1.3 Borehole layout

Whenever possible boreholes should be sunk close to the proposed foundations This is important where the bearing stratum is irregular in depth For the same reason the boreholes should be accurately located in position and level in relation to the proposed structures

Where the layout of the structures has not been decided at the time of making the investigation a suitable pattern of boreholes is an evenly spaced grid of holes For extensive areas it is possible to adopt a grid of boreholes with some form of zn-situ probes, such as dynamic or static cone penetration tests, at a closer spacing within the borehole grid EC 7 recommends, for category 2 investigations, that the exploration points forming the grid should normally be at a mutual spacing of 20—40 m Trial pits for small foundations, such as strip foundations for houses, should not be located on or close to the intended foundation position because of the weakening of the ground caused by these relatively large and deep trial excavations

The required number of boreholes which need to be sunk on any particular location is a difficult problem which is closely bound up with the relative costs of the investigation and the project for which it is undertaken

Obviously the more boreholes that are sunk the more is known of the soil conditions and greater economy can be achieved in foundation design, and the risks of meeting unforeseen and difficult soil conditions which would greatly increase the costs of the foundation work become progressively less However, an economic limit is reached when the cost of borings outweighs any savings in foundation costs and merely adds to the overall cost of the project

For all but the smallest structures, at least two and preferably three boreholes should be sunk, so that the true dip of the strata can be established

Even so, false assumptions may still be made about stratification

The depth to which boreholes should be sunk is governed by the depth of soil affected by foundation bearing pressures The vertical stress on the soil at a depth of one and a half times the width of the loaded area is still one-fifth of the applied vertical stress at foundation level, and the shear stress at this depth is still appreciable Thus, borings in soil should always be taken to a depth of at least one to three times the width of the loaded area

In the case of narrow and widely spaced strip or pad foundations the borings are comparatively shallow (Fig 1 1(a)), but for large raft foundations the borings will have to be deep (Fig 1 1(b)) unless rock is present within the prescribed depth Where strip or pad footings are closely spaced so that there is overlapping of the zones of pressure the whole loaded area becomes in effect a raft foundation with correspondingly deep borings (Fig 11(c)) In the case of piled foundations the ground should be explored below pile-point level to cover the zones of soil affected by loading trans- mitted through the piles EC 7 recommends a depth of five shaft diameters below the expected toe level It is usual to assume that a large piled area in uniform soil behaves as a raft foundation with the equivalent raft at a depth of two-thirds of the length of the piles (Fig 1 1(d))

Figure 1.1 Depths of boreholes for various foundation conditions The 'rule-of-thumb' for a borehole depth of one and a half times the foundation width should be used with caution

Deep fill material could be present on some sites and geological conditions at depth could involve a risk of foundation instability

Where foundations are taken down to rock, either in the form of strip or pad foundations or by piling, it is necessary to prove that rock is in fact present at the assumed depths Where the rock is shallow this can be done by direct examination of exposures in trial pits or trenches, but when borings have to be sunk to locate and prove bedrock it is important to ensure that boulders or layers of cemented soils are not mistaken for bedrock This necessitates percussion boring or rotary diamond core drilling to a depth of at least 3 m in bed-rock in areas where boulders are known to occur On sites where it is known from geological evidence that boulders are not present a somewhat shallower penetration into rock can be accepted In some areas boulders larger than 3 m have been found, and it is advisable to core the rock to a depth of 6 m for important structures

Mistakes in the location of bedrock in boreholes have in many cases led to costly changes in the design of structures and even to failures.

It is sometimes the practice, when preparing borehole records, to define rockhead or bedrock as the level at which auger or percussion boring in weak rock has ceased and coring in stronger rock has commenced

This practice is quite wrong The decision to change to core drilling may have nothing to do with the strength of the rock It may depend on the availability of a core drill at any given time or on the level at which the borehole has reached at the end of the morning or after- noon's work Rockhead or bedrock should be defined as the interface between superficial deposits and rock, irrespective of the state of weathering of the latter

Direct exposure of rock in trial pits or trenches is preferable to boring, wherever economically possible, since widely spaced core drillings do not always give a true indication of shattering, faulting, or other structural weakness in the rock Where rock lies at some depth below ground level, it can be examined in large-diameter boreholes drilled by equipment described. Because of the cost, this form of deep exploration is employed only for important structures EC 7 recommends that where the possibility of base uplift in excavations is being investigated the pore-water pressures should be recorded over a depth below ground-water level equal to or greater than the excavation depth Even greater depths may be required where the upper soil layers have a low density When boreholes are sunk in water-bearing ground which will be subsequently excavated, it is important to ensure that they are backfilled with concrete or well-rammed puddle clay If this is not done the boreholes may be a source of considerable inflow of water into the excavations

In a report on an investigation for a deep basement structure in the Glasgow area the author gave a warning about the possibility of upheaval of clay at the bottom of the excavation, due to artesian pressure in the under-lying water-bearing rock After completing the basement the contractor was asked whether he had had any trouble with this artesian water The answer was that 'the only trouble we had with water was up through your borehole' In another case, large bored piles with enlarged bases were designed to be founded within an impervious clay layer which was underlain by sand containing water under artesian pressure The risks of somewhat greater settlement due to founding in the compressible clay were accepted to avoid the difficulty of constructing the piles in the underlying, less compressible sand However, considerable difficulty was experienced in excavating the base of one of the piles because of water flowing up from the sand strata through an unsealed exploratory borehole

Site investigations of foundation failures.

From time to time it is necessary to make investigations of failures or defects in existing structures The approach is somewhat different from that of normal site investigation work, and usually takes the form of trial pits dug at various points to expose the soil at foundation level and the foundation structure, together with deep trial pits or borings to investigate the full depth of the soil affected by bearing pressures A careful note is taken of all visible cracking and movements in the superstructure since the pattern of cracking is indicative of the mode of foundation movement, e g by sagging or hogging It is often necessary to make long-continued observations of changes in level and of movement of cracks by means of tell-tales Glass or paper tell-tales stuck on the cracks by cement pats are of little use and are easily lost or damaged The tell-tales should consist of devices specially designed for the purpose or non-corrodible metal plugs cemented into holes drilled in the wall on each side of the crack and so arranged that both vertical and horizontal movements can be measured by micrometer gauges Similarly, points for taking levels should be well secured against removal or displacement They should consist preferably of steel bolts or pins set in the foundations and surrounded by a vertical pipe with a cover at ground level The levels should be referred to a well-established datum point at some distance from the affected structure, ground movements which may have caused foundation failure should not cause similar movement of the levelling datum

The Building Research Establishment in Britain has developed a number of devices such as tiltmeters and borehole extensometers for monitoring the movements of structures and foundations

A careful study should be made of adjacent structures to ascertain whether failure is of general occurrence, as in mining subsidence, or whether it is due to localized conditions

The past history of the site should be investigated with particular reference to the former existence of trees, hedgerows, farm buildings, or waste dumps The proximity of any growing trees should be noted, and information should be sought on the seasonal occurrence of cracking, for example if cracks tend to open or close in winter or summer, or are worse in dry years or wet years Any industrial plant in which forging hammers or presses cause ground vibrations should be noted, and inquiries should be made about any construction operations such as deep trenches, tunnels, blasting, or piling which may have been carried out in the locality