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GROUND COMPACTION - URETEK Deep-Injection

EVALUATION CRITERIA

NATURE OF THE INQUIRY

Whatever the nature of an inquiry, the processes of evaluation are similar, involving due consideration of all relevant, available and obtainable information.

Evaluation of the extent of the ground to be treated, as well as the method of injection and the pattern and depth of each injection, is made on a specific, case by case basis. Factors to be considered include the reason for intervention, the soil classification, the estimated compressibility of the soil and the presence of any ground water table, as well as the structure of the building itself.

In support of this evaluation there is a considerable wealth of knowledge and experience to draw on, accumulated over years through the collaboration of URETEK Worldwide and eminent external specialists: Geologists, Geotechnical and Structural Engineers.

Proposed increase in foundation loading
Where significant increases in loading are proposed to be added to existing foundations (e.g. another storey) URETEK will consult with the client's engineers to establish design parameters. In addition, knowledge of the existing structure and geotechnical information will be required.

Settlement problem
Before correction, it is important that the cause of a settlement problem be investigated and, if necessary - and possible, remedied.

URETEK Sales engineers, familiar with many Causes of Subsidence, liaise with the client and/or his engineer, to establish the cause and ensure URETEK is the appropriate solution.

INVESTIGATION TECHNIQUES

Examination of foundation plans
In the case of settlement, knowledge of the foundation structure is fundamental to understanding the problem and recommending the correct solution. The type of footings, their material composition, dimensions and layout are all significant.

Available as-built plans are always carefully examined by URETEK staff. However, as is often the case, when plans are not available it may be necessary to directly survey the foundation structure, sometimes involving internal or external hand excavations.

Visual inspection of site
One way or another, site conditions are the cause of settlement, so careful evaluation of them is obligatory in the formulation of a repair plan. Not all conditions are obvious; some require geotechnical investigation but there are many to be spotted by the observant eye. Some affect the ground at depth, some have only upper level consequences. For instance:

A tree too close to the building: perhaps a garden over (or under) watered.
Flooding from a vehicle washdown area.
Condensate from an air-conditioning unit saturating the ground.
Broken rainwater downpipe discharging onto the ground instead of into a drain.
A "Cut and Fill" building platform.
Proximity to heavy traffic or railway line.
Lateral movement of an excavation or retaining wall.

Visual inspection of a damaged structure
Cracks in a building can provide information as to the nature of the subsidence that has occurred. Knowledge of the degree, form, inclination and evolution of the cracking in load-bearing and dividing walls and across floors, is important for an accurate interpretation of the dynamics of settlement.

The cracks must be considered in relation to the size of the building, the materials used in its construction and the type of structure.

Most buildings have a certain degree of elasticity which allows them to adjust, without damage, to minor settlement of the foundations. It is when settlement assumes significant proportions that the structure is no longer able to absorb the redistribution of forces and tends to fracture.

The cracks first occur at the points of least resistance and then spread to other parts of the building, causing a permanent weakening of its structure.

A new equilibrium is achieved when the foundations finally stabilise. However, as subsidence often occurs gradually over a very long period, it is sometimes difficult to be certain that an equilibrium has been reached, even when the cracks seem to be stable.

Further, cracks in one part of a building can result from movement in other parts, resulting from differential structural tension. (Remedial foundation work applied to areas where cracks were apparent would be wasted, if the subsidence had actually occurred several metres from the point demonstrating the distress.)

In most cases careful observation and the use of appropriate instrumentation can ensure that the area of the building where subsidence has originated can be identified with an adequate degree of confidence.

When a building begins to show settlement cracks, one can generally expect them to become worse. Prompt action will limit further damage and minimise costs.

Geotechnical investigation - Checking the characteristics of the ground
Whilst both cohesive and non-cohesive soils can be treated, it is cohesive soil that needs the greatest consideration. Is it under, or over, consolidated? What will be the effect of a change in moisture content?

Put in a basic way, to confirm the suitability of URETEK Deep-Injection, there are three basic knowledge requirements. The soil profile, its (varying) bearing capacity and its inherent stability under changing moisture content.

Soil profiles can be obtained by hand digging or bore hole investigation; bearing capacity by either Pressuremetric or Penetrometric tests; and stability generally by laboratory investigation.

Bore hole sampling
This is conducted by specialist contractors and may involve augering or core drilling. Experienced engineers interpret auger progress and tailings, as they come to the surface, or examine core samples retrieved.

Core samples may be sent to a laboratory for further analysis.

Pressuremetric tests
Such tests are also carried out by specialist contractors and involve large equipment. The tests depend on inducing stresses into the ground with an expandable, cylindrical probe which is introduced into a pre-drilled hole.

The basic principle of Pressuremetric tests is the expansion of a cell inside a cylindrical cavity, in order to measure the relationship between the pressure and deformation of the ground, so a pressure-deformation diagram can be drawn.

Penetrometric testing
There are various types of Penetrometers available for different situations, some only used by specialist contractors. They are further described in the Technical section of this web-site.

This form of testing is very common, as it allows numerous determinations at a relatively modest cost.

In brief, Dynamic Penetrometer testing, involving driving a rod into the ground by the impact of a falling weight, is used to determine the penetration resistance offered by predominantly granular soils. For fine grain soils, a Static Penetrometer, a rod is driven into the ground by a hydraulic ram and the resistance measured, can be more appropriate.

Correlation of penetration resistance against industry or proprietary tables, provides an estimate of the bearing capacity of the ground.

Portable URETEK Penetrometers
URETEK Deep-Injection crews carry portable dynamic penetrometers, so they can carry out before and after analysis. They can be used outside, inside or under buildings and otherwise inaccessible locations. Hand Penetrometers may be used to one or two metres. Small hydraulic units use a falling mass of 30 kg (66 lb) with a fall of 20 cm (7,9") and have the ability to record to some 12 metres (36 feet).

The same equipment is available to URETEK sales engineers

Laboratory investigations
These are primarily used to determine the physical and mechanical properties of cohesive, or clay, soil. URETEK specifically concentrates on Atterberg limits and Edometric tests. (See Technical section.)

SCOPE OF WORKS

Based on the above investigations, the URETEK scope-of-works is defined along with any responsibilities of the client or others.

Having established the client's needs, the works are programmed for a suitable time.
In commercial situations, work can be carried out overnight but this is generally unnecessary as the intrusion of URETEK operations is minimal.

COST OF CORRECTION

With the above information it is possible to estimate the amount of time and material likely to be required to complete the works.

As URETEK material does not have wetting-out characteristics, the ground will only accept such material as is able to expand to fill any voids and compact inadequate strata, before lifting occurs. Thus minimal material usage is assured and costs controlled.

URETEK SUBMISSION Statements in relation to all of the above, along with any responsibilities of the client or others, are set out in a written submission to the client.

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