soil strength and stiffness of the soil surrounding the EB can be improved significantly. In fact, the EB behaves as a large-scale pressuremeter, which provides informa t ion about s t r e s s - s t r a in characteristics of the soil adjacent to the pile toe, which is used to verify toe capacity. During inflation and with increasing diameter, the EB shortens slightly in length; as such, the soil below the base of the EB can be post-grouted after inflation during a subsequent grouting phase. This post- grouting process further increases the stiffness of the soil and increases the bearing capacity of the pile. Therefore, the bearing capacity of each pile can be tested and verified. The EB model now consists of a 1 to 2 m (3.3 to 6.6 ft) long cylindrical tube of folded steel and is watertight. The uninflated cylin- drical tube has an initial diameter of about 12 cm (4.75 in) and can be expanded to a diameter ranging from 0.4 m to 0.8 m (1.3 to 2.6 ft), resulting in an increase of about 330 to 700%. The increase in the diameter of the EB is directly related to the volume of injected grout; as such, the cross-sectional area (thus, the volume per unity of length) is increased by about 1,100 to 6,600%. As the injection process is registered, it is possible to determine the actual EB volume from the calibration curve for each EB model. Furthermore, the effective diameter and cross-sectional area of the EB can be calculated given the injection pressure and grout volume, which can be compared with the design assumptions. Type EB 410 EB 610 EB 612 EB 615 EB 815 EB 820 Pre-inflation Diameter m [ft] 0.12 [0.4] 0.12 [0.4] 0.12 [0.4] 0.12 [0.4] 0.12 [0.4] 0.12 [0.4] Length m [ft] 1.0 [3.3] 1.0 [3.3] 1.2 [3.9] 1.5 [4.9] 1.5 [4.9] 2.0 [6.6] Geometric parameters of different EB sizes Lateral expansion of the EB is one of the main reasons for the significant ground improvement effect, which occurs in the soil surrounding the EB. In cohesionless soils, the lateral expansion of the EB produces two main effects: (1) increase in density and (2) increase of inter-granular stresses, where both effects produce an 88 • DEEP FOUNDATIONS • MAY/JUNE 2018 increase in soil stiffness out to about 3.5 diameters from the center of the EB. The increase in stiffness is directly related to the initial soil stiffness and the final diameter of the EB; that is, the lower the initial stiffness, the greater the relative increase will be (i.e., final stiffness / initial stiffness). The grouting process of the liquid-tight EB occurs under controlled conditions, where it is possible to measure the gradual increase in EB volume and corresponding injection pressure. All relevant parameters (e.g., flow rate, pressure and grout volume) are recorded by a computer-controlled data acquisition system, which is an integral part of the quality control program. The applied grout pressure reflects the soil resistance during the expansion of the EB and is a measure of soil stiffness and soil strength. The grout- ing record is obtained for each pile and offers a measure of quality control and assurance. The injection of the EB can be done right after concreting of the pile or some time later at an appropriate time depending on the hardening progress of the concrete. Tension forces are induced in the EB during expansion and the initial fracturing of the concrete requires relatively low pressures as the concrete surrounding the EB is comparatively thin. The expansion of the EB forces the concrete surrounding the EB outward into the soil. Furthermore, EBs have been injected up to four months after construction of a pile with no difference in bearing capacity compared to piles on the same project where the EBs were injected right after concreting. After inflation and expansion Diameter m [ft] 0.4 [1.3] 0.6 [2.0] 0.6 [2.0] 0.6 [2.0] 0.6 [2.0] 0.8 [2.6] Length m [ft] 0.86 [2.8] 0.76 [2.5] 0.96 [3.1] 1.26 [4.1] 1.26 [4.1] 1.76 [5.8] Behavior in Different Soil Conditions In loose to medium-dense sands and in silty sands, drilled displacement piles are an efficient method to increase the side resistance of the pile. Combining drilled displacement piles with the EB concept has Toe Area sq m [sq ft] 0.13 [1.4] 0.28 [3.0] 0.28 [3.0] 0.28 [3.0] 0.50 [5.4] 0.50 [5.4] EB prior to installation and inflation resulted in a pile with large side resistance and large end bearing capacity because the soil is compressed and moved radially, thereby increasing horizontal soil stress and stiffness. The full expansion achieved by the EB allows the soil to achieve its maximum horizontal stress, which can be observed from the injection curves (volume vs. pressure). As the EB functions similar to a pressuremeter, monitoring of the EB expansion process is an in-situ soil test. The initial part of the pressure-volume curve provides information pertaining to the Skin Area sq m [sq ft] 1.10 [11.8] 1.43 [15.4] 1.83 [19.7] 2.38 [25.6] 3.17 [34.1] 4.42 [47.6] Volume cu m [cu ft] 0.11 [1.2] 0.21 [2.3] 0.27 [2.9] 0.36 [3.9] 0.63 [6.8] 0.88 [9.5] initial soil conditions prior to EB inflation. The shape of the curve depends on the geotechnical conditions (e.g., in-situ stress, strength and stiffness) of the soil. In loose granular soils, the typical shape of the grouting curve usually indicates an initially slow increase in grout pressure, which cor- responds to low soil resistance. After
