NATIONAL UNIVERSITY OF SINGAPORE

Department of Civil and Environmental Engineering

CE2112 Soil Mechanics

Take-Home Assignment 2 2013/14:

An Introductory Exercise into Geotechnical Forensics

1 Problem Statement

You are a geotechnical engineer in a consulting company. You have just been contacted by a

potential client who is the developer-owner of a cluster of flatted factory buildings around the

coast. Cracks have been developing progressively in some of the buildings over the past 20 years.

At first, they were thought to be due to nothing more than shrinkage of plaster in the wall and

were plastered over. However, new cracks continued to develop and the situation is now looking

increasingly serious with cracks in the beam-column joints, ground floor slabs while new cracks

continue to appear on the walls right down to ground level.

You took a visit to the site to see things first-hand. During this visit, you noticed that the cracked

buildings were clearly in structural distress and many of the cracks were not just plaster cracks

but they were structural cracks. There were also other things which you noticed, which were as

follows:

(a) In some areas, especially around buildings where cracks are observed, the road level is as much as 600mm below the building apron level; this elevation difference is much larger

than normal aprons and roads. Enquiry with the owner suggested that the road in some

parts of the site have settled appreciably over the past 20 to 30 years.

(b) Water pipes and other services into and out of the cracked buildings have been re-routed several times in the past as the surrounding ground settles and pipes broke up.

Digging back into the history of this area, you found out that this area is one of the coastal areas

that were reclaimed about 40 years ago. The borelog data show highly variable ground conditions.

All of the boreholes showed a layer of sand fill with a thickness of approximately 5m; this is the

reclamation fill. In some areas, the sand fill is underlain by very stiff residual soil of the Old

Alluvium formation. In other areas, the sand fill is underlain by soft, normally consolidated

marine clay of the Kallang Formation, with thickness varying from 0m to 20m. The coefficient of

consolidation of the marine clay is about 0.5m2/year and its bulk density is about 16kN/m

3. There

are no signs of coastal erosion, so this can be ruled out as a cause.

All the buildings are supported on 250mmx250mm square-section reinforced concrete piles,

driven to refusal (i.e. until the piles refuse to go in further into the hard stratum). The pile length

varies significantly over the site. In areas where there is no marine clay, piles are relatively short.

This is to be expected since refusal was likely to have been encountered at a shallow depth. In

areas where there is a thick layer of marine clay, the piles are long (~25m) and all of them are

socketed into stiff Old Alluvium Formation which underlies the marine clay. All of the buildings

showing cracks and structural distress are sitting on top of a thick layer soft marine clay, but are

supported by the piles as described above.

Based on the above information, write a short preliminary report on what you think is the cause

of the cracks and structural distress and how much longer you think the problem is likely to

persist. Your findings may be supported by simple hand calculations, but no computer analysis is

needed. The text in your report (excluding calculations and figures) should not exceed 1000

words. Overlength reports may be penalized.

In thinking through the problem, please remember the following:

(a) You should strive to be as specific and clear as possible in your geotechnical forensic deductions.

(b) You should avoid giving hand-waving, general or ambiguous possibilities such as soft clay, low strength etc.

(c) You should also avoid raising many possibilities in the hope that one or some will hit the mark. If you do so, it shows that you do not know the real reason(s) and marks will be

deducted.

(d) Marks will be given for reports which show a specific and clear cause-to-effect relationship(s).

(e) Useful tip: when we say a building is supported on piles, this usually means that the entire weight of the building rests on the piles, not the soil surface. This is usually

done when the soil is deemed to be too soft to support the building.

Marks will be awarded for reports which demonstrate deep, logical argument, rigorous forensic

deduction in pinning down the smoking gun, together with an ability to take into the evidence at hand and apply the concepts which you have studied in your CE2112 module. Calculations

which are relevant and add value to your case will also be given marks.

As a client, I will be able to clarify any uncertainties which you may have with regards to site

conditions i.e. I will be able to elaborate more, if required, on the building and ground surface

conditions, as I see them. However, I will not be able to make any forensic deductions since I am

not supposed to be a geotechnical engineer.

[20 marks]

2 Administrative Details

The preferred method of submission of your answers is via soft copies onto IVLE into the

workbin Assignment02 - submissions. If your answer comes in several files, e.g. pdf, xls etc.., you should Winzip all the files into a zip file and name the file with your matriculation

number. Please indicate only your matriculation numbers (not names) on your solutions. The deadline for submission is Tuesday 1 April 2013 11.59pm. All IVLE submissions are

automatically logged in.