Past exam paper theory questions

Question 1

Describe the role of soil element testing in geotechnical design

Answer 1

Need geotechnical parameters for use in design calculations.

 One method of obtaining these parameters is to perform element testing on undisturbed (or reconstituted)
soil samples.
 e.g. form a link between soil identification and design calculations

Question 2

What considerations are required when specifying an element test?

Answer 2

Need to obtain suitable parameters for the types of calculations required (e.g. different parameters for settlement calculations and slope stability problems)

Type of parameters depend on type of design:

• What geometry is being investigated
• Whether the event is drained or undrained.

Need to choose appropriate element test to get these parameters:
(oedometer for 1D -compression/consolidation)

(triaxial - for strength/stiffness)

What soil state:
- voids ratio,
- stress history
- effective stress level during testing
drainage conditions

Question 3

rotatory core techinque

Answer 3

In hard soils and rocks core drills are often used.
 Consists of a small diameter hollow tube, fitted at lower end with a coring bit.
 Core barrel rotates at speed (600-1200 rpm), a control pressure is applied and water-based flushing fluid
is circulated through the bit.
 Fragments removed in the annular cut are brought to surface with fluid as core fills the barrel.
 Drill run of 1-3m made before raising barrel and removing core.
 Standard sizes of core barrel in range 30-100mm.
 Flush could be air, water or foam

Question 4

u100 clay proccess

Answer 4

In clay soils it is normal to sample using a U100 driven sample (or a UT100 thin walled sampler under EC7) to
obtain samples in clay. This is undertaken by driving a 450mm long, 100mm sample tube into the ground using a
sliding hammer system. The Sampler normally has a cutting shoe in advance of the U100 and may have some kind
of core catching system included. Normally at the head of the sampler/based of the sliding hammer is a non-return
valve that allows any trapped water or air to escape. U100 samples are then sealed and recovered back to the
laboratory for element testing eg Triaxial or oedometer etc. Guidance suggests that this method is typically
deployed

Question 5

u100 sand proccess SPT

Answer 5

SPT

In sands U100 samples are not taken but an insitu testing device called the standard penetration test (SPT) is
typically deployed. This involves driving a 450mm long split spoon (thick walled sampler) in increments of 75mm
and counting the number of blows required to advance every 75mm. The sampler is driven from the surface by a
drop hammer system that drops a known weight, a set height onto an anvil that is connected to the sampler at depth
by a series of rods. The split spoon sampler only recovers disturbed samples that are only useful for identifying the
material the test was undertaken in. In gravels the entry to the split spoon is blocked of using a cone end piece and
no sample is recovered.

Question 6

in situ vs element testing

Answer 6

Insitu testing has benefits in determining design parameters hard to sample: sand gravel.

insitu measures insitu stress regimes, lab does not.

Element testing disturbed.

Element testing allows the determination
directly of parameters for design.

Question 7

insitu testing advantages in relation to boring

Answer 7

Relatively quick

Soil properties measured insitu ie undisturbed and insitu stress state

Highly mobile rigs

Accurate profiling possible and indicate thin layers that may be missed during sampling during boring

Question 8

boring advantages in relation to in-situ testing

Answer 8

Boring for sample recovery
Recovers soils samples for further lab testing

Physically observe soils for identification

Samples will be disturbed to some extent and will not reflect insitu soil state

Relatively slow
May allow installation of monitoring devices

Question 9

Describe why it is important to select drained or undrained element tests:

Answer 9

Soil testing parameters will only be relevant where field and laboratory drainage conditions correspond

We must consider the rate at which changes in total stress are applied in relation to rate of dissipation of excess
pore water pressure. For instance high construction rates in low permeability soils may lead to undrained
conditions, increased pore pressures and low effective stresses.

Triaxial testing must be undertaken with appropriate drainage conditions (e.g. drained or undrained) or
alternatively drained parameters can be derived from undrained tests where pore water pressure measurements are
made.

Question 10

How to use light percussive and what is used (generic)

Answer 10

The following are items that may be mentioned and discusses

• Cable percussive rig consists of a lightweight derrick and winch

• Various tools can be advanced into the ground (clay cutter, bailer, chisel) by rasing and lowering the tools
and impacting the ground. The tools allow material to be brought to the surface and the borehole to be
advanced

• In water bearing or non self supporting ground the borehole may be supported using casing
• The clay cutter is normally used to advance in clay
• The chisel is used to prove rock and smash/move hard obstructions
• The bailer with flap valve is typically used in water bearing soils

Question 11

light percussive for undisturbed (u100) method

Answer 11

• U100 tube normally driven 450mm using sliding hammer.

• U100 sampler incorporates a cutting shoe at the end, a core catcher to return the sample and a non-return
valve at the top to avoid the trapping of air or water at the top of the sample. This also creates a vacuum to
aid sample recovery.

• When sample raised to surface cutting shoe removed and ends of U100 sealed with wax to retain sample
moisture

Question 12

light percussive (disturbed) method

Answer 12

Disturbed samples normally a result of use of clay cutter or bailer

• Sample depth may not accurately be know and sample disturbed

Question 13

SPT test (how to use, when)

Answer 13

Hammer of known mass is used to drive a split spoon sampler a known depth (450mm) and the blows
required are recorded

• First 150mm of drive considered a seating blow, blow count recorded but not used to calculate N value
• Maximum number of seating blows 25
• Blow count for 450mm drive recorded in 75mm increments
• N-value calculated from number of blows recorded for 300mm of penetration after the seating blow

Question 14

how to use a triaxial device and how it works

Answer 14

Sample is enclosed in a membrane which will enhance strength. Corrections are available for this.

Change in area assumes that the soil barrels shape or changes in radius in relation with each strain
increment.

The soil sample does not change shape uniformly with less area change near the end caps due to end effects. This leads to approximation in calculation of the stress applied to the soil.

The loading applied to the sample was measured via a rod extending from the base of the loading ring to
the top of the sample.

o-rings seal tight and provide friction.

Question 15

Describe for site investigation: suitabilty, design, construction and site choice

Answer 15

Suitability: To assess general suitability of site and environs for proposed works including effects of previous
use and contamination.

2) Design: To enable adequate and economic design to be prepared, including design of temporary works.

3) Construction: To plan best method of construction, to foresee and provide against difficulties during
construction due to ground and other local conditions. To select sites for waste disposal.

4) Site Choice: Where alternatives exist, to advise on the relative suitability of different sites or different parts of
the same site.

Question 16

pre site methods of site investigation

Answer 16

1, Aerial photographs, modern and historic; satellite imaging

2, Topographic maps

3, Historical maps

4, Geological maps, solid & drift

5, Previous investigations/boreholes, BGS in UK

6, Mining records

7, Site walkover

Question 17

why budget for a site investigation

Answer 17

Desk studies should be allowed for when budgeting for site investigation because they are a very cost effective and
efficient method of investigating a site.

This is predominantly because such investigation can be undertaken by a
consultant utilizing office based staff that have ready access to the required resources.

Once plant and equipment are sent to site for intrusive investigation costs may escalate significantly. Thus they
should mobilize appropriate staff and equipment.

For instance appropriate to the geology ie there is no point
mobilizing rock coring equipment (only) where rock is not likely to be encountered. Such information can be
gained from geological maps or previous investigation records.

Having a feel for the existing geology and topographic features will allow selection of:
appropriate techniques
the extent (depth) of investigation
frequency of investigation
specific features to investigate

Question 18

Describe the use of drainage as a slope remediation technique including how it
operates and give examples of types of drainage systems.

Answer 18

Three different modes of operation:

(a) control surface water (shallow/surface drains)
(b) modify seepage pattern in ground (deep drains)
(c) short term reduction of undrained pore pressure

Types of drains

• trench drains
• toe drains
• drainage wells / bores / galleries

Question 19

Slopes normally become unstable due to a change in the groundwater regime affecting the slope this could be
caused by:

Answer 19

• Changes in seasonal weather
• Poor maintenance of track side drainage
• Sudden devegetation of slopes
• Burst water supplies

Question 20

hazards working at base of wall or inside shaft

Answer 20

Working at the bottom of the shaft may expose operatives to hazards associated with flooding, gas and problems
with emergency access and egress.

The location also may be hazardous in terms of material and equipment
overhead lifting.

In order to mitigate the hazards the working area may be classed as a “confined space” and as:

such have an appropriate working method with only confined space trained workers allowed access to the base of
the shafts.

Gas monitoring may be required as well as forced ventilation. Operative may have access to self-rescuer
sets and be trained in their use.

Cranes should have beepers to alert operatives below of movements. All staff to be properly trained in slinging and banksman in attendance.

Question 21

types of temporary retaining walls for construction

Answer 21

Sheet pile wall
• Diaphragm wall
• Contiguous or secant pile walls

Question 22

what cpt can do and why use it

Answer 22

cone penetration test (CPT)
• Insitu test that gives rapid accurate profiling of soil layers
• Many correlations that allow determination of insitu relative density and friction angle
• CPT rig can be used to install monitoring devices eg piezometers and standpipes (gas monitoring) for
determining water table level, water and gas sampling for contamination.
• When used with MOSTAP sampler could be used to recover sand samples for lab testing.
• Other specialist cones could be deployed in contaminated ground eg video cone or gas chromatograph
cone.

Question 23

why would be we use shell and augur, in sand?

Answer 23

Cheap, high mobility
• Used with SPT to give insitu information
• Many correlations that allow determination of insitu relative density and friction angle
• Only disturbed samples in sand but may provide enough material for reconstitution in the lab
• Depth profiling less accurate
• Allows installation of instrumentation as above