2025.9.17 Memo1 Geotech Report - Retaining Wall.pdf

Memo

TO: RS&H Architects-Engineers-Planners, Inc.

FROM: Terracon Consultants, Inc. (Texas Firm Registration No. F-3272)

Joshua C. Miles, P.E.

Daniel B. Mabirizi, E.I.T

Haijian Fan, Ph.D, P.E.

DATE: September 17, 2025

RE: NASA JSC EA Ph1a – Soil Parameters for Cast-In-Place Wall

Terracon Report No. 91235100.Memo1

This memorandum provides the design recommendations for the cast-in-place wall for the referenced project. The retaining wall is part of NASA JSC EA Ph1a project. External stability analysis and global stability analysis are beyond the scope of the letter.

Wall Layout and Geometries

According to the site plan (Sheet C-70-9, dated July 29, 2025) issued by RS&H, the proposed retaining wall is on the east side of Building 70. The wall is approximately 400 ft long. The top of wall is at Elev. +20.75 while the top of footing in the range from Elev. +13.15 to Elev.

+14.1, implying that the maximum wall height should be approximately 8 feet. About 8 feet behind the wall is Building 70. It is relatively flat behind the wall.

Recommended soil parameters to be used in design of the proposed retaining wall are as follows:

Shallow Footings – Design Parameters

Shadow spread/strip footings may be utilized to support the proposed retaining wall planned at this site. The following design parameters are applicable for spread footings.

Item Description

Minimum Embedment Depth 1 2 feet below the finish grade

Allowable Bearing Pressure 2 1,500 psf

Approximate Post-Construction

Settlement 3 Approximately one inch or less

Estimated Post-Construction

Differential Settlement 4

Approximately ½ of post-construction settlement

Allowable Frictional Resistance 5 150 psf allowable cohesion (native clays)

1. The footings should extend through the fill soils and bear upon the native undisturbed clay soils.

Memo for Cast-in-Place Wall

NASA JSC EA Ph1a | League City, Texas

September 17, 2025 | Terracon Project No. 91235100.Memo1

Item Description

2. This pressure assumes that any soft/weak soils, existing unsuitable materials, if encountered, will be undercut and replaced with properly placed and compacted select fill or lean concrete.

3. The estimated post-construction settlement of the shallow footings is based on proper construction practices being followed.

4. The post-construction differential settlements may result from variances in subsurface conditions, loading conditions, and construction procedures. The settlement response of the footings will be more dependent upon the quality of construction than upon the response of the subgrade to the foundation loads.

5. To be utilized on the base of the footings. For clay soils, lateral resistance using cohesion should not exceed ½ the dead load.

Shallow Footings – Construction Considerations

Excavation for the shallow foundations should be performed with equipment capable of providing a relatively clean bearing area. The bottom 6 inches of the excavations should be performed using a smooth-mouthed excavation bucket or by hand labor. The excavations should be neatly excavated and properly formed. Disturbance of the bearing area of the foundations should be minimized during the excavation operations. Soft zones observed during construction should be over-excavated to a firm and undisturbed soil layer and all loose materials in the excavation bottom should be removed before placement of concrete. Water should not be allowed to accumulate at the bottom of the foundation excavations. To reduce the potential for groundwater seepage into the excavations and to minimize disturbance to the bearing area, we recommend that steel and concrete be placed as soon as possible after the excavations are completed and properly cleaned. Excavations should not be left open for more than 24 hours. The bearing surface of the foundations should be evaluated immediately prior to placing concrete.

A thin seal slab (approximately 2 to 4 inches thick) should be placed at the bottom of the footing excavation to protect the bearing surface of the footing from disturbance if the footing cannot be poured within 24 hours following excavation.

Lateral Earth Pressures

Structures with unbalanced backfill levels on opposite sides should be designed for earth pressures at least equal to values indicated in the following table. Earth pressures will be influenced by structural design of the walls, conditions of wall restraint, methods of construction, and/or compaction and the strength of the materials being restrained. Two wall restraint conditions are shown in the diagram below. Active earth pressure is commonly used for design of free-standing cantilever retaining walls and assumes wall movement. The at-rest condition assumes no wall movement and is commonly used for below-grade walls or other walls restrained at the top. The recommended earth pressure coefficients do not include a factor of safety.

Lateral Earth Pressure Coefficients

Earth Pressure Condition 1 Coefficient for Backfill Type 2, 3

Active (Ka) Fine Grained – 0.41 (onsite clays)

At-Rest (Ko) Fine Grained – 0.74 (onsite clays)

1. For active earth pressure, wall must rotate about base, with top lateral movements 0.002 H to

0.004 H, where H is wall height. For passive earth pressure, wall must move horizontally to mobilize resistance. Fat clay or other expansive soils should not be used as backfill behind the wall.

2. Uniform, horizontal backfill, with a maximum unit weight of 120 pcf for cohesive soils and compacted to at least 95 percent standard Proctor (ASTM D698) maximum dry density.

3. It has been assumed in the evaluation of the coefficient of earth pressure that the ground surface behind the wall is level. The coefficient of earth pressure could be higher when there is a backslope. In that scenario, Terracon should be contacted to review the wall design and the coefficients of earth pressure.

Backfill placed against structures should consist of granular soils or low plasticity cohesive soils

(with plasticity index of 30 or less).

Other loads bearing on backfill behind walls may have a significant influence on the lateral earth pressure. Placing footings within wall backfill and in the zone of active soil influence on the wall should be avoided unless structural analyses indicate the wall can safely withstand the increased pressure.

The lateral earth pressure recommendations given in this section are applicable to the design of rigid retaining walls subject to slight rotation, such as cantilever, or gravity type concrete walls. These recommendations are not applicable to the design of modular block - geogrid reinforced backfill walls (also termed MSE walls). Recommendations covering these types of wall systems are beyond the scope of services for this assignment.

Retaining Wall Backfill

Backfill should be compacted to at least 95 percent of the Standard Effort (ASTM D698) maximum dry density. Backfill within 5 feet of the below grade walls should be compacted with hand tampers or small equipment to reduce horizontal pressure on the wall. To achieve 95 percent of the maximum dry density with hand or small equipment in this area, placement of the fill in 4-inch lifts may be necessary. The fill should be placed on prepared surfaces in lifts not to exceed 8 inches loose measure, with compacted thicknesses not to exceed 6 inches.

The on-site clay soils should be moisture-conditioned to between the optimum and +4 percent above the optimum moisture content. Imported clay soils with plasticity index less than 30 and sand backfill should be moisture-adjusted to within 2 percent of the optimum moisture content. Prior to any filling operations, soil samples should be obtained for moisture-density testing. A qualified technician should perform in-place density tests to verify compaction levels.

Subsurface Drainage for Below-Grade Walls

A perforated rigid plastic drain line installed behind the base of walls and extends below adjacent grade is recommended to prevent hydrostatic loading on the walls. The invert of a drain line around a below-grade building area or exterior retaining wall should be placed near foundation bearing level. The drain line should be sloped to provide positive gravity drainage to daylight or to a sump pit and pump. The drain line should be surrounded by clean, free-draining granular material having less than 5% passing the No. 200 sieve, such as TxDOT 247 aggregate. The free-draining aggregate should be encapsulated in filter fabric. The granular fill should extend to within 2 feet of final grade, where it should be capped with compacted cohesive fill to reduce infiltration of surface water into the drain system.

In closing, we believe this information is sufficient for you to design the proposed wall. Feel free to contact us if you have any questions regarding the information in this letter.

Sincerely, Terracon Consultants, Inc.

Texas Firm Registration No.: F-3272

Daniel B. Mabirizi, E.I.T. Haijian Fan, Ph.D, P.E.

Project Manager Senior Engineer

Joshua C. Miles, P.E.

Senior Engineer