Andrew Chan (2003) - Observations from excavations - a reflection

The process of using observed data to determine the properties of the ground and the behavior of structures, often used to validate design assumptions and predictions.

Sites comprising a significant thickness of soft silty clay are common in the coastal cities of Southeast Asia.

Well-controlled deep excavations in urban Hong Kong are likely to cause maximum lateral displacements of the retaining walls that will be between 0.15% to 0.3% of the final excavation depth.

A 250m tall building located in the Lu Wan District of Shanghai, involving excavation of a 3-level basement and constructed in two phases to reduce ground movements near the Shanghai MRT Line 1 Huang Pi Station and running tunnels.

A structural system that is used to retain soil and resist lateral pressure. It is constructed by excavating a narrow trench that is then filled with concrete.

A construction method where the excavation is carried out from the top downwards, often used in urban areas to minimize disruption to the surroundings.

A 0.8m thick diaphragm wall was used to support the retained soil during the excavation at Newton Station, Singapore.

A type of underground structure that extends several levels below the ground surface, often used for parking, storage, or other commercial purposes.

Sites with a high proportion of clay that is easily deformable under load, posing challenges for construction and foundation design.

Back analysis of excavation lateral movements in Hong Kong shows that for movement prediction purposes, reasonable and conservative estimates of displacement are likely to be achieved by using an Young’s Modulus (E) for soil of 1.5N(MPa) for fill and alluvium and a value of E = 2N(MPa) for completely decomposed granite. Higher E/N values are relevant for CDG/HDG with high 'N' values.

A metro station in Shanghai located at a distance of about 10m from the New World Centre site boundary, where large ground movements can occur particularly in soft Shanghai clays.

A type of soil that gains strength over time. It was observed to have movements that differed from the predicted displacements, leading to a back analysis procedure using FREW to revise soil stiffness parameters.

A type of retaining wall made of steel sheets driven into the ground to provide lateral support and prevent soil movement.

A structure designed to resist the lateral pressure of soil when there is a desired change in ground elevation that exceeds the angle of repose of the soil.

The method of excavation used at Newton Station, Singapore, involving permanent concrete floor slabs cast as the excavation progressed.

Information gathered from previous experiences and current conditions, which may not be sufficient for design without validation and back analysis.

A construction technique used at the New World Centre site to limit ground movements and deflection, particularly during Phase II excavation for the building podium area.

A line in Taipei with several stations (BL13 to 16) designed between 1989 and 1991 and constructed in the early to mid-90's.

A type of soil found in the project area of the Nankang Line, described as part of the Sungshan Formations by Woo and Moh (1990).

Soil that remains in place after the natural development of the land, often consisting of weathered rock material.

Sedimentary material that has been deposited by marine processes such as waves, currents, and tides.

The process of using observed data to determine the properties of the soil and the behavior of the structure during the excavation process.

Detailed analysis carried out to evaluate the interaction between the tunnel, walls, and station stanchions, as well as the effects of excavation within the station box, as seen in the MRTA Northern Section project in Bangkok.

The process of making observations from previous similar projects in similar ground conditions to gain confidence in design procedures, enabling the extrapolation of design experience into more innovative and economical construction methods.

Specific data collected from each location that enhances the confidence in the ability to predict wall movements during excavations.

The vertical distance or level below the ground surface.

Unconfined Compressive Strength, a measure of soil strength.

A structural element used to resist longitudinal compression in a diaphragm wall.

Machines used to excavate tunnels for metro construction projects, such as the MRTA Northern Section in Bangkok.

A method used to adapt to unforeseen ground behavior during construction. In this case, it was applied to eliminate the buried prop in the latter part of the excavation.

A construction method where the station excavation progresses from the top down, used in the MRTA Northern Section project in Bangkok.

Material used to reinforce tunnel 'eyes' at the end walls of the station box in the MRTA Northern Section project in Bangkok.

A type of testing used in site investigations to determine the small strain stiffness of the soil profile, as carried out for the design of the MRTA Northern Section project in Bangkok.

Clay deposits with a high silt content, which can cause relatively large wall movements during excavations.

The ratio of Young's Modulus (E) to the SPT N value, used to estimate lateral displacement in soil profiles.

The process of improving the ground conditions to provide stability for construction.

A ground treatment method involving the injection of grout to improve soil properties.

Common Diaphragm Wall/Heavy Duty Diaphragm Wall - types of diaphragm walls used in construction.

A procedure used to determine the soil stiffness parameters by comparing observed displacements with predicted displacements. In this case, it was used to revise the soil stiffness parameters to match the observed displacement.

A reinforced concrete wall constructed in the ground using a special technique to support the excavation and prevent the collapse of soil.

A type of retaining wall constructed from a thick mixture of water, soil, and cement to support the sides of deep excavations.

Excavations that involve digging to significant depths below the ground surface, often requiring retaining walls for support and stability.

Sites characterized by the presence of soft clay, commonly found in coastal areas of Southeast Asian cities, including Singapore, Bangkok, Taipei, Shanghai, and parts of Hong Kong.

The process of designing structures and systems based on an understanding of the properties and behaviors of the earth materials involved, incorporating observations from previous similar projects in similar ground conditions to refine design processes and remove unnecessary conservatisms.

A measure of the stiffness of a material, representing the ratio of stress to strain in the elastic range of deformation.

The use of the design process to extrapolate previous experience, gained from observations, to enable innovative and more economical construction methods to be employed.

Strengthening elements or supports, such as precast concrete props, used to achieve a maximum wall movement ratio (δ max/H) of about 0.2% or below in deep excavations.

The shear strength of soil when it is loaded without being allowed to drain.

Supporting elements used during construction to provide temporary lateral support to retaining walls or other structural elements.

Instruments used to measure the angle of slope or tilt of an object with respect to the force of gravity.

A 3m thick raft introduced below the excavation level to reduce ground movements and protect sensitive buildings for station BL14 of the Nankang Line.

A reinforced concrete wall constructed in a narrow trench using a top-down method, often used as a retaining wall for deep excavations.

A measure of the safety margin between the actual base heave and the allowable base heave, often expressed as a ratio, where a higher value indicates a greater safety margin.

A computer program used for analyzing the behavior of retaining walls and soil structures, taking into account factors such as soil stiffness, groundwater pressures, and soil stresses.

Analysis using the Finite Element Residual Strength (FREW) method with a set of soil parameters from standard measurements and empirical correlations, which can provide good predictions of maximum movement and profile.

Records of past experiences and observations related to deep excavations, which are valuable for understanding and predicting wall movements.

Standard Penetration Test (SPT) N value is a measure of the resistance of soil to the penetration of a standard sampler driven by a standard mass under standard conditions.

Material used to raise the ground level or support a structure.

A jet grout raft was installed prior to commencing the excavation at Newton Station, Singapore, to reduce the movement caused by the presence of deeper clay.

The process of extending design methods and principles from known conditions to larger or more economical designs with some level of confidence.

The resistance of soil to deformation under stress, often represented by the modulus of elasticity (E value) for different soil types.

A software used for back-analysis and validation of local case histories on projects in the same area as station BL14 of the Nankang Line.

A test conducted to assess the hydraulic properties of an aquifer by pumping water from a well and observing the effects on water levels in the surrounding area.

The number of blows per 300mm required to drive a standard sampler into the ground, used as an indicator of soil density and stiffness.

Cemented Decomposed Granite, a type of soil with specific engineering properties, often encountered in deep excavation projects.

The amount of displacement or bending that a structural element experiences under an applied load.

A ground improvement technique using cement to create piles for stabilizing the ground.

Related to the sea or ocean; often used to describe soil conditions near bodies of water.

A systematic approach to the design and construction of geotechnical engineering works that involves monitoring and adjusting the design based on observed performance during construction.

Walls 1.2m thick used to support the 17.7m excavation for station BL14 of the Nankang Line.

The horizontal movement of the excavation wall, often measured as the maximum lateral wall displacement divided by the depth of excavation (δ max/H).

A parameter equal to the wall EI value divided by the unit weight of water and the strut spacing to the fourth power, used to characterize the stiffness of the excavation support system.

An excavation method where the upper part of the structure is constructed first, and then the excavation progresses downwards while the lower part is supported by the already constructed upper part.

The coefficient representing the ratio of the lateral stress to the vertical stress in the soil, particularly important in modeling the movement of the wall during the excavation process.

The ratio of maximum wall deflection to excavation depth.

A device used to measure the deflection of retaining walls or piles.

The process of extending established design methods to produce innovative and economical solutions in problems involving deep excavations.

Mixed Diaphragm Wall - a type of diaphragm wall construction.

The process of validating and gaining confidence in design methods by analyzing the actual performance of a structure or system.

Excavations with a depth that is significantly greater than the width, often requiring lateral support systems to prevent collapse.

A reinforced concrete wall constructed to support and protect the soil and adjacent structures during the excavation process.

The physical and geological characteristics of the soil and rock at a specific location, including factors such as soil type, water content, and strength.

The ratio of the elastic modulus (E) to the effective stress (N) of the soil, used to model the movement of the wall during the excavation process.

Rules based on practical experience and observation, such as the Clough and O'Rourke (1990) rule, which provide a reasonable first estimate of maximum wall movement.

The process of analyzing field measurements to investigate the selection of suitable soil parameters, especially the soil stiffness empirically correlated with SPT 'N' values, for residual soil profiles such as those found in Hong Kong.

Methods such as jet grouting before excavation, which become necessary to address movement limits in soft clay profiles.

A reinforced concrete wall constructed in a narrow trench using a trench cutting machine.

Temporary supports used to provide stability to structures during construction or repair.

A reinforced concrete wall constructed in the ground using a special trenching technique which allows for deep excavations and provides support to surrounding structures.