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