Thursday, December 13, 2012

Soil Investigation.

A soil survey can range from a few trial pits inspected by the designer and the soil untested by laboratory analysis to an extensive borehole investigation with deep and numerous bores and extensive sampling and testing of the soil usually by specialist investigation contractors.

The factors affecting the investigation are the amount of existing information available, the known uniformity or likely variability of the sub-soil in the area, the foundation loading and the type of structure, the general topography and likely groundwater conditions of the site.

Subsidiary factors such as the amount of time and money available, the site access and other matters should not inhibit the planning of a thorough (and as reliable as is  reasonably possible) investigation.

No matter what kind of investigation is carried out, the authors, from experience, recommend the digging of  trial pits as a first stage. Trial pits have over the past few decades fallen into almost contemptuous dismissal by some  with the increased sophistication of boring techniques, increased cost of labour in digging pits and increased awareness of the limitations of pits (e.g. they do not detect underlying soft soils which can be affected by foundation loading). But during the same period there has been increased adaptability, mobility, etc., of relatively small excavators. Such machines can easily excavate and backfill a dozen pits, or trenches, in a day to a depth of 3–6 m and can be hired on a daily basis at cost-effective rates. The cost of replacing services damaged during excavation can be substantial, especially in the case of optical fibre cables, and the responsibility for adequate insurance cover should therefore not be overlooked.

1 Borehole layout  Three bores are the minimum necessary to determine the dip of a plane strata (where known with confidence to be plane) and as a rough guide this is the minimum for a proposed investigation (it is almost self-evident not to have too many!)...

2 Trial pit layout  Trial pits should be located near to the proposed or existing foundations but not so close as to adversely affect foundation excavation or to disturb existing underground services and drains. They should straddle the proposed site of the building to give...

3 Hand augers  Hand augers are sometimes used in preliminary reconnaissance since the equipment is light, cheap and immediately available, and so that overall, time can be saved in planning a full survey. They can, in soft to firm soils, bore a hole about 150 mm diameter to a depth of 3–4 m and provide disturbed samples of the soil. They can be used in restricted spaces, which is useful in investigating foundation failure below a confined basement. However the work can be physically hard, somewhat slow and very difficult, or impossible, in stony clays and gravels.

4 Boring  Most bores are carried out using light cable percussion plant backed up, when necessary, with rotary coring and other equipment and attachments. The cable percussion rig commonly uses an 8 m high tripod and employs a friction winch to raise and lower the boring tubes and tools. Rotary coring is used when hard shales, boulders or rock strata are encountered.

There is an increasing variety of plant, sampling methods and tools, with particular advantages in cost, quality of sampling, speed of operation, use in conditions of limited access or headroom, etc., and the choice of rig is affected by the likely soil conditions to be encountered.

5 Backfilling of trial pits and boreholes  If bores and particularly pits are positioned sufficiently close to the proposed structure so as to affect foundation excavation then they should be carefully backfilled. A strip footing founded on firm clay and passing over an inadequately compacted backfilled trial pit is effectively passing over a soft-spot. A borehole can sometimes act as an artesian well or as a seepage point. Trial pits or trenches should be backfilled in layers with controlled compaction. Boreholes should be backfilled, as the casing is withdrawn, with selected excavated material and punned with a weighted shell. Grouting boreholes is sometimes necessary with 4 : 1 cement : bentonite. The quality of backfilling of trial pits is however often unreliable and if the pits are close to the foundation they should be re-excavated along with the foundation excavation and backfilled again after completion of foundation construction.

6 Soil sampling  Samples of the soil are taken from boreholes and trial pits so that the soil can be described and tested. There are two types of samples:

Disturbed samples. Samples taken from boring tubes or hand excavated from the sides and bottom of trial pits where the soil structure is disturbed  i.e. broken up, cut, pressed, etc. These samples are placed in airtight jars (similar to screw lid jam-jars), labelled to identify the borehole or pit number, the position of the sample, the number given to it in the records, and the date taken.

Failure to label samples in standard format will obviously lead to confusion at the laboratory so the label must be secure and the information noted on it must be legible and written in waterproof ink.

Disturbed samples are tested to determine, mainly,  the type and description of the soil. The sampling and
testing of disturbed samples is relatively inexpensive and the test results are used to determine the test
programme of undisturbed samples.

If the disturbed samples are to be used to determine the moisture content of the soil it is important that the
sample jar should be completely filled by the sample to prevent it drying out. As a further precaution the air- tight cap should be wound round by a water-resistant

• Undisturbed samples. The term undisturbed  is somewhat  of a misnomer for even with refined equipment it is difficult to obtain a true undisturbed sample. Certainly, undisturbed samples are generally superior to disturbed samples in representing more closely the actual in situ structure and moisture content of the soil.

The soil structure and moisture content are important factors in soil strength and behaviour under load. Disturbed soil is trimmed from the ends of the sample tubes, the ends are then covered by foil and waxed before screwing on the tube cap or lid. Labels, giving the same information as for disturbed samples, should be placed both inside  the cap and outside the tube.

Undisturbed samples are tested to determine mainly the strength and behaviour of the soil. Undisturbed  samples are relatively expensive to obtain and test and it is generally not necessary to test all the samples. Nevertheless it is advisable to obtain at least one sample for each stratum at each borehole. The test programme is fully determined after study of borehole logs and soil profiles.

7 Storage of samples  Preferably samples should be sent to the testing laboratory immediately – and this, of course, is not always possible.  If they are just left lying around the site they could be sub- ject to drying out, impact, etc. so they should be carefully stacked and stored in a cool and somewhat moist site hut or container box.

8 Frequency of sampling  The soil investigation engineer, preferably with the design engineer’s report on site study, reconnaissance and trial pit findings if available, can decide on an economic frequency
of sampling. Generally undisturbed soil samples should be taken at 1.5 m intervals and at change of stratum level and disturbed samples taken at 1 m intervals. This is not a rigid rule and should be varied to suit soil and foundation conditions. When trial pits have not been excavated, these intervals should be halved from ground level to 2–3 m below the anticipated depth of foundation excavation. It is at or near ground level that the soil is usually most variable due to exposure to weather, change in moisture conditions and
variations in the water-table level.

The foreman driller should keep a log noting the type (classification) of soil, its depth, change of stratum level,
position of obstructions, changes of soil conditions within a stratum, groundwater level, seepage and similar information. Experienced and reliable foremen drillers are becoming, unfortunately, rarer and it is essential that the soil survey investigator backs up the foreman’s observations by adequate inspection visits by site supervision engineers.

The log should give a continuous description of the soil in the borehole from ground level to base of bore. It is important that the foreman is aware of the standard classification and description used in References 3 and 4 and does not solely employ (the often colourful) local terms such as cowbelly, sludge, mucky clay, cobbly clay. While these terms may be well-known to local engineers they can be unfamiliar and totally misleading to others. The local terms are often an invaluable guide to experienced local engineers in describing the soil and its properties and it would be a pity in some cases if these were to die out. Where there is a mixture of clay, silt and sand the MIT (Massachusetts Institute of Technology) classification should be used (see Fig. 3.3).

Fig. 3.3 The MIT classification for clay, silt and sand.

9 Appointment of specialist soil investigator  Most design offices do not have sufficient demand for soil
investigations to warrant the capital costs of obtaining site and laboratory equipment, nor the current costs of employing site and laboratory personnel. It is therefore generally necessary to appoint specialist firms – and this may not always be as easy as it might appear.

The work should be carried out by competent soil survey specialists of good reputation, staffed by experienced engineers (and drillers) who will not only supervise the borings but also the testing and can be relied upon to report accurately and advise soundly on their findings. The specialist firm should carry adequate indemnity. In the past a number of excellent firms have been driven out of business by cut-throat competition from ‘cowboy’ firms savagely undercutting sensible rates. This is a deplorable situation which could cost the client, in the end, far more than has been  saved by employing such firms. (On more than one occasion the authors’ practice has been asked to investigate foundation failures and found that borehole logs are a  complete fabrication – because they were not done!)

There should be detailed discussion between the design engineer and the soil specialist on the survey specification, cost and time. Soil specialists may not have wide experience of foundation design, behaviour of structures, economics of alternative designs, construction difficulties, etc., so the discussion is essential for reliable investigations.

It is also strongly advisable for the design engineer in  person to inspect the boring during progress to see firsthand the condition of the soil samples and sampling methods.

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