Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 3 IN-SITU GEOTECHNICAL TESTING
C N . = Depth Correction Factor - (Should not be greater than 1.7)
= (1/σ’ vo ) 0.5 - (Liao and Whitman 1986)
= 2.2/ (1.2 + σ’ vo / Pa) - (Kayen et. Al, 1992)
σ’ vo = Effective overburden pressure (γt - γw).z in tons / sq ft
Pa = 1 tons/sq. ft (95KN/m2)
C E = Correction Factor for Energy Ratio of 60%. = ER /60
ER = Energy Ratio for drill rigs (Table below)
Country Hammer Releases ER (%)
USA Safety 2 turns of Rope 55
Donut 2 turns of Rope 45
Japan Donut Tombi 78 -85
Donut 2 turns of Rope 65 – 67
China Automatic Trip 60
Donut Manual 55
UK Automatic Trip 73
Additional correction has been proposed by (Skempton, 1986, Robertson and Wride, 1998) for
hammer type (donut and safety), borehole diameter rod lengths and sampler.
3.2 CONE PENETRATION TEST (CPT)
The cone penetration test is quickly becoming the most popular type of in-situ test because it is fast,
economical, and provides continuous profiling of geostratigraphy and soil properties evaluation.
The CPT can be used in very soft clays to dense sands, yet is not particularly appropriate for gravels
or rocky terrain. The pros and cons are listed in Table 3.2 below. As the test provides more accurate
and reliable numbers for analysis, yet no soil sampling, it provides an excellent complement to the
more conventional soil test boring with SPT measurements.
Table 3.2 Comparison between Advantages and Disadvantages in CPT
Advantages
Fast and continuos profiling
Economical and productive
Results not operator-dependent
Strong theoretical basis in interpretation
Particularly suitable for soft soils
Disadvantages
High capital investment
Requires skilled operator to run
Electronic drift, noise and calibration
No soil samples are obtained
Unsuitable for gravel or boulder deposits
except where special rigs are provided and /
or additional drilling support is available.
Samples of various cone penetrometers are illustrated in Figs. 3.4, 3.5 and 3.6.
March 2009 3-5