Detailed investigation is required for a preloading project which may involve penetration testing (may include piezocone), boring, sampling and extensive laboratory experiments. The especial emphasis should be given to
• Presence of microlayer that are permeable i.e. stratification of ground underneath project
• Drainage conditions below and above the consolidation susceptible clay layer
• Stress-strain and strength characteristics i.e. stress history
• Permeability and consolidation characteristics both in horizontal and vertical directions.
Horizontal co-efficient of consolidation, Ch is required to design vertical drains. Ch determination usually is not considered part of regular foundation investigations.This can be determined in laboratory or in field. The in-situ methods are:
• Piezocone test introduced by Torstensson (1975)
• Permeability test: constant/falling head tests can be used to determine both Cv and Ch (Wilkinson,1968)
• Consolidation (Clarke et al.,1979)
The laboratory test methods are:
Standard consolidation/ triaxial test: Developed by Uriel & Escario (1961), Marsh (1963). Equipment is modified to allow radial drainage alternatively cutting specimen properly.
While preloading with or without drains some uncertainties are found during installation due to wrong assumption about soil based on disturbed sample and different non-ideal conditions during installation of drains. Trail embankment is constructed to find out feasibility of the preloading project considering disturbance and malfunctioning or misusing of installing equipment of vertical drain.
While construction of trail embankment, one portion of it may be intentionally overbuilt to produce shear failure; But good knowledge is very important about site conditions in order to determine representative test location for whole project.
A minimum monitoring for six month of trail embankment is required, as per Forrester (1982). If it is not possible, the construction of trail embankment is considered worthless. According to report of Forrester, following recommendations can be concluded for trail embankment:
1. Location of trail embankment has to be selected in such way that it becomes a part of earth structure that will be used as final construction. Thus extra costs are saved and project becomes economical.
2. The shape of trail embankment has to be selected so that it follow the assumption of drainage and strain conditions of final structure
3. When trail embankment is to check feasibility of land reclamation project, the fill have to be placed in such way that increase in linear stress is imposed over wide area. This is achieved by providing width across top of embankment equal or more than depth of underneath soft soil. But where it is not possible or feasible, analysis of results of preloading test to predict future performance of structures of different sizes may become more complex.
4. Height of fill for trail embankment has to be ideally equal to height of actual embankment to be constructed. This is done to render similar stress condition on soft soil as co-efficient of consolidation and permeability of it reduced with increase in applied load.
5. Slip-circle analysis and bearing capacity have to check to ensure safety of trail embankment to avoid any type of failure.
6. Instrument for monitoring should include
a) Settlement gauges
b) Settlement plates
c) Alignment stakes
d) Leveling points
e) And piezometers.
They should response rapidly to take some remedial action. This instruments measure different parameters of trail embankment. The variables to be investigated depend on available time for monitoring and limitation imposed by costs. In most trail embankment, vertical drains are constructed to determine economical and practical spacing.