measure. Dig a 3-inch-deep, 5-inch-square hole in the floor at the foot of the wall every 12 inches along the width of the wall, using a spade. Stand a 5-inch-square steel I beam as tall as the basement in each hole. Fill the holes with concrete around the I beams. Repeat for the opposite wall.
Measure the width of the basement. Cut as many pieces of 2-by-10-inch lumber as you have steel I beams, using a circular saw. Lift one lumber piece up to the top of each I beam, with the 10-inch-wide facing vertical. Bolt the 2-by-10 pieces to the top ends of the I beams using two lag bolts per piece. Repeat for the opposite wall.
Prepare a small batch of grout. Fill the gap between the parallel bars of the I beam and the wall with grout, using the grouting tool. Mix up more grout as you go rather than prepare it all at once so it does not begin to set before you use it all. Repeat this process for the opposite wall. Allow the grout 24 hours to set before decorating or covering up the braces.