H. J. Liao

Cavity expansion and cone penetration resistance in anisotropic clay

Abstract Due to strength anisotropy, the undrained shear strength of naturally deposited clay can vary with its position in the coordinate. When subjected to a spherical expansion, the undrained shear strength of clay yields the lowest value in the horizontal direction and the largest value in the vertical direction. To represent the nature of clay, a model based on the anisotropic undrained strength criterion has been developed in this paper to determine the anisotropic undrained shear strength under a spherical expansion condition. To simplify the model, the effects of in‐situ initial stresses, anisotropy, and the anisotropy of soil deformability are not taken into account here. Combining the spherical cavity expansion theory and the anisotropic undrained shear strength under spherical expansion conditions, a simplified limit pressure of spherical cavity expansion in anisotropic clay has been established. The magnitude of limit pressure is related to its position in the spherical cavity. Based on the limit pressure in spherical cavity expansion derived from anisotropic strength, the cone factor of an advancing cone has been derived to correlate the cone resistance and undrained shear strength of clay. The calculated cone factors uniquely correspond to the undrained shear strengths determined from different tests and are interrelated to each other in terms of strength anisotropy ratio Ar of clay. The accuracy of the calculated cone factors has been satisfactorily verified with laboratory and field CPTU results. However, it has also been found that the effect of strength anisotropy only becomes obvious when clay has a low rigidity and high strength anisotropy. If the strength anisotropy of clay is not considered, a less than 15% error on the value of the cone factor will result.

Case studies on bermed excavation in Taipei silty soil

Bermed excavation (deep excavation supported by an excavation wall and a berm) can be an economical method for basement construction in urban areas if ground movement around the excavation is not the primary concern and the excavation site is large and (or) irregular in shape. This paper reviews three bermed excavation projects carried out in the Taipei Basin. One project was successful, one was successful after near failure, and one was a failure. Ironically, extensive ground improvement work had been done for the near-failure and failure cases; but no ground improvement was done for the successful case. It is of interest to find the similarities and differences among these bermed excavation projects. The causes of instability that result in unsuccessful bermed excavation projects and the applications of limit equilibrium analysis to the stability of bermed excavation are also discussed here. These three cases show that the existence of grout columns does not help to effectively increase the stability of...

Modeling the effect of ground improvement on reducing movement during bermed excavation in clay

Abstract To evaluate the effect of ground improvement on bermed excavation induced soil movement in clay, a 1‐g model test is proposed while keeping the base stability number (= γH/su ) equal to the full scale excavation in the field. The undrained shear strength of soil used in this model test is reduced according to size reduction from the prototype to the proposed model test. To verify the suitability of model test results, a three‐dimensional explicit finite difference program is adopted to perform numerical analyses for deducing the relationship between prototype and the proposed model test. In addition, a field case is compared with this model test. By comparing ground surface settlement associated with different ground improvement layouts, the buttress type arrangement has shown a better effect on controlling bermed excavation induced surface settlement than the column type arrangement. It is found that an increase of improved zone dimension will show more significant impact on reducing soil movement than an increase of improved soils shear strength. Finally, apparent shear strength of buttress or column type improvement is determined by an empirical equation in terms of the unconfined compressive strength of treated material and the undrained shear strength of in‐situ clay.

Base Stability of Grout Pile–Reinforced Excavations in Soft Clay

Grout piles are used to reinforce base soil to increase the base stability of excavations in soft clay and to reduce excavation-induced ground movements. To propose a model for the base undrained stability of deep excavation in clay reinforced with grout piles, this paper presents an anisotropic strength criterion for clay reinforced with grout piles and the upper-bound bearing capacity theory for excavation. The proposed model is capable of determining the factor of safety against base heave for an excavation with or without ground improvement for various-strength anisotropy ratios of clay and clay reinforced with grout piles. The suitability of the model is verified with five field excavation cases. Among them, four were with no ground improvement, and one was with ground improvement. Generally, the larger the ground improvement rate, the more the material strength of a grout pile can be mobilized. The factor of safety also becomes larger when the ground improvement rate is higher. However, the factor o...

Ground improvement piles induced shear strength increase in normally consolidated clay

Abstract Ground improvement piles, are usually installed by injection or mixing rather than by driving. So, soil around a ground improvement pile is mainly subjected to lateral displacement and is similar to that of soil around an expanding cavity. A ground improvement pile not only generates excess pore water pressure in the soil but also causes subsequent soil strength change if drainage of excess pore water pressure is possible. This paper presents the results of a large scale laboratory model test on the grout bulb induced undrained shear strength increase in normally consolidated clay. If sufficient time is allowed for pore water pressure dissipation (t > 3tp, tp is the time needed to complete the primary consolidation), the vane shear strength of soil can be increased by more than 170% for the soil right outside the expansion body. This decreases gradually to the initial vane shear strength at a distance of 7rcc (rcc = radius of expansion body). Based on the theory of the cylindrical expansion cavity and the model test results, a semi‐empirical model is developed to estimate the change of undrained shear strength ratio of in‐situ clay before and after pile installation at different distances from an expansion body. For normally consolidated clay, a two‐fold increase in undrained shear strength ratio can result from ground improvement pile use.

The Development and Application of The Slope Management System

In an effort to manage the tedious slope maintenance works of mountain highways in Taiwan, a Slope Management System is developed based on a well designed MIS system in combination of the Geography Information System. The Slope Management System contains four major components: the inventory database, historical hazard information database, maintenance record database, and the expert system for mitigation strategy. All the information stored in the database and analyzed results can be visualized by taking the advantage of powerful display functions of GIS. In this paper, the detailed program structure of Slope Management System as well as operation example of a demo project will be introduced. Purpose of this paper is to introduce a helpful tool for slope mitigation and maintenance management.

Simple Method to Measure the Long-Term Load Change of Ground Anchors

This paper introduces a simple and reliable method for measuring the long-term load changes of ground anchors. The method operates in a similar fashion to that of the tell-tale load cell in principle. However, in contrast to the tell-tale load cell, the proposed method is highly durable and inexpensive. It only requires the components used in a typical anchor assembly and consists of only one extra strand as the reference strand, which is not engaged by the anchorage. For this method, when the anchor-load changes, the anchorage moves and a relative deformation between the reference strand and anchorage is generated. Based on the measured relative deformation and the effective free-strand length, the change in anchor load can be approximately calculated. The suitability of this method was confirmed by stressing tests performed on field anchors. Although its accuracy may not be as high as that of electrical load cells, it is still satisfactory for evaluating the long-term stability of anchored slopes.

Properties of Hydraulically Dumped Coal Ash after Soil Mixing Improvement

The coal ash dump site studied here consists of various proportions of fly ash, bottom ash, and slag. The materials deposited have poor physical strength properties and the site has high groundwater levels. The potential for liquefaction is high in this area during a strong earthquake. Hence, it is not a suitable site for building foundations. Due to the porous and crushable nature of the material particles, coal ash is not easily compacted by normal methods or by vibration methods such as sand compaction piles or dynamic compaction. Therefore, two soil mixing methods were adopted here to improve the mechanical properties of in-situ coal ash: the CLSM method for shallow depth improvement and the WILL method for intermediate depth improvement. The former uses a on-site mixing plant to mix the coal ash with binder; the latter uses a special mixing arm to mix in-situ coal ash with binder and is capable of constructing a grid type layout of ground improvement. Test results indicate that both methods can effectively increase the in-situ compressive strength of coal ash to over 1 MPa with the mixing proportion of 100 kg binder per cubic meter of coal ash and with the water/cement ratio of 1.67. However, the WILL method can provide a much more homogeneous improved ground.