User:Cmp fuller/Controlled Modulus Columns

Controlled Modulus Columns
Controlled Modulus Columns (CMCs), a patented ground improvement technology, are pressure grouted auger displacement elements that are installed using a specially designed tool at the working end of a high torque, high down-pressure drilling machine.

General Description of the CMC Technology Some amount of settlement occurs when any structure is built and put into service. The settlement is only problematic when it exceeds the tolerance for the structure. The amount that a structure will settle is dependent on the loads and the soil properties (both elastic stiffness and long term settlement parameters) of the underlying soils. The amount that a structure settles is not always uniform for the entire structure as the substrata may vary significantly.

In addition to excessive settlement, bearing capacity failure may occur where weak soils are present. This occurs when the underlying soils cannot carry the load from the structure and the ground shears or ruptures, causing either rotation or punching. Although bearing capacity problems are less common than settlement problems, when bearing capacity failures occur, they can be catastrophic in terms of the performance and servicability of the structure.

Deep foundations are rigid structural elements that are used to transfer the load from the structure to competent layers below (bedrock, dense or stiff soils) by “bridging” the compressible soft soils. Because the loads are highly concentrated on the elements, the elements need to have direct contact with the structure to be able to transmit these loads either through end-bearing, skin-friction, or a combination of the two. Deep foundation systems are designed to allow minimal settlement and when used to support structures, they require the use of a structural mat, or pile caps to connect the tops of the piles together to provide a foundation for the structure. The structural mats and pile caps add significant costs to the project.

While more “deformable” ground improvement solutions (e.g., stone columns or aggregate piers) are often very economical compared to a deep foundation system, the expected and observed settlements are typically greater than that of rigid deep foundations. In the case of stone columns for example, the ratio of stiffness between the soil and the stone column determines the ratio of load shared between the soil and the element. Settlement is a factor of the stress carried by the stone columns and the soil and the respective compressibility of column material and the surrounding soil. Stone column design is based on the assumption that the column and the surrounding soil are compressed, or settle, equal amounts. When ground improvement is used, the load concentration in each element is significantly reduced as compared to a deep foundation system, and the structure need not be as rigid. With ground improvement, structures can be designed as if it they are founded on competent ground with a slab-on-grade and spread and strip footings.

Controlled Modulus Column™ (CMC) technology bridges the gap between these two different approaches (deep foundations and deformable inclusions) by reducing the global deformability of a soil mass using semi-rigid soil reinforcement columns. The soil–CMC mass behaves as a composite mass of greater stiffness than the initial untreated ground, reducing settlements induced by the weight of the structure to within allowable ranges. CMCs are not intended to directly support the loads imposed by the structure, but to improve the global response of the soil in order to control settlement. The dimensions, spacing, and composition of the CMCs are based upon the development of an optimal combination of support from the columns and the surrounding soil to limit settlements for the project within the allowable range, and to obtain the required value for the equivalent composite deformation modulus of the improved soil.

Some features of the CMC technology include: Material is grouted in place with the use of a displacement auger in order to reinforce the ground; Deformation modulus of the CMC elements is 50 – 3,000 times that of the soil (weakest stratum); A load transfer platform of generally granular fill (LTP) is placed over the CMC reinforced ground that has a modulus less than that of the CMC elements which can be partially penetrated by the inclusions to promote strain compatibility/ load sharing between all the components; In granular soils, densification due to the lateral displacement may occur between the columns by virtue of the displacement drilling process; Virtually no spoils are generated by the drilling process which eliminates the need to manage spoils and the potential unearthing of contaminated soils.

The tops of CMCs are typically installed 1 to 3 feet below the bottoms of the shallow foundations. A layer of compacted granular material referred to as the load transfer platform (LTP) is installed above the top of the CMCs and below the structure following installation of the CMCs. The main purpose of the LTP is to transfer the load from the structure to the CMCs without using pile caps between the structure bottom and the CMCs. The load is transferred to the CMCs through arching within the high phi-angle LTP and through side friction below the top of the CMCs. The system is generally designed to transfer 50 to 95% of the load to the CMCs while the remainder of the load is transmitted to the soils between the CMCs. The ratio of load sharing is dependent upon the type and stiffness of the soils between the CMCs as well as the allowable settlement for the structure.

The CMC technology is very well suited for very soft soil conditions such as organic clays, peat and wastes. Compared to stone columns that require a significant degree of lateral confinement to avoid bulging when loaded, CMCs have no such limitations due to the relatively high stiffness of the column material.

The installation of CMCs does not generate vibrations so the technology is ideally suited for construction in urban areas (working close to sensitive structures). CMCs are commonly used to support structures such as storage tanks, buildings, warehouses, industrial facilities, culverts and pipes, as well as platforms, embankments, retaining walls, and bridge abutments. The CMC columns typically range from 12 to 18 inches in diameter. Installation of CMCs CMCs are installed using a specially designed displacement auger that displaces the soil laterally without generating spoils or creating vibrations. The displacement auger is hollow, which allows for continuous placement of the grout as the auger is withdrawn. The grout for the CMC element is placed with enough back pressure to avoid collapse of the displaced soils during auger withdrawal (typically the static head of grout plus less than 100 psi is necessary). The installation process allows for the creation of a column with the diameter that is at least as large as that of the auger. CMCs are installed with drilling equipment that has large torque capacity and high static down thrust. Upon reaching the desired depth, grout is pumped through the hollow stem of the auger and into the soil bore as the auger is withdrawn at a pre-defined rate that is calibrated to avoid necking.

With a conventional continuous flight auger, “negative displacement”, stress relief, or even lateral mining around the auger is inevitable. This creates a movement of the surrounding soils which are loosened by the augering process toward an active (Ka) condition. This condition creates a risk of necking. On the contrary, with the CMC displacement auger, the effect is opposite: the soil adjacent to the auger is displaced laterally by the displacement stem portion of the auger and brought to a denser passive (Kp) state of stresses. Stress relief does not occur and the risks of necking the CMC are quasi-inexistent, except with operator error. Quality control of the CMC and monitoring to catch any operator error is done with real time monitoring of the following installation parameters: speed of rotation; rate of advancement and withdrawal of the auger; torque, down-thrust (crowd) during the drilling phase; depth of element; time of installation; grout pressure in the line at the top of the drill string; volume of grout as a function of depth.

The grout pressure is monitored by a sensor located at the top of the concrete line above the swivel attached to the mast drilling head. The CMCs are usually installed using a target overbreak of 5 to 10% of the volume of grout. During the grout phase, pressure readings are kept to a moderate positive pressure. Any loss in pressure can reveal a soft or loose soil zone that may not have been detected during the geotechnical investigation.

A significant benefit of the recordation of installation parameters is that changes in subsurface conditions can be detected in the field, and more importantly, column depths can be adjusted based on the encountered conditions as detected by the response of the drilling equipment. The recorded drilling parameters of down pressure, speed and torque are readily interpreted in the field during drilling and changes in stratigraphy can be sensed based on ease or difficulty of drilling. This ability to adjust column lengths in the field offers a significant advantage over most other forms of column installation.

Other forms of QC include monitoring fluid grout properties for consistency with the expectations of the design mix, and sampling, curing and testing of samples for grout strength. Load testing (ASTM D1143) is routinely done when there is no previous experience with elements capacities in the subject strata. Other in-situ testing such as PIT (Pile Integrity Tests), dynamic loads tests (ASTM D4945) or rapid load tests (ASTM D7383) have also been used.

The CMC Design Concept The behavior of an individual inclusion is predicated on reaching equilibrium under loads (Combarieu, 1988). While the inclusion is being compressed by the load, negative skin friction is acting in its upper part and positive skin friction in its lower part. When the equilibrium is reached, the stresses acting on the inclusion can be divided into four components: 1) The vertical load, Q at the top of the inclusion; 2) The negative skin friction acting on the upper portion of the inclusion; 3) The positive skin friction acting at the lower portion; and, 4) The vertical reaction at the tip. The load of the structure is usually distributed to a network of inclusions by the LTP. The load distribution between CMCs and surrounding soil is based on reaching an equilibrium between deformations of the CMCs and the surrounding soils. The design of a network of inclusions is thus based on a good knowledge of the distribution of stresses and deformations in the soil and the inclusions.

While calculation methods have been proposed by various authors (see Combarieu), with the development of more powerful computers, finite element method (FEM) analysis has quickly become the method of choice when designing a network of CMCs.