Contrastación de hipótesis

There are numerous techniques available to physically investigate a site. It is rarely obvious at the start of an investigation which methods will be suitable to gather the information needed by the project designers. It is preferable, therefore, that all but the smallest ground investigations be carried out in phases. The first phase may consist of an initial pattern of boreholes and testing across the whole site. Preliminary results from the first phase will allow second and subsequent stages to be designed; such stages may include more closely spaced boreholes in areas where ground conditions are unclear, or perhaps specialist testing such as a pumping test.

Ground investigation usually involves the use of some combination of the following methods: boring, drilling, probing and trial pitting; in situ testing;

geophysics; and laboratory testing. These ground investigation methods are briefly outlined below, mainly in relation to British practice as outlined in BS5930 (1999) and BS1377 (1990). Factors particularly relevant to investi-gations for dewatering projects are highlighted.

In Britain the most common form of boring is light cable percussion drilling, colloquially known as ‘shell and auger’ drilling (Fig. 6.1). Soil sam-ples may be recovered from the borehole (for description of soil type), or cer-tain types of in situ test may be performed within the borehole. A key point to note is that at various stages during drilling water may be added to the borehole by the driller, or may be removed by the action of the boring tools.

This can lead to natural groundwater levels and inflows being masked dur-ing bordur-ing operations.

Rotary drilling (using a water or air-based flush medium with polymer or foam additives) is also widely used (Binns 1998), particularly in relatively intact rock strata, but also in uncemented drift deposits. Core samples can be recovered from the boreholes (for soil and rock description), and certain types of in situ tests carried out. Because the borehole is generally kept full of the flush medium, groundwater levels and inflows can be difficult to determine during drilling.

The older technique of wash boring is rarely used in Europe but is still employed in countries where labour is cheap. The basic rig is a winch tripod.

The associated equipment consists of an outer pipe with a chisel bit at the lower end and a swivel head at the upper end of the wash pipe, and incor-porating a water pressure hose connection with a weight for driving the cas-ing into the ground. A pump passes water down the wash pipe to slurify the soil at the bottom of the outer casing. The return washings are not regarded as reliable for identification of soil types – though recordings of wash-water colour changes should be noted. This method is suitable for use in sands and silts, but progress in clayey soils is likely to be slow. Groundwater levels and inflows are masked in a similar way to rotary drilling.

Hollow stem continuous flight augers are suitable for use in cohesive soils but are of limited use in water-bearing granular soils; indeed, in granular soils this technique is often unworkable. The drilling spoil brought to ground surface gives only an approximate indication of soil types and hori-zons. Drive-in samplers can be inserted through the hollow stem to obtain strata samples at convenient depth intervals.

As an alternative to boring or drilling, in recent years probing methods have been developed. A wide range of equipment exists and is used, but all have the objective of determining a profile of penetration resistance with depth. Most methods were developed as a low-cost and rapid alternative to drilling and boring. Two of the most commonly used methods are dynamic probing and static probing by the ‘cone penetration test’ (commonly known as the CPT).

Figure 6.1 Light cable percussion boring rig (courtesy of WJ Groundwater Limited).

Dynamic probing involves a percussive action to drive the probe into the ground, producing output in the form of blows per unit depth of penetration (see, e.g. Card and Roche 1989). Window sampling is a variant on the dynamic probing method that allows soil samples to be obtained via sam-pling tubes driven into the ground.

Static probing by CPT (also known as ‘Dutch cone’ testing after the coun-try where the method was developed) is more sophisticated than dynamic probing. The cone is pushed continuously into the ground, using reaction from the test truck, producing an output of resistance against depth (see Meigh 1987; Lunne et al. 1997). Piezocone testing is a variant of the CPT method, where pore water pressures are measured in addition to resistance parameters; this can allow estimates of permeability to be obtained in low per-meability soils.

Trial pitting is a simple and widely used method for investigation of shal-low strata. A pit is dug, exposing the sub-soil for inspection and sampling.

Groundwater inflows and seepages can normally be clearly identified. During excavation it may be possible to form an opinion as to appropriate methods of full-scale excavation, if relevant.

Trial pits are normally dug by mechanical excavator (Fig. 6.2). Small backhoe loaders can normally excavate to a depth of around 3 m, and larger excavators may be able to work to a maximum depth of around 5 m.

Figure 6.2 Trial pitting using a mechanical excavator.

Trial pitting is a potentially hazardous exercise. Trial pits of greater than 1.2 m depth should only be entered if adequately shored and supported.

Even pits of less than 1.2 m depth may be unstable. Each pit should be assessed before entry, and if any doubt exists the pit should not be entered.

Soils can be described from the surface, and samples taken from the spoil in the excavator bucket. In difficult locations or where ground disturbance must be kept to a minimum, it may be possible to excavate pits by hand excavation. However, this method is very slow, and such excavations must not be taken deeper than 1.2 m without employing timbering or some form of proprietary side support system. Safety in pits and trenches is discussed by Irvine and Smith (1992).

Further details on all these methods can be found in Clayton et al. (1995).

Their relative merits are outlined in Table 6.1. The ground investigation stage Table 6.1 Advantages and disadvantages of methods of boring, drilling, probing and trial

pitting

Method Advantages Disadvantages

Drilling Suitable for a wide range of soils Progress can be difficult if cobbles or and boring Allows soil and groundwater boulders are present

samples to be obtained Some methods require specialist Can allow in situ permeability equipment

tests to be carried out Allows observation wells

(standpipes and standpipe piezometers) to be installed

Probing Provides information on soil Does not provide soil or groundwater

profile samples

Piezocone can provide Some methods require specialist information on soil permeability equipment

Can allow simple standpipe

observation wells to be installed Needs to be used in conjunction with Some methods allow soil boreholes to enable correlation of

samples to be obtained soil types

Penetration depth is limited in stiff materials and coarse granular soils Does not allow installation of

stand-pipe piezometers

Trial pitting Suitable for a wide range of soils, Depth limited to around 5 m including very coarse soils May be difficult to progress below Allows stability and ease of groundwater level in unstable soils

excavation of soils to be directly Does not allow installation of

observed standpipe piezometers

also includes the installation and monitoring of groundwater observation wells; this is discussed further in Section 6.5.

Various methods of in situ testing can be carried out as part of boring, drilling, probing and trial pitting. The most relevant of these to groundwa-ter lowering works are permeability tests; these are described in Section 6.6.

Geophysical methods are sometimes used in investigations for civil engineering works, but these methods are much more widely used in the oil, mineral extraction and water resource fields. In general, geophysical methods are used to provide information on changes in particular properties of strata beneath a site, and can be used to provide information between widely spaced boreholes. The use of boreholes in combination with geophysics is important, because the borehole data can be used to ‘correlate’ or ‘calibrate’

the geophysical results for the site in question. Geophysical methods used for civil engineering investigations are described in Clayton et al. (1995), Chapter 4 and McCann et al. (1997). Geophysical methods used in hydro-geological and water resource investigations are described by Barker (1986) and Beesley (1986). On a number of occasions when geophysics has been used in investigations the results have been perceived to be disappointing or inconclusive. This is probably a reflection on an inappropriate choice and specification of method, rather than a systematic drawback with the use of geophysics. To get the most out of geophysical surveying, it is essential that engineering geophysicists are involved at an early stage of planning and thereafter; otherwise the method will not achieve its potential.

Samples of soil or rock obtained from boreholes or trial pits may be tested in the laboratory. The purpose of testing can be as an aid to soil description and classification, or can be to determine soil properties for engineering design. Properties routinely tested for include strength, compressibility, per-meability (see Section 6.6) and chemical characteristics. Samples of ground-water recovered during investigation may also be chemically tested in the laboratory.

6.3.3 Reporting

To be useful to the designers and managers of the project, the site investiga-tion must be reported in an organized, concise and intelligible manner.

Ideally, reporting should be carried out by geotechnical specialists who have been involved with the investigation since its inception. The minimum reporting requirement is for a ‘factual report’ which presents the data gath-ered during the desk study and site reconnaissance, and the borehole logs, trial pits logs, test results and groundwater monitoring data from the ground investigation. Such reports do not usually comment on the implications of the data gathered.

In many cases, particularly on large or complex projects, an ‘interpreta-tive report’ is produced in addition to the factual report. Again, written by

geotechnical specialists, this should review the ground and groundwater conditions at a site. It should include discussion of the effect of the antici-pated conditions on the proposed design and construction methods. At the time the interpretative report is produced the project design may not be finalized, but the report should discuss the geotechnical aspects of the full range of design options current at that stage. If particular potential prob-lems in design and construction are highlighted, one of the report’s conclu-sions may be to recommend further or specialist investigations.