63.4 Geotechnical Design Report (GDR)
COMMENTARY ON 63.4
The following list, which is by no means exhaustive, indicates the topics on which advice and recommendations are often required. The level of comment required here varies but an underlying need is the identification of ground-based risks and hazards. Given the availability of a wide and ever-changing range of proprietary systems, all of which interact with the ground In subtly different ways, it is important that the report writer does not overstate their level of knowledge.
A robust ground model which is then interrogated and evolved through the whole investigation process is critical to the success of the investigation process. By identifying the uncertainties in the knowledge about the ground, sensible decisions can be made as to the need for further investigation. This further investigation might include additional sub-surface investigation, examination of exposures within the construction or by monitoring of the structure during and after its completion. Monitoring can be part of an observational approach to the design, when the performance is checked against design assumptions.
The GDR should provide all parties with the information needed to assess the suitability of various options and the design of the works. It should include the following information.
- a) Spread foundations: level, either in terms of a depth or to a stated stratum; safe or allowable bearing capacity; estimated total and differential settlements; possible alternative types of foundation; possible ground treatment; effects of nearby trees.
- b) Piles: types suited to the ground and groundwater profile and environment-estimated safe working loads, or data from which they can be assessed; estimated settlements of structures; ease of installation such as for driven piles; need for pile tests.
- c) Retaining walls: lateral pressures or data from which they can be derived; wall friction; bearing capacity; groundwater conditions.
- d) Basements: comment on the possibility of flotation; estimate the rise of the basement floor during construction and groundwater levels.
- e) Ground anchorages: bearing stratum and estimated safe loads, or data from which they can be calculated.
- f) Chemical attack: most commonly takes the form of recommendations for protecting buried concrete against attack from sulfate-bearing soils and groundwater. The results are usually evaluated by reference to BRE Special Digest 1 [N1]. Also to be considered is the possibility of corrosion of steel in saline waters or in the presence of sulfate-reducing bacteria. The effect of acidic or highly alkaline soils might also need to be considered. The presence of contaminated soils, especially those containing high concentrations of organic chemicals, should be taken into account for their effects on all building materials, including effects on services. These factors should also be evaluated with regard to health and safety during construction and in subsequent use of the structure.
- g) Pavement design: assessment of appropriate design parameters or California Bearing Ratios; type and thickness of pavement; possibility of using soil stabilization for forming pavement bases or sub-bases; recommendations, where appropriate, for sub-grade drainage; comment on susceptibility of soil at formation level to frost heave.
- h) Slope stability: recommendations on temporary and permanent slopes for
- excavations, including, where appropriate, drainage measures. Comment should be made, where relevant, on the possibility of weathering of rock faces and the available methods of dealing with this hazard. Recommendations for the monitoring of unstable slopes might also be required.
- i) Mining subsidence: description of the workings; voids and stability; possible recommendations for methods of filling known cavities near the surface; the design of structures to withstand movements without damage or measures to limit the damage and simplify repairs.
- j) Natural cavities: description of the cavities; voids and stability; possible
- recommendations for methods of filling known cavities near the surface; the design of structures to withstand movements without damage or measures to limit the damage and simplify repairs.
- k) Tunnels and underground works: a description of the ground through which the tunnel is to be driven; possible covering of the following points: methods and sequence of excavation; whether excavation is likely to be stable without support; suggested methods of lining in unstable excavations; potential use of rock bolting; likelihood of encountering groundwater and recommendations for dealing with it; special features for pressure tunnels; risk of encountering ground or water contamination; possibility of natural or man-made gases.
- l) Safety of neighbouring structures: an assessment of the likely amount of movement caused by adjacent excavations and groundwater lowering, compressed air working, grouting and ground freezing or other geotechnical processes. The possibility of movement due to increased loading on adjacent ground might also need to be considered.
- m) Monitoring of movements: comment on the necessity for measuring the amount of movement taking place in structure and slopes, together with recommendations for the method to be used; recommendations for taking photographs before the commencement of works (see Annex H).
- n) Embankments: comment on stability of embankment foundations; assessment of amount and rate of settlement and the possibility of hastening it by such means as vertical drains; recommendations for side slopes; choice of constructional materials and methods; parameters for control of earthworks.
- o) Drainage: comment on possible drainage methods during construction for works above and below ground; general permanent land drainage schemes for extensive areas.
- p) Planning requirements: enable the discharge of planning conditions.
Where calculations have been made, they should be included as an annex, or a clear indication of the methods used should be given. The calculation methodology might form a significant part of the GDR.
63.4.2 Construction expedients
Comments and recommendations are often needed on the points listed below and safety aspects should be included where appropriate.
- a) Open excavations: method and sequence of excavation; what support is needed; how to avoid boiling and bottom heave; estimated upward movement of floor of excavation. Comment on relative merits of sheet piling and diaphragm or bored pile walls where appropriate.
- b) Underground excavations: method and sequence of excavation and the need for temporary roof and side support; dealing with gases.
- c) Groundwater: likely flow, head and quantity and how to deal with it.
- d) Driven piles, bored piles and ground anchors: methods of driving or construction suited to the ground profile, environment and neighbouring buildings.
- e) Grouting: types of grouts likely to be successful in the ground and recommended method of injection.
- f) Mechanical improvement of soil below ground level. Comment on the suitability of techniques for the consolidation of loose soils.
- g) Contamination: known or suspected contaminants and gases in soil, groundwater and any cavities. Comment on health and safety aspects both during and after construction, see BS 10175.
- h) Monitoring during construction works, e.g. stability of deep excavations, vibrations and movement near other structures.
- i) The need for sustainability: this can include selecting appropriate materials from appropriate sources as well as in the methods to be used for construction. Geotechnical structures can also be used to access shallow geothermal energy sources for building heating and cooling. The design of such structures needs to be considered at an early phase of the investigation process and appropriate testing carried out in the investigation.
63.4.3 Sources of materials for construction
The following sources of materials for construction should be taken into account.
- a) Fill: possibility of using excavated material for this purpose with an assessment of the proportions of usable material; methods and standards of compaction; possible off-site sources of fill; bulking factor.
- b) Aggregates: in areas where no commercial sources are available, the possibilities of winning and processing materials available locally.
- c) Groundwater as a construction material.
NOTE In many parts of the world, and even parts of the UK, sources of water for use in construction cannot be assumed to be available in the quantities required, and this requires consultation with the water supply authorities through the use of piped, surface or ground water sources.
63.4.4 Parameter characterization
COMMENTARY ON 63.4.4
Methods of analyzing ground data and applying them to the solution of engineering problems are not covered in this British Standard. For guidance on this, see BS 5493, BS 6031, BS 7361-1, BS 8002, BS 8004 and CP 2012-1.
The report should include comment on the compatibility of existing information and new information, and in particular draw attention to anomalies and proposed further investigation to address such matters.
The report should also present the current status of the ground conditions as incorporated in the ground model; by this stage the model ought to be mature and should provide a comprehensive identification of ground-based hazards and risks to the design and construction. Any uncertainties that remain should be discussed, and any plans for their resolution presented; it might also be the case that further investigation to resolve anomalies has been decided against for cost benefit reasons; these should also be discussed.
A clear statement should be made about the data on which the analysis and recommendations are based, particularly any limitations and variability. Any concerns/issues should be recorded in the geotechnical risk register and suggestions for resolution presented.
NOTE The information comes under two separate headings, as follows.
- a) The information related to the project and usually supplied by the designer. For example, for buildings and other structures this should include full details on the loading, split into dead and live; column spacing, where appropriate; and depth and extent of basements and details of neighbouring structures. For earthworks, heights of embankments and the materials of which they are to be made, together with the depths of cuttings, are all relevant to the interpretation.
- b) Ground parameters, selected from the summary of ground conditions report by the engineer making the analysis and preparing the recommendations.
63.4.5 Supervision, monitoring and maintenance report
COMMENTARY ON 63.4.5
It is important to note that investigation is not complete once the design of the geotechnical structure is completed. Investigation is an on-going process through the construction with observations being made to confirm assumptions made in the design and to look for any deviations from the expected ground or groundwater conditions. The design of the structure might also require measurements to be made of settlements or other movements.
The frequency and locations of checks should be identified in the GDR and communicated to those involved with construction on site. In addition, it should be made clear who is to make these checks and there should be a procedure for review and action in place, if necessary.
In accordance with BS EN 1997 an extract from the GDR containing the supervision, monitoring and maintenance requirements for the completed structure should be provided to the owner/client.
63.5 Geotechnical Baseline Report (GBR)
The GBR should set out the anticipated geology, soil and rock profiles and groundwater conditions. The GBR should be agreed between all parties to the contract before the construction works commence and it should be agreed which party is liable for any deviation from the identified conditions. The GBR should then be used during construction to measure the conditions actually encountered and to operate the changed physical conditions clause in the construction contract.
NOTE The intention of the GBR is to place an equitable, safe and economic balance for the risk for the ground between the parties. The GBR may include factual elements of the GIR.
Where a ground investigation has been undertaken in an attempt to identify the cause of failures, the following points should be taken into account.
- a) Foundations: the nature and dimensions of the foundations, identification of the type and cause of failure and, where appropriate, an estimate of the amount of settlement or heave that has already occurred, together with an assessment of how much more is likely to occur and its probable effect on the structure; cause of excessive vibrations of machine foundations; recommendations for remedial measures.
- b) Landslides or slope instability: classification of the type of movement and location of the failure surfaces; recommendations for immediate stabilizing expedients and long-term measures; recommendations for monitoring.
- c) Embankments: identification of whether the seat of failure lies within the embankment itself or the underlying strata, the probable cause and suggested method of repair and strengthening.
- d) Retaining walls: comment on cause of failure or excessive deflection; forecast of future behaviour of wall and recommendations where appropriate for strengthening it; recommendations for monitoring.
- e) Pavements: determination of whether the failure is within the pavement itself or the sub-grade and recommendations for repairs or strengthening or both; recommendations for monitoring.
- f) Differential ground movement: provide summary of details for main types, including natural and anthropogenic such as karst, formation, dissolution of soluble ground, mining; summarize effects on development.
- g) Mining or natural cavity related failures: determine likelihood of further movement; recommendations for repairs or strengthening or both and any monitoring.
63.7 Geotechnical Feedback Report (GFR)
A Geotechnical Feedback Report (GFR) should be prepared where required.
NOTE The contents of this report might include the topics detailed in BS 8002 and BS 8004 and in the contract specification.