AS 8700:2011 – Execution of prefabricated vertical drains.
The scope of the application of vertical drainage is to handle and solve problems associated
with the following aspects of geotechnical engineering:
(a) Consolidation settlement in low-permeability soils (resulting from surface loading or groundwater lowering).
(b) Stability (of structures and embankrnents).
Other applications of vertical drainage include treating soils exposed to the effects of dynamic and cyclic loading (e.g. in seismic regions or under rail tracks), as a result. he impacts of which can be reduced as well as the effects of vibrations on structures reduced. Vertical drainage may also be used for reniediation of contaminated ground and for mitigation of liquefaction potential.
Vertical drainage design encompasses two phases, as follows:
(i) Functional dcsign Functional design is the first phase and the need for vertical drainage has to he quantified. This phase defines the loading and drain spacing necessary to produce the desired effects on rate of consolidation and settlements. Treatment of soft soils with vertical drainage followed by prcloading provides ground improvement by accelerated consolidation resulting in gain in the undrained shear strength of the soil. It results in a reduction, post-development, of in-service settlements under design loads and also in creating satisfactory drainage paths for pore waler in the case of liquefaction.
(ii) Pro’ss design Is the second phase, the method of drain installation and its functioning in practice has to be designed. This phase accounts for effects of drain installation on the ground, geometry, the nature and the dimensions of the drains and for effects such as possible buckling in case of excessive strains in some soil layers.
Vertical drainage may be used for different purposes as outlined in Clause 3.1; however, the process of designing vertical drainage generally follow’s the operations listed in Figure 3.1. The objective (design basis) and the ground properties (first row ol’boxes) interact with the settlement and stability analyses to satisfy the requirements put on the targeted outcome from the effect of the drains: that is to reach a given degree of global and”or local consolidation within a specific period of time.
Ground treatment by vertical drainage and the associated subsequent preloading shall be designed and executed in such a manner that the structure, embankment or paved area supported by the treated ground, during its intended life and with an appropriate degree of reliability and cost-effectiveness, will remain lit for the intended use and sustain all actions and influences that are likely to occur.
The serviceability requirements for the development of the soft soil site shall he specified by the client. The observational method, which involves adapting the design in a planned manner, is an important part of the design.
The design shall take into account the loads that could occur during Construction and service. It shall account for the known effect of the drain installation on the properties of the ground and the effects of the initial and longer term groundwater levels and the impacts such levels may pose on the preloading design.
The design shall consider the toiloing:
(a) Suhurfcwe charucfrrs:cmvn Generally using field and laboratory invcstigations. special attention shall be given to asse’s the preconsolidation stress. drainac boundary conditions and the consolidation characteristics of the soils
NOTE: It may be worthwhile to carry out trials at an early stage to assess the performance of PVDs and thc soils.
(b) Developed land use and associated loadings Account shall be taken of the final ground level after development and the long-term loading conditions.
(c) Current and future groundwater levels The groundwater level is essential to determine the effective stress of the soils a the current state as well as in the future. Spccial attention shall hc given to the groundatcr level if it is located within or intrudes into the fill placed for preloading operations.