Stiff Clay Layer Research Articles

An investigation on a failed double-wall cofferdam during construction

To mitigate floods, a 400 m long double-wall cofferdam was constructed enclosing a river mouth. The 6–25 m wide cofferdam, braced using steel struts and walers welded to sheet piles inside the cofferdam, was constructed with 18 m long steel sheet piles penetrating a very soft marine clay and terminating in a medium stiff clay layer. However, the cofferdam failed after a heavy rainfall. A portion of the sheet piles’ head deflected about 2 m toward the retention pond, which had been dredged just before the final depth of 4.2 m below the top of the cofferdam. Site inspection revealed that some of the struts were detached from the walers welded to the sheet pile wall abutting the pond. A study was launched to investigate the failure of this cofferdam. This paper presents the background, the failure, the results of the post-failure finite element analyses, and finally, pin-pointed the actual causes of the failure.

Evaluation of air permeability in layered unsaturated materials

Field estimation of air permeability is important in the design and operation of soil-vapor extraction systems. Previous models have examined airflow in homogenous soils, incorporating leakage through a low-permeability cap either as a correction to the airflow equation or as a boundary condition. The dual leakage model solution developed here improves upon the previous efforts by adding a leaky lower boundary condition, allowing for the examination of airflow in heterogeneous layered soils. The dual leakage model is applied to the evaluation of pump tests at a pilot soil-vapor extraction system at the Savannah River Site in South Carolina. A thick, low-permeability, stiff clay layer divides the stratigraphy at the site into two units for evaluation. A modified version of the previous model, using the water table as the impermeable lower boundary, is used to evaluate the permeability of the low-permeability stiff clay layer (3.2 × 10 − 10 cm 2) and permeable sand (7.2 × 10 − 7 cm 2) beneath it. The stiff clay permeability estimate is used in the evaluation of the shallow unit. Permeability estimates of the shallow sand (3.8 × 10 − 7 cm 2) and kaolin cap (1.5 × 10 − 9 cm 2) were obtained with the dual leakage model. The shallow unit was evaluated using the previous model for comparison. The effects of anisotropy were investigated with a series of model simulations based on the shallow unit solution. The anisotropy sensitivity analysis suggests that increased anisotropy ratio or decreased axial permeability has a significant impact on the velocity profile at the lower boundary, especially at high values of the anisotropy ratio. This result may increase estimates of SVE removal rates for contaminants located at the interface of the lower boundary, typical of chlorinated solvent contamination.

Geotechnical Analysis of Mizuki Embankment Remains

ABSTRACT This paper presents the investigations of the historical remains of the Mizuki embankment, which was built in 664 AD with wide moats and a steep slope facing on one side. Considering the historical and local topographical features, it is thought that the Mizuki embankment was mainly used as a defence facility. The in-situ investigation reveals that the embankment was built using a Hanchiku (layer-compaction) method. Based on the slope stability analysis, it was found that brushwood reinforcement with a loading berm construction method proved the stability. Compacted inclined layers of soil and sand have been found in the embankment, and these layers might have been constructed to drain infiltrated water from the top of the embankment, thereby preventing the potential failure induced by heavy rainfall. The installation of the timber waterspouts across the embankment used a cut and fill method. Field investigation shows that below the wooden pillar at the west gate of Mizuki embankment, there was a compacted stiff clay layer. The seismic performance of the wooden pillar-clay system is shown to be effective by numerical analysis using program FLUSH.

Construction of shaft tunnel by slurry pipe jacking through sedimentary ground under high water pressure

Pipe jacking technology has been developed rapidly in recent years to satisfy demand for distances of more than 1 km or curves of less than 10 m radius. A shaft tunnel driven by pipe jacking, another type of development, was adopted in order to provide ventilation to the Imaigawa underground reservoir tunnel under the national highway in Yokohama, Japan, which has been temporarily in service since 2001. Downward excavation of the 2000 mm ID ventilation tunnel starts from ground surface and reaches to the crown of the existing 10,800 mm ID reservoir tunnel, which is located 87.8 m deep, through the stiff clay layer. Depression angle of the tunnel is set as 75.6 degrees since the ventilation building is constructed in the existing park along the highway. Pore water pressure at the arrival is 0.66 MPa. This project is one of the largest in this type of tunnels in Japan. This paper focuses on equipment and facilities adopted on the project and construction methodology of the inclined pipe jacking. Specially-designed equipment and facilities were planned and employed to overcome anticipated difficulties; namely shield machine, jacks and reaction frame, lift-preventer, inclined elevator, and survey system. Slurry shield machine was specially designed considering the high water pressure, stiff clay excavation, accurate directional control, connection work to the existing reservoir, and simple dismantling. It shall be noted that the machine can not access the existing reservoir tunnel and the dismantled motors of the machine had to be removed through the tunnel driven. The thrust jacks and reaction frame were designed to facilitate easier setting of pipes and ground anchors were used against the reaction force of up to 8000 kN. To prevent the lift due to buoyancy to the shield machine and pipes during excavation, which was significant when the thrust jacks are released, the lift-preventer with power casing jack and ground anchors were developed. Inclined elevator, which is used for pipe connection and maintenance, was designed in such a way that it moves to give space to the pipe setting. Since accuracy was vital for the tunnel drive, survey system with laser theodolite and non-prism laser distancemeter (with automatic target recognition), which enables constant monitoring, was adopted. Relatively complicated construction sequence was developed bearing smooth operation and safety in mind. It was however completed successfully due to the well-prepared sequence. This method is considered effective for inclined shaft construction, such as ventilation shafts or access tunnels. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.