In Hong Kong deep excavations …

11 November 2018

In Hong Kong deep excavations are often used for construction of underground transportation networks, buildings basements and water distribution system. The deep excavation methods always causing adverse ground movement and result in damage of the structure of the adjacent property particularly in soft ground situation. Thus, excavation and lateral support (ELS) system is very important to prevent the excessive ground movement. Some of the researchers have studies the ground deformation created by excavations in soft clay, however seldom of researchers have studies the deformation behaviour during excavation in reclaimed Land. In this Dissertation, a deep excavation case using diaphragm wall to support the mixed ground for studying the ground movement behavior will be shared.

Nowadays, numerical analysis is the major method to estimate the ground settlement and deformation induced by excavations, while it is quite difficult to select a suitable model and difficult to determine the soil parameters. Review of the soil parameters is the most important process before carrying out the numerical analysis. Hence, back-analysis will be conducted to define the parameters for excavations in mixed ground. The proposed Dissertation report mainly focuses on the behaviour of Diaphragm Wall during deep excavation. This progress review report mainly to discuss the literature review and method to analysis the behavior of Diaphragm Wall for deep excavation process.The proposed Route 6 network development in Hong Kong comprise of Central Kowloon Route (CKR), Trunk Road T2, and Tseung Kwan O – Lam Tin Tunnel. Route 6 will offer an east-west express link between West Kowloon and Tseung Kwan O and resolve the existing heavily congested urban road network in the central and eastern Kowloon areas [CEDD, 2015].

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Contract No. KL/2014/03 – Kai Tak Development Stage 3 is required to build a Trunk Road T2. The Length of T2 is about 420m underground vehicular tunnel by using of a supporting underground structure (SUS) which consist of diaphragm walls, pre-bored H-piles, tunnel top and bottom slabs, temporary end walls for CKR connection and ventilation adits buried underneath the existing Shing Cheong Road and Cheung Yip Street. Road T2 is the entrustment works of the part of Central Kowloon Route development [CEDD, 2015].Due to widespread volcanism and plutonism beneath the earth’s surface in ancient, the geological profile generally in Hong Kong are volcanic and granitic materials [Sewell 2000]. The subtropical and monsoonal climate with cool dry winters and hot and wet summers in Hong Kong has accelerate weathering processes in these kinds of rocks. The profiles are usually from few meters to several tens of meters in thickness depend on the intensive of the scrubbing processes [Fyfe 2000].

The rocks cover with Quaternary surface sediments, ranging in thickness from lamellae to several tens of meters. Quaternary superficial deposit comprises many local deposits and extensive collapse of alluvium. Marine sediments may exist in some areas above the alluvial layer. In developed areas, a few meters thick of fill soil located above the sediments surface. Groundwater level is generally higher, just a few meters below the surface. Hong Kong mainly is hilly geometry land thus lack of flat land for urban development. The past century, the extensive reclamation has been carried out at the coastal line of Hong Kong Kowloon Peninsula [Fyfe 2000].

In Hong Kong, there are 6 type of decomposition levels to classify the soil and the rock, table 4 shown in Geoguide 3 are usually used to describe the volcanic and granitic materials in Hong Kong [Geoguide 3, 2017]. The soil mechanics terminology of “rock” define as the material of decomposition levels from Grades I to III and Grade I is fresh rock, Grade II ; III slightly and moderately decomposed rock. Grades IV to VI decomposition material define as “soil.” Soil is equal to “rock” of decomposition Grades IV and V or namely highly decomposed and completely decomposed materials. Grade VI, defined as “residual soil,” which is not common found in Hong Kong and usually in a thin layer.It is not easy to obtain undisturbed granular granitic and volcanic samples deeper than 20 meters for carry out testing in laboratory. Standard Penetration Test (SPT) is the most common method used in Hong Kong to verify its geotechnical characteristics.

It has recently been found that shear modulus of granitic rock is highly nonlinear even at very small strains. In-situ measurements of shear modulus of granite soil at very small strains Go by compression wave and shear wave (P-S) velocity logging tests conducted at Kowloon Bay [Ng 2000], where N is number of blow count from SPTs. A range of SPT N values correlated with Young’s modulus such as E = 200 to 3000N (kPa) was proposed by Geotechnical Engineering Office (GEO) of Civil Engineering Development Department (CEDD) of The Government of the Hong Kong Special Administrative Region (HKSAR) [Geoguide 1, 2017]. Also, the relationships between SPT N values and shear strength parameters (i.e., effective cohesion, c’ and angle of friction, I ;’) and fine contents are given by [Pun and Ho 1996]. The range for I ;’ varies from 33° and 44° and c’ from 0 to 6 kPa related to the proportion of fine contents and SPT N values.

The geological feature of Kai Tak airport is a reclamation area, it is using granular fill material from the seabed to develop Kai Tak airfield by hydraulic fill method from Victoria Harbour [CEDD web site]. During the feasibility stage of the proposed Central Kowloon Route, CEDD of HKSAR has conducted ground investigation for the proposed underground trunk road development. The geological ground condition of the area generally comprises of around 10~14m thick of fill, 2~11m thick of marine deposit, 12~22m thick of alluvium, 8~48m thick of completely decomposed granite and moderately decomposed granite (Bed Rock).Throughout ground investigation process on site and laboratory testing of soil sample, most of the important soil parameters could be defined. These information is important for design the deep excavation system and the proposed works such as:In dense urban city, space for new construction site is limited thus deep excavation for substructure work will induce adverse ground movement to the adjacent property. Throughout the past 30 years diaphragm wall was widely used for deep excavations in Hong Kong such as the Mass Transit Railway, Express Rail Link, Basement of Mega High-Rise Building and Harbour Area Treatment Scheme (HATS) that provide and extra underground space for development. Diaphragm Wall is reinforced concrete wall panel constructed in the ground using bentonite slurry trench technique during excavation, slurry has unit weight around 10.

5 to 10.8 kN/m3. Obstructions such as boulders may be presented during the excavation, hydrofraise machine ore pre-boring will be used to overcome the hard material. Water-stop will be provided in the vertical joint between the reinforced concrete wall panels.The D-walls are supported by props such as concrete floor slabs for top-down construction method or temporary props such as steel struts in both top-down and bottom-up construction method. In a complex underground structure, tie-back anchors may be used which provided more working space then the traditional strutting system. The advantage of top-down method is to speed up construction process and to minimize ground deformation, while the bottom-up method is often used when working space is allowed.

According to the findings from [Long 2001], the wall deflection using the top-down construction method is not smaller than the bottom-up construction method. In some case, the wall deflection using the top-down construction method is found greater than the bottom-up construction method such as the case in Singapore Nicholl Highway.Inject grout curtain to the toe of D-wall is a normal practice which to control seepage of groundwater entering into an excavation site and to prevent the groundwater drawdown and ground settlement outside the D-wall. A pumping test is performed to ensure water tightness of diaphragm wall.Bottom-up method for construction of the underground vehicular tunnel was adopted in this project, “hit and miss” diaphragm walls with strutting as the temporary support system for stages excavation works. The layout and details of the ELS are presented in the following:Most of the excavation are carried out in dense urban areas, so it is very important to check the stability of the works which may induce adverse effect to the adjacent structures during the design and construction period. It is necessary to prepare well in advance to predict the excessive ground deformation phenomenon of deep excavation in design stage and review during the construction stage to ensure the safety and serviceability of the adjacent buildings and facilities.

Inappropriate planning and design of ELS system can result in substantial deformation of soil and damage to the adjacent properties. Based on the available information of site, preventive measures will be developed for ELS works. The key elements need to be identified by finite element simulation. Therefore, numerical analysis helps to design the appropriate measures suitable for construction. The accuracy of soil deformation estimation depends on the correct interpretation of the numerical tool, the appropriate soil parameters, water pressure and properties of the lateral support system.There are three categories methods for soil deformation prediction, they are numerical methods, the analytical solutions and, empirical methods. Those methods have their own advantage and disadvantage on application which depends on the specific site constrains.

Particularly, the numerical analysis can provide analysis of non-linear soil behaviour and complex construction procedure, this method needs proper understanding of soil parameters and properties of the lateral support system.Two-dimensional (2D) finite element analysis is used by Engineers since 1969, owning to the availability of computer facilities and simply for modelling of different site constrains and construction methods. In most cases, 2D analysis is conservatively predicts the deformation behaviour. This conservative prediction due to correct interpretation of the soil parameters, location and nature of fictitious boundary conditions, effect of surcharge from adjacent structures, sequence of excavations, etc.Some researchers have studied the effect of diaphragm wall on the stress distribution and deformation characteristics of ground below adjacent structure at different locations. Idealized processing of typical building loads, analysis of their effects on excavation and lateral support systems, bending moments, shear forces and displacement of D-walls. The results of the study indicate that the use of D-wall can greatly limit the ground movement under the building and safely for excavation.

The horizontal and vertical displacements and the shear stress in the soil decrease linearly with the distance from the D-walls.In this research, an effort will be made to compare the soil deformation pattern in design stage and construction stage owning to deep excavation. The Trunk Road T2 is still in construction progress. The predicted deformation modelling of the mix ground soil by the computer program “Plaxis” will be used and also compared with the actual site monitoring measured data such as ground settlement marker, vertical inclinometer, strain gauge and piezometer during the construction.Finally, back calculation will be adopted to evaluate the accuracy of estimated soil parameters. Even under similar ground conditions, soil parameters can change significantly due to size, loading history, and excavation methods. However, these soil parameters will be followed the guidelines in Geoguides.

This method can prove to be the best practice in any geotechnical energineering, as well as the initial field and laboratory investigations. Therefore, it can minimize the risk of construction and damage to adjacent structures. However, how to overcome the challenge of accurately predicting ground deformation does not have much guidance. The key difficulties of predicting ground movements from the comparison results will be suggested and recommended the improvement for the prediction throughout on the design consideration and inspection process.

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