This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
GROUND IMPROVEMENT
SITE INVESTIGATON
November 2009
Lankelma ltd Cold Harbour Barn Cold Harbour Lane Iden, East Sussex
Biotechnical Stabilisation – brush matting; bush layering
Geosynthetics – geotextiles; geogrids and geomeshes.
A number of these techniques readily lend themselves to investigation and
compliance testing with the CPT.
2. Role of the CPT
For non-cohesive sands and silty sands requiring densification by the techniques outlined above, the CPT has been found to be one of the best methods to monitor and document the effect of densification due to the continuous and repeatable nature of the CPT process and data (see figure 1).
For shallow compaction the CPT can also be useful in checking the variability of a fill compacted in layers, or in checking whether unsatisfactory material has been left below a fill.
Figure 1
● before compaction
o after compaction
qc (MPa)
0
1
2
3
4
5
6
7
8
9
Depth
(m
)
4
In the improvement of cohesive soils by means of surcharge, with or without
vertical drains, the primary task is the monitoring of the rate of dissipation of
generated excess pore water pressures, as well as the assessment of general
variations of hydraulic conductivity of the soil. For these activities a cone
penetrometer with an additional pressure transducer (piezocone) is required.
A selection of guidelines to the zone of soil behaviour where vibrocompaction
techniques are most applicable are given on the CPT soil behaviour charts
presented in appendix A.
3. Compliance Testing
The required effect of any deep compaction technique can be set directly in terms
of measured cone resistance, or in terms of “equivalent relative density”.
Other analytical approaches that utilise the CPT platform for compliance testing of
deep compaction projects include the full displacement push pressuremeter and
the seismic cone.
The shear wave velocity obtained from the seismic cone, like the pressuremeter,
can be directly related to the small strain shear modulus (Go), and is therefore a
direct measure of the soil stiffness. Hence, a compaction specification of
compliance criterion could also include a minimum normalized shear wave
velocity, or the pressuremeter limit pressure.
A further advantage in earthquake prone geographic regions, is that the shear
wave velocity can be used as an additional measure of liquefaction potential,
especially in silty sands.
5
4. Settlement of Deep Compaction Fills
The settlement analysis is fundamental to the design of most compaction applications. The analysis requires a knowledge of the soil compressibility, that is, the soil modulus and preconsolidation stress. Since the Factor of Safety against bearing capacity failure is usually high for foundations on coarse grained soil, the designer is interested in a modulus, E25, for an average applied stress limited to a value equal to about 25% of the estimated ultimate bearing resistance. The modulus can be obtained directly from the seismic cone shear wave velocity, or the full displacement push pressuremeter, or indirectly from the average cone tip resistance as follows:
E25 = α qt where E25 = secant modulus for a stress equal to about 25% of the ultimate stress. α = an empirical coefficient qt = cone resistance
A simple approach promoted by the Canadian Foundation Engineering Manual (CFEM 1992) states that the ratio between E25 and qt is a function of both soil type and compactness and is presented on Table 1. Table 1.
α = E25 / qt from static cone penetration tests.
The above values of E25 apply to a settlement analysis that can be
assumed to behave as linearly elastic media.
Soil Type α = E25 / qt
silt and sand 1.5
compact sand 2.0
dense sand 3.0
sand and gravel 4.0
6
5. CPT Plant
A variety of CPT units are available for deep compaction verification and
compliance testing purposes. Plant type selection should assess whether
wheeled or track mounted units are appropriate for the surface traffic conditions.
Appendix A
Guideline for soils suitable for vibrocompaction techniques
Soil classification for deep compaction based on the Eslami-Fellenius chart
Soil classification for deep compaction based on CPT data
Appendix B
Influence of compressibility on normally consolidated, uncemented, unaged
predominantly quartz sands (after Jamiolkowski et al 1985)
Appendix C
All Lankelma CPT units can deploy the following devices:
Cone penetration testing
Friction cone - cone tip and friction sleeve resistance
Piezocone - cone tip and friction sleeve resistances and porewater pressure
Seismic cone - as the piezocone, but with the measurement of shear wave velocity and hence the small strain shear modulus, Gmax
Soil moisture probe - as the piezocone, but with the additional in-situ water content, temperature and soil conductivity measurement
Environmental probes - a variety of probes are available including fuel fluorescence detection, resistivity, conductivity and temperature
Shear vane equipment - Lankelma can deploy a Geonor penetration shear vane for the assessment of in-situ undrained shear strength
Push full displacement pressuremeter - to assess in-situ soil stiffness