Geotechnical evaluation for metro lines in ho chi minh city

Geotechnical Evaluation for Metro Lines in Ho Chi Minh City Tony J Barry FICE FCIHT CEng CGeol CEnv PMP September 2017 Introduction Ho Chi Minh is currently developing a number of metro lines as shown on Figure 1 Figure Planned MRT Lines in Ho Chi Minh This note summarises the general geology of Ho Chi Minh City, the challenges facing underground MRT works, and the requirements for investigation of detailed geology and engineering geology to enable design and construction of new works http://www.think-railways.com/wp-content/uploads/2015/04/vietnam_metroline.jpg PAGE Geotechnical Evaluation for Metro Lines in Ho Chi Minh City The Geology of Ho Chi Minh Ho Chi Minh lies in the flood plain of the Saigon River and the near surface deposits consist of unconsolidated deposits of clay, silt and sand The general nature of the deposits is shown on Figure 2, though the material shown as sandstone is believed to be unindurated deposits Figure Superficial Geology of Ho Chi Minh showing MRT Lines There have been attempts to develop a ground model for Ho Chi Minh, collecting available information into a comprehensive set of data which could be referred to in further infrastructure development Since Metropolitan Authority for Urban Rail (MAUR), the authority responsible for the metro development, are considering developing a BIM based approach to data management this may be an area they will support or lead in the future Quaternary geology has been studied for some parts of Vietnam as shown for example in Figure 3 based on Dinh et al, Mapping ground subsidence phenomena in Ho Chi Minh City through radar interferometry technique using ALOS PALSAR data, Remote Sensing, 7, 2015 © Tony Barry, September 2017 page | of 10 Geotechnical Evaluation for Metro Lines in Ho Chi Minh City Figure Example of mapping of Quaternary deposits Hydrogeology As shown, the ground level is only at most some 8m above sea level, and the city suffers from flooding from the river and its tributaries Consequently groundwater is generally near the surface, though Dinh et al (op cit) identify partially confined aquifers and extensive groundwater pumping which have depleted these A consequence of the groundwater depletion is the consolidation of the superficial deposits and the subsidence of the surface However the reclamation of low-lying areas is also identified as contributing to surface subsidence This is discussed further below Information Requirements for an MRT Issues relevant to MRT Some of the issues that may require considering are: Cut and cover vs bored tunnelling: generally this is not driven by the nature of the ground conditions, but by planning and land take requirements leading a general preference for bored tunnels The tunnel vertical alignment will aim to be shallow to limit costs and inconvenience of deep stations This tends to favour twin running tunnels rather than a single bored tunnel, and side by Tran Nghi, Nguyen Thanh Lan, Dinh Xuan Thanh, Pham Nguyen Ha Vu, Nguyen Hoang Son, Tran Thi Thanh Nhan , Quaternary sedimentary cycles in relation to sea level change in Vietnam, VNU Journal of Science, Earth Sciences 23 (2007) 235-243 © Tony Barry, September 2017 page | of 10 Geotechnical Evaluation for Metro Lines in Ho Chi Minh City side tunnels over stacked configuration However the pressure to limit land take means the two bores must be close together and some ground treatment may be needed to avoid issues during construction Cut and cover is likely to be required for stations and cross-overs and will generally be expected to use diaphragm walling and top-down construction Achieving a cut-off for the diaphragm wall excavations is challenging so there may be requirements for base grouting and for dewatering with recharge Disposal of spoil is a challenge for these city centre sites, and the need to find suitable disposal areas The option of treating the spoil to make it suitable for reuse as engineering fill needs to be considered Contamination of the ground may influence its use and method of disposal, and this is discussed further below Seismicity Ho Chi Minh lies quite distant from the main source of earthquakes affecting Vietnam, which are on the northern border area However there are some events recorded offshore the city as shown on Figure 4 Figure Seismic events in Vietnam and region © Tony Barry, September 2017 page | of 10 Geotechnical Evaluation for Metro Lines in Ho Chi Minh City The seismic design code TCXDVN 375:20064 identifies relatively low base ground acceleration but amplification will be caused by deep soft deposits The MRT structures need to be assigned to appropriate hazard exposure groups in accordance with disaster planning strategies, and using FEMA or other appropriate standard Liquefaction The soils of the type found in Ho Chi Minh are susceptible to liquefaction under strong earthquakes It may be that the intensity of earthquakes in Ho Chi Minh is not sufficient to give any cause for concern, but this needs to be considered Figure 5 shows the consequences of buildings founded on shallow foundations on loose alluvial fill material after a major earthquake, and Figure 6 shows the amount of settlement that can occur Figure Niigata Earthquake 1966 Figure Settlement due to liquefaction Ambraseys has however identified that liquefaction appears to occur only within a certain distance of earthquake epicentres as shown on Figure 7 Procedures for assessing liquefaction potential based on ground investigation data are discussed by Barry (2015)6 TCXDVN 375:2006, Design Of Structures For Earthquake Resistance, Vietnamese Building Standards http://www.npr.org/2011/04/07/135181474/in-japan-shaken-soil-turned-soft-after-quake DPWH Guidelines Volume 2A: Geohazard Assessment, Department of Public Works and Highways, Republic of the Philippines, January 2015 © Tony Barry, September 2017 page | of 10 Geotechnical Evaluation for Metro Lines in Ho Chi Minh City (Kramer, 1996 after Ambraseys) Figure Estimate of Zone of Liquefaction Subsidence Dinh et al2 examined remote sensing data and ground control information to estimate the rate of ground subsidence in the city, and concluded that it was progressing over the period 2006 to 2010 at a rate of up to 30mm per year, as shown on Figure 8 Groundwater extraction and lowering of groundwater level are believed to be a major cause of this, and measures to control groundwater extraction may be able to limit long-term continuation of this However it needs to be taken into account both for flood level selection for station access and also for consideration of differential settlement in buried structures and utility connections © Tony Barry, September 2017 page | of 10 Geotechnical Evaluation for Metro Lines in Ho Chi Minh City Figure Surface subsidence Contamination An understanding of the amount and nature of any contamination in the ground is necessary both for the health and safety of workers during construction and for any consequences for spoil disposal Dioxon is perhaps the contaminant of greatest concern in Vietnam It is generally found in only specific locations associated with former US military activity Some traces were found at one location at Tan Son Nhat Airport which Line 5 is routed through or near and therefore it may be necessary to undertake further investigations specific to this chemical Other contaminants are likely to be found in the alluvial sands and silts including petroleum products from underground tanks and other chemicals from general industrial activities UXO USA records for Vietnam are quite comprehensive but believed to be generally not applicable to HCM as bombing in the city was much less severe than elsewhere However UXO has been found in a tunnel thrust-bore crossing of Saigon Rover It has been indicated that HCM is clear of UXO but this may refer only to normal activities and shallow depth © Tony Barry, September 2017 page | of 10 Geotechnical Evaluation for Metro Lines in Ho Chi Minh City Ambient air quality The city lies some 50km from the sea and is not therefore seriously affected by chloride contamination, though saline intrusion of the groundwater has been identified7 However in order to identify suitable protective measures for concrete, including tunnel linings, an understanding of air quality and its likely variation over the life of the structures is necessary8 Geotechnical Investigations Development of a Database Extensive data have been obtained for the many reclamations, roads, bridges and buildings constructed in Ho Chi Minh However this information is not likely to be easily accessible for two reasons: • Some of the data are reported to be held by the private sector and therefore not able to be released • Much data is held in hard copy, and/or is unindexed so that accessing it is resource intensive Dealing with the first point, since all developments require approval of the City, it should not be difficult to require that all geotechnical investigation data provided as part of a development be made available This might be dealt with by a Regulation9 The second point can be addressed in a staged approach: • Adoption of a standard format for geotechnical data would allow it to be incorporated into a database for ease of access The AGS10 format is one appropriate system • Data could then be linked to the GIS which is being developed in the City • Historical data could be captured into the system as resources allow • The system should be accessible to anyone that is involved with development, land-use planning, engineering or other design activities Web-based access might be appropriate in the long-term Minh Thien Ngo, Jae Min Lee, Hyun A Lee, Nam Chil Woo, The sustainability risk of Ho Chi Minh City, Vietnam, due to saltwater intrusion, Geosciences Journal, September 2015, Volume 19, Issue 3, pp 547–560 Barry A J, Testing of tunnel segment concrete, Technical Note, December 2016, Unpublished As part of the initial feasibility studies for the MRT in Singapore the Author drafted a bye-law requiring all development within one hundred metres of the planned MRT corridor to submit geotechnical reports in a standard format, for inclusion in a city-wide database A similar Regulation, but covering the whole of the City of Ho Chi Minh, would be appropriate 10 The AGS data format is a now industry-standard in UK and many UK influenced countries It is an opensource file standard for geotechnical and gee-environmental data There are then a good choice of programs for data entry, for generation of initial reports and for analysis including generating cross sections, contoured plots and three dimensional plots These can then be interfaced with CAD programs for use in the BIM © Tony Barry, September 2017 page | of 10 Geotechnical Evaluation for Metro Lines in Ho Chi Minh City It is understood that a geological model for Ho Chi Minh has been developed in recent years, but it appears not to be maintained as new information comes available; nor is it called upon generally as first source when designing ground investigations Desk Study Procedure The geotechnical survey design is often specified to be prepared as the first deliverable; this overlooks the requirement for an adequate desk study, which will substantially improve the quality of any field investigations required Regulations also require that the Construction Survey Plan be prepared as the first stage in the investigations These plans usually refer to the intention to collect existing data; however in most reports seen there is no reference to any existing data except for the geological map The Desk Study should be undertaken before the geotechnical survey design is finalised It should include collection of existing surveys, as well as aerial photography and mapping A short report summarising the data and the conclusions from the study should be prepared This could be included in the Construction Survey Plan and should be made use of to design the survey Improvements in Survey Methods Areas that should be considered for improvement are: a) Logging of materials to identify geological facies, with standard soil units adopted b) Adoption of a standard weathering scale for logging materials c) Use of cone penetration testing (with pore pressure measurement) to improve the interpretation of the ground conditions and obtain improved parameters for design d) Adoption of a stationery piston sampler for obtaining high quality samples of soft clay for laboratory testing e) Installation of piezometers as part of surveys to obtain realistic data on groundwater levels f) Chemical testing of the aggressivity of soils g) Review of laboratory testing procedures, including handling of soft clay samples and their testing h) Introduction of the concept of precision in reporting test results Analysis of Data Results from laboratory testing are generally grouped into a series of soil units specific to the site, and then averaged Firstly this does not allow any differences within the presumed soil unit to be seen Secondly many soil properties vary with depth, as a result of variation in effective stress and age of the deposit These effects are completely masked by the taking of averages © Tony Barry, September 2017 page | of 10 Geotechnical Evaluation for Metro Lines in Ho Chi Minh City Data from laboratory testing should be analysed in a progressive manner Firstly index tests should be used to identify, along with the detailed borehole logging, soil units appropriately - related to the lithology - and their variation with depth Only then can strength and compressibility parameters be assessed for specific soil units; these latter engineering design parameters can then also be compared with index data to obtain a more comprehensive database for the City The Author has also seen very little evidence of in-depth back analysis of field data, such as ground treatment monitoring or pile testing If undertaken in detail these types of analysis can provide considerable improvements in reliability of designs Back analysis of this type should be a requirement of all significant projects, and the results should be incorporated into the database described above to be made generally available Building condition/foundation type surveys This survey may be considered as separate from the geological and geotechnical survey, but the outputs require a full understanding of the ground conditions Since the route of the MRT lines through the city is in the central part through heavily built up areas there is a need for detailed investigation of the existing buildings Information on these buildings is of variable quality, and there appears to be no central database from which the information can be extracted; hence this survey will require significant resources and may be time-consuming Contract Forms and Value Engineering The scope of the geological and geotechnical investigations should be considered in relation to the nature of the construction contracts Typically Ho Chi Minh is adopting design and build contracts for the MRT works and consequently the scope of the surveys should allow for any likely form of construction adopted by bidders Though it has been suggested that bidders could undertake their own investigations during the bid process if they need further information this is not practical Consequently it is in the Employer's interest to provide as much detail as possible of the ground conditions to bidders Value engineering is allowed for in the FIDIC contract terms usually adopted, and contractors can undertake further investigations at the detailed design stage if they wish to develop the information further This does not need to be considered in the FEED stage In any event the contactor will be required to comply with the approved fundamental design so opportunities for value engineering are very limited Ì © Tony Barry, September 2017 page | 10 of 10 .. .Geotechnical Evaluation for Metro Lines in Ho Chi Minh City The Geology of Ho Chi Minh Ho Chi Minh lies in the flood plain of the Saigon River and the near surface deposits consist of... 10 Geotechnical Evaluation for Metro Lines in Ho Chi Minh City It is understood that a geological model for Ho Chi Minh has been developed in recent years, but it appears not to be maintained as new information comes available; nor is it called upon... 10 Geotechnical Evaluation for Metro Lines in Ho Chi Minh City Ambient air quality The city lies some 50km from the sea and is not therefore seriously affected by chloride contamination, though saline intrusion of the groundwater has been identified7