Many older homes have foundations made of field stone that may over time have sagged or bowed from the elements pushing on the stones. Since no mortar as we know it today was used, the stones can easily shift. If there has been a serious total collapse of the stone wall, total substitute is necessary. If the problem is less harsh, a new concrete reinforcing wall may be in order. If you have an earthen basement floor the job is already easier. Starting on the inside of the basement you must first excavate a trench alongside the existing wall to install a new footing for the reinforcing wall. I would make the footing at the minimum two feet wide but three feet wide is much better. Make the footing one foot thorough. The top of the new footing should match the bottom of the stone wall and the new floor will sit on top of the footing. Be careful not to upset the soils under the stones. There will be no footing under a stone wall. Once you are done excavating, form the footing and install the reinforcing bars. Use at the minimum 5/8′ diameter bars. Run them the complete length of the footing and in a three foot wide footing there should be three continuous bars. Now take pieces cut four feet long and bend a one foot bend at one end making an ell shape. These bars will be tied to the inner and outer most footing rebar to form two rows of dowels running the complete length of the footing, no more the thirty-two inches apart. When the footing is poured you will see only the vertical part of the bars sticking out. Once the footing concrete is dry, strip all the forms leaving no wood behind for termites to latch on to for dinner.
Now using the same size rebar, install horizontal bars starting three inches up from the footing and place one every twelve inches running the complete length of the new wall. Do this on both rows of dowels. These when completed are called wall mats. Using some small scraps of rebar, tie them with wire between the two mats to keep them separated when you pour the heavy concrete. The new wall thickness will depend upon how thick your stone wall is and how badly bowed the stone wall is. My last wall was basically twenty four inches thick but twelve inches thick is a minimum you should build. Your rebar wall mats want to be as close to three inches from both faces of the wall as possible. The inside mat may be a bit more depending on the amount of bow in the stone wall. I used Symonns concrete form panels from my rental store for ease of forming the new wall but you can also build the forms out of wood. The advantage of concrete form panels is they come in all sizes of heights and widths, have clips for easy fastening, have pre-made anchors and so on. You must remember that this is a one sided form. The toughest kind by far to build. You must brace it to death and then add some more braces to keep it from moving under the weight of wet concrete. Installation of what is called a whaler brace will keep the forms from bowing out but add kickers galore. On concrete pour day you will need some additional help. Pouring by a basement window is no joke as the guys inside cannot see the guy outside in the ready mix truck.
One man has to stay outside and yell orders by the window. Faster, slower, more, too wet and so on. Inside one man has to do nothing but watch the forms. If something moves he has to closest stop the pour. Being buried under wet concrete and smashed form panels is no joke. Another person has to vibrate the forms with a mechanical vibrator to assure there are no voids in the concrete. however another person can tap the forms with a hammer to assure there is concrete in all the spaces and watch down by the top of the form to assure they are filling up as level as possible. This eliminates putting too much pressure in one identify of the forms at once. Excess pressure can cause the forms to fail. Wet concrete exerts what is called hydraulic pressure sideways against the form work and can amount to tons of pressure. I have seen forms fail that were blown a hundred feet away from the work site. Depending upon how thick your new wall is a two day wait for form removal is more than enough. Twenty four hours is better nevertheless. Any spilled concrete will get super hard and make form removals that much harder if you let the concrete sit too long. Again strip all remnants of wood from the work. Back fill the footing and level the floor area to prep it for the new basement floor.
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