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Improving prospects

Report on the East Midlands Geotechnical Group meeting 'Ground improvement techniques'held at the University of Nottingham on 22 November 1999, by Harriet Miller, Department of Civil and Structural Engineering, Nottingham Trent University.

Introduction

Chairman of the East Midlands Geotechnical Group Russell Jones introduced Barry Slocombe, the speaker for the first of four meetings to be held. Slocombe works for geotechnical contractor Keller Ground Engineering and has over 25 years experience of ground improvement methods.

Background

Site investigations are vital for any engineering project to evaluate the load bearing potential of the ground. Where a site investigation encounters difficult ground conditions there are a number of options. A reasonable solution might be to dig past the weak layer and make use of the stronger ground below, eg using piles. Another solution would be to remove the unsuitable soil and replace it.

Alternatively, the properties of the ground can be enhanced or modified to make it suitable for supporting the intended loads. As more and more land becomes subject to urban and industrial development, good construction sites and borrow areas are becoming harder to find and frequently improvement techniques become the best option.

Improvement techniques were discussed, concentrating on the design approach and some technical details, with specific reference to two key methods: vibro stone columns and dynamic compaction.

Vibro stone columns Vibro stone columns (formed by vibro replacement or vibro flotation) can be used in granular materials to densify the deposit and in cohesive soils to reinforce the ground.

The vibrator has eccentric weights mounted on a vertical shaft directly linked to a motor. Energy is applied directly to the ground through the casing which has fins to prevent twisting. The resulting vibratory motion is horizontal, with typical frequencies of 30Hz or 50Hz to suit electric or hydraulic power cycles. Treatment depths are usually between 3m and 6m but can reach depths of 35m. The vibrating machines are often provided with automatic systems for continuous recording of power demand related to time and depth of penetration. Records of power needed to vibrate the soil provide a high level of control and good contract records, allowing a check that the soil is as expected and, if it is not, it can be investigated.

Vibro techniques were first used in Germany in the 1930s. Various techniques for air and water flushing have been developed since for different ground conditions.

In the conventional top feed process, a hole is formed using a vibrator suspended by a crane. The vibrator is withdrawn and stone is placed in the hole and compacted in stages, assisted by compressed air (Figure 1). The stone is forced outwards and tightly interlocked with the surrounding ground to build up a very compacted column.

The bottom feed process is a dry method (Figure 2). The main advantage is that the vibrator remains in the ground during installation, making the technique ideal for unstable ground and high groundwater levels. The stone is discharged from skips into the chamber at the top of the vibrator and placed at depth and the method is generally dry, fast and clean.

The wet top feed process is used in fully saturated and very weak soil. Water jetting removes soft materials and stabilises the hole allowing stone backfill to reach the bottom of the vibrator. The advantage of the wet flush method is that a large diameter hole can be flushed out but it also washes out fines and contaminants and stone columns are limited to 1. 5m centres due to the instability introduced through wetting the soil. The new BRE review document on ground improvement gives details and guidance on vibro compaction and top and bottom feed stone column methods.

These techniques provide the soil with better settlement values and increased bearing capacity.

In granular soils, the particles can be rearranged and therefore lower void ratios can be achieved through the vibration process. In cohesive soils, the particles do not respond to compaction in the same way. However, these soils confine the dense stone column, which can provide a higher bearing capacity relative to the surrounding the soil.

The benefits of the techniques are also dependent on the ground material. In a granular fill for example, settlements can be reduced from 5mm to 15mm and bearing capacities of between 100kN/m 2can be achieved. Cohesive fills are slightly harder to control, with settlements reduced to 5mm to 25mm with bearing capacities of 100kN/m 2produced.

The techniques also have limitations. The ground to be treated must be basically inert. It cannot be heavily contaminated with refuse/ degradable matter or have soluble chemical waste present.

The technique is also not usually suitable for materials that are heavily voided or for recently placed clay fill. Ideally, details on the age of the fill and how well it was placed should be known. Also sites that are likely to have problems with future inundation are not suitable because collapse settlement may occur as the water saturates the ground, especially if the soil has high void ratios. Since the columns will support the structural loads, they must not be likely to collapse. In these situations dynamic compaction may be more suitable as it treats the soil as a whole.

Failure mode The columns are designed to avoid shear failure. In fact, shear failure of stone columns is rare. More commonly, bulging occurs due to overloading, which causes excess settlement. Standard factors of safety can be employed to reduce bulging effects. Punching failure can also occur, usually as a result of the column not being taken down to sufficient depth. The stone columns are irregular in shape and therefore shed a lot of load along the length of the columns. Generally, if the column is nine times longer than it is wide, end bearing can be avoided. Standard factors of safety of 2. 5 - 3 apply for these modes of failure.

Design of vibro replacement The design spacing of the stone columns must take account of soil conditions, size of columns and friction characteristics of the stone, as well as the loading and settlement requirements of the structure to be supported. Columns are put in all corners of the buildings and are generally laid at 2m centres to avoid sagging of foundations.

The safe working load is found using:

s. sFwheres = 1 + sinf (. . . + 4c + p - u) s= stress on stone column at failure f= friction angle of stone . BH = overburden pressure at level of critical layer c = cohesion of critical layer A s= area of stone column F = factor of safety u = excess pore water pressure - negligible, except under rapid loading or slope stability problems From research, f is about 45degrees for stone. A change in friction angle makes a large difference in the 1+ sinf factor and therefore in the factor of safety. 1 - sinf The stone needs to be well compacted to keep f as high as possible. As can be seen from Figure 3, the improvement factor, n, increases with values off. The value gained for the friction angle is an indication of the workmanship.

The vibrating techniques should be used as an identification tool while the job is in progress. If anomalies are found this may indicate that the site investigation was not thorough enough and that ground conditions vary from expected. In this way vibro-replacement is a very useful tool in assessing the ground conditions.

Depth of treatment depends on the competence of the soil at depth. Where deep deposits of weak material are present, the depth of the column is designed to accommodate the majority of the additional stresses imposed by the new structure. This can be applied where predicted settlements are low and unlikely to contribute to differential settlement of the structure. The disadvantage of using partial depth columns is that any variations in the thickness of the soil layers may not be accounted for. There may be problems from a deeper depth well below the stress bulb of the new building, which should be taken into account when assessing the treatment depth and foundation design.

As well as supporting loads from structures, these stone columns can be used to improve slope stability, with columns placed through the likely slip plane to avoid movement. This technique can be cheaper than digging out the soil.

And in ground susceptible to earthquake induced liquefaction, vibro techniques can be used to densify the soil before piling in the treated ground. Good workmanship is essential in all these applications.

Vibrated concrete columns Vibrated concrete columns (VCCs) are ideal for weak alluvial soils (such as peats and soft clays) overlying competent founding stratum such as sands, gravels and soft rocks. Before penetrating the ground the system is charged with concrete. In common with stone column techniques, the ground is pushed laterally to produce a hole in which to pour the typically low slump concrete. The founding layer, if granular, is then compacted by the vibrator.

Concrete is pumped out of the base, with the poker occasionally raised by 1m and then put back down to displace the concrete into a bulb until a set resistance is achieved (Figure 4). In soft soils like peat, a significant overbreak can be experienced. Advantages over conventional piling are that required depth is reduced due to the lateral vibrating technique and therefore costs can be reduced by about half. The advantage over stone columns is that once the columns are set they should not deform any further.

A VCC scheme was used near the toll site of the M4 for the second Severn crossing. Geogrids were used to further reinforce the base of the embankment, with large loads applied to the VCCs with little load in between, giving significant ground improvement. The area has not moved since the improvement work was completed.

Dynamic compaction

This technique was used as long ago as Roman times, and was also used during the American Civil War. However, the technique has only been seriously applied on a wide scale since 1970. It is used to compact weak soils and results in increased bearing capacity and reduced settlement of the ground as a whole. Marginal sites have been improved so that shallow foundations can be used rather than resorting to deep excavation or piles.

The ground is subjected to repeated surface tamping using a heavy steel or concrete weight (Figure 5). Some 8t is dropped from a height of15m, giving impact speeds of about 64km/h, the effect of which normally reaches 6m to 7m depth and repeated blows drive energy to depth. The result is a series of coned areas of improvement (Figure 6). Middle layers are then improved using weights dropped from a lower height and a final 'ironing pass' is applied to the top 2m to 3m which helps to re-compact any loosened shallow layers.

It is important to have a clear idea of the required bearing capacity and settlement characteristics. Thorough understanding of the soils prior to treatment is also essential. Close geotechnical control should be maintained throughout the work, so that anomalies from expected results can be spotted and investigated.

Better performance is obtained on granular soils as the particles will rearrange under pressure. The technique is not so effective for weak soils that will fail on impact and therefore become even weaker.

An empirical relationship for the depth of influence has been developed from field data where: Dmax = 0. 5 v(WH) Dmax = depth of influence in metres W = mass of weight in tonnes H = height of drop in metres A disadvantage of this technique is the ground surface vibrations created from the impact of the falling weights. These vibrations can cause damage to buildings and therefore dynamic compaction cannot be carried out close to developments and sites of antiquity.

Summary

Good reliable site investigation data is vital before deciding on the best remedial strategy.

Workmanship is also important and the monitoring available with stone columns and VCCs allows good quality control.

Questions

Asked if bulging failure was avoided using known data or empirically, Slocombe replied that largely empirical suitable factors of safety were needed.

He was also asked about guidance on vibration parameters. Various codes are available including:

BS7385 Part 1 (1990) and Part 2 (1993). Evaluation and measurement for vibration in buildings.

DIN 4150 (1975) BS5228 (1992). Noise control on construction and open sites - Part 4 Code of practice for noise and vibration control applicable to piling operations.

BS6472 (1992) Evaluation of human exposure to vibration Ted Murray suggested that there can be problems with fractures of VCCs if necking occurred during placement. It was suggested that either reinforcement or a hit and miss approach may be valid to solve this problem.

Slocombe was asked whether the shape of compacting blocks had much influence on performance of the dynamic compaction technique. He suggested the shape probably had little influence on this crude method that works very well.

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