Stabilizing Poor Bearing Soils

Grout injection pipes will be divided to a firm layer of soil with a hand driver or by a mechanical driver for greater depths. Once a firm layer of soil is reached, a cement based grout consisting of portland cement, fly cement, fly ash and admixtures will be injected into the soil. Sand will be added to the grout mixture when large voids or extremely soft soil is found to warrant its use.

One pound of pressure is normally used for each lineal foot of depth. This pressure is sufficient enough to force the water out of the soil to be stabilized and to penetrate voids in the soils. Pressures will be adjusted depending on the type of soil being pressure grouted.

The grout mix proportion will be adjusted at each point of injection and from batch to batch during the process in order to obtain optimal grout take and penetration.

Primary grout insertion pipes will be driven on eight (8) foot centers and a record will be kept of the blow counts per foot it takes to drive the injection pipes. Once a firm layer of soil is reached, injection pipes will be pumped at one (1) foot intervals to the bottom of the concrete slab or footing. The amount of grout placed in each pipe is also recorded.

After the first stage of the grouting procedure in completed, secondary grout injection pipes are then driven between the first stage grout injection pipe locations. The blows required to drive the secondary pipes and the amount of material placed in each pipe is then recorded. This information will indicate to what extent the grout has penetrated the soil and to what degree the bearing capacity of the soil has been improved. If necessary, additional grout injection pipes will be driven in order to achieve the desired result.

The theory of pressure grouting for stabilization is to improve the soils load bearing value and to spread the load on the soil over a larger area by improving the strength of the soils structure.