7 Design considerations
7.1.1 Jet grouting can be applied in either temporary or permanent works for different purposes. For example:
- — providing foundations for structures to be erected (Fig.5 a);
- — underpinning existing foundations (Fig.5 b);
- — creating low permeability barriers;
- — creating retaining or supporting structures;
- — complementing other geotechnical works;
- — reinforcing a soil mass.
7.1.2 The design of jet grouting works should specify that borehole collars are to be located above the water table.
7.1.3 Wherever the borehole collars are located below the water table or artesian head, special measures shall be taken into account, in order to avoid piping through the boreholes.
7.1.4 The purpose of jet grouting works shall be clearly defined in the design. The specified physical properties and geometry of jet grouted elements or structures shall be appropriate for this purpose.
7.1.5 The technical conditions which may influence the working sequence of the elements shall be specified.
7.1.6 When relevant, the detailed execution sequence should be shown on the execution drawings.
7.1.7 If the design of the jet grouting works cannot be completed due to deficient site investigation, and it is not possible to obtain the missing information by further soil investigation, an appropriate preliminary field test shall be specified.
7.1.8 Where no comparable previous experience is available representative preliminary field test shall be carried out, using the equipment, materials and techniques proposed for the main jet grouting works.
7.1.9 To ensure a consistent result in the preliminary in-situ trial, at least one jet grouted element should be constructed with a given working procedure. In significantly heterogeneous soil conditions more trial elements and different working procedures should be necessary for approval.
7.1.10 Where the jet grouted material is to be subjected to onerous conditions, such as high stresses or resistance to aggressive environments, the preliminary in-situ tests should be preceded by specified laboratory tests on samples of the soil to be treated and the grout, mixed in proportions encompassing the working range envisaged.
7.1.11 For mechanical tests on jet grouted material the conditions for testing the samples and the criteria for acceptance shall be specified.
7.1.12 Tolerances on specified performance parameters should take into account the accuracy of the proposed test methods, especially when these methods are indirect, as quoted in Annex C.
7.1.13 Whenever acceptance is defined on the basis of tests on core samples, the criteria for positioning and timing of the proposed corings in the jet grouted structure shall be specified in the design.
7.1.14 Design parameters should take account of construction difficulties which may reduce the effectiveness of the jet grouting.
7.1.15 Sequence and rate of execution, setting and hardening time, and diameter of columns shall be considered in order to avoid local soil failures or unacceptable settlement or uplift.
7.1.16 The design should define the acceptable limits of settlement, heave, distortion of structures and services likely to be affected by the jet grouted works.
7.1.17 The design of jet grouting works shall comply with Annex A.
7.2.1 The cross-sectional dimensions of a jet grouted element depend not only on the jet grouting system selected and on the parameters employed, but also on the soil type and its heterogeneity.
7.2.2 The design shall take into account the construction tolerances indicated in 8.4.3, 8.4.4, and 8.5.6, or shall propose different values for them.
7.2.3 The following shall be clearly shown on design drawings:
- — minimum cross sectional dimensions of jet grouted elements to be achieved in the different soil layers that will be encountered;
- — tolerances on the position and inclination of the element axes (see 8.4.3 and 8.4.4).
7.2.4 Maximum cross sectional dimensions should also be considered and related to the element spacing to ensure the integrity of a jet grouted structure.
7.2.5 If any identified underground obstruction cannot be removed, the relevant area shall be marked on the design drawings, and the adjacent jet grouting treatment shall be designed so as to avoid unacceptable damage.
7.3 Strength and Deformation Characteristics
7.3.1 The strength of a jet grouted material depends both on the jet grouting system selected and on the parameters employed, and also on the soil type and its heterogeneity.
7.3.2 In underpinning applications stability and deformation in the temporary condition, prior to set, of the jet grouted columns beneath foundations shall be taken into account.
7.3.3 Where deformations of jet grouted elements or of jet grouted structures are an important concern, the design shall indicate the parameters to be measured in specified acceptance tests, the range of their values and the relevant mean values (mean on the element, on larger parts of the structure, on the whole structure).
7.3.4 The required statistical minimum strength of the mass should be established at the design stage, taking into consideration the variability of the soil conditions.
7.3.5 In many cases a practical degree of scatter will be unable to be set due to limited previous experience. In these circumstances a scatter can only be identified by on site tests.
7.3.6 Where indirect testing is to be used for checking the results the acceptance criteria shall be expressed in the design in terms of the actual parameters to be measured.
7.3.7 The jet grouted columns can be reinforced by high strength elements (bars, pipes, beams), installed during or after jet grouting operations.
7.4.1 Where jet grouting works are employed to control groundwater, the results depend on the geometric accuracy of the elements and on the permeability of the jet grouted material.
7.4.2 Permeability requirements shall be expressed in the design in terms of limit values of parameters actually measured in specified acceptance tests.
The design shall specify the conditions of measurement of the jet grouted material permeability, and of the overall jet grouted structure.
7.4.3 The overall permeability of a jet grouted structure should be evaluated taking into account the effects of deformation due to excavation or to loading conditions at full scale.