GB 50478-2008Standard for investigation of geotechnical engineering of geothermal power plant (English PDF)

1 General provisions

1.0.1 The document states that it was drawn up to put the geotechnical investigation of geothermal power plants on a regular footing, so that the work is safe and suitable, technically advanced and economically reasonable, and so that quality is assured and the environment protected.

1.0.2 It applies to the geotechnical investigation of newly built, extended and reconstructed geothermal power plants.

1.0.3 This clause is one of the provisions declared mandatory in the ministerial announcement. Before a geothermal power plant is designed and built, a geotechnical investigation must be carried out following the basic construction procedure.

1.0.4 The geotechnical investigation is to follow the requirements of each investigation stage of the project, to reflect the engineering geological conditions, to identify adverse geological actions and geological hazards, to be carried out and analysed with care, and to produce an investigation report whose data are complete and whose assessment is correct.

1.0.5 Besides this document, the geotechnical investigation of geothermal power plants is to comply with the relevant current national standards.

2 Terms

2.0.1 geothermal power plant - a power plant that generates electricity using the heat energy carried by a geothermal fluid.

2.0.2 geothermal - the natural heat energy that comes from the deep part of the earth crust and is stored in underground rock and in the pores and fissures of rock.

2.0.3 geothermal reinjection - the process of injecting used geothermal fluid or another water source back into, or letting it flow into, the reservoir interval through a reinjection well, in order to keep the reservoir pressure, to use the energy fully and to reduce the pollution caused by discharging geothermal fluid directly.

2.0.4 engineering geological condition - the sum of the conditions directly or indirectly related to the construction works: landform, stratigraphic lithology, geological structure, engineering properties of the soil and rock, hydrogeology, environmental geology and natural construction materials.

2.0.5 geotechnical engineering analysis and evaluation - the collective name for the work of analysing and calculating on the basis of engineering geological mapping, exploration, testing and monitoring, of selecting the geotechnical parameters, of demonstrating the stability and suitability of the site, of the foundation soil and of geotechnical structures, of proposing workable schemes and recommendations for the use, remediation and improvement of the ground, and of predicting and monitoring the geotechnical problems that may arise during construction and operation, together with the corresponding countermeasures, measures and recommendations.

2.0.6 investigation during construction - for building foundations with complex geotechnical conditions or with special service requirements, the investigation needed during construction to add missing information or carried out when the geotechnical conditions found during foundation works do not agree with the investigation report.

3.1 Basic requirements - Basic technical principles

3.1.1 The geotechnical investigation of a geothermal power plant is to identify the adverse geological actions and the geological hazards that affect the siting of the plant, to reflect correctly the engineering geological conditions bearing on design and construction, to analyse and assess the stability of the site, to carry out the geotechnical analysis and evaluation and to put forward sound geotechnical recommendations.

3.1.2 This clause is one of the provisions declared mandatory in the ministerial announcement. A geothermal power plant is strictly forbidden to be sited on ground affected by serious landslides, collapses, debris flows, ground fissures or ground subsidence, or on a Holocene active fault zone.

3.1.3 The geotechnical investigation is to be planned with a technical scheme suited to the depth of work required by the corresponding investigation stage.

3.1.4 The investigation is to use single or combined investigation and testing methods chosen to suit the engineering geological conditions of the site. All original data, records and test data must be true and reliable.

3.1.5 Where the effect of ground remediation and the construction conditions have to be checked, or where the remediation scheme is to be optimized, a full-scale field test is to be carried out.

3.1.6 Geotechnical analysis is to run through the whole of the investigation, and qualitative and quantitative methods are to be used according to the object analysed.

3.1.7 The classification and identification of soil and rock, engineering geological mapping and survey, exploration, in-situ testing, sampling, laboratory testing and the assessment of the corrosiveness of water and soil are to follow the relevant provisions of the current national standard GB 50021 on geotechnical investigation.

3.1.8 New techniques and new methods are to be taken up in the geotechnical investigation of geothermal power plants.

3.1.9 Field checking is to be carried out during the geotechnical construction works of a geothermal power plant.

3.2 Basic requirements - Classification of building sites

3.2.1 The complexity of a building site may be divided into complex site, moderately complex site and simple site.

3.2.2 A site may be rated complex when any one of six listed conditions is met: complex landform with more than three geomorphological units; many stratigraphic layers, with uneven distribution and widely varying properties of the foundation soil and rock; foundation soil consisting of special soil or rock that is seriously collapsible, saline, contaminated, expansive, frost-heaving or thaw-settling; complex geological structure with strongly developed adverse geological actions; a peak ground acceleration at or above 0.40 g, or ground dangerous for the seismic behaviour of buildings; complex hydrogeological conditions with several groundwater layers affecting the works.

3.2.3 A site may be rated simple when five listed conditions are met together: fairly level terrain with a single geomorphological unit; simple stratigraphic structure with uniform soil and rock properties and no special soil or rock; groundwater buried deep enough not to affect the works; simple geological structure without adverse geological actions; a peak ground acceleration below 0.10 g, or ground favourable for the seismic behaviour of buildings.

3.2.4 Sites outside the conditions listed in 3.2.2 and 3.2.3 may be rated moderately complex.

3.3 Basic requirements - Principles for the division of investigation stages

3.3.1 The geotechnical investigation of a geothermal power plant is to be carried out in stages, and the division of the investigation stages is to match the design stages. Four stages are named: preliminary feasibility study stage, feasibility study stage, preliminary design stage and construction drawing design stage.

3.3.2 The clause lists the circumstances in which the investigation stages may be adjusted. The first item, the only one on the page read, concerns the geotechnical investigation of extended or reconstructed geothermal power plants, which is to be based on the existing investigation data.

4.1 Investigation at the preliminary feasibility study stage

4.1.1 The investigation at this stage is to give a preliminary assessment of the stability and of the foundation conditions of the candidate sites, to state whether a site is suitable for the plant, and to recommend two or more site schemes that are relatively stable and have better engineering geological conditions.

4.1.2 The investigation should collect seven kinds of material: topographic maps at scales from 1:5000 to 1:50000; regional geological data; regional seismic and seismogeological data; hydrogeological, geotechnical and geological hazard data for the region of the site; geothermal field exploration and geothermal tail water reinjection data; the distribution and exploitation of mineral resources and the distribution and protection grade of antiquities and important fossil groups; local building experience and the current national standards.

4.1.3 The main tasks of the stage are listed: to learn the regional geological structure, the active faults and the seismogeological data of the site area, to determine preliminarily the basic ground motion parameters and to assess preliminarily the tectonic stability of the site area; to survey the adverse geological actions at and near the site, to analyse how damaging they are, to assess preliminarily the stability of the site and to propose avoidance or control measures; to survey the landform, the stratigraphic lithology and the groundwater burial conditions of each site area and to assess preliminarily the type of foundation that may be used; to learn the distribution, exploitation and planning of the geothermal and mineral resources at and near the site; to analyse preliminarily the environmental geological problems of the site and their effect on the works; and, when the peak ground acceleration is at or above 0.10 g, to analyse preliminarily the possibility of seismic liquefaction of saturated sand and saturated silt at the site.

4.1.4 The work of this stage is to consist mainly of collecting material and of field reconnaissance; where necessary, engineering geological survey or mapping, geophysical prospecting and a suitable amount of drilling and pit exploration are to be carried out.

4.1.5 The assessment and recommendation of the site at this stage is to be based on three conditions: the stability of the site, the possibility of avoiding the adverse geological actions and how difficult they are to treat; the ground motion parameters and the effect of the site on the seismic behaviour of buildings; the type of foundation proposed, how difficult the ground treatment is, and the effect of the relief on the use or levelling of the site.

4.2 Investigation at the feasibility study stage

4.2.1 The investigation at this stage is to give a final assessment of the stability of each site, to assess the engineering geological conditions of the site and the type of foundation, to predict the environmental geological problems that the works may cause, to recommend the site with the better engineering geological conditions, and to make sure that the investigation of the later stages will not reach an opposite conclusion.

4.2.2 Besides the material listed in 4.1.2, the stage is to collect four further items: design data on the proposed scale of the works, the capacity of the units and the general planning concept; the terms of reference for the geotechnical investigation of the feasibility study stage; drawings showing the general layout of the plant area within the outline of the site, the water intake and cooling system plan and the geothermal extraction and reinjection system plan; and the seismic safety evaluation of the site, the assessment of the risk of geological hazards and the survey of overlain mineral deposits.

4.2.3 The main tasks of the stage are listed and include the analysis of the regional geological structure and the use of the seismic safety evaluation to assess the effect of active faults at and near the site and to reach a final assessment of the tectonic stability of the site area; the identification of the adverse geological actions at and around the site, the analysis of how damaging they are and how they are developing, the final assessment of the stability of the site and a preliminary control scheme; the preliminary identification of the origin and age of the strata, of the distribution of lithology and of the physical and mechanical properties of the main soil and rock layers, together with the geological structure, the groundwater burial conditions and the corrosiveness of water and soil inside the site; the analysis and prediction of ground subsidence, swamping, salinization, freeze-thaw effects, engineering landslides and other environmental geological problems that geothermal extraction, reinjection and construction may cause; the survey of overlain and worked mineral deposits and their effect on the works; the supply of the basic ground motion parameters, the determination of the site class, the division of the ground into dangerous, unfavourable, favourable and ordinary for the seismic behaviour of buildings and the assessment of the possibility of landslide, collapse or subsidence under seismic action; the assessment of the liquefaction grade of saturated sand and saturated silt when the peak ground acceleration is at or above 0.10 g; and the analysis of the foundation type, with a demonstration of the ground treatment or pile foundation scheme where needed.

4.2.4 For sites in mountainous and hilly areas the investigation is to make full use of engineering geological mapping and survey. Mapping should be carried out for complex sites, mapping or survey as needed for moderately complex sites, and a survey for simple sites. The mapping is to cover the site and the surrounding area, and the scale of the topographic map for the mapping should be between 1:1000 and 1:5000.

4.2.5 The clause fixes the exploration work in the plant area at this stage: exploration points may be laid out on a grid that also takes account of the general layout, and the grid should extend somewhat beyond the outline of the proposed plant area; in mountain sites exploration points are to be laid out in every geomorphological unit, and points are to be added where geomorphological units meet and where the thickness of the overburden changes markedly; the spacing and number of exploration points are to follow the complexity of the site, with a stated spacing range and a minimum number of points per site for complex sites and another for moderately complex and simple sites; the depth of the control and ordinary exploration points in Quaternary strata is fixed, with an increase for soft soil sites and an adjustment when bedrock is met within the intended depth, the control points entering moderately to slightly weathered bedrock by a stated amount; and where bedrock is exposed or shallowly buried, some exploration pits or trenches should be laid out.

4.2.6 Representative undisturbed soil samples and groundwater samples are to be taken at this stage for laboratory tests of physical and mechanical properties and for groundwater quality analysis. The samples of each main soil layer should not be fewer than six.

4.2.7 At the feasibility study stage, the investigation of the water intake structures, of the geothermal wellhead and of the reinjection site is to consist mainly of engineering geological mapping or survey, with a certain amount of exploration work where necessary.

4.2.8 The geotechnical investigation of a geothermal power plant with respect to active faults is to follow the provisions of section 6.1 of the document.

4.2.9 When a natural foundation scheme is hard to sustain, the ground treatment methods and the choice of pile type are to be analysed and demonstrated, and a ground treatment or pile foundation scheme is to be recommended.

4.2.10 The comparison of sites at this stage is to follow clause 4.1.5, and the site with the better engineering geological conditions is to be recommended.

4.3 Investigation at the preliminary design stage

4.3.1 The investigation at this stage is to identify the engineering geological conditions of the site further, to assess and recommend the foundation scheme of the main buildings and the remediation scheme for the adverse geological actions and the environmental geological problems, and to put forward recommendations on the general layout of the buildings.

4.3.2 The stage should include the terms of reference for the geotechnical investigation of the preliminary design stage; a general layout drawing of the buildings showing the design level of the ground floor; and the shape, size and depth of the foundations preliminarily proposed for each building, together with the unit load and the total load preliminarily determined for the foundations of the main buildings. The list continues on a page that is not among the extracted images.

Note on the title change made by the 2024 partial revision Change of title, number unchanged

Announcement No. 108 of 2024 of the Ministry of Housing and Urban-Rural Development, dated 27 June 2024 and reproduced in the volume, approves the partially revised provisions of the national standard GB 50478-2008 and puts them into force from 1 October 2024. The same announcement changes the title of the standard from Code for investigation of geotechnical engineering of geothermal power plant to Standard for investigation of geotechnical engineering of geothermal power plant.

The number of the standard is not changed: the cover of the volume examined still carries GB 50478-2008, so the document remains a mandatory national standard. The cover carries the new Chinese title, the English title Standard for investigation of geotechnical engineering of geothermal power plant and the marking 2024 edition, while the issue date of 15 December 2008 and the implementation date of 1 August 2009 of the original edition are kept at the foot of the cover.

Announcement No. 195 of 2008, also reproduced in the volume, is the announcement of the original edition and names clauses 1.0.3, 3.1.2, 6.2.1 and 6.3.1 as the mandatory provisions of the document.

The body of the document has been brought into line with the new title: clauses 1.0.1, 1.0.2 and 1.0.5 call the document a standard.

No line beginning with the character for replaces appears on the front matter of the volume, and no earlier edition is named in the two announcements. The supersedes field is therefore left empty.

Note on the table of contents Why the contents entries carry no page numbers

The contents page of the volume is not among the extracted images: the front matter available runs from the cover to the end of the foreword, and the body pages available run from page 1 to page 10 of the printed text.

The entries listed in the table of contents above are therefore the headings actually read on the body pages, and they are given without page numbers. Clause 4.2.8 refers to section 6.1 of the document and the 2008 announcement names clauses 6.2.1 and 6.3.1, so chapter 6 exists, but its headings were not read and are not listed.