§ 86-12. Subgrade design requirements.

The following shall be requirements of subgrade design:

(1)

Descriptions.

a.

The subgrade shall consist of the top portion of the finished grade for pavement and/or curb and gutter construction. Subgrade preparation will be required for that portion of subgrade under pavements and one foot outside the back of curbs.

b.

Subgrade soils shall be classified for this purpose under three classifications:

1.

P.I. of nine or less;

2.

P.I. of more than nine but less than 20; and

3.

P.I. of 20 and above.

The plasticity index (P.I.) shall be determined by an independent laboratory acceptable to the City of Haltom City.

(2)

Subgrade preparation for soils with a P.I. of nine or less.

a.

Wherever the preexisting subgrade soil has a P.I. of nine or less, the subgrade shall be at least six inches in depth.

b.

When excavation or fill has been constructed to subgrade elevation, the city will have the laboratory take samples and determine the characteristics of the soil and the density requirements at optimum moisture.

c.

The subgrade shall then be scarified and wetted to optimum moisture (-1% to +2%). The moisture shall be thoroughly mixed into the subgrade and approved by the engineer. A stabilizing agent approved by the engineer shall also be mixed into the subgrade.

d.

The contractor shall then proceed to compact the subgrade to a value of 95 percent of standard proctor.

e.

When density tests have been made and it is determined that the subgrade is to the proper required density, the contractor will smooth the subgrade to obtain smooth densely compacted finish at grade ( 0.10 feet).

f.

The contractor may now proceed to set forms for either concrete pavement or curb and gutter, whichever has been previously agreed upon with the city.

(3)

Subgrade preparation for active soils.

a.

Soil P.I. of more than nine but less than 20. Wherever the preexisting subgrade soil has a P.I. of more than nine, but less than 20, the subgrade shall be prepared by mixing thoroughly into the top eight inches of soil one of the following stabilizations to the degree indicated:

1.

Lime—Six percent by weight.

2.

Portland cement—Four percent by weight.

b.

Soil P.I. of 20 or more. Wherever the preexisting subgrade soil has a P.I. of 20 or more, the subgrade shall be prepared by mixing thoroughly into the top eight inches of soil one of the following stabilizations to the degree indicated:

1.

Lime—Six percent by weight.

2.

Portland cement—Eight percent by weight.

c.

Preparation and standards.

1.

Hydrated lime and lime slurry where allowed shall meet the requirements of TxDOT specification no. item 264. Portland cement shall meet the requirements of TxDOT specification no. item 272. In all cases when lime is used as the subgrade stabilizing agent, it shall be applied at a minimum of ten lbs. per square yard for each two inches depth of subgrade; i.e., a subgrade depth of eight inches will require, as a minimum, 40 lbs. per square yard of lime.

2.

When the laboratory has decided the optimum percentage of lime or Portland cement, the contractor shall scarify the subgrade and spread the required amount of lime or Portland cement thereon. The contractor shall then thoroughly mix and blade the lime or Portland cement uniformly throughout the subgrade with the addition of water to ensure thorough mixing. Next, the contractor shall begin mixing operations until the lime or Portland cement is thoroughly incorporated in the subgrade, water being added as required.

3.

The subgrade thus prepared shall be left to cure a minimum of three days for lime and seven days for Portland cement while moisture is being added as needed and as directed. After the minimum curing period, the contractor will be required to remix the lime stabilized subgrade soil again until it is thoroughly fluffed and of uniform consistency.

4.

After the subgrade has been left to cure and has been remixed, the contractor shall compact the subgrade to a value of 95 percent standard proctor density at optimum moisture (-1% to +2%).

5.

When the subgrade has been thoroughly stabilized and compacted, the finished subgrade shall be carefully bladed to within one-quarter inch of finished grade and compacted and rolled and approved by the engineer.

6.

The subgrade now having been prepared, the contractor may proceed with setting the pavement or curb and gutter forms.

(4)

Minimum depth of subgrade. The subgrade shall always, at a minimum, equal the same thickness as the pavement section above it.

(Code 1973, § 18-17; Ord. No. O-50-94, § 1, 1-9-95)