9 Supervision, testing and monitoring (p. II)

9.4 Testing on the constructed elements

9.4.1 Testing to assess geometry Visual inspection and direct measurement are the most effective ways of assessing the dimensions. This requires large excavations preferably down to the full depth of the element and therefore is rarely feasible on elements to be incorporated in the work. Wherever visual inspection is not possible, information about the cross-sectional dimensions of an element can be obtained from coring or drilling with measurement of drilling rate, inclined to the element axis. The length of an element can be detected by coring or drilling or penetration testing parallel to its axis. This is increasingly difficult with slenderness, and is practically excluded where the ratio of length to diameter exceeds 15. When coring is performed, the inclination of the coring axis should be measured, and the location and inclination of the jet grouted element axis should previously have been determined. Coring shall be performed only after sufficient hardening time is elapsed. The method of coring, the equipment used and the size of the cores shall be such as to ensure that representative samples are obtained. Special precautions are necessary when coring jet grouted elements formed in clayey/silty soils or in heterogeneous soils (containing cobbles for instance) or if the jet grouted material is of low strength.

9.4.2 Mechanical tests Where in situ tests are used to measure the mechanical characteristics of the jet grouted material (penetrometer, pressuremeter or any other test requiring to drill through), the position of the measuring tool shall be defined referring to the element geometry and lay-out. Compression tests shall be performed on samples with height to depth ratio of 2,0. Where relevant, the compressive strength of the jet grouted structures should be assessed by testing four samples taken from the structure for each 1 000 m3 of its volume, if not otherwise specified by the design. Where mechanical properties are obtained from tests on cores, care should be taken to allow for the effects of sampling, trimming and test procedures. The tendency of the strength and modulus to increase with time is strongly dependent on the soil type, with longer development for higher fines content. Brazilian traction tests1) and shear tests can also be performed on cores if required by the specific application. In situ loading tests of the jet grouted elements can be appropriate where they are to be used as deep foundations. Mechanical tests shall be performed at appropriate times after set of the jet grouted material, taking into account both the construction requirements and the influence of the nature of the soil on the hardening time of the jet grouted material. Samples taken for mechanical tests should be stored under controlled moisture and temperature conditions.

9.4.3 Permeability tests The overall watertightness of jet grouted structures should be assessed by pumping tests and/or piezometric readings. The overall watertightness of a jet grouted structure around and/or below an excavation in geotechnical categories 2 and 3 shall be assessed by pumping tests and piezometric readings before any excavation below the original ground water level is undertaken. permeability of the elements can be measured by borehole water tests.

9.5 Monitoring

9.5.1 Monitoring of the jet grouting parameters during the performance of jet grouting works is essential for the quality control of the results.

9.5.2 For applications in geotechnical categories 2 and 3, the following parameters shall be continuously recorded for all elements, except for short periods during unavoidable equipment faults:

  • — pressures and flows of the fluids;
  • — translation and rotation speed of the monitor.

9.5.3 When jet grouting works have to be performed in situations where there is a high risk of unacceptable deformations of adjacent structures, monitoring and alarm systems shall be employed.

9.5.4 For underpinning applications the buildings to be underpinned should be monitored by repeated levelling or by automatic settlement sensors.

EN 12716:2001 Execution of special geotechnical works — Jet grouting