Pressure (AEP) diagram. Schnabel Foundation Company (Schnabel) used an AEP diagram for this project that was developed based on its 60 years of company experience. Experience gained from using the Schnabel AEP, which was based on thousands of constructed ERS projects, has proven the Terzaghi & Peck (T&P) diagrams to be slightly conservative in certain situations. The main differences between the Schnabel T&P AEP diagrams are the magnitude of the overall load and the concentration of load around the tieback or strut. The concept that the load is transferred to the stiffer parts of the structure has also been proven through documented monitoring of constructed ERS, where arching of the soils behind the wall allows for this to happen in relatively flexible systems. A hinge was assumed at subgrade, as is typical with other AEP diagrams, which results in zero moment below the bottom of the excavation. The additional load (i.e., surcharge) from the existing structure was also considered in the design of the jet grout wall. The surcharge was transferred directly to the jet grout columns, which were installed so that the columns extended directly beneath the existing footings and were in intimate contact with the bottom of the existing footings. Using this process, the load was transferred directly from the existing structure and into the jet grout columns, allowing for the loads from the footings to be transferred to the soils below the subgrade of the adjacent excavation. The application of the load directly onto the jet grout columns required the design analysis to consider the effects of combined bending and axial loading. The internal stresses of the jet grout columns were analyzed per ACI-318 for unrein- forced concrete given the loading con- ditions. Based on the magnitude of the earth pressure that exerted the bending stress and the axial loading from the existing building, it was determined that the jet grout columns could be installed without the need for reinforcement within the jet grout columns. In addition, based on the results of the analyses, the minimum required unconfined compressive strength of the jet grout columns was established as 1,000 psi (6,895 kPa). Apparent Earth Pressure (AEP) diagrams (Schnabel et al, 2002): (a) Schnabel, (b) Terzaghi & Peck sand and (c) Terzaghi & Peck clay Jet grouting beneath an existing footing is a method of underpinning. As such, it requires careful sequencing because there is a temporary condition during which the jet grouting is still fluid and there is little- to-no support directly beneath part of an existing footing. Before the jet grout material sets and hardens, there is a temporary reduction in the factor of safety for bearing capacity. This reduction (and increased risk exposure) is managed by working below only a small portion of the footing at a time. Jet grout underpinning was performed on this project beneath both continuous strip footings and isolated spread footings. Quality Control and Construction On almost every project, a test program is performed prior to the start of the production work to verify the many different parameters to be used and to confirm the achievable diameter and strength of a soil-cement column. To confirm the diameter of the column, it is best to excavate around a test column and to visually examine and measure the constructed geometry. However, this is usually difficult, if not impossible, when working in limited space conditions and deep columns. As an alternative, one method of verifying column diameter would be to drill and retrieve samples from the centroid of a three-column layout. In this scenario, three individual test columns are installed in a triangular layout in which all three columns overlap slightly in the middle. A borehole can be drilled in the centroid of this three-column layout and continuous split spoon samples can be retrieved. Standard penetration testing (SPT) blow counts obtained from driving the split spoon can be compared with soil borings taken prior to jetting to confirm that the ground has been strengthened. In addition, phenylethane indicator can be sprayed onto the retrieved samples to indicate the presence of cement by turning a bright color if present. These are but two ways of confirming that the column diameter has been achieved. Coring is sometimes used to retrieve samples for visual observation and lab testing. However, coring of relatively low strength material has proven to be unsuc- cessful in obtaining intact samples, especially in granular material containing gravel and/or cobbles. The rotary-wash action of the coring tends to erode low strength soil-cement, and larger pieces of sand or gravel, if present, can get caught on the core barrel teeth causing the sample to crumble. DEEP FOUNDATIONS • JAN/FEB 2020 • 97
