foundation system of burj al arab

Settlement Observations of Embankment Soil Under Preloading

Monitoring of embankments constructed over soft ground for preloading purpose is very important to determine the behavior of ground; this helps us by following means: 

• Prevent sudden or unexpected failures, 
• Change in rate of soil consolidation can be recognized, 
• Help to verify or evaluate design parameters, 
• Provide an option to adjust construction schedule. 

Performance evaluation can also help us to improve construction efficiency and settlement predictions for future projects. The application of some instruments in determining settlement behavior will be discussed here, but details of instrumentation are not provided.

Basic instruments required to monitor can embankment include:

• Settlements plates, leveling points, settlement gauges
• Alignment stacks
• piezometers

Settlement gauges like gauges of hydraulic type, used to measure long-term deformation at actual ground surface which lies below permeable blanket provided during construction. These gauges are usually placed just after installation of vertical drains.

Following figure typical layout of basic instrumentation installed during embankment construction on ground that is modified by vertical drains. Depending on existing site conditions, different configuration may be required. The requirement of monitoring is also influence this configuration.

Settlement plate is installed to record readings of initial settlement. A benchmark is required in this purpose and it is set up conveniently on the stable ground at a suitable distance from fill.

Settlement plate is the most popular settlement monitoring instrument. The construction of it is very simple and reading can be taken manually.But automated recording with electrical sensor can be installed which can be monitored from remote place.

Settlement plate offer us whether-

• Settlement is occurring,
• If occurring, rate and magnitude of it,
• Settlement completed or not,
• If completed total magnitude of it,
• Time required to complete it

Settlement plates may be used to monitor heave too. It may be installed alone or may together with other instruments like piezometer, alignment stakes.

As base platform of settlement plate generally plywood is used but concrete pad or steel plate may be used. A riser pipe known as reference rod is attached to platform; a threaded connection at the end of reference rod is used. This threaded connection allows to extend pipe with the progress of filling.

This plate can be placed at any elevation of filling of our interest but generally they are placed above existing ground. A protection pipe of PVC materials can be installed around riser pipe where extra protection or high accuracy of reading is required.

Level points are installed at the top of embankment after completion of construction. Thus settlement can be taken at any interval of our interest, say weekly or monthly when settlement rate is high under the influence of vertical drains.

When settlement rate is low, say preloading without any type of drains, settlement record is less frequent. Settlement value can be computed by using the Asaoka method.

Let’s discuss about alignment stakes, they are installed when construction of fill is started. There are placed parallel to slope of embankment.They offer us a simple way to monitor lateral displacement of foundation soil while progress of construction.

They could provide us early warning of imminent bearing failure, even under visual inspection of construction site. The rate of this movement is related to rate of change in pore pressure below the embankment.

A better alternative of alignment stakes is inclinometer. This sophisticated instrument can measure lateral displacement in borehole.

Piezometer cannot directly measure settlement, but pore pressure record is required to analysis stability in respect to effective stresses especially when surcharge is applied on foundation soil to achieve consolidation.

Determination of Cracks Width in Reinforced Concrete Member

A reinforced concrete flexure member cracks, usually, under loading well below its service load and surprisingly often even before subjected to loading; this is due to shrinkage under retrain condition.

Now consider reinforcing steel, embedded in concrete, which has also contribution in restraining cracks. But to be effective to its desire strength, embedded concrete have to be cracked. Before formation of cracks, steel is only stressed n times the stress in the surrounding concrete.

We know concrete is very weak in tension, this is only a fraction of compressive strength of it. Near modulus of rupture, concrete is stressed only by 500 psi and if modular ratio n=8, the stress in embedded steel is 500X8=4000psi. The usual yield strength of reinforcing steel (40~75) ksi; using heat treatment and changing constituents, its strength can be increased even more. So 4 ksi is only a fraction of its yield strength. 

Thus reinforcement is not effective before concrete is cracked. Where modular ratio, n =Es/Ec. In common practice with usual materials n=8.

In properly designed flexural member like beam, cracks due to flexure are fine and often defined as hairline cracks which usually not allow corrosion to reinforcement, if allow, it will be very little.

When loads are increased gradually exceeding cracking load (exceeds modulus of rupture), number of cracks is also increased; the width of cracks is also increased which reached around 0.016 in (typical width) at stress corresponding service load. A further increase in load accompanied by further increase in crack width, though number remains more or less equal.

As discussed in previous post formation and progression of crack is highly variable as controlled by many factors. Considering this difficulties, cracks widths are now determined based on observations confirmed by test. The equations developed to predict crack width.

Now we will discuss about two equations for determining crack width. It should keep in mind that the equations were produced based on analyzing experimental data. Analysis means application of statistical approach which offers us 90% probability of matching actual crack width.

Let’s explain more easily; about 90% cracks formed in flexure member have width below the value calculated by these equations. However, individual cracks may have width exceeding twice the width calculated.

ACI code considered two expressions as prominent to develop its crack control provisions; these are expression developed by

• Lutz and Gergely in 1968
• Frosch in 1999

Two expressions determine maximum width of cracks at tension face of beam.

The respective equations are:

Equations to determine crack width

w  =maximum crack width, thousandth inches,
fs;= steel stress corresponding to load for which crack width have to be measured, in ksi
Es= modulus of elasticity of steel, in ksi.

The parameters used in equation dc, β, A and S are shown in following figure and are explained as follows:

dc =thickness of concrete measured from tension face to center of rebar.In determining center of rebar, outermost layer of rebar is considered; not considered center of all rebar.
β factor for determining concrete crack width

h1 =Distance between neutral axis and centroid of steel
h2 =Distance between neutral axis and tension face
A= Area of concrete surrounding each bar. This is measures by dividing total effective concrete tension area surrounding reinforcement having identical centroid by total number of bars (in2)
S= maximum spacing of bar (in)

Both the equations (1) and (2) inly applicable for beams that are reinforced with deformed bars. The β factor is added to consider the effect of distance from the tension face to neutral axis;