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Published By Lankelma
Lankelma is the foremost contractor for onshore in-situ soil testing in the UK. An acknowledged
specialist in CPT, Lankelma also offers a worldwide consultancy and training service.
A.P. van den Berg develops, designs and manufactures geotechnical and environmental soil
investigation equipment for onshore and offshore applications. Specialists in CPT systems and equipment.
Gardline
Gardline Geosciences offers worldwide marine geotechnics, in-house consutancy and services with marine
investigations ranging from nearshore to full ocean depth (down to 3000m).
About the Author
Hans Brouwer studied civil engineering at Delft University in The Netherlands. He has
worked as a part-time lecturer at Amsterdam Polytechnic and was senior partner in a structural
engineering consultancy. He has written a standard textbook in Dutch about the design of
building foundations. He now lives in England where he writes technical textbooks in
English, hopefully to reach a bigger readership.
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Chapter 7
Onshore testing
Fieldwork
| FIELDWORK 7.2 Before the cone penetration tests start, the client should define (on paper) the aim of the tests. Are the tests for stratification, pile design or settlement prediction of soft layers etc? A clear strategy for executing the tests should be determined in discussion with the contractor. Existing data from boreholes or geotechnical maps should be made available. The locations of the test should be marked, and the coordinates and altitude of all the test locations should be recorded. If possible, the groundwater levels should be measured beforehand. The day rate for a thrust machine exceeds £1000; it is therefore very important to undertake all the necessary preparations prior to its arrival on site to ensure maximum production time. General preparations |
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| Accessibility of test location (by road/through fields? locked farm gates? has permission of the site owner been obtained? etc) | if in doubt, ask for a site visit before cone penetration company's staff arrive |
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| Is the test location clearly marked, numbered and the co-ordinates taken? | CPT number: Co-ordinates: x = y = z = Grid reference: |
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| Level of groundwater | Metres BGL: |
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| Stratum from borehole: (if available) Anticipated termination of CPT: | Layer Depth Soil type Unit weight 1 2 3 |
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| Location of borehole | No Co-ordinates x = y = z = Grid reference: |
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| CPT tests To set up tests the following steps are taken: |
| 1 | Position rig over test location |
| 2 | Level thrust machine using the hydraulic jacks |
| 3 | Load the computer with test number, client's name etc |
| 4 | Enter on the computer if there is pre-drilling |
| 5 | Connect cone penetrometer to first thrust rod |
| 6 | Lower the penetrometer by means of the hydraulic rams to just above ground level |
| 7 | Check the zero readings for loads, inclination and depth |
| 8 | Commence the test attaining a rate of 20 mm/sec |
| 9 | Stop thrust every metre to connect the next rod |
| 10 | Finish test at scheduled depth or refusal (inclination, tip resistance, sleeve friction too high, or total force too much due to hard rock) |
| 11 | In case of cobbles or boulders resulting in a refusal, move rig 1m and record new position, before repeating the test |
| 12 | Remove the penetrometer, disconnecting and restacking the rods |
| 13 | Check the zero readings just above ground level |
| 14 | Save test data in computer |
| 15 | Enter test details and results on day report |
| 16 | Remove rig |
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| Dissipation tests
- The purpose of a dissipation test is to evaluate the factor of
consolidation (settlement). To do this, the level of the groundwater table must be known - Dissipation tests are only useful in clays. It is therefore necessary to
research the stratum, first, by using a CPT or CPTU - When only one dissipation test will be made at a location, the best
position to do the test is vertically midway in the clay layer - In soft or loose soils, pre-drilling should be taken down to the
groundwater table. The pre-drilled hole should be filled to ground level with water if the pore pressure is to be measured using a watersaturated system. If the groundwater table is located at great depth, the pore pressure system should be saturated with glycerine or silicone oil - The choice of the filter location is between U1 and U2. A filter at U3 is
used only in combination with U2. Both filter locations U1 and U2 have advantages and disadvantages: ♦ U1 gives larger positive porewater pressure, even in heavily over- consolidated clays, but the filter is more prone to damage and wear ♦ with U2, the filter is much less prone to damage, but in heavily over-consolidated clays, very low (or even negative) porewater pressure may be produced - At present the location of the filter is not standardised. The ISSMGE
reference test procedures[1] refer to the location behind the cone, U2, as the preferred location - During the dissipation test the push rods can be locked or unlocked
(in the push/pull clamp). The ISSMGE reference test procedures do not give a preferred procedure.
For t50, the test can be stopped when the pore pressure is dissipated until a pressure ut = 0.5(ui + u0) is obtained. Where u0 is the groundwater pressure and ui is the pore pressure at the start of the test. Where u0 is unknown, it is safe to stop at 0.5 ui. Figure 65 shows typical test results for U1 and U2. Since procedure (locked or not locked) and location of filters U1 and U2 influence the test results, it is important that the client gives written instructions.
Figure 66 shows an example of how to calculate and where to stop the dissipation test for t50.
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| | Figure 65 Typical dissipation curves
Figure 66 Dissipation test (t50) ui = initial pore water pressure U1 or U2 uo = in-situ porewater pressure before penetration When uo is unknown, test should be stopped at 0.5ui Example ui = 250 kPa uo = 50kPa (5m of water) Test to be stopped at 0.5 (250 + 50)= 150kPa or at 0.5 x 250 = 125 kPa if uo is unknown |
| Maintenance |
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| Checking routine | Start of project | Start of test | End of test | Every 3rd month |
| Verticality of thrust machine | | x | | |
| Penetration rate | | x | | |
| Safety functions | x | | | x |
| Push rods | x | x | | |
| Wear | x | x | x | |
| Gaps and seals | x | x | x | |
| Filter | | x | | |
| Zero drift | | x | x | |
| Calibration | x | | | x |
| Function control | x | | | x |
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| Performance | |
| CPT and CPTU | 100-200 m/day for CPT |
| Dissipation test, t50 | 0.5-1.0 hours, depending on permeability |
| Mostap soil sampling | 0.5-1.0 hours per metre-sample, depending on depth |
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