Construction
Monitoring
For Earth Dams
Reasons for Construction
Monitoring
Ensure proper materials are used
Ensure proper construction and design is
followed
Quickly modify design and construction
practices based on encountered site conditions
IT IS IMPERATIVE TO HAVE FULL TIME SITE
INSPECTION
Inspection Requirements
Must not hinder or slow down contractor
Must work with contractor
Must consider contractor construction practices
Construction Monitoring
MATERIALS
Grain size distribution analysis of materials
Core, filters, drains
Make sure material installed meets specifications
Make sure that the borrow materials do not change…
Construction Monitoring
MATERIAL tests
Triaxial extension/shear – filter and core
Consolidation – core m
v
Hydraulic conductivity
Lab tests:
filters - Constant or falling head
core – triaxial
Field clay:
Double ring infiltrometer
Centrifuge permeanometer
Construction Monitoring
Proctor Tests
Source materials in borrow pit
Materials hauled to site
Field Compaction
Uncompacted layer thickness (300mm max)
Compaction equipment is suitable
Moisture content and Maximum dry density
Nuclear Density, sand cone, rubber balloon
Make sure Nuclear density is calibrated
Goal of Compaction
Place loose soil in the field and compact it to
make soil strong as possible
Maximum shear strength
Very little settlement
Low hydraulic conductivity
Find soil lowest emin ……highest dry unit weight
Knead Clay Chunks
Sheepsfoot roller
Soil compacted wet
optimum will be ductile
and self healing
Soils compacted dry of
optimum will be brittle
and suspectible to
cracking
Specify optimum plus 2%
for clay cores
Soil Compaction Measurement
Soil Compaction Measurement
Use dry sand with known
dry density and specific
gravity
Use dry sand to get
volume of hole
Quick and reliable
method
Soil Compaction Measurement
Use radioactive material
to get moisture content
and soil density
Quick method
Reliable if calibrated
Radioactive device
therefore special
transportation and rules
must be followed
Compaction Specification
Insitu dry unit weigh t
Compaction level (%) x 100
Max. dry unit weigh t (Proctor)
Standard Proctor Specification
95 to 100 percent of MDUW
Modified Proctor Specification
92 to 98 percent of MDUW
Compaction Specification
Make sure compacted
soil same as Proctor
material (grain size
distribution analysis)
Add water to soil if too
dry
95% Field
Specification
Field Instrumentation
Measure performance of structure during
construction
Long-term monitoring of structure behaviour and
health
Must not impact structure performance
Geotechnical instrumentation can reduce undesirable
consequences from construction. These consequences
may be the results of adverse performances, damage to
the adjacent facility and/or delays.
Justification for Instrumentation
Engineers should developed justifications for
geotechnical instrumentation program on their
projects
In practice such programs are used to save lives,
save money and/ or reduce risk of failure
In concept, these are simple and easy to understand
benefits but in practice it is difficult to quantify
Reasons to Install Instrumentation
Indicate impending failures
Provide a warning
Reveal unknowns
Evaluate critical design assumptions
Assess contractor's means and methods
Minimize damage to the adjacent structures
Control construction
Control operation
Provide data to help select remedial methods to fix problems
Documents performance for assessing damages
Inform stakeholders
Satisfy regulators
Reduce litigation
Advanced state- of – knowledge
Field Instrumentation
Piezometers
Excess pwp in core during compaction
Uplift pressures
Foundation head loss
Core pheatic surface
Inclinometers
Stability of slopes and foundations
Settlement gauges
Extensometers
Total earth pressures (soil arching)
Suggested Piezometer locations
Earth fill dam:
1- Control placement of fill, monitor pwp to find shear strength and measure uplift pressure
2- Control placement of fill, monitor pwp to find shear strength and measure uplift pressure
and monitor seepage
3- Control placement of fill and monitor seepage.
Piezometer
Inclinometers:
Monitor lateral earth movements in embankment e.g. detect movement
of D/S of earth fill dam, particularly during impounding. Determine type of
shear and zone in foundation. Monitor stability of U/S slope during and
after impounding. Determine depth, direction, magnitude and rate of
movement
Inclinometer
system
(Courtesy of N. Sivakugan,
James Cook University,
Australia)
Inclinometer:
Embankment:
•Locate shear zone and help identify whether shear is planner or circular
•Measure the movement at the shear zone. Determine whether the
movement is constant, accelerating or slowing.
Inclinometer must be founded into solid foundation
5- Tilt meter: Monitor changes in the tilt of the structure. Activities such as dewatering,
tunnelling, excavation causes settlement or lateral deformation. Placement of
surcharge and pressure may cause heaves. Dam impounding, excavation beyond
diaphragm wall etc.
Monitor differential settlement
Dewatering
6- Settlement cell: Pneumatic settlement provide a single point measurement of
settlement. They can be read from central location and arte particularly useful where
asses is difficult. Monitor consolidation during construction and long term
settlement in the foundation of the fill.
Earth fill dam
Settlement cell
Monitor long term settlement and consolidation in the
foundation of embankments
Settlement cell
7- Bore hole extensometer: Monitor settlement heaves, convergence, and lateral deformation in the
soil and rock
For vertical settlement profile
Borehole extensometer:
Earth fill dam:
Monitor vertical settlement in the toe of the dam ( Magnetic
extensometer)
Borehole extensometer
Embankment:
Monitor settlement to determine when construction can continue. ( Magnetic
extensometer used inside the inclinometer)
8- Total pressure cell: Measured combined pressure of effective stress and pwp
Embankment dam
Verify assumptions and warn of the soil pressures in excess of those a structure
is designed to withstand. It determine distribution, magnitude and direction of
the total stress.
Earth pressure cell
(Courtesy of N. Sivakugan, James Cook University, Australia)
Total pressure cell
Rock fill dam
SM: Strong motion accelerograph ( for monitoring earth tremors) TS: temperature
sensor EX: Extensometer( Identify movement of dam base and ground at base) PZ:
piezometers PC: Pressure cell W: V-Notch weir SC: settlement cell WL: water level
meter
Earthfill dam
SM: Strong motion accelerograph ( for monitoring earth tremors) TS: temperature
sensor EX: Extensometer( Identify movement of dam base and ground at base) PZ:
piezometers PC: Pressure cell W: V-Notch weir SC: settlement cell WL: water level
meter
Key Components for Design of
Instrumentation
Put in redundancy
Instruments will get lost due to construction activities
Equipment will stop working
Protect equipment from contractors
Put in safe areas
Mark equipment
Protect it during installation and post installation
Spend money so can remotely monitor and collect data
Consider data analysis cost
Key References
Geotechnical Instrumentation for Monitoring Field
Performance by John Dunnicliff 1993 Wiley & Sons
Rock Slope Engineering by Hoek & Bray 3rd Edition – Can
be downloaded from web. By searching Evert Hoek
US Corps of Engineers- Instrumentation of Embankments
Dams and Levees (posted on course website)
