b. When displacement limit is reached, initial grouting is stopped and stage grouting may be done. (Note that when excessive shaft movement occurs, it is an issue of insufficient side resistance. Stage grouting may be performed but still may not solve the issue.) c. Criterion resets for each subsequent stage (displacement limit is not cumulative for grout stages). 3. Grout Volume a. A minimum volume value must be established to assure grout lines are not blocked, and grout is reaching the base of the shaft. b. An upper limit volume value is sometimes specified. If this upper limit is reached, stage grouting is initiated. (Note: this is rarely used in practice. It is always desirable to achieve the required pressure during the first stage). c. Large grout takes may indicate hydro-fracturing of the ground or grout flow up the side of the shaft. Monitoring instrumentation typically includes an oil-filled Bourdon pressure gauge, electronic pressure transducer, digital survey displacement measuring system, conventional survey level (for redundancy), grout holding tank volume measurement and grout flow meters. Grout monitoring systems which plot the grouting measurements on multi-axis plots in real time (Figures 3 and 4) are encouraged on all production shafts. Optional instruments such as strain gauges and telltales provide valuable insight, but a precise value from these devices should not serve as a specified acceptance criterion. Strain gauge/telltale data are used to FHWA Post-Grouting of Drilled Shafts Study A significant FHWA-sponsored evaluation and devlopment project is being conducted by a team of investigators organized through ADSC. The first phase of work included a literature review, consultation with practitioners involved with post-grouting, collection and interpretation of results from full-scale load tests on post-grouted and conventional drilled shafts, numerical modeling to investigate and evaluate load transfer and improvement mechanisms for post-grouted drilled shafts, and development of design and construction recommendations. The second phase of work includes installation, grouting and testing of full scale shafts. This second phase work is ongoing, and the final report is expected in fall 2016. This study has identified improvement mechanisms that occur during post-grouting: pre-mobilization and ground improvement, and asserts that considering these mechanisms separately can improve predictions and allow better post-grouting execution in field. These mechanisms have been described by Marinucci and Nichols (2016). Ground improvement through post-grouting occurs through densification, permeation and the enlargement of the shaft tip. Pre-mobilization is improvement in shaft performance that results from load reversal during grouting and subsequent top- down loading (Fleming, 1993). Acceptance Methods The following were agreed upon regarding acceptance methods for post-grouting of drilled shafts: • Grout pressure, shaft uplift (vertical displacement) and grout volume are the primary parameters to be measured during post- grouting. • Grout pressure is considered to be most important, followed by shaft uplift and then grout volume. In general, parameter monitoring trends must be stable. • The design or target grout pressure should be a design element and a function of skin friction, shaft weight, soft layers, hydrofracture potential, soft/loose areas and water table. Figure 3. Grout pressure and grout volume (Muchard, 2015) provide a relative indication that the shaft tip is being stressed by the applied grout pressure. Typical strain response near the shaft base during grouting is shown in Figure 5. This multi-axis plot of grout pressure and strain measurements during grouting indicates typical eccentricity, but average strain shows excellent trending with the applied grout pressure. • To date, a maximum shaft uplift has been used as a limiting criterion to prevent a presumed potential for deterioration. However in side resistance it is recognized that movement at the top of the shaft (or from telltales in deep slender shafts) also provides assurance that grout is being distributed across the base sufficiently enough to result in a bi-directional load causing upward movement and mobilization of side resistance; thus, some minimum amount of upward movement is desirable. • A minimum top of shaft displacement requirement is not always feasible because high side resistance may prevent shaft movement. Shaft movement is important to measure, and instrumentation at the surface or within the shaft is required to ensure upward shaft movement. 88 • DEEP FOUNDATIONS • SEPT/OCT 2016
