Placing the pressuremeter probe in the ground
C.1 General considerations
Pressuremeter testing and the production of the pressuremeter series of pockets shall be considered together. The quality of each pocket wall governs the quality of each test. In order to place the probe in the ground and to obtain valid Ménard pressuremeter parameters the pressuremeter pocket formation techniques shall be adapted as listed below to the type of soil (see Table C.2). When the soil conditions are unknown, equipment for several different techniques shall be brought to the site so as to cover unexpected cases.
If any placing technique other than those listed in C.2 or C.3 is used, the operating organization shall be able to demonstrate upon request that the technique yields pressuremeter results of satisfactory quality (see D.2.2).
C.1.1 Spacing between tests and the minimum depth of probe in the ground
In any pressuremeter sounding, the minimum spacing between two successive tests shall not be less than 0,75 m. The spacing between the locations of the central section of the probe for two successive tests should be 1 m.
The minimum depth zc below the ground level for a test in a pressuremeter sounding shall be 0,75 m
The probe shall be placed in the pocket so that the top of its expanding length is at least 0,5 m from the pocket entry.
If the pressuremeter pocket is created from the bottom of a larger borehole no test shall take place at a depth of the central cell less than 0,75 m below the conventional borehole foot.
The bottom of the expanding length of the probe shall also not be closer than 0,3 m from the bottom of the pocket.
C.1.2 Maximum length of the drilling stage before placing pressuremeter probe
When the pocket is obtained by drilling, the pressuremeter probe shall be installed in the pocket as soon as possible after drilling has been completed (see C.1.3). Drilling or driving should be advanced between every test. However, drilling or driving stages enough for several tests may be allowed if ground conditions and time permit, as shown in Table C.1.
|Soil type||Maximum continuous drilling or tube driving stage length (m)|
|Adapted rotary drillingb||Rotary percussive drillingb||Smooth tube pushing, driving and vibrodrivingc|
|Sludge and soft clay, soft clayey soil||1a||—||1a|
|Firm clayey soils||2||2||3|
|Stiff clayey soils||5||4||4|
— above ground water table
— below water table
|Loose sandy soils:
— above ground water table
— below water table
|Medium dense and dense sandy soils||5||5||4|
|Coarse soils: gravels, cobbles||3||5||3|
|Coarse soils with cohesion||4||5||3|
|Loose non-homogeneous soils, other soils not specified above (e.g. tills, etc.)||2||3||2|
|Weathered rock, weak rock||4||5||3|
|a Or the required interval between two successive tests.
b Refer to Table C.2 for acceptable techniques.
c Not applicable to STDTM technique (see C.2.6.3).
C.1.3 Time between forming the test pocket and testing
When pockets are obtained through drilling or tube pushing, pressuremeter testing shall be carried out immediately after the test pockets have been drilled and during the same working shift.
When the pressuremeter probe is directly driven or pushed into the ground inside a slotted tube, two ways to carry out the tests are permitted:
- after stopping driving at each depth of test, or
- after completing the driving or pushing, by lifting the string of casing or rods between tests.
NOTE 1 The first way implies that some delay will possibly be required between the end of driving or pushing and the start of the test to ensure equilibration of the pore water pressure.
NOTE 2 The second way is only possible if the casing string diameter is the same as that of the slotted tube. This technique helps equilibration of the pore water pressure for the upper tests without further delay.
C.2 Probe placement techniques without soil displacement (p. I)
When a test pocket is drilled, the primary concern shall be the quality of the pocket wall obtained. The second concern is that this pocket diameter shall be adapted to the pressuremeter probe diameter. For any requirement apart from soil sampling techniques, soil sampler features and borehole diameters, reference to ISO 22475-1 is mandatory
The guidelines given in Table C.2 shall be considered when selecting the proper method and the appropriate equipment.
When selecting the method and equipment, it shall be considered that the wall of the test pocket shall be as smooth as possible and that its diameter shall be as constant as possible over the length of the test pocket.
NOTE If the test pocket diameter varies significantly, because of ravelling for example, or if the pocket is not cylindrical, the quality of the test will be impaired.
C.2.2 Cutting tool diameter for the pocket
When determining the diameter of the necessary cutting tool for a bored test pocket, three factors shall be considered:
- the diameter of the pocket required;
- the over cutting of the pocket resulting from either wobble of the cutting tool or wall erosion by the mud circulation or both;
- the inward yielding that occurs between the removal of the cutting tool and the probe placement. Inward yielding or swelling can be reduced by the use of an appropriate drilling fluid.
When selecting equipment for the site, several bits of various sizes should be available so as to adjust the size of the tool depending on whether over cutting or inward yielding occurs.
One of the following techniques may be used to prepare the test cavity for the pressuremeter probe, depending on the type of ground (see Table C.2)
C.2.3 Rotary drilling (OHD)
C.2.3.1 Open hole drilling
Rotary open hole drilling consists in rotating a cutting bit, applying a downward force from the ground surface with a drill rig and washing the resulting cuttings to the surface with a flow of fluid.
The selected bits should be drag bits or rock roller bits with specially designed axial bottom discharge nozzles.
Above water table, a hand auger may be used to drill the test pocket. It consists of two tubular steel segments with a cutting edge, or auger blades, welded at the top to a common rod to form a nearly complete tube, but with diametrically opposed longitudinal slots. The auger blades are connected at the bottom by a helical point or tapered screw. The blades also block the escape of the contained soil. A handle is fitted to the top extension rod.
Hand augering (HA) gives good results in soft and medium stiff soil.
NOTE Depending on the stiffness and the grading of the soil, the use of a hand auger can become difficult. The pocket walls can be damaged by too many removals of the cutting tool. This technique is used for testing at shallow depths (4 m to 6 m).
Conversely, a hand auger with axial bottom discharge of slurry (HAM for "hand auger with drilling mud") may be used to stabilize the pressuremeter pocket wall too, It may be used in clays exhibiting limit pressures lower than 0,5 MPa, as long as:
- the auger blades are very sharp, and
- the auger diameter is slightly larger than the pressuremeter probe diameter, but still smaller than 1,08 times the probe diameter.
Care shall be taken that the auger does not simply displace very soft soil.
C.2.3.2 Advancement specifications
The rotating drill bit shall be advanced into the soil while satisfying the following conditions:
- low vertical pressure on the drilling tool, slow rotation (less than 60 rotations per minute) and
- low and controlled drilling fluid flow appropriate for the material being drilled.
The drilling fluid shall cause the minimum damage to the pressuremeter pocket wall. The fluid should have a viscosity high enough to remove the cuttings at low pumping rates.