Chapter 6 Test Results
6.1 Static Load Test
To separate the densification effects due to static and cyclic loadings, in this section, the surface of four 0.15 m-thick soil lifts was compressed with the static vertical loading only. Effects of soil densification such as the surface settlement, change of relative density, vertical and horizontal stresses and cone resistance in the compressed fill are investigated.
6.1.1 Surface Settlement Due to Static Load
The surface settlements of the four 0.15 m-thick compressed soil lifts due the static
weight of the compactor were discussed. The initial relative density of the loose fill was 35.5% (see Fig. 4.12). The applied static normal stress was q = 9.2 kPa. To achieve a uniform settlement, the vertical static loading was applied on the surface with the 5×5 formation (see Fig. 5.12 (a)). Fig. 6.1 showed the settlement measurement was carried out with the laser distance meter placed on top of the steel beam. The surface settlement measured at the centers of disc loading disc was shown in Fig. 6.2. For the four 150 mm-thick soil lifts, the accumulated minimum and maximum settlements were 15.0 and 22.3 mm. The average settlement was 19.0 mm, which was about 3.2% (volumetric strain εv = 3.2%) of the soil thickness. It is obvious that static vertical loading is an effective method to compact the loose fill. To limit the scope of this thesis, only q = 9.2 kPa was used throughout this study. It should be mentioned that the vertical strain distribution in the soil lift may not be uniform.
6.1.2 Density Change Due to Static Load
To investigate the density distribution in the compressed fill, density cups were buried in the soil mass at different elevations and locations in the four 0.15 m-thick soil lifts (see Fig 5.9). For the un-compacted loose soil, the average relative density was about 35.5%. In Fig. 6.3, Chen (2011) reported that, for a 0.6m-thick fill, after the application of static vertical load q, the density increase more near the surface and the density increase less at greater depths. The induced density change decreased with increasing depth. Fig. 6.4 shows, after applying the static vertical load 9.2 kPa on each lift, the relative density of fill increased. This static vertical loading represents the weight of the cyclic torsional shear compactor. On the average, the relative density increase was about 26.4% from 35.5% to 61.9%. It should be mentioned that the distribution of density is not uniform with depth. However, the relative density
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achieved Dr = 61.9% is not enough to achieve the target of dense sand (Dr = 70%-85%).
6.1.3 Stress Change Due to Static Load
For comparison purposes, at the beginning of this study, experiments were conducted to investigate the stresses in an uncompacted loose fill. Air-pluviation method was adopted to prepare the fill and the relative density achieved for the loose sand was 35.5 %. Fig. 6.5 shows the location of soil pressure transducers to measure the distribution of vertical and horizontal stress with depth. SPT2, SPT3, SPT6 and SPT7 were buried in the soil mass to measure
v and SPT1, SPT4, SPT5 and SPT8 were used to measure h. The vertical stress v measured in the soil mass was shown in Fig. 6.6. In this figure, the vertical stress v increased with increasing depth z. Test data were in fairly good agreement with the traditional equation v = z. In this study, unit weight was 15.6 kN/m3 for the loose sand. The distributions of horizontal earth pressure h with depth were shown in Fig. 6.7. In the figure, the earth pressure profile induced by the 0.6 m-thick loose fill was approximately linear and was in good agreement with the Jaky’s equation. Mayne and Kulhawy (1982), Mesri and Hayat (1993) reported that Jaky’s equation was suitable for backfill in its loosest state. From a practical point of view, it was concluded that for a loose fill, the vertical and horizontal earth pressure in the soil mass can be properly estimated with the equation
v = z and Jaky’s equation, respectively.To investigate the change of stresses due to static load, the loose fill was placed in four 0.15m-thick lifts as shown in Fig. 6.5. Static load q was applied each lift on the surface with the 5×5 formation (see Fig. 5.12) and then removed. Fig. 6.6 shows the vertical stress profile after the static vertical loading. It is clear in the figure that the vertical overburden pressure v can be properly estimated with the equation v = z.
As compared with the v for loose sand the measured,v increased slightly, probably because the compressed fill had a slightly higher density (see Fig. 6.4). It is clear in the figure that the static vertical load did not result insignificant residual stress in the vertical direction. It may be concluded that the effect of static loading on the vertical pressure v was insignificantly. Fig. 6.7 shows the horizontal stress was also increased slightly after the application of the static vertical loading.
6.1.4 Cone Resistance Change Due to Static Load
Cone penetration tests were conducted to investigate the change of soil properties due to static loading on the loose fill. The fill is 600 mm-thick as shown Fig. 6.5. The bottom of the soil bin is a solid steel plate. Due to boundary effects, the cone resistance may suddenly increase, when the penetrometer approached the bottom of the soil bin. For this study, cone penetration was conducted for z = 0 to 400 mm. For Fig. 6.8 shows the cone resistance for the loose soil varied from 0 to 300 kPa. After static vertical loading, the cone resistance varied from 0 to 1681.2 kPa. It is obvious that static vertical loading significantly increase the tip resistance of the compressed fill.
Jamiolkowski et al. (1985) were reported that could make cone resistance converted to relative density. Fig 6.9 shows the cone resistance of static load transformed to the relative density by Jamiolkowski theory. It could be found the test result and the theory were agreement in the loose fill. However, it was not agreement after compressed. It was probability that the soil mass not big enough.