4.3.2 Rocks and rock masses

COMMENTARY ON 4.3.2

The engineering properties of rock relevant in design are controlled by the extent and orientation of the bedding planes and joints within the rock mass, together with the water pressures on the discontinuity planes. The site investigation needs to establish the strength and orientation of the discontinuity planes.

Weak rocks, particularly weakly cemented sandstones, fissured shales and chalk, are often difficult materials to sample and test.

4.3.2.1 The identification and classification of rock should conform to BS EN ISO 14689-1.

4.3.2.2 Rock properties should be determined in accordance with BS EN 1997-2 and BS 5930, as well as BS EN 1997-1:2004+A1:2013, 3.3.2, 3.3.8 and 3.3.9.

4.3.2.3 Characteristic rock parameters for intact rock should be selected in accordance with BS EN 1997-1, based on the results of field and laboratory tests, complemented by well-established experience.

4.3.2.4 Design parameters for the rock mass should take into account the properties of the intact rock and any discontinuities. See BS EN 1997-1:2004+A1:2013, 3.3.8.

4.3.2.5 The following non-destructive tests may be used to determine rock mineralogy and composition in order to predict the rock's performance during excavation (in particular, if it will slurry or smear rather than be broken up into smaller pieces):

  • microscope;
  • X-ray computerized tomography; and
  • spectroscopy.

NOTE 1 Guidance on rock description in engineering practice can be found in Soil and rock description in engineering practice [19].

NOTE 2 Information about the behaviour of rocks can be found in the ICE manual of geotechnical engineering (2012), Volume I, Chapter 18 [2].

NOTE 3 Information about mudrocks, clay, and pyrite (and their issues) can be found in the ICE manual of geotechnical engineering (2012), Volume I, Chapter 36 [2].

4.3.3 Fill

COMMENTARY ON 4.3.3

The term «fill» refers to artificially deposited material, for example, in an excavation or as ground made by human activity. These materials are also termed «artificial ground».

«Non-engineered» fill is material that is dumped with little control and in deep lifts. It is often poorly compacted, and thus in a loose state, and has varying geotechnical properties, both horizontally and vertically. Non-engineered fill is commonly referred to as «made ground».

«Engineered» fill is material that is placed with some degree of control to ensure that its geotechnical properties conform to a predetermined specification.

Engineered fill is commonly referred to as just «fill».

On geological maps, «made ground» refers to material placed above and «fill» to material placed below original ground level. On recent British Geological Survey (BGS) maps, these terms have been altered to «made up ground» and «infilled ground».

4.3.3.1 All materials intended to be placed behind foundations should be properly investigated and classified.

4.3.3.2 Non-engineered fill, such as industrial, chemical, and domestic wastes, should not be placed beneath foundations.

4.3.3.3 Engineered fill comprising selected coarse granular soils, such as well-graded small rockfills, gravels, and sands, may be placed beneath foundations.

4.3.3.4 Engineered fill should be appropriate to the intended application. The fill should be classified and an earthworks specification provided detailing acceptability criteria, compliance testing, and compaction requirements. The general specification of earthworks fill should conform to BS 6031.

NOTE 1 Guidance on the description of made ground can be found in Soil and rock description in engineering practice, Chapter 14 [19].

NOTE 2 Information about non-engineered fills can be found in the ICE manual of geotechnical engineering (2012), Volume I, Chapter 34 [2].

NOTE 3 Information about fill formation and deposits (covering opencast mining backfill, colliery spoil, pulverized fly ash, industrial and chemical wastes, urban fill, domestic refuse, infilled docks, pits, and quarries, and hydraulic fill) can be found in BRE Report 424 (2nd edition), Chapter 2 [31].

4.3.4 Earthworks

The properties of earthworks should be determined in accordance with BS 6031.

4.3.5 Groundwater

4.3.5.1 Groundwater pressures should be determined by considering hydrological, hydrogeological, and environmental information.

4.3.5.2 To conform to BS EN 1997-1:2004+A1:2013, 2.4.6.1(6)P, design values of groundwater pressure at the serviceability limit state should be the most unfavourable values that could occur during normal circumstances.

4.3.5.3 To conform to BS EN 1997-1:2004+A1:2013, 2.4.6.1(6)P, design values of groundwater pressure at the ultimate limit state should be the most unfavourable values that could occur during the design lifetime of the structure.

NOTE The relationship between characteristic and design water pressures can be determined by applying a geometrical margin.

4.3.5.4 If suitable statistical data is available, then design values of groundwater pressure at the serviceability limit state should be chosen with a return period at least equal to the duration of the design situation; and design values at the ultimate limit state should be chosen such that there is a 1% probability that they are exceeded during the design situation.

4.3.5.5 Where groundwater pressures are not hydrostatic, the design should take into account:

  • the worst credible combination of heterogeneity and anisotropy of permeability;
  • effects of layering, fissuring and other heterogeneity;
  • any geometrical features that could cause pressures to concentrate (such as in corners of excavations).

4.3.5.6 Long-term changes in groundwater that are likely to occur during the design working life of the structure (including those due to climate change and rising groundwater) should be taken into account.

4.3.5.7 Increased groundwater pressures owing to burst pipes and other failures of engineered systems should be classified as accidental actions if the event that causes the increase in groundwater pressure is unlikely to occur during the design working life of the structure.

4.3.5.8 The design of a foundation should be based on the most adverse water pressure conditions that can be anticipated.

NOTE Guidance on the selection of water tables and seepage forces can be found in CIRIA Report C580 [32].

4.3.5.9 Design water pressures should take into account the effect of tides on water levels in the ground.

NOTE Guidance on tides and water level variations can be found in BS 6349-1-3.

4.3.5.10 If the equilibrium level of the water table is well defined and measures are taken to prevent it changing during heavy rain or flood, the design water pressures can be calculated from the position of the equilibrium water table, making due allowance for possible seasonal variations. Otherwise, the most adverse water pressure conditions that can be anticipated should be used in design.

4.3.5.11 Equilibrium water levels in fine soils should be determined from piezometric readings taken over an adequate length of time.

4.3.5.12 Allowance should be made in undrained (i.e. total stress) analyses for water pressures due to the temporary filling of cracks in fine soils.

4.3.5.13 Water pressures used in drained (i.e. effective stress) analyses should be determined for the groundwater regime in the vicinity of the structure.

4.3.5.14 Where a difference in water pressures exists on opposite sides of a foundation, allowance should be made for seepage around the wall. Where layers of markedly different permeability exist, the water levels relevant to each permeable stratum should be taken into account.

4.3.5.15 The distribution of pore water pressures may be determined from a flow net, provided it adequately represents the hydraulic and permeability conditions in the vicinity of the structure.

4.3.6 Concrete

4.3.6.1 Concrete incorporated into foundations should conform to BS EN 1992-1-1, BS EN 206, and BS 8500-2.

4.3.6.2 Concrete incorporated into foundations should be specified in accordance with BS EN 206 and BS 8500-1.

4.3.6.3 Steel reinforcement for concrete foundations should conform to BS EN 10080 and BS 4449.

4.3.7 Steel

4.3.7.1 Steel incorporated into foundations should conform to BS EN 1993-1-1, BS EN 1993-5, and BS 8081, as appropriate.

4.3.7.2 The values of steel parameters should be determined in accordance with BS EN 1993-1-1 and BS EN 1993-5, and their UK National Annexes.

4.3.7.3 Hot rolled steel products should conform to BS EN 10025.

4.3.7.4 Hot rolled steel products manufactured to a different standard than BS EN 10025 may be used if it can be demonstrated by appropriate additional testing that the products meet the requirements of BS EN 10025 that are relevant to the foundation.

4.3.7.5 Cold formed hollow steel sections should conform to BS EN 10219.

4.3.7.6 Cold formed hollow steel sections manufactured to a different standard than BS EN 10219 may be used if it can be demonstrated by appropriate additional testing that the sections meet the requirements of BS EN 10219 that are relevant to the foundation.

4.3.8 Timber

4.3.8.1 Timber incorporated into foundations should conform to BS EN 1995-1-1.

4.3.8.2 The values of timber parameters should be determined in accordance with BS EN 1995-1-1.

4.3.9 Masonry

4.3.9.1 Masonry incorporated into foundations should conform to BS EN 1996-1-1.

4.3.9.2 The values of masonry parameters should be determined in accordance with BS EN 1996-1-1.

4.3.9.3 The following masonry units should conform to the relevant part of BS EN 771:

  • clay masonry units (Part 1);
  • calcium silicate masonry units (Part 2);
  • aggregate concrete masonry units (Part 3);
  • autoclaved aerated concrete masonry units (Part 4);
  • manufactured stone masonry units (Part 5); and
  • natural stone masonry units (Part 6).

4.3.9.4 The dimensions of clay and calcium silicate brick of special shapes and sizes should conform to BS 4729.

4.3.9.5 Mortars used in masonry foundations should conform to PD 6697.

4.3.9.6 Damp proof courses used in masonry foundations should conform to BS 8215.

4.3.9.7 Wall ties should conform to PD 6697.

4.3.10 Pipes

Pipes used in drainage systems for foundations should conform to one of the following standards, as appropriate:

  • vitrified clay pipes – BS 65 and BS EN 295;
  • concrete pipes – BS 5911 and BS EN 1916;
  • glass reinforced plastics (GRP) pipes – BS 5480;
  • cast iron – BS 437;
  • ductile iron – BS EN 598;
  • unplasticised polyvinyl-chloride (PVC-U) – BS 4660 or BS 5481 or BS EN 1401-1;
  • polypropylene (PP) – BS EN 1852-1;
  • polyethylene (PE) – BS EN 12666-1;
  • thermoplastics structure wall pipes – BS 4962;
  • geotextile wrapped land drains – BS 4962.

BS 8004:2015 Code of practice for foundations