TECHNICAL REFERENCE 205

BACKFILL COMPACTION FOR VOLCLAY® WATERPROOFING:

An Essential Ingredient for a Successful Project

 

For several years, Allco Agencies has required that soil backfill placed against a foundation wall with Volclay waterproofing must be compacted to 85% Modified Proctor density. Unfortunately, this recommendation has caused some confusion amongst those parties responsible for achieving this requirement. The purpose of this technical note is to explain more fully the reasons for Volclay’s backfill compaction requirement and to present an alternative recommendation that can be implemented in the field.

Figure 1. Cross-sectional schematic showing the difference between a poorly confined and a well-confined Volclay waterproofing membrane.

 

The Importance of Backfill Confinement

Most of Volclay’s waterproofing products (Voltex, Swelltite and Ultraseal) are non-adhered when installed on a freestanding foundation wall. They are held in place with fasteners at regular intervals, without adhesives or primers as required with other waterproofing membranes. Volclay products have the additional benefit of being self seaming when overlapped. However the performance of the products depends on their compression or confinement against the wall to which they have been fastened. A lack of confinement can cause two problems (Figure 1). First, the overlapped seams of Volclay’s products will not be squeezed shut, allowing water to short-circuit the membrane. Second, the membrane will not be in “intimate contact” with the concrete wall, allowing water which has entered through the overlap to travel across a broad area of the wall in search of an entry point to the building interior. The result of these problems is leakage, which is often difficult and expensive to remedy.

To put it simply, inadequate confinement will cause leakage problems.

If, however, the waterproofing system is properly confined, the overlaps of the membrane will self-seal, and the membrane itself will be in intimate contact with the concrete. The system will function as intended. But how is confinement obtained in the field? And how can the quality of confinement be evaluated to ensure proper performance? Compaction of the soil backfill is the answer to these questions.

 

Confinement Through Compaction.
Compaction is the densification of soils by the application of mechanical energy. It may also involve modification of the water content of the soil. Compaction can be achieved by the use of hand-operated vibrating plates, motorized rollers, rubber-tyred equipment and free falling weights.

When soil backfill is properly compacted against a Volclay waterproofing membrane, confinement occurs. Void spaces in the backfill soil settlement are removed, and the potential for settlement is greatly reduced. When (or if) the swellable component of the membrane is hydrated, the resulting swelling pressure reacts against the compacted soil layer and forces the membrane against the wall. Intimate contact is achieved and the potential for lateral flow is eliminated.

 

What is 85% Modified Proctor Density?
Basic soil compaction theory was developed by R.R. Proctor in the 1930’s, and the standard laboratory test which he also developed is still referred to as the “Proctor Test”. In this procedure, a hammer of a certain weight is dropped from a specified height for a specified number of times onto a soil sample to simulate compaction. The test is repeated at various soil moisture contents, and the density of the sample (weight per unit volume) is measured after each test. ASTM D 698 is often called the “Standard Proctor Test” as it was the first such test to be developed and was widely used for decades. ASTM D 1557 is often called the “Modified Proctor Test” because the weight of the hammer was modified (increased) relative to the older method to allow more aggressive compaction, which is believed to be more representative of the capabilities of modern compaction equipment. The modified Proctor test is now the most common means by which soil compaction is specified.

Density testing results are plotted graphically to create a compaction curve as shown in Figure 2. In almost all cases, a distinct peak in compacted density will occur at a specific moisture content, identified as the “optimum” moisture content or OMC. All other density values can be described as a percentage of this peak value. For example, an engineer may specify that a foundation soil shall be compacted to 95% of its peak density value, or “95% of Modified Proctor density”. Referring to Figure 2, this value would be 95% of 117 lbs/ft³ (1,874 kg/m³) or 111 lbs/ft³ (1,778 kg/m³). Volclay’s standard recommendation is for compaction to 85% modified Proctor density, meaning that the soil backfill must have an in-place density that is 85% of its maximum density as determined by ASTM D 1557.

Figure 2. Compaction curves showing the density difference between the “standard” and “modified” test methods.

It should be noted that 85% compaction is NOT an especially demanding requirement. Foundation soils are routinely specified to be compacted to at least 95% of their modified Proctor density, and rarely, if ever, is a density requirement specified at less than 90%. Allco’s 85% requirement is readily achievable with modest compactive effort.

 

The Value of Allco’s Compaction Requirement.
Including this compaction statement in Volclay’s installation guide requires the contractor to actually make an effort to compact the backfill rather than simply dropping it in place from the top of the excavation. It also requires the contractor to use soils that are capable of being compacted, as well as free from rubbish and debris that can damage the waterproofing membrane or promote void formation. Finally, it requires that there is a third-party means by which the backfill placement and compaction effort can be measured. All of these restrictions are essentially “embedded” into the general requirement for 85% modified Proctor density.

At the end of the project, it is hoped that compliance with this requirements will result in backfill that is uniform in consistency, placed in even lifts all the way to the top of the foundation wall, and compacted to the extent needed to engage the waterproofing membrane into “intimate contact” against the wall, without void spaces. To the best of our knowledge, the current compaction requirement addresses all of these issues in one fairly simple statement.

 

Alternative Guidelines for Backfill Placement.
It is recognised that, in some cases, and especially in residential projects, that it may not be possible to verify whether the compaction requirement has been achieved. Whether this is because soils data is absent or for some other reason, not every project will undergo the level of scrutiny that Allco prefers in terms of backfill placement. In such cases, the following guidelines should be applied:

1)

Backfill soils should be clean compactable, free of rubbish, construction debris, and other materials exceeding one inch (25mm) in diameter and/or which cause puncture or damage to the waterproofing membrane during placement and compaction. Third-party inspectors should verify the acceptability of the proposed soil backfill prior to work; Contact a Volclay Approved Applicator or Allco Agencies Ltd.

2)

Backfill should be placed in lifts not to exceed 12 inches (300mm) in thickness. Each lift should be compacted with a walk-behind vibratory compactor or some other mechanical soil compaction device to the extent that foot traffic on the lift does not leave indentations greater than one-half inch (12mm). Care should be taken to ensure that the compactor does not touch the waterproofing membrane. The width of the compacted soil should be at least enough to support the compaction equipment. The compacted backfill should extend to the wall of excavated soil to create a continuous soil zone outward from the membrane.
If aggregate has been specified:
If coarse aggregate backfill is used (over 25mm), a protection layer must be applied over the membrane. Aggregate backfill must be compactable and must have a maximum grain size less than three quarters of an inch (20mm). It must be poorly graded such that fines are present to infill voids between the larger particles to ensure uniform confinement. “Washed” aggregate such as pea gravel is not acceptable for this reason.

3)

Backfill soils can be pushed into the excavation one lift at a time, provided that care is taken not to disturb the waterproofing membrane. Only as much soil should be pushed as can be compacted in one lift. Use only soils that are capable of being compacted.

4)

Visual observation and photographs should be taken during the backfill process to ensure that Volclay’s recommendations are performed properly.

 

Conclusion.
Backfill placement is a critical step in the installation of certain of Volclay’s waterproofing membranes. Proper placement in accordance with Volclay’s 85% compaction rule, or in accordance with the procedures outlined earlier, will greatly increase the chances for a successful waterproofing project.

 

Note.
See Volclay waterproofing detail AQ200W for drainage detail example. Volclay Aquadrain may be used in place of aggregate drainage.